PERGAMON MATERIALS SERIES VOLUME 6
Multinuclear Solid-State NMR of Inorganic Materials
PERGAMON MATERIALS SERIES Series Editor: Robert W. Cahn VRS Department of Materials Science and Metallurgy, University of Cambridge, Cambridge, UK
Vol. 1 Vol. 2 Vol. 3 Vol. 4 Vol. 5 Vol. 6
C ALPHAD by N. Saunders and A. P. Miodownik Non-Equilibrium Processing of Materials edited by C. Suryanarayana Wettability at High Temperatures by N. Eustathopoulos, M. G. Nicholas and B. Drevet Structural Biological Materials edited by M. Elices The Coming of Materials Science by R. W. Cahn Multinuclear Solid-State NMR of Inorganic Materials by K. J. D. MacKenzie and M. E. Smith
A selection offorthcoming titles in this series: Underneath the Bragg Peaks: Structural Analysis of Complex Materials by T. Egami and S. L. J. B illinge Phase Transformations in Titanium- and Zirconium-Based Alloys by S. Banerjee and P. Mukhopadhyay Thermally Activated Mechanisms in Crystal Plasticity by D. Caillard and J.-L. Martin Nucleation by A. L. Greer and K. F. Kelton Non-Equilibrium Solidification of Metastable Materials from Undercooled Melts by D. M. Herlach and B. Wei The Local Chemical Analysis of Materials by J. W. Martin Synthesis of Metal Extractants by C. K. Gupta Structure of Materials by T. B. Massalski and D. E. Laughlin Intermetallic Chemistry by R. Ferro and A. Saccone
P E R G A M O N MATERIALS SERIES
Multinuclear Solid-State NMR of Inorganic Materials by Kenneth J.D. MacKenzie School of Chemical and Physical Sciences, Victoria University of Wellington, and N e w Zealand Institute for Industrial Research and D e v e l o p m e n t
Mark E. Smith Department of Physics, University of Warwick, U K
2002
PERGAMON An Imprint of E l s e v i e r S c i e n c e A m s t e r d a m - Boston - London - New Y o r k - Oxford - P a r i s S a n D i e g o - S a n F r a n c i s c o - Singapore - Sydney - Tokyo
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Preface Although the concept of solid state nuclear magnetic resonance with magic angle spinning (MAS NMR) has been with us for many years, it is only over the last two decades that the technique has become increasingly used in other disciplines. The advantages of being able to probe the atomic environment of poorly crystalline materials were quickly appreciated by mineralogists and zeolite chemists, who exploited the fact that the two nuclides of major interest to mineral studies (29Si and 27A1) are readily amenable to MAS NMR study. Other subject areas, including ceramics, glasses and cements, have now followed this lead, and today MAS NMR is being applied to most of the major topics in the area of inorganic materials research. In the early stages of its development, MAS NMR tended to be the preserve of specialist spectroscopists and theoreticians, but, as its possibilities have become better known, it has become very much a technique used by engineers and scientists to solve real problems. The literature, which was once found mainly in fundamental and highly technical texts, is now just as likely to be found in applied science journals, as increasing numbers of non-specialist researchers turn to MAS NMR to solve problems in real and often non-ideal systems. Arising from questions directed to us by students and colleagues wishing to use MAS NMR in just such studies, especially of inorganic materials, for some time we have been conscious of the need for a book addressed to non-specialist researchers. Ideally, such a book would provide accessible answers to the most common questions about the theory and practice of MAS NMR asked by novices, and it should also provide a more specialised and up-to-date treatment of the most important areas of inorganic materials research to which MAS NMR has application. Such a book would bring together all the theory necessary to appreciate the scope (and limitations) of the technique, together with practical details and hints which are often not published elsewhere because they seem too self-evident and mundane to established practitioners. In short, the book we have envisioned is precisely that which we would have found invaluable when we first began to use MAS NMR to study inorganic materials, and into which we would continue to delve for background information as the need arose to branch out into the study of new systems. This, then, is the philosophy behind the present book, which we hope above all will prove useful to MAS NMR users whatever their level of expertise, and whatever inorganic materials they wish to study. MAS NMR is constantly being extended to a more diverse range of materials, pressing into service an ever-expanding range of nuclides including some previously considered too intractable to provide usable results. At the
vi
Preface
same time, new developments in both hardware and software are being introduced and refined. In this book we have tried to cover the most important of these new developments without spreading ourselves too thinly over the subject matter. For this reason, the book does not include any of the extensive literature on carbon-based polymers which is already catered for by comprehensive texts. Materials science is very much an interdisciplinary exercise, and its practitioners are likely to come from a variety of scientific backgrounds. It is expected that the readers of this book will be at least slightly acquainted with some aspects of physics, chemistry, mineralogy or engineering, and some may be deeply knowledgeable in one or more aspects of these subjects. Because of the diverse nature and range of inorganic materials which have been subjected to NMR investigation (from cements, catalysts and glasses to superconductors and metals), it is beyond the scope of this book to treat in depth the most advanced chemical and physical aspects of the various materials. We have therefore confined ourselves to discussion and illustration of how solid state NMR techniques have been applied to a wide range of systems, and have provided an extensive bibliography to allow a reader with particular interests a convenient entry into the literature to gain more advanced information on any specific topic. The plan of the book is straightforward; the second chapter lays the theoretical groundwork for an understanding of the NMR experiment, including the factors which must be taken into account both when establishing the appropriate experimental conditions and when interpreting the data. Particular attention has been given to the theoretical basis of new experimental techniques such as multiple-quantum MAS NMR, which holds considerable promise for a wide range of inorganic solids. The third chapter covers the practical aspects of solid state NMR spectroscopy, while in the following chapters, the foregoing principles are applied to specific nuclides, with particular emphasis on applications of the technique to solving problems in inorganic materials science. A range of illustrative examples has been included, together with tabular data summarising the present state of the literature on the various nuclides in inorganic materials. Finally we should note that this book is possible only because of the ingenuity of the NMR researchers who have contributed to the rapid development of the subject. We are especially grateful to the many NMR colleagues who have shared their enthusiasm and expertise with us, and to members of our research groups both past and present, whose efforts have been very important. The copyright holders who gave their permission to use figures are gratefully acknowledged. MES is grateful for the support of funding bodies such as EPSRC, the Royal Society and the Leverhulme Trust, and particularly thanks Susan Holmes for her support and understanding during the preparation of this book. KM is indebted to the Royal Society of New Zealand for a James Cook Research Fellowship allowing two years to be spent in Oxford during which time the book became a reality, and to the Materials Department of Oxford
Preface
vii
University for being gracious hosts and providing the necessary facilities for this undertaking. We also wish to record our gratitude to the Series Editor, Prof. Robert Cahn, and the Editorial staff of Elsevier Science for their assistance in bringing this book to fruition. KENNETH MACKENZIE Email: kenneth.mackenzie@ vuw. ac.nz MARK SMITH Email: m.e.smith, [email protected] Oxford and Warwick, October 2001
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Contents Preface CHAPTER 1 INTRODUCTION 1.1. Methodology of Materials Characterisation by NMR 1.2. Historical Aspects of NMR Spectroscopy 1.3. Brief Description of the NMR Experiment 1.3.1 General Principles 1.3.2 Overcoming NMR Spectral Broadening in Solids by MAS 1.3.3 Other NMR Experiments used with Solids 1.3.3.1 Decoupling 1.3.3.2 Cross-Polarisation (CP) 1.3.3.3 Spin-Echo Experiments 1.3.3.4 Two-Dimensional Experiments 1.3.4 Nuclei Suitable for NMR Spectroscopy 1.4. Further Reading References
3 6 7 7 10 11 12 12 12 12 13 17 18
CHAPTER 2 PHYSICAL BACKGROUND 2.1. Fundamental Interaction with External Magnetic Fields 2.1.1 A Quantum Mechanical Description of the Zeeman Interaction 2.1.2 Bulk Magnetisation 2.1.3 The Rotating Frame and the Application of RF Pulses 2.1.4 Observation of the NMR Signal 2.2. Internal Interactions 2.2.1 The Dipolar Interaction 2.2.2 Scalar Coupling 2.2.3 Paramagnetic Coupling 2.2.4 Chemical Shielding 2.2.5 Knight Shift 2.2.6 Quadrupole Interaction 2.2.7 Nature of Interactions
23 25 26 29 34 35 37 40 43 44 48 50 57
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2.3.
58 59 59 61 63 64
One Dimensional Methods for Improving Resolution 2.3.1 Magic Angle Spinning and First-Order Effects 2.3.1.1 Physical Principles 2.3.1.2 Formation of Spinning Sidebands 2.3.2 Magic Angle Spinning and Higher-Order Effects 2.3.2.1 MAS of Second-Order Quadrupole Effects 2.3.2.2 Residual Coupling Effects due to Quadrupolar Nuclei in MAS Spectra 2.3.2.3 Nonequivalent Homonuclear Spins 2.3.3 Variable Angle Spinning 2.3.4 Double Angle Spinning 2.3.5 Multiple Quantum Transitions 2.3.6 Ultrasonically-Induced Narrowing 2.4. Dipolar Decoupling 2.4.1 Heteronuclear Dipolar Decoupling 2.4.2 Homonuclear Dipolar Decoupling 2.5. Spin-locking 2.6. Cross-Polarisation 2.7. Two-Dimensional Methods 2.7.1 Dynamic Angle Spinning 2.7.2 2D MQMAS 2.8. NMR Relaxation 2.8.1 Introduction to Relaxation 2.8.2 Mechanism for Relaxation Processes References
71 74 74 75 77 78 78 78 79 83 85 90 92 93 98 98 101 105
CHAPTER 3 EXPERIMENTAL APPROACHES 3.1. Basic Experimental Principles of FT NMR 3.2. Instrumentation 3.2.1 Overview of a Pulsed FT NMR Spectrometer 3.2.2 Magnets 3.2.3 Shimming 3.2.4 Transmitters 3.2.5 Probes 3.2.6 Connection of the Probe 3.2.7 Signal Detection 3.2.8 Additional Equipment
111 112 112 113 115 116 120 122 124 127
Contents
3.3.
Practical Acquisition of NMR Spectra 3.3.1 Processing the FID to Produce a Spectrum 3.3.1.1 Window Functions 3.3.1.2 Shifting of the Time Origin and Linear Back Prediction 3.3.1.3 Zero Filling 3.3.1.4 Phase Correction 3.3.1.5 Baseline Correction 3.3.2 Complications in Recording Spectra 3.4. Static Broad Line Experiments 3.4.1 Pulsed Echo Experiments 3.4.2 Stepped Experiments 3.5. One-Dimensional High Resolution Techniques 3.5.1 Magic Angle Spinning (MAS) 3.5.2 Extraction of Parameters from MAS NMR Spectra 3.5.3 Suppression of Spinning Sidebands 3.5.4 Special Considerations for MAS of Quadrupolar Nuclei 3.5.5 Magic Angle Spinning Observation of Satellite Transitions 3.5.6 Double Angle Rotation of Quadrupolar Nuclei 3.5.7 Practical Implementation of CRAMPS 3.6. Two-Dimensional Experiments 3.6.1 Nutation NMR 3.6.2 Off-Resonance Nutation 3.6.3 Order-Resolved Sideband Spectra 3.6.4 Dynamic Angle Spinning (DAS) 3.6.5 Two-Dimensional Sequences Developed from Solution NMR 3.6.6 Multiple Quantum Experiments in Dipolar Coupled Systems 3.6.7 Multiple Quantum NMR Experiments of Non-Integer Spin Quadrupolar Nuclei 3.6.8 2D XY Correlation Methods 3.6.9 Correlation of Tensor Information- Separated Local Field Experiments 3.7. Summary of Approaches for Examining Quadrupole Nuclei 3.8. Multiple Resonance 3.8.1 Cross-Polarisation (CP) 3.8.2 SEDOR, REDOR and TEDOR 3.8.3 TRAPDOR and REAPDOR 3.9. Techniques for Determining Relaxation Times and Motional Parameters 3.9.1 Measurement of T1
xi 127 128 128 129 129 130 130 130 133 133 136 138 138 143 143 144 149 150 152 153 153 154 155 156 157 160 161 168 170 172 172 173 178 182 183 183
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3.9.2 Other Spin-Lattice Relaxation Times (TI~, T1D) 3.9.3 Transverse Relaxation Times (Y2) 3.9.4 Molecular Motion 3.9.5 Diffusion Measurements 3.10. NMR Under Varying Physical Conditions 3.10.1 Variable Temperature NMR 3.10.2 High Pressure Experiments References
CHAPTER 4 298I NMR 4.1. General Considerations 4.1.1 Broadening Effects in 298i Spectra 4.1.2 Relaxation Effects in 298i Spectra 4.1.3 Effect of Structure on 298i Spectra 4.2. Si-O Compounds 4.2.1 Relationships between 29Si NMR Spectra and Structure/Bonding 4.2.2 Four-Coordinated Si-O-Compounds 4.2.3 Tetrahedral 298i Chemical Shifts in Silicates 4.2.4 29Si Chemical Shifts in Aluminosilicates 4.2.5 Effects of Other Nearest Neighbours on the 298i Shift 4.3. Order-Disorder Effects in Minerals 4.4. Identification of Silicate Minerals 4.5. Thermal Decomposition of Silicate Minerals 4.6. Relationships between 298i Chemical Shift (d) and Structure 4.6.1 Relationships between 6 and the Si-O Bond Length 4.6.2 Relationships between 6 and the Si-O-Si Bond Angle 4.6.3 More Complex Relationships between 6 and the Structure 4.7. Five and Six-Coordinated Si-O Compounds 4.8. Cross-Polarisation (CPMAS) Experiments 4.8.1 Cross-Polarisation between ~H and 298i 4.8.2 Cross-Polarisation between 19F and 298i 4.8.3 Other Cross-Polarisation Experiments with 298i 4.9. Glasses, Gels and Other Amorphous Materials 4.9.1 Silicate Glasses 4.9.2 Deconvolution of 298i NMR Spectra 4.9.3 Connectivities in Glass 4.9.4 Chalcogenide Glasses
184 185 186 187 187 187 189 190
201 201 202 204 205 205 205 205 206 208 208 212 214 217 218 219 223 225 227 227 229 229 230 231 235 236 238
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4.9.5 Gels 4.9.6 Other Amorphous Materials 4.10. Si-N and Si-N-O Compounds 4.11. Si-A1-O-N Compounds 4.11.1 fl-Sialon, Si6_zAlzOzNs_z 4.11.2 O-Sialon, Si2_xAlxOl+xNz_x 4.11.3 X-Sialon, nominally Si12AllsO39Ns 4.11.4 Polytypoid Sialons, (Si,A1)m(O,N)m+l 4.11.5 oL-Sialons,MxSilz_(m+n)Alm+nOnN16-n 4.12. Other Metal Silicon Nitrides and Oxynitrides 4.13. Si-C, Si-C-O and Si-C-N Compounds 4.13.1 Silicon Oxycarbide Species 4.13.2 Silicon Carbonitride Species 4.14. Other Materials 4.14.1 Biologically Compatible Glasses 4.14.2 Cements 4.14.3 Inorganic Polymers References
240 242 244 247 247 25O 251 253 253 253 255 256 257 257 257 257 259 260
CHAPTER 5 27AL NMR 5.1. General Considerations 5.2. Chemical Shifts in 27A1 Spectra 5.2.1 27A1Chemical Shifts in A1-O Environments 5.2.2 27A1Chemical Shifts in Aluminosilicates 5.2.3 Relationships between 27A1 Chemical Shift (giso) and Structure 5.3. Five-Coordinated A1-O 5.3.1 A1(v) in Well-Defined (Crystalline) Environments 5.3.2 A1(v) in Non-Crystalline Environments 5.3.3 A1(v) in Zeolites 5.4. Aluminium Oxides 5.5. Amorphous Aluminium Compounds 5.5.1 Aluminate Gels 5.5.2 Glasses 5.5.3 Other Amorphous Systems 5.6. Aluminophosphates 5.7. Aluminium Borate and Molybdate 5.7.1 Aluminium Borate 5.7.2 Aluminium Molybdate
271 272 273 274 279 281 281 283 287 291 294 294 299 303 304 307 307 307
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Aluminium Fluorides 5.9. Thermal Decomposition Reactions 5.10. Cements 5.11. Nitride and Oxynitride Compounds 5.12. Sialon Compounds 5.12.1 Polytypoid Sialons 5.12.2 [3-Sialons 5.12.3 O-Sialons 5.12.4 X-Sialons 5.12.5 oL-Sialons 5.12.6 Sialon Glasses References
308 310 313 316 317 317 318 320 321 322 323 324
CHAPTER 6 170 NMR 6.1. Introduction 6.2. Background 6.2.1 Enrichment Schemes 6.2.2 Experimental NMR Methodology 6.2.3 Relationships between NMR Parameters and Structure 6.3. Binary Oxides 6.3.1 Crystalline Materials 6.3.2 Sol-Gel Produced Samples 6.4. Crystalline Ternary Ionic Systems 6.5. Silicates and Germanates 6.5.1 Crystalline Materials 6.5.1.1 Silica and Germania 6.5.1.2 Ternary Silicates 6.5.1.3 Silicates and Germanates of Zirconium and Titanium 6.5.2 Amorphous Materials 6.5.2.1 Silica and Germania 6.5.2.2 Metal Silicate and Germanate Glasses 6.5.2.3 Gel-Based Silicates 6.6. Aluminium- and Gallium-Containing Systems 6.6.1 Alumina and Aluminates 6.6.2 Crystalline Alumino- and Gallosilicates 6.6.3 Amorphous Aluminosilicates 6.7. Boron-Containing Systems 6.7.1 Borates
333 334 334 337 346 349 349 352 355 359 359 359 361 365 366 366 367 369 372 372 375 379 381 381
5.8.
Contents
6.7.2 Ternary and Quaternary Systems 6.8. Other Systems 6.9. Hydrogen-Containing Samples 6.9.1 Crystalline Hydroxides and Other Hydrogen-Containing Materials 6.9.2 Hydrous Gels and Glasses 6.10. High Temperature Ceramic Superconductors References
CHAPTER 7 NMR OF OTHER COMMONLY STUDIED NUCLEI 7.1. 23NaNMR 7.1.1 General Considerations 7.1.2 23NaNMR Spectra of Sodium Compounds 7.1.3 Relationships between the 23Na Chemical Shift and Structural Parameters 7.1.4 23NaNMR of Crystalline Materials 7.1.5 23NaNMR Studies of Thermal Reactions 7.1.6 23NaNMR of Glasses 7.1.6.1 Silicate and Aluminosilicate Glasses 7.1.6.2 Sodium Borosilicate Glasses 7.1.6.3 Sodium Borate, Germanate and Tellurite Glasses and Melts 7.1.6.4 Phosphate Glasses 7.1.6.5 Miscellaneous Glass Studies 7.1.7 23NaNMR of Zeolites 7.2. llBNMR 7.2.1 General Considerations 7.2.2 11B NMR of Crystalline Compounds 7.2.3 I~B NMR of Glasses 7.2.4 ~B NMR of Zeolites 7.3. 31PNMR 7.3.1 Relationships between 31p NMR Parameters and Structure 7.3.2 31p NMR of Glasses 7.3.2.1 Binary Phosphate Glasses 7.3.2.2 Phosphosilicate Glasses 7.3.2.3 Alkali Borophosphate Glasses 7.3.2.4 Borosilicophosphate Glasses 7.3.2.5 Phosphoaluminosilicate Glasses 7.3.2.6 Alkali Phosphoaluminoborosilicate Glasses
XV
382 384 386 386 387 388 390
399 399 399 403 406 412 413 413 414 415 415 416 418 420 420 421 424 431 432 438 441 441 443 445 445 446 447
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7.3.3 7.3.4 References
Contents
7.3.2.7 Phosphorus Chalcogenide Glasses 31p NMR of A1PO4 Molecular Sieves 31p NMR of Biomaterials
CHAPTER 8 NMR OF LOW-~/NUCLIDES 8.1. General Considerations 8.1.1 Problems Associated with Low-y Nuclei 8.2. NMR of Spin-1/2 Nuclei 8.2.1 89y NMR 8.2.2 l~ and l~ NMR 8.2.3 183WNMR 8.3. Quadrupolar Nuclei 8.3.1 laN NMR 8.3.2 25Mg NMR 8.3.3 33S NMR 8.3.4 35C1and 37C1 NMR 8.3.5 39K NMR 8.3.6 43CaNMR 8.3.7 47Ti and 49Ti NMR 8.3.8 67ZnNMR 8.3.9 91ZrNMR 8.3.10 95Mo and 97Mo NMR 8.3.11 135Baand 137Ba NMR 8.3.12 Other Miscellaneous Low-y Nuclei References
CHAPTER 9 NMR OF OTHER SPIN-l/2 NUCLEI 9.1. Introduction 9.2. Abundant High-~/Nuclei 9.2.1 ~H NMR 9.2.1.1 Background to Proton Studies in Inorganic Materials 9.2.1.2 Studies of Stoichiometric Protons in Crystalline Materials 9.2.1.3 Non-Stoichiometric Proton Environments in Crystalline and Glassy Materials
447 448 450 452
461 461 462 462 469 473 475 475 479 488 491 495 502 505 511 514 516 522 525 526
535 536 536 536 539 542
Contents
9.3.
9.2.1.4 1H NMR of Hydrous Glasses 9.2.1.5 Biomineral-Related Materials 9.2.2 19F NMR 9.2.2.1 Introduction 9.2.2.2 Simple Inorganic Fluorides 9.2.2.3 More Complex Fluorides 9.2.2.4 Applications to Fluoroapatite Studies 9.2.2.5 Fluorine in Aluminosilicate Minerals and Related Materials 9.2.2.6 Surface Interaction of Fluorine with Silica- and Alumina-Based Materials 9.2.2.7 Fluorine in Alumino- and Gallophosphates 9.2.2.8 Fluorine in Oxygen-Containing Glasses 9.2.2.9 Fluoride Glasses 9.2.2.10 Fluorine in Other Materials 9.2.2.11 Fluorine as a Source of Cross-Polarisation 9.2.2.12 Summary of 19F Shift Trends and Other NMR Properties Dilute or Medium-~/Nuclei 9.3.1 13C NMR 9.3.1.1 13C NMR of Elemental Carbon 9.3.1.2 Silicon Carbide 9.3.1.3 Other Binary Carbides 9.3.1.4 Ternary and Quaternary Carbides 9.3.1.5 Carbonates 9.3.2 15NNMR 9.3.2.1 Nitrides 9.3.2.2 Silicon Aluminium Oxynitride Ceramics and Glasses 9.3.2.3 Nitride Ceramics from Polymeric Precursors 9.3.2.4 Nitrates and Nitrites 9.3.3 77Se NMR 9.3.4 111Cd and 113CdNMR 9.3.5 ll5Sn, ll7Sn and ll9Sn NMR
9.3.6
9.3.5.1 Crystalline Oxygen-Containing Materials 9.3.5.2 Oxide Solid Solutions and Glasses 9.3.5.3 Non-oxide Materials 123Te and 125TeNMR 9.3.6.1 Crystalline Tellurides 9.3.6.2 Crystalline Tellurites and Tellurates 9.3.6.3 Glassy Tellurium-Containing Materials
xvii 545 550 550 550 551 554 555 556 557 559 559 560 562 562 562 563 563 563 568 570 572 572 574 575 576 579 582 583 587 591 591 594 595 598 598 599 601
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9.3.7 129Xe NMR 9.3.8 195pt NMR 9.3.9 199Hg NMR 9.3.10 2~ and 2~ NMR NMR 9.3.11 2~ 9.3.11.1 Correlations between 2~ Chemical Shifts and Structure 9.3.11.2 2~ NMR of Crystalline Lead Compounds 9.3.11.3 2~ NMR of Lead-Containing Glasses 9.3.11.4 2~ in Sol-Gel Prepared Ceramics References
CHAPTER 10 NMR OF OTHER QUADRUPOLAR NUCLEI 10.1. 6Li and 7Li NMR 10.1.1 General Considerations 10.1.2 6'7LiNMR of Crystalline Solids 10.1.3 Relation between 6Li Chemical Shifts and Structure 10.1.4 6'7LiNMR of Fast Lithium Ion Conductors 10.1.5 6'7LiNMR of Glasses 10.2. 9Be NMR 10.3. 5iV NMR 10.3.1 General Considerations 10.3.2 5iV NMR of Vanadium Oxides and the Vanadates 10.3.3 5~V NMR of Zeolites and Catalysts 10.4. 63CHand 65CHNMR 10.4.1 63CHNMR of Superconductors and Superfast Ionic Conductors 10.5.
10.6.
10.7. 10.8.
69Ga and 71Ga NMR 10.5.1 General Considerations 10.5.2 69'71GaNMR of Crystalline Compounds 10.5.3 69'71GaNMR of Other Compounds 87Rb NMR 10.6.1 General Considerations 10.6.2 87RbNMR of Crystalline Compounds 10.6.3 87RbNMR of Rubidium Fullerides 93Nb NMR 133Cs NMR 10.8.1 General Considerations
601 603 604 604 607 607 609 613 615 616
629 629 630 634 636 638 639 642 642 642 646 649 650 653 653 655 657 658 658 658 661 662 665 665
Contents
10.8.2 10.8.3 10.8.4
133CsNMR of Crystalline Caesium Compounds
133CsNMR of Minerals and Zeolites 133CsNMR of Fullerides, Superionic Conductors and Semiconductors 10.9. 139LaNMR References
xix 666
669 673 674 678
CHAPTER 11 SOLID STATE NMR OF METALS AND ALLOYS 11.1. Introduction 11.2. Experimental Approaches 11.3. Metallic Elements 11.4. Intermetallic Alloys 11.5. Phase Transformations, Ordering and Defect Sites 11.6. Phase Composition and Precipitation 11.7. Atomic Motion References
687 689 691 693 696 698 700 701
SUBJECT INDEX MINERAL INDEX
703 721
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Chapter 1
Introduction 1.1. Methodology of Materials Characterisation by NMR 1.2. Historical Aspects of NMR Spectroscopy 1.3. Brief Description of the NMR Experiment 1.3.1 General Principles 1.3.2 Overcoming NMR Spectral Broadening in Solids by MAS 1.3.3 Other NMR Experiments used with Solids 1.3.3.1 Decoupling 1.3.3.2 Cross-Polarisation (CP) 1.3.3.3 Spin-Echo Experiments 1.3.3.4 Two-Dimensional Experiments 1.3.4 Nuclei Suitable for NMR Spectroscopy 1.4. Further Reading References
3 6 7 7 10 11 12 12 12 12 13 17 18
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Chapter 1
Introduction Solid state NMR spectroscopy is a powerful technique capable of providing information both about the structure of materials and about the dynamics of processes occurring within those materials. The focus of this book is on the application of NMR spectroscopy to structural studies of materials rather than their dynamics. NMR spectroscopy has been applied to the study of a wide range of materials including polymers, organic compounds, organometallics and foodstuffs, but these are outside the scope of this book, which has been restricted to the range of diverse inorganic materials amenable to multinuclear NMR spectroscopy in its various forms.
1.1. METHODOLOGY OF MATERIALS CHARACTERISATION BY NMR
A number of experimental techniques exist for investigating the structures of materials (Figure 1.1). On the largest scale, materials are characterised by measurements of their bulk properties including density, surface area, chemical composition and thermal properties. At the next level, the macroscopic and mesoscopic properties can be probed by optical and electron microscopy, with small angle X-ray and neutron scattering (SAXS, SANS) also used to probe the mesoscopic properties. In the medium range of atomic ordering, structures can be probed by conventional X-ray, neutron or electron diffraction. Diffraction of amorphous materials yields the radial distribution function which is being increasingly used to characterise such materials. Structures are investigated on the atomic scale by vibrational spectroscopies (infrared, ultraviolet-visible, Raman), by X-ray absorption (EXAFS, XANES) or by the various multinuclear NMR techniques. Materials can exist in an almost infinite variety of states of structural disorder and heterogeneity (Figure 1.2). Unlike techniques such as X-ray diffraction which conventionally require the presence of long-range atomic order, NMR spectroscopy is almost equally useful for probing the atomic environments of the most disordered (melts and colloid gels) as for the most ordered single crystal systems. NMR is thus able to monitor the changes occurring in the atomic environment when materials change from one state of structural disorder or heterogeneity to another and has proved to be a powerful technique for studying the transition from glasses to melts, nucleation of polycrystals from glasses and the formation and growth of crystalline phases from colloids or gels. NMR is also an ideal method for studying the intermediate phases formed when minerals react or transform to other phases by heating or
Multinuclear Solid-State NMR of lnorganic Materials
Atomic scale probes
Bulk measurements
Multinuclear NMR:
MAS, CP, MQ
i
X-ray absorption: EXAFS, XANES
>
Density Surface area Chemical composition Thermal analysis
I
Vibrational spectroscopy: IR, UV-visible, Raman
Macroscopic and mesoscopic probes
Medium range probes X-ray diffraction Neutron diffraction Electron diffraction Radial distribution function
Electron microscopy Optical microscopy
Mesoscopic probes Small angle X-ray scattering Neutron scattering
Figure 1.1. Relationship between the various methods for investigating the structures of materials.
Melt
Colloid (Gel) /
I Glass transition
/
Agglomeration
O oN
Glass
Nanocrystal /
,4~
Nucleation ~
~//Crystal growth
Polycrystai
i / ~ Single Crystal
Composite
Structural heterogeneity Figure 1.2. Schematic relationship between disorder and heterogeneity of complex materials. mechanical grinding, which often lack the long-range order necessary for conventional diffraction studies. An important element in the use of NMR techniques to determine the structural characteristics of materials is to establish relationships between the experimental
Introduction
5
NMR spectrum and recurring structural motifs. In its simplest form this is done by a "fingerprinting" procedure in which an extensive database of NMR spectra is established for related materials of known composition and structure, and the characteristics of an unknown material are deduced by comparison, (shown schematically in Figure 1.3, pathway 3). The parameter most readily determined from an NMR spectrum is the position of the resonance peak, and although this may not necessarily represent the true value of the isotropic chemical shift ~iso (see below), empirical relationships have been developed for a limited range of similar compounds between the resonance position and some characteristic feature of the structural unit (e.g. the mean bond angle or bond length). To put these empirical relationships on a sounder theoretical basis and make them applicable to a wider range of compounds, other spectral parameters such as the nuclear quadrupolar coupling constant (• or the chemical shift anisotropy (CSA) may have to be invoked; these are deduced from a more in-depth analysis of the NMR spectrum, in some cases making measurements at more than one magnetic field and using specialised techniques to improve the resolution of the spectral lineshapes. Another approach to deducing the structures of materials from their solid state NMR spectra is to make an ab initio theoretical calculation of the electric field gradient at the nucleus of an atom in a known crystal structure environment (Figure 1.3, pathway 1), from which the nuclear quadrupolar parameters and hence the expected NMR spectrum can be calculated and compared with the experimental spectrum (Figure 1.3. pathway 2). The difficulties inherent in making such ab initio calculations are such that relatively few real compounds have so far been solved completely, but developments in ab initio theory, and mathematical algorithms and computing techniques should make this approach much more widely accessible in future.
I
' Sam le 'i P I Structure I
(~
~- Nuclear l, Spin / Hamiltoniau
.... J
] xperlmeut INMR ~ Spectrum J Figure1.3. Relationship between experimental and theoretical approaches for establishing the structural characteristics of materials by NMR spectroscopy.
Multinuclear Solid-State NMR of lnorganic Materials
1.2. HISTORICAL ASPECTS OF NMR SPECTROSCOPY This book is about an experimental technique which has been a mainstay of solution chemistry for many decades. Since very well resolved and structurally informative 13C NMR spectra can readily be obtained from solutions at modest magnetic field strengths, organic chemists have traditionally been the principal beneficiaries. In earlier times inorganic chemists attempting to apply the technique to solid samples did not fare well, as these spectra were too broad to provide much useful information. Furthermore, the interesting nuclei are often quadrupolar (e.g. 27A1), ideally requiring much higher magnetic fields. The need for higher field strengths was eventually satisfied by the development of superconducting magnets, the field of which could not, however, readily be varied. Thus the traditional technique of irradiating the sample with a continuous source of radiofrequency radiation while scanning over a range of the magnetic field had to be modified; the field was now kept constant and the radiofrequency was applied as a sequence of pulses. An important breakthrough in NMR spectroscopy of solids came when spectroscopists in the UK (Andrew et al. 1959) and America (Lowe 1959) deduced that some of the factors causing broadening in these materials could be minimised by rapidly rotating the sample at a particular angle to the magnetic field axis (the magic angle, 54.73~ However there were still difficulties associated with observing materials of interest such as ~3C in organic polymers as the sensitvity of ~3C was low and the relaxation time was long. In addition the ~H-~3C dipolar coupling was too strong to be narrowed by MAS at the spinning speeds then available. A number of other experimental breakthroughs were necessary. The concept of polarisation transfer from an abundant to an insensitive nucleus was demonstrated by Hartman and Hahn (1962). This was combined successfully with decoupling for ~H-13C (Pines, Gibby and Waugh 1973). The final piece of the jigsaw was to combine cross-polarisation, decoupling and MAS (Schaefer and Stejskal 1976) to reveal the fine structure in ~3C NMR spectra of solid materials. These developments opened the way for commmercial probe development to allow usefully narrow solid state NMR spectra to be obtained from nuclei such as 298i and 27A1, a fact rapidly exploited by mineralogists and materials scientists working on zeolite catalysts. The study of the latter received considerable impetus and funding from the oil crisis of the 1970s, resulting in a great deal of detailed work being done to establish practical relationships between the structural units of the silicates and zeolites and their 298i NMR spectra (summarised in Engelhardt and Michel 1987). Although 298i NMR has historically received the greatest amount of attention for solid state NMR of inorganic materials because of its ubiquity and its technical importance in materials science, other nuclei (especially quadrupolar nuclei such as 27A1, 23Na and ~B) have become increasingly accessible with the availability of higher magnetic fields, faster spinning speeds and new methodologies for improving resolution and
Introduction
7
offsetting the effects of quadrupolar broadening (double rotation, double angle spinning and multiple quantum techniques). All these aspects are treated in depth in the following chapters.
1.3. B R I E F D E S C R I P T I O N O F T H E N M R E X P E R I M E N T
1.3.1 General principles Many nuclei possess a quantised property called nuclear spin which can be adequately described only by quantum mechanics (see Chapter 2) but can usefully be thought of as being caused by the physical spinning of the nucleus. Nuclei with even mass number and even charge (e.g. 12C, 160) have zero spin and are therefore of no use for NMR spectroscopy. The angular momentum of a spinning nucleus is a function of its spin quantum number I which can have either integer or half-integer values. When such a nucleus is placed in a strong magnetic field, the energy levels between the various spin states are split (the Zeeman interaction). The differences between the various energy levels are small compared with spectroscopies involving electronic energy states, and transitions are only possible between adjacent energy levels, with the absorption or emission of a photon in the radiofrequency (rf) range. It is the frequency of this rf radiation which is measured in an NMR experiment. The nuclei in different structural environments in a solid may experience slightly different magnetic fields because they are shielded by the surrounding electrons and consequently absorb photons of slightly different frequency. Since the absolute NMR frequencies are difficult to measure with sufficient accuracy, the resonance frequencies are normally reported as chemical shifts (~) relative to an external standard compound. Unless the nucleus is in a very symmetrical environment, it experiences anisotropic shielding which will be reflected in its ~ value. A more useful parameter for comparison purposes is the isotropic chemical shift ~iso, the average shift which is experienced by the nucleus. The goal of all NMR experiments is to determine the change in the separation of the energy levels for different environments. In its simplest form, an NMR experiment consists of three parts, the preparation of the nuclear spin system by placing it in an external magnetic field, its perturbation by applying a pulse of rf radiation, and the detection of phenomena accompanying its return to the initial state when the perturbation is removed.
Preparation. When the nuclear spin system is placed in the external magnetic field Bo, its magnetic moment precesses about the magnetic field axis similar to the manner in which a gyroscope precesses about the orientation of a gravitational field (Figure 1.4A). In the NMR experiment we are dealing with many spins simultaneously and the observed response is an average of the behaviour of the individual spins. The
Multinuclear Solid-State NMR of Inorganic Materials
Z
Z' M
"~Xx
A. Preparation
B. Perturbation
C. Detection
(evolution)
N
/ \
/ \
I \
/ /
N N
/
Figure 1.4. Schematic representation of the three stages of a simple NMR experiment. In the preparation stage the magnetic moment of the nucleus precesses about the principal axis of the applied magnetic field Bo. In the evolution stage the sample is irradiated with plane-polarised rf radiation at the Larmor frequency containing a magnetic field component Bo which causes the net magnetisation M to incline, becoming oriented perpendicular to Bo in the case of a -rr/2 pulse. In the detection stage the spin system coherently precesses in the transverse plane inducing a voltage in the coil producing a signal which is detected (the FID). frequency of the precession is called the Larmor frequency and is a characteristic of the particular nucleus (since each nuclide has its own characteristic Larmor frequency, it follows that in any particular experiment the technique acquires information only about the nuclide for which the system is specifically tuned). The magnetic moments of all the nuclei in a particular sample can be visualised as forming a cone about the z-axis. yielding a net magnetisation M parallel to the Bo axis. When viewed in a frame rotating about the z-axis at the Larmor frequency (the so-called rotating frame), the spin system appears to be stationary. When thermal equilibrium is attained between the nuclear spin moments and their surrounding lattice, an equilibrium magnetisation is obtained, which is, however, only a very small fraction of the maximum possible value if complete alignment of the nuclear moments with Bo was achieved.
Perturbation. The sample is now irradiated with a pulse of plane-polarised rf radiation at the Larmor frequency. Plane polarised electromagnetic radiation consists of electric and magnetic fields oscillating in fixed planes perpendicular to each other and to the direction of the radiation. The nuclear spin packets which are already in thermal
Introduction
9
equilibrium with the static external magnetic field interact with the magnetic field component of the radiation, causing the equilibrium magnetisation to incline with respect to the applied magnetic field (Figure 1.4B). In the simplest experiment, the angle of inclination is 90 ~ and provided the pulse is of short duration compared with the time taken for the spin packet to spiral into the 'new' field direction, the precession can be assumed to remain fixed at 90 ~ Such a pulse, which is long enough to rotate the spin packet through 90 ~ is described as a 7r/2 pulse (a 7r pulse is one which rotates the spin packet through 180~
Detection. When the rf field is turned off at the end of the simple 1-pulse experiment,
the spin system dephases in the x-y plane with a characteristic relaxation time T2, and returns to thermal equilibrium along the z-axis with a characteristic relaxation time T1. The transverse magnetisation after the pulse induces a voltage in the coil which is recorded as a function of time and is called a Free Induction Decay (FID) (Figure 1.5) containing frequency information. This can be extracted mathematically by a process called Fourier Transformation (FT) giving a plot of amplitude vs. frequency (Figure 1.5) showing which frequencies would have to be added together with what relative amplitudes to reproduce the time-dependent shape of the FID. Thus, Fourier transformation of a "rr/2 single-pulse NMR experiment gives exactly the same information as would be obtained from a continuous wave spectrum, allowing the entire frequency range to be covered in the time taken to acquire a single FID. Usually the sample is irradiated with many pulses and the resulting signals added together to give an improved signal/noise ratio. A clear and simple description of the processes involved in pulsed NMR has been given by Schwartz (1988). Many of the advances in the application of NMR to solids have involved the development of more complex pulse sequences for manipulating the spin systems in order to improve the spectral resolution or extract other information (bond lengths, atom connectivities, etc.)
~
,~
FT
>
t
v
FID
NMR spectrum
Figure 1.5. Schematic representation of Fourier transformation of a FID in the time domain to an NMR spectrum in the frequency domain.
10
Multinuclear Solid-State NMR of lnorganic Materials
1.3.2 Overcoming NMR spectral broadening in solids by MAS The NMR spectra of solids suffer from broadening due to various interactions between the dipole moments of the nuclei, between the quadrupole moments of quadrupolar nuclei and the electric field gradient (EFG) at the nucleus, and by anisotropy of the electronic shielding at different sites in the structure. These interactions are dealt with in detail in Chapter 2. These broadening effects do not arise in liquids because their atomic motion is faster than the interaction frequency, allowing all the nuclei in a particular atomic environment to experience the same average magnetic field, thus producing extremely narrow NMR spectra. A number of the interactions giving rise to line broadening in solids can be cancelled out or at least reduced by spinning the sample very rapidly (typically 10-15 kHz or 10,000-15000 revolutions per second) at an angle of 54.74 ~ to the axis of the applied magnetic field (the so-called magic angle, Figure 1.6A). Broadening in spin I = 1/2 nuclei arises mainly from interactions between the magnetic dipoles of adjacent nuclei which are anisotropic in powder samples. In quadmpolar nuclei where the spin I > 1/2 (a category containing more than two-thirds of all the NMR-active nuclei) the non-zero nuclear electric quadrupole moment results from the fact that the charge distribution is non-spherically symmetric. Interaction of this quadrupole moment with the electric field gradient at the nucleus causes the broadening associated with these nuclei. As discussed in detail in Chapter 2, some of these interactions contain terms in (3cos 2 0-1) the second-order Legendre polynomial (P2(cos0)), where 0 is the angle between the axis of the applied magnetic field and the principal axis of the interaction. At the magic angle, cos0 = 1/~/3 and the term P2(cos0), becomes zero. Magic angle spinning (MAS) thus removes the dipoledipole and chemical shift anisotropy (CSA) interactions, as well as the first-order quadrupolar interactions, narrowing the resonance lines from both spin I - 1/2 and quadrupolar nuclei to reveal spectral details (Figure 1.6B). Lines equispaced on each side of the central resonance (spinning side bands) are caused by modulation of the interaction by the physical act of spinning the sample, and, since they are harmonics of the spinning speed, they can be distinguished from tree peaks because their position changes with the spinning speed, moving away from the central peak at higher speeds. It should be noted that although the well-resolved NMR spectra of solids obtained by MAS may resemble solution spectra, they are normally broader due to distributions of the isotropic chemical shifts in solids. There is also another important difference between liquid and solid MAS NMR spectra. Unlike the random molecular motion in liquids, magic angle spinning is a coherent averaging process, introducing the possibility of synchronising the pulse sequences with the rotor speed to selectively reintroduce anisotropic interactions and allowing the measurement of internuclear distances. MAS removes the broadening of the spectra of quadrupolar nuclei resulting from first-order quadrupolar interactions, but not broadening due to higher-order interac-
11
Introduction
A
~;o
|
~;o
!
i
o
!
!
-lOO
!
-~'oo
~7AIshift (ppm) w.r.t AI(H20)~3+
Figure 1.6. A. A typical magic angle spinning probe illustrating the orientation at the magic angle of the sample within the stator/coil assembly with respect to the axis of the applied magnetic field B0. B. Typical solid state 27A1 NMR spectra of a sintered mixture of A1203 and Y203. Upper spectrum unspun, lower spectrum spun at the magic angle. The peaks marked with asterisks are spinning side bands.
tions which have angular dependences other than the (3 C O S 2 0 - 1) term eliminated by MAS (see Chapter 2). In complex spectra of samples containing several overlapping sites, the removal of higher-order broadening may be important, and can be accomplished by spinning at more than one angle either simultaneously as in double rotation (DOR) or sequentially as in double angle spinning (DAS) (see Chapter 3). A more recent and extremely promising method which does not require complex and fragile probe hardware involves the excitation of higher order quantum coherences, exploiting differences in their associated evolution under different interactions. This method, called multiple quantum or MQMAS NMR, is described in detail in Chapter 2.
1.3.3 Other NMR experiments used with solids Although MAS NMR with a simple pulse sequence is the technique most generally used to study solid materials, there are a number of other experiments which may be used with or without MAS to provide specific information or improve the quality of the spectrum. Since these are described in Chapters 2 and 3 and will be encountered in many of the practical examples given in the subsequent chapters, they are only treated briefly here.
12
Multinuclear Solid-State NMR of Inorganic Materials
1.3.3.1 Decoupling. In some cases the broadening arising from interactions between the dipole moments of two different nuclei (heteronuclear broadening) is too large to be removed completely by MAS. Typically the second nucleus may be 1H, in which case the protons in the system are continuously irradiated at their Larmor frequency, which continuously modulates their spin state, cancelling the effect of their dipole moment. The spectrum of the other nucleus is collected during this decoupling period and is thus freed of heteronuclear broadening.
1.3.3.2 Cross-Polarisation (CP). In systems containing two nuclei, one with more abundant spins than the other, magnetisation can be transferred from the more abundant (often 1H) to the less abundant nucleus by irradiating both nuclei at their correct Larmor frequencies in fulfillment of the Hartmann-Hahn condition (Hartmann and Hahn 1962) (see Chapter 2). The spectrum of the less abundant nucleus can then be collected at a higher magnetisation level determined by the protons, and, since the relaxation time T~ is now that of the protons, a greater number of scans can be collected in a given time, giving a better signal/noise ratio. Further, since the signal from the less abundant nuclei located in structural sites in closest proximity to the protons is preferentially enhanced, additional structural information can be gained by comparing CP and non-CP spectra of the same sample.
1.3.3.3 Spin-echo experiments. Spectra containing broad resonances can suffer from artifacts and distortion introduced during the first few microseconds of instrumental dead time at the beginning of the FID. To overcome this, the spin system can be given an initial pulse and is then subject to another pulse (or pulse sequence) which causes the dephasing spin system to rephase (the refocussing pulse) and is sometimes termed a spin-echo (Hahn 1950). The FID is then collected and Fourier transformed in the usual way. Spin-echo experiments are also often used for determining relaxation times. 1.3.3.4 Two-dimensional experiments. A wide range of experiments are now available in which a series of FIDs (and hence spectra) are collected, each with a different value of some time-dependent variable (e.g. the evolution period in a multiple-pulse experiment) that modulates the FID. This has been termed two-dimensional spectroscopy and was first suggested by Jeneer (1971) and implemented practically a few years later (Aue et al. 1976). The spectra are first Fourier-transformed in the usual way, then again with respect to the second time variable, and may be plotted as a contour with the chemical shift along one axis and the effects related to the evolution period along the other. 2D experiments can be used to provide a range of structural information, including the identification of sites which are directly connected, and are discussed in detail in Chapter 2.
Introduction
13
1.3.4 Nuclei suitable for NMR spectroscopy To produce an N M R spectrum, a nucleus must possess a nuclear spin. Nuclei with odd mass numbers (e.g. 29Si, 27A1) have half-integer spins and are of most interest for solid state NMR. Nuclei with even mass numbers and odd charge (e.g. 2H, 14N) have integer spins, and although subject to difficulties they can still be useful N M R nuclei. Of the 120 nuclei suitable for NMR, 9 have spin I - 1, 31 are spin I - 1/2, 32 are spin I = 3/2, 22 are spin I = 5/2, 18 are spin I = 7/2 and 8 are spin I = 9/2. One factor influencing the usefulness of an N M R nucleus is its natural abundance, which can range from 100 percent down to the vanishingly small. In the latter case, it may only be possible to acquire an N M R spectrum if the sample has been artificially isotopically enriched, as is normally done for 15N and 170 NMR. The natural abundances of the most useful spin I = 1/2 nuclei are listed in Table 1.1 and those of the quadrupolar nuclei in Table 1.2. The standard substances against which the chemical shifts of the various nuclides are quoted are listed for the spin I -- 1/2 nuclei in Table 1.1 and for the quadrupolar nuclei in Table 1.3. Tables 1.1 and 1.2 also provide information about the N M R sensitivities of the various nuclei, listed as their receptivity relative to Si, with the absolute sensitivity defined by Harris (1984) as @COO + 1)) where C is the natural abundance of the nucleus and ~/is its gyromagnetic ratio. The relative receptivities of the quadrupolar nuclei in Table 1.2 have been adjusted by taking into account the fractional contribution of the central transition.
Table 1.1. NMR properties of the spin I = 1/2 nuclei. Nucleus
Natural abundance (%)
Vo at 7.05 T (MHz)
1H 3He 13C 15N
99.985 1.3 • 10 - 4 1.108 0.37 100 4.70
300.00 228.546 75.435 30.408 282.282 59.601
2.71 • 1.56 • 4.77 • 1.04 ;< 2.26 • 1
100 2.19
121.440 9.714
1.80 • 102 2.01 • 10 -3
7.58 100 100 51.82 48.18 12.75 12.26 0.35
57.213 14.751 9.516 12.141 13.569 63.645 66.579 98.580
1.47 3.22 • 10 -1 8.56 • 10 -2 9.43 • 10 -2 1.33 X 10 -1 3.33 3.66 3.36 • 10 -1
19F 29Si 31p 57Fe
77Se 89y
l~ 1~ l~ 111Cd 113Cd 115Sn
Relative receptivity* 103 10 -3 10 -1 10 -2
103
Standard substance TMS TMS MeNO2 CC13F TMS H3PO4 Fe(CO)5
Me2Se Y(NO3)33aq mer-[RhC13(Sme2)3] Ag+aq Ag+aq CdMe2 CdMe2 Me4Sn
Multinuclear Solid-State NMR of lnorganic Materials
14
Table 1.1. (Continued) Nucleus
Natural abundance (%)
Vo at 7.05 T (MHz)
Relative receptivity*
Standard substance
ll7Sn l l9Sn
7.61 8.58 6.99 26.44 100 14.31 14.40 1.64 33.8 16.84 29.50 70.50 22.6
106.896 111.870 94.647 83.430 24.810 52.830 12.483 6.846 64.242 53.733 173.100 172.899 62.760
9.46 1.22 • 10 6.07 1.54 X 10 1.53 2.12 2.87 X 10 .2 5.42 X 104 9.19 2.66 1.54 X 102 3.79 X 102 5.45
Me4Sn Me4Sn Me2Te XeOF4
125Te ~29Xe 169Tm 171yb
183W 1870S 195pt 199Hg 2~ 2~ 2~
WF6 OsO4 [Pt(CN)6] 2MezHg T1NO3aq T1NO3aq Me4Pb
* relative receptivity normalised to 298i
Table 1.2. NMR properties of the quadrupolar and integer-spin nuclei. Nucleus
I
Natural abundance
Vo (MHz) at 7.05 T
(%) 2H 6Li 7Li 9Be l~ liB 14N
170 21Ne 23Na 2SMg 27A1 33S 35C1 37C1
39K 41K 43Ca
45Sc 47Ti 49Ti 51V
1 1 3/2 3/2 3 3/2 1 5/2 3/2 3/2 5/2 5/2 3/2 3/2 3/2 3/2 3/2 7/2 7/2 5/2 7/2 7/2
1.5 x 10 -2 7.42 92.5 100 19.58 80.1 99.63 0.037 0.27 100 10.0 100 0.75 75.77 24.23 93.26 6.73 0.135 100 7.28 5.51 99.75
46.051 44.146 16.67 42.20 32.239 96.32 21.671 40.71 23.71 79.44 18.39 78.27 23.06 29.44 24.51 14.02 7.70 20.23 73.03 16.95 16.95 79.05
Quadrupole Relative moment receptivity* (mb)
Quadrupole Stemheimer broadening antishielding factor** factor (~)
7.35 x 102 3.76 x 10
-0.808 -40.1 52.9
2.28 X 10-1 1.10
0.2 0.2
3.59 x 10 2 5.22 2.93 x 10 -2 1.80 x 10 -2 2.51 x 10 2 7.29 x 10 -1 5.61 x 102 4.62 x 10 -2 9.71 1.79 1.29 1.46 x 10 -2 2.35 x 10 -2 8.19 x 102 4.16 x 10 -I 5.61 x 10-1 1.04 x 103
40.6 20.44 -25.6 101.6 104 199.4 146.6 -67.8 -81.7 -64.6 58.5 71.0 -49.0 -220 302 247 -52
2.83 x 10 -1 NA 6.40 x 10 -2 7.21 2.64 8.60 1 3.30 3.75 2.80 4.20 11.3 1.39 X 10 -1 1.12 21.40 6.08 5.78 X 10 -2
0.19 -6.7 -13.8 -9.5 -5.5 -4.1 -3.6 -52.2 -42.0 -42.0 -21.8 -21.8 -18.8 -23.1 -9.0 -9.0 -7.6
15
Introduction Table 1.2. (Continued) Nucleus
I
Natural abundance
Vo (MHz) at 7.05 T
Relative receptivity*
(%) 53Cr 55Mn
59Co 61Ni
63Cu 65Cu 67Zn 69Ga 71Ga 73Ge 75As 79Br 81Br 83Kr 85Rb 87Rb 878r 91Zr 93Nb 95Mo
97Mo 99Ru lOlRu lO5pd 113in 115in
121Sb 123Sb 127I 131Xe
133Cs 135Ba 137Ba 139La 141pr 143Nd 145Nd 1478m 1498m 151Eu 153Eu 155Gd
157Gd
3/2 5/2 7/2
3/2 3/2 3/2 5/2 3/2 3/2 9/2
3/2 3/2 3/2 9/2
5/2 3/2 9/2 5/2 9/2 5/2 5/2 5/2 5/2 5/2 9/2 9/2
5/2 7/2 5/2 3/2 7/2
3/2 3/2 7/2 5/2 7/2 7/2 7/2 7/2 5/2 5/2 3/2
3/2
9.50 100 100 1.14 60.11 39.89 4.11 60.1 39.9 7.73 100 50.69 49.31 11.5 72.16 27.84 7.00 11.22 100 15.92 9.55 12.7 17.0 22.33 4.3 95.7 57.36 42.64 100 21.2 100 6.59 11.23 99.91 100 12.18 8.30 15.0 13.8 47.8 52.2 14.8 15.65
17.00 74.56 71.04 26.87 79.65 85.32 18.82 72.24 91.79 10.50 51.57 75.46 81.34 11.59 29.08 98.56 13.06 28.02 73069 19.65 20.07 13.78 15.45 13.80 66.02 66.17 72.30 39.15 60.47 24.79 39.64 30.02 33.58 42.73 91.90 16.35 10.07 12.51 10.31 74.63 32.95 9.28 12.18
2.34 x 10 -1 4.85 x 102 7.57 x 102 1.11 x 10 -1 1.75 X 102 9.59 x 10 3.19 x 10 -1 1.14 X 102 1.55 x 102 2.96 X 10 -1 6.90 X 10 1.09 X 102 1.33 x 102 5.95 x 10-1 2.88 x 10 1.34 x 102 5.16 x 10 -1 2.88 1.32 x 103 1.41 9.03 x 10 -1 3.89 x 10-1 7.35 x 10 -1 6.84 x 10-1 4.09 x 10 9.20 x 102 2.54 x 102 5.39 x 10 2.59 x 102 1.62 1.31 X 102 8.92 X 10 -1 2.13 1.64 x 102 9.09 X 102 1.12 1.78 x 10 -1 6.17 x 10 -1 3.18 X 10 -1 2.32 X 102 2.19 x 10 6.00 • 10 -2 1.40 x 10 -1
Quadrupole moment (mb) -150 330 420 162 -220 -204 150 171 107 - 196 314 313 261.5 259 276 133.5 335 -176 - 320 -22.0 255 79.0 457 660 799 810 -360 -490 -616 -114 -3.4 160 245 200 -59.0 -630 -330 -260 75.0 903 2410 1270 1350
Quadrupole Sternheimer broadening antishielding factor**
factor (Y)
21.9 5.81 4.20 16.18 10.06 8.08 4.76 6.70 2.06 3.36 31.67 21.50 13.92 5.32 10.42 3.00 7.91 6.02 1.28 0.98 12.88 1.80 53.75 125.5 8.9 9.13 8.98 10.36 24.95 8.68 4.93 x 10 -4 14.12 29.60 1.58 1.51 X 10 -1 41.02 18.28 9.13 0.92 43.45 700.9 2878 2478
-6.6 -5.8 -4.7 -25.2 -25.2 -21.9 - 17.0 -17.0 -8.7 -80 -80 - 85.5 -52.8 -52.8 -47.8 -26.6 - 23.0 -20 -20 -28 -28 - 162 -110 -110 -110 - 71 -
Multinuclear Solid-State NMR of lnorganic Materials
16
T a b l e 1.2. (Continued) Nucleus
I
Natural abundance
Vo (MHz) at 7.05 T
Relative receptivity*
Quadrupole moment (rob)
72.12 10.33 14.46 64.07 8.66 14.61 34.28 12.18 7.65 36.40 68.50 69.21 23.64 5.4 5.87 5.22 20.07 49.09 5.85
1.88 X 102 2.46 X 10 -1 8.75 X 10 -1 5.54 X 102 3.11 X 10 -1 5.89 X 10 - l 8.25 X 10 7.07 X 10 -1 2.03 X 10- l 1.02 X 102 1.41 X 102 2.43 X 10 2 1.07 3.04 X 10 -2 6.79 X 10 -2 8.13 X 10 -2 5.32 X 10 - l 3.92 X 102 2.93 X 10 -3
1432 2507 2650 3580 3570 2800 4970 3360 3790 3170 2180 2070 856 816 751 547 386 -516 4936
(%) 159Tb 161Dy 163Dy 165Ho 167Er 173yb 175Lu 177Hf 179Hf 18~Ta 185Re ~87Re ~89Os 191Ir 193Ir 197Au 2~ 2~ 235U
3/2 5/2 5/2 7/2 7/2 5/2 7/2 7/2 9/2 7/2 5/2 5/2 3/2 3/2 3/2 3/2 3/2 9/2 7/2
100 18.9 24.9 100 22.95 16.12 97.41 18.606 13.629 99.988 37.40 62.60 16.1 37.3 62.7 100 13.18 100 0.72
Quadrupole Sternheimer broadening antishielding factor** factor (~/) 470.9 2419 1931 338 2487 2134 1218 1566 1728 466.5 275.9 246.1 513.3 2042 1591 949 122.9 4.99 7038
--73.8 -
* relative receptivity normalised to 298i ** quadrupole broadening normalised to 27A1 Data from Smith and van Eck (1999) and references therein, and Pyykk6 (1992, 2001)
T a b l e 1.3. Standard reference substances for the quadrupolar and integer-spin nuclei, with compounds commonly used as secondary references and their shift relative to the primary reference. Nucleus
Reference substance
Secondary reference compound
2H 6'7Li 9Be l~ 14N 170 23Na 25Mg 27A1
TMS LiCl(aq) BeO(s) BF3.Et20 CH3NO2 H20 NaCl(aq) MgSO4(aq) Al(H20)63+
NaBH4 NH4C1 NaCl(s) MgO(s) Y3A15OI2 (octahedral peak)
33S
CS 2
NH4AI(SO4)2.12H20
35'37C1
NaCl(aq)
CaS KCl(aq)
Shift from the primary compound (ppm)
3.2 342.4 7.2 26 0.7 333 -28.5 3.07
17
Introduction
Table 1.3. (Continued) Nucleus
Reference substance
39'41K 43Ca
KCl(aq) CaC12(aq) 47'49Ti TIC14 51V VOC13 63'65Cu CuC1 67Zn Zn(NO3)2(aq) 69,71Ga Ga(H20)63+ 85'87Rb RbCl(aq) 91Zr Cp2ZrBr2 93Nb NbC15in wet acetonitrile 95'97Mo Na2MoO4(aq) 131Xe Xe(g) at zero pressure 133Cs CsCl(aq) 135Ba BaClz(aq) , 139La LaC13(aq) 2~ (CH3)zHg
Secondary reference compound
Shift from the primary compound (ppm)
CaO(s) SrTiO3 0.16M NaVO3(aq) ZnSe ' _ BaZrO3 Nb2Os(s) Mo(CO)6 BaZrO3 -
128 - 843 -574.4 276 _ 208.1 - 1240 - 1854 279 -
A further factor to be taken into account when assessing the relative ease with which a quadrupolar nucleus may be observed is the width of the line which is determined for a given electric field gradient by the quadrupole moment (also listed in Table 1.2). Often only the central transition is observed, for which the linewidth is determined by its second-order quadrupolar broadening and is proportional to [Q2(I(I + 1)-3/4)2]/ [ ( 2 I - 1)2]. These quadrupolar broadening factors listed in Table 1.2 have been normalised to 27A1. The actual linewidth will be given by this factor multiplied by the electric field gradient and although this property is not easily defined for a particular nucleus, it is amplified by the Sternheimer antishielding factor (Table 1.2) which must therefore also be taken into account when comparing nuclei. Hence to get a feeling for the observability of the central transition of a given quadrupolar nucleus, the secondorder broadening factor should be multiplied by the square of the Sternheimer antishielding factor. Values of the latter are given in Table 1.2 only for cases where meaningful calculations have been carried out for ions with a closed shell structure, e.g. A13+, C1-. Where possible, these values are quoted for a lattice (Schmidt et al. 1980), otherwise they pertain to the free ions.
1.4. F U R T H E R R E A D I N G
This book is intended to be a self-contained introduction to the background of solidstate NMR, its experimental techniques and applications to inorganic materials.
18
Multinuclear Solid-State NMR of lnorganic Materials
Nevertheless there are a number of excellent books dealing with different aspects of NMR spectroscopy and solid state NMR with a different emphasis and perspective. Some of these are listed below, and the interested reader is encouraged to consult them. The background to the physics of NMR can be found in the excellent books by Abragam (1983), Slichter (1990) and Munowitz (1988), the latter providing a very readable quantum mechanical description of solid state NMR experiments. Harris (1983) provides a nice introduction to the chemical aspects of NMR techniques, and the physical background of multidimensional NMR techniques is developed in detail by Ernst et al. (1988). High-resolution solid state NMR is dealt with by Haeberlen (1976) and Mehring (1983). A book devoted to pulsed solid state NMR has been written by Gerstein and Dybowski (1985). Although 13C NMR of organic materials is not dealt with in the present book, there is ample literature on this subject, including the books by Stejskal and Memory (1994) and Fyfe (1983), the latter providing numerous applications of 13C NMR. Schmidt-Rohr and Spiess (1996) present a comprehensive background to modem NMR techniques for characterising solid polymeric materials. Applications to silicates and zeolites were dealt with comprehensively by Engelhardt and Michel (1987). On practical matters, the authors find it useful to have copies of the books by Fukushima and Roeder (1981) and Derome (1987) in the laboratory. Last but not least is the Encyclopaedia of NMR, published in 1996 by Wiley.
REFERENCES
Abragam, A. (1983) Principles of Nuclear Magnetism, Oxford University Press, Oxford. Andrew, E.R., Bradbury, A. & Eades, R.G. (1959) Nature 183, 1802. Aue, W.P., Bartholdi, E. & Ernst, R.R. (1976) J. Chem. Phys. 64, 2229. Derome, A.E. (1987) Modern NMR Techniques for Chemistry Research, Pergamon, Oxford. Engelhardt, G. & Michel, D. (1987) High-Resolution Solid State NMR of Silicates and Zeolites, Wiley, Chichester. Ernst, R.R., Bodenhausen, G. & Wokaun, A. (1988) Principles of Nuclear Magnetic Resonance in One and Two Dimensions, Oxford University Press, Oxford. Fukushima, E. & Roeder, S.W.B. ( 1981) Experimental NMR Spectroscopy, Addison-Wesley, Reading, Mass. Fyfe, C.A. Solid State NMR for Chemists, CRC Press, Guelph. Gerstein, B.C. & Dybowski, C.R. (1985) Transient Techniques in NMR of Solids, Academic Press, Orlando, Florida. Haeberlen, U. Advances in Magnetic Resonance, Supplement 1, Academic Press, San Diego. Hahn, E.L. (1950) Phys. Rev. 80, 580. Harris, R.K. (1984) NMR Spectroscopy, (Pitman, London). Hartmann S.R. & Hahn, E.L. (1962) Phys. Rev. 128, 2042. Jenner, J. (1971) Amp6re International Summer School, Basko Polje, Yugoslavia.
Introduction
19
Lowe, I.J. (1959) Phys. Rev. Lett. 2, 285. Mehring, M. (1983) High Resolution NMR of Solids, Springer-Verlag, Berlin. Munowitz, M. (1988) Coherence and NMR, Wiley, New York. Pines, A., Gibby, M.G. & Waugh, J.S. (1973) J. Chem. Phys. 59, 569. Pyykk6, P. (1992) Z. Naturforschung 47a, 189. Pyykk6, P. (2001) Mol. Phys. 99, 1617. Schaefer, J. & Stejskal, E.O.J. (1976) J. Amer. Chem. Soc. 98, 1031. Schmidt, P.C., Sen, K.D., Das, T.P. & Weiss, A. (1980) Phys. Rev. B 22, 4167. Schmidt-Rohr, K. & Speiss, H.W. (1996) Multidimensional Solid-State NMR and Polymers, Academic Press, San Diego. Schwartz, L.J. (1988)J. Chem. Educ. 65, 752. Slichter, C.P. (1990) Principles of Magnetic Resonance, Springer-Verlag, Berlin. Smith, M.E. & van Eck, E.R.H. (1999) Prog. Nucl. Magn. Resort. Spectrosc., 34, 159. Stejskal, E.O. & Memory, J.D. (1994) High Resolution Solid State, NMR Oxford University Press, New York.
This Page Intentionally Left Blank
Chapter 2
Physical Background 2.1.
Fundamental Interaction with External Magnetic Fields 2.1.1 A Quantum Mechanical Description of the Zeeman Interaction 2.1.2 Bulk Magnetisation 2.1.3 The Rotating Frame and the Application of RF Pulses 2.1.4 Observation of the NMR Signal 2.2. Internal Interactions 2.2.1 The Dipolar Interaction 2.2.2 Scalar Coupling 2.2.3 Paramagnetic Coupling 2.2.4 Chemical Shielding 2.2.5 Knight Shift 2.2.6 Quadrupole Interaction 2.2.7 Nature of Interactions 2.3. One Dimensional Methods for Improving Resolution 2.3.1 Magic Angle Spinning and First-Order Effects 2.3.1.1 Physical Principles 2.3.1.2 Formation of Spinning Sidebands 2.3.2 Magic Angle Spinning and Higher-Order Effects 2.3.2.1 MAS of Second-Order Quadrupole Effects 2.3.2.2 Residual Coupling Effects due to Quadrupolar Nuclei in MAS Spectra 2.3.2.3 Nonequivalent Homonuclear Spins 2.3.3 Variable Angle Spinning 2.3.4 Double Angle Spinning 2.3.5 Multiple Quantum Transitions 2.3.6 Ultrasonically-Induced Narrowing 2.4. Dipolar Decoupling 2.4.1 Heteronuclear Dipolar Decoupling 2.4.2 Homonuclear Dipolar Decoupling 2,5, Spin-locking 2.6. Cross-Polarisation 2.7. Two-Dimensional Methods 2.7.1 Dynamic Angle Spinning 2.7.2 2D MQMAS 2.8. NMR Relaxation
23 25 26 29 34 35 37 40 43 44 48 50 57 58 59 59 61 63 64 71 74 74 75 77 78 78 78 79 83 85 90 92 93 98
2.8.1 2.8.2 References
Introduction to Relaxation Mechanism for Relaxation Processes
98 101 105
Chapter 2
Physical Background 2.1. FUNDAMENTAL INTERACTION WITH EXTERNAL MAGNETIC FIELDS
The nuclei of interest to the NMR experiment possess a nuclear spin angular momentum ] and consequently a nuclear magnetic moment ~. A classical description of this magnetic moment placed in a static magnetic field B shows a torque z being exerted of
z-ExB
(2.1)
The magnetic dipole moment ~ is related to the spin angular momentum via
= ?'hi and I~[- 17[h4I(I + 1)
(2.2)
where ~/is the nuclear gyromagnetic ratio (units of rads-lT-1). The torque in Eq. 2.1 causes the magnetic moment to change direction such that
dt
=7~xB
(2.3)
If the moment was inclined at 0 to B this would cause a precession of the moment in a conical path at constant 0 about B at an angular frequency O~ogiven by O)~ -?'[B I
(2.4)
COois known as the Larmor or resonance frequency. Strictly this is the angular resonance frequency but the literature uses this term interchangeably with the direct frequency which is of more immediate experimental relevance, Vo = eOo/2~r. This means NMR is a radiofrequency spectroscopy so for protons at B = 1 T, Vo - 42 MHz. ~/can be either positive or negative, although the consequences of this are usually ignored. Levitt (1997) has explored this issue and the consequences it has for phase shifting and detecting the signal. Classically, a magnetic dipole moment in a magnetic field has an energy of
E--~.B~ o --Th[.B
o
(2.5)
which not only depends on ~/, h and the magnitude of B o but also on the relative orientation of ~ and B o. In NMR spectroscopy the energy difference between different spin 23
24
Multinuclear Solid-State NMR of Inorganic Materials
levels (i.e. effectively different orientations of I) is of interest. This is the well known Zeeman interaction and is the basis of magnetic resonance spectroscopy. Although the magnetic moment has been treated classically, and indeed many NMR effects can be conveniently described classically, the spin is quantised. This means that its values and components can only take discrete values. The energy levels can be calculated using the spin Hamiltonian which is an operator giving the energy of the system. In principle all energy levels in a system (nucleus, system of coupled nuclei, molecules) are given by the time-dependent Schr6dinger equation: ih ~)l/t _ HI// (2.6) ~)t where H = Hamiltonian and qJ is the wave function. In NMR the way the system is interrogated (e.g. with an rf pulse) is assumed not to alter the energy levels of the system so that it can be assumed that the Hamiltonian is time-independent. Then the wave function that solves Eq. 2.6 will be of the form
= qg. exp(-iEt / h)
(2.7)
where ~ is time-independent. If these two equations are combined then q~ will satisfy
Hq~= Eq~
(2.8)
where q~ is the eigenfunction of the Hamiltonian H with eigenvalue (energy) E. The Hamiltonian gives all possible energy levels of a system, including rotations, vibrations, bonding etc. However the NMR energy splittings are --~ 1.24 X 10 -6 eV (= 300 MHz = 2 x 10 -25 J = 0.01 cm-~ in various spectroscopic units) which is sufficiently different from other interactions that it is possible to split the Hamiltonian, and hence the resulting wavefunctions, into those associated with the nuclear spin and those with the other interactions (e.g. vibrations). Wave functions that represent the interactions of the nuclear spin alone will give an equivalent equation to 2.8 but relating only to the nuclear spin. This removes all the interactions that do not influence NMR experiments. The Hamiltonian H will henceforth in this book be taken to be the nuclear spin Hamiltonian and the eigenfunction q~refers to this Hamiltonian. The eigenfunctions of the Hamiltonian are defined in terms of a complete set of orthogonal spin functions q~n. A general eigenfunction q~ can now always be written as a linear combination of the spin functions q~n where the coefficients Cn vary for different Hamiltonians (interactions). q9 - Z C,~0n B
(2.9)
25
Physical B a c k g r o u n d
Table 2.1. Nuclear spin operators. Operator
Description
Operator II,m > =
I
Nuclear Spin
~/I(I + l)h l I, m >
12 ---Fx -/-Fy -/-Fz
Square of the total nuclear spin
I ( I + 1)h 2 I I, m >
Iz
z-component of nuclear spin
mhlI, m>
I+ --Ix + ily
Raising Operator
~/I(I + 1 ) - m ( m + 1)h l I, m + 1 >
1_ : Ix- ily
Lowering Operator
~/I(I + 1)- m(m-
1)h l I, m -
1>
In NMR the q~n are characterised by the quantum numbers I and m where I is the spin quantum number (0, 1/2, 1, 3/2,...) and m is the z-component of I with values of m = - I, - I + 1. . . . . I - 1, I. The spins of nuclei are given in Tables 1.1 and 1.2. The state of a nucleus would be described by the pair of quantum numbers I and m and is usually written I I, m > in Dirac bra and ket notation. This text is not intended as a primer in quantum mechanical operators but to understand papers that interpret interactions and develop new pulse sequences it is worth noting some of the most important results. It is immediately apparent from Table 2.1 that II, m > is an eigenfunction of both I 2 and Iz so that II, m > is unchanged. In contrast the raising and lowering operators, I+ and I_, change the spin function so that I I , m > is not an eigenfunction of I+ or I_. The raising and lowering operators describe changes in the z-component of the spin state corresponding to a transition of a spin between energy levels.
2.1.1 A quantum mechanical description of the Zeeman interaction The classical Zeeman interaction between a magnetic moment and an applied magnetic field was defined above. In a quantum mechanical description the operators for the quantities need to be used, such as the nuclear spin I. With an externally applied magnetic field B o the Zeeman Hamiltonian is given by (2.10)
H z =-yh[.B_ o
The usual convention is to let the direction of the externally applied static magnetic field B o define the z-direction in the laboratory frame. Then the Zeeman Hamiltonian can be written as
H z - -~,'h Iy
9
- _~,hBoI z
Bo
(2.11)
26
Multinuclear Solid-State N M R of Inorganic Materials Zero field Bo = 0
Bo finite E = YhBO 2
(,,
./,.,-' """ x\
AE= y Bo
E - -TfiBO
~
I~-~>t I~.~) J
Figure 2.1. Energy level diagram of a spin-l/2 nucleus showing the Zeeman interaction. The Zeeman Hamiltonian can be used to calculate the energy difference between the nuclear spin states. For a nucleus the energy eigenvalues can be obtained by taking the Hamiltonian operator given in Eq. 2.11 so that H_ I I, m >= E m I I, m >= -COol_ I I, m >= - m h ~ o I I, m >
For I
=
1/2, m can be
_+ 1/2
E% - - ~ h
(2.12)
which gives the eigenvalues Em of
COo(m - ~ ) and E_,/: = ~ h COo(m - - ~ )
(2.13)
The separation (Figure 2.1) of m - _+ 1/2 levels is AE = E - l / - El/ - h O~o which is 2 2 . the energy of the m - _+1/2 transition and matches the classical expression given in Eq. 2.4.
2.1.2 Bulk magnetisation A sample on which an NMR experiment is carried out will usually contain a large number of nuclei. They will all precess around the direction of B o but the phase of precession of each individual spin will vary (the so-called random phase approximation). Consequently, in a unit volume the total nuclear magnetisation M is obtained by summing the individual contributions. The components of magnetic moment perpendicular (transverse) to B o cancel leaving M pointing parallel to the applied magnetic field. M acquires a non-zero equilibrium value Mo in this longitudinal direction since the applied field B o tends to align the magnetic moments preferentially in the B o direction through a normal susceptibility mechanism. It is pertinent at this point to reflect on the different ways of describing magnetic fields. They can be characterised by the magnetic induction (or flux density) designated by B and measured in Tesla (T) or by the magnetic field intensity H measured in Am-1. The difference between these two vector fields is that H is a measure of the field from free current alone whereas B depends on the material. B is the more fundamental quantity and is what is meant when the term "magnetic field" is used in this book. The magnetisation (M) results from the
27
Physical Background
additional internal field provided by the sample itself and is also measured in Am-~. The three vector fields are related by B
H = ,~ - M ~
~0
(2.14)
~
Where ~o = 4w x 10 -7 Hm -~ is the permeability of a vacuum. The amount of magnetisation generated by a sample when placed in magnetic field is characterised by the susceptibility Xv. The literature on magnetic susceptibility is littered with confusing units and careful attention needs to be paid to the units being used. Magnetisation is effectively the amount of magnetic moment per unit volume so the most directly useful measure is the volume susceptibility thus
Zv -
~M
(2.15)
~H
In a linear isotropic material B = / z H where p~ is the permeability of the material. The relative permeability ~r = P~/~o or = 1 + • is a measure of how much magnetisation a material will generate. The molar and specific susceptibilities are also quoted in the literature. The equilibrium bulk magnetisation comes from the net moment on summation of contributions from individual spins. A sample containing spin-1/2 nuclei has two states (energy levels) with the nuclear spins distributed between these according to the Boltzmann distribution which gives the probability of occupation of the different states. N1/2
AE = e kT
~-. 9~,~
N-1/2
The total number of spins is given by N = N1/ + N_ and the difference in population is n = N1/z-N 1/2. Then N1/2 -- (N-n)/2 aZnd N_ 1/2' (N+n)/2. Substituting into 1/2 - Eq. 2.16 gives AE
N-n
~ - - e
AE
kT
1 - e kv n - N ~
(2.17)
N+n
l+e
k~
which in the high temperature limit (nearly always a good approximation in NMR) AE AE gives ~ << 1 meaning that the population difference is n - N . From Eq. 2.13 kT 2kT AE = ~/hBo. The z-component of the magnetic moment is given by P~z = ~/hm and
28
Multinuclear Solid-State NMR of Inorganic Materials
hence per spin the average z-component of the magnetic moment is (/xz) --/xz (m) P(m) where P(m) is the probability of m, given by the Boltzmann distribution. Then
(~: ) - ThlB~( ThlB~"
(2.18)
where B~ is the Brillioun function. Then
M~176
)
(2.19)
In the high temperature approximation
Bt
ThlBo ) _ ThlB o I(I + 1) kT kT 31
(2.20)
so that the equilibrium magnetisation is
Mo = N ,v 2Bo~ 2I,,I t +1) 3kT
(2.21)
and the volume susceptibility is
OM Z~, = 11o OB
~oN~'2h2I(l + 1) 3kT
(2.22)
which is termed Curie-like. It can be seen immediately from the form of the magnetisation that the strongest signal will be obtained from samples with many spins (large N), at high magnetic field and at low temperatures. The Boltzmann factor also gives a clue as to what lies at the core of one of the major weaknesses of NMR, namely sensitivity. The population difference, which is the source the magnetisation, depends on the energy separation AE compared to the thermal energy (kT). Hence the NMR signal is produced by a very small fraction of the total number of nuclei in the sample, so that NMR is unfortunately a relatively insensitive technique. For example at 9.4 T at room temperature for 1H only 3.2 • 10-3% of the nuclei contribute. This gets very much worse for "low-~/" nuclei with small magnetic moments such as 89y, where this fraction is reduced to only 1.6 • 10-4%. In spectroscopy involving electromagnetic radiation both spontaneous and stimulated events can occur. NMR is a relatively low frequency electromagnetic spectroscopy so that spontaneous events are very unlikely (-~ 10-22s - l ) and stimulated events therefore dominate. Hence NMR is termed a coherent spectroscopy meaning that the excess population can be made to work in a concerted way. This is achieved via the application of additional radiofrequency (rf)
29
Physical Background
pulses. To understand how the magnetisation is manipulated it is often convenient to observe the effects in the so called rotating frame rather than the laboratory frame and this is introduced next. This provides a classical description of what happens to the magnetisation.
2.1.3 The rotating f r a m e a n d the application o f RF pulses At any instant the total magnetisation M is given by
M-
(2.23)
M x i+My ~+M z k
The change of the magnetisation with time is then
- = dt
dt
i+ -
j+ dt -
/
+ M X - +My - +M z dt dt
dt
(2.24)
In a rotating frame of reference the magnitude of the unit vectors remains the same but not the vector directions di - = ~x/, dt
dJ - = cox j, dt - -
dk - = cox _k, dt -
(2.25)
so that dM
d M- I
dt
dt
lab
+ cox (Mxi + My j+ M~k) - -dM_ ~
rot
dM
~
=
dt rot
~
-
~
- e x M = y M x B + M x
+ (ox M
(2.26)
rot
(2.27)
lab
CO
= yM • (B+ ~ ) - y M • B~ff Y
(2.28)
CO
where Beff= B+ - is the effective magnetic field in the rotating frame. If the frequency Y and direction of the rotating frame are chosen to coincide with those of the magnetic moments
30
Multinuclear Solid-State NMR of Inorganic Materials - ~0 - - T B 0
(2.29)
so that
B~eff-
-7B o ~B~q 7 - B0 "[- ~ 2 , =0
(2.30)
- 7M • B~ - 0 (co - COo)
(2.31)
dM dt
rot
then M is stationary in a frame rotating at the Larmor frequency. Now consider what happens if a second, smaller magnetic field B1 is introduced which is perpendicular to the Bo field and is rotating at a frequency ~ ( - to k). This frequency need not be equal to the Larmor precession frequency tOo, The B1 field is given by B - B e -;~o, ~1
(2.32)
1
In a rotating frame with the same frequency as the 01 field, the 01 field will appear stationary and the effective field in that frame will be given by (Figure 2.2) o)
%---- 00+ -~- + B1
(2.33)
As B1 is usually applied perpendicular to Bo the strength of the effective field is given by
_
[
Laboratory. frame
2 +B 7
2
(2.34)
.1 Rotatin~ frame
z)
Z
B.0 +z
y
x/"
coo
-B1
y,
xi,
Figure 2.2. Representation of fields in the laboratory and rotating frames.
Physical Background
31
The change of the magnetisation with time is still
dm dt
- ?'M x By,
(2.35)
rot
so that M precesses about B eff in the rotating frame. If the frequency to of rotation of the B~-field (and hence of the rotating frame) is the same as the Larmor frequency too, then %
(2.36)
#er 0+7 +
Now the effective field lies in the xy plane (say along the x'-axis for convenience) in the rotating frame. The magnetisation will now precess around the x'-axis with a frequency of ~1 = -3/B 1
(2.37)
If M = M k at t = 0 then M is tipped towards the - y ' axis through an angle, termed the tip angle 0 = eOlt p - -'yBltp. In almost all NMR experiments the rf field is applied close enough to the Larmor frequency that it is effectively on resonance. Also all values of tip angle are used but there are a few special cases of the duration tp that are most commonly used. These are 0 = 90 ~ which tips M onto the - y ' axis, and 0 = 180 ~ when the magnetisation ends up along - z ' - a x i s (Figure 2.3A, B). The above has adopted a classical picture for describing the interaction between an rf field and the magnetisation. However, just as for the interaction with the main static applied magnetic field there is an analogous quantum mechanical description. The interaction between the rf magnetic field and the nuclear spins is simply another Zeeman interaction. The difference for this field is that it is time-dependent. In practice, the sample is irradiated with a linearly polarised if-field of strength 2B1, frequency tOrf and phase oL. A
iiiii I
B
Z'
,.'""--
M ~
i/
Z'
1""'" y?
Figure 2.3. The precession of the magnetisation in the rotating frames for 90~ and 180~pulses (assuming that ~ is positive).
32
Multinuclear Solid-State NMR of Inorganic Materials
(2.38)
B(t) = 2B 1cos(co,ft + a)
Then the Hamiltonian for an if-field applied along the x-axis is
/ H,u = 2hooll x cos(co,it + ~)
(2.39)
(2.40)
This Hamiltonian contains the spin operator Ix and in Table 2.1 above this can be rewritten in terms of the raising and lowering operators as
H,f - -?'fiB12 (t+e" -i~o~,, + I e i~
I~ - - ~1 [ i + + i _ ] so that (2.41)
These operators induce transitions between different spin states so that by applying an if-field to nuclear spins in the presence of a large static magnetic field close to the Larmor frequency, the spin distribution between the energy levels is perturbed away from thermodynamic equilibrium. In pulsed NMR experiments the spin system is excited with a short rf pulse near resonance and the system is measured afterwards. The total external Hamiltonian from the applied fields is then H = H z + H,f = hOgol.- + 2ho9~I x cos(og,ft + a)
(2.42)
The induced transitions change the populations of the different energy levels. After a 90 ~ pulse the populations are equalised, and after a 180 ~ pulse the populations are inverted so that for a spin-l/2 nucleus the + 1/2 state now contains the population excess. To keep track of the populations of the different levels and how pulses affect the magnetisation, the most complete and powerful formalism is to use the density operator or density matrix (Farrar 1990, 1990a). This approach will not be adopted here but ideas that have come out of this can be extremely informative, allowing an understanding of some of the more complex pulse sequences adopted today. A central idea is that of coherences and coherence order (Munowitz 1988, Ernst, Bodenhausen and Wokaun 1988, Keeler 1990). Transverse magnetisation is a concept that can be readily visualised and can be represented by a classical vector. As the rf pulses are applied as
Physical Background
33
described above, the spins change their magnetic quantum number m by + 1 and are termed single quantum coherences because the change corresponds to absorption (or emission) of one quantum. It is only this coherence that gives rise to a directly observable signal and the sign convention adopted is to regard Am = - 1 as the observable magnetisation. However it is possible that other coherences are generated. For example for nuclei with spin higher than 1/2 it is possible to excite transitions that cause spins to undergo a change Am > 1, which is also possible for systems of coupled spins which flip in unison. The higher order coherences cannot be observed directly but if a series of pulses is applied the higher order coherences will influence the final transverse magnetisation. It is worth noting that a zero-order quantum coherence corresponds to a net change Am -- 0 but that longitudinal magnetisation is not a coherence. Then in a two-dimensional (2D) NMR experiment this influence is recorded as a function of a second time period. Such ideas have been long used in 2D sequences applied to liquids. 2D sequences are now more commonly used in solids, and multiple quantum techniques are at the heart of generating higher resolution NMR spectra from noninteger spin quadrupolar nuclei. To follow the effects of a pulse sequence a coherence level diagram can be helpful. The coherence values are shown and the effects of the pulses of a sequence are then shown by the coherence pathway line (Figure 2.4). Such a sequence starts with the longitudinal magnetisation at m = 0. The changes in the mvalue occur when there are pulses present. Often when a pulse is applied there is no clean generation of a single change of coherence order but different coherence changes occur together with differing probabilities. However the desired coherence pathway can be selected by the appropriate phase cycling of the pulses. In Eq. 2.40 there was an arbitrary phase oLwhich, in the rotating frame, corresponds to the direction in which the magnetisation points. Spectrometers are capable of generating such phases. This phase
2 t2
I" "1 3
,/
10 Longitudina magnetisati~
3Qcoherence /
-1 -2
~~
Observable magnetisation
,,
-3 Figure 2.4. An example of a coherence level diagram for a 3Q experiment with symmetrised pathways and a z-filter pulse.
34
Multinuclear Solid-State NMR of lnorganic Materials
often needs to be rapidly set (--~ Ixs). A decade ago spectrometers for solids came with four hard-wired channels with the four phases + x and _+ y always available. Modem spectrometers are capable of setting the phases for the pulses to 0.09 ~ in less than 200 ns. The phases of the pulses can be used to discriminate between the coherence orders and hence pathways since if the phase of a pulse is changed by ot then a coherence order change Am will experience a phase change of otAm (Ernst et al. 1988).
2.1.4 Observation of the NMR signal The magnitude of the transverse magnetisation produced by the pulse is proportional to Bo (from the Boltzmann factor) and ~2, with one ~/arising from the Boltzmann factor and the other from the magnetic moment. It is also possible to detect magnetisation directly by using a SQUID (superconducting quantum interference device) and development of NMR probes based on such an approach is on-going. However the vast majority of NMR experiments use a coil to apply the perturbing rf pulse and then to record the signal. The maximum signal is generated after all the initially longitudinal magnetisation becomes transverse (i.e. after a 90 ~ pulse). The pulse is turned off and then in the laboratory frame (where the NMR coil is located) this transverse magnetisation precesses through the torque exerted by the Bo field as M ~ • B o so that the precession is proportional to ~/Bo. Through Faraday's Law of electromagnetic induction the induced voltage is proportional to the rate of change (~ ~/Bo) of magnetisation (~ ~/2Bo), producing an induced signal S given by 3
2
S - NVc~' B~
+ l)
(2.43)
3kT where Vc is the volume of the sample. Since there are now no rf fields perturbing the system this is called the free induction decay (FID) signal. Typically induced voltages are --~ ~V although there is significant variation with ~/. The influence of these factors in determining the signal intensity can be seen by comparing the intrinsic sensitivity (per magnetic nucleus) of ~3C which is 133 times greater than 89y (this assumes that the sensitivity is proportional to ~/3 which is generally accepted although some analysis has shown that the variation is proportional to @/2). Even though 89y is 100% naturally abundant compared to only 1.1% for 13C, the ~/-factor results in 89y displaying a relative sensitivity of only 69.6% of 13C. The relative receptivity (R) (Harris 1983) takes into account the nuclear factors, including natural abundance (C), on the sensitivity via R = ]yl 3 CI(I + 1). Equation 2.43 indicates that other factors can be used to enhance signal intensity. A larger sample increases Vc, the experiment can be carried out at lower temperature and at higher applied magnetic field. Also, if the natural abundance of an isotope is low, isotopic enrichment will help sensitivity. Papers should be read carefully as solid state NMR reports especially of low abundance
Physical Background
35
nuclei often use enriched samples, making the experiment much more tractable than at natural abundance. The most significant change in experimental technology is the wider availability of higher magnetic fields (i.e. >- 11.7 T). Detailed analysis indicates that the signal increases with applied magnetic field as Bo3/2-Bo 7/4. The detail is not important, but as sensitivity rises rapidly with field the study of many nuclei in solids is now much more feasible than even just a few years ago.
2.2. INTERNAL INTERACTIONS The total interaction energy of the nucleus is the sum all the individual Hamiltonians it experiences. The external magnetic fields applied allow the spectroscopist to manipulate the spin. The information about the atomic scale surroundings of a nucleus is contained within the interactions between the nucleus and those surroundings. These interactions are summarised in the internal Hamiltonian H~nt -- H D + Hcs + H K + Hj + Hp + HQ ~1) + HQ ~2) + - . .
(2.44)
In diamagnetic insulating solids spin-1/2 nuclei experience a range of interactions that include magnetic dipolar (HD), interaction through space with nearby nuclear magnetic moments, chemical shielding (Hcs), modification of the magnetic field at the nucleus due to the surrounding electrons and indirect spin-spin coupling (Hj), interaction of nuclear magnetic moments mediated by intermediate electron spins. In materials that contain paramagnetic centres the unpaired electrons can interact strongly with the nuclei (Hp) and possibly cause very large shifts and severe broadening of the NMR signal. The fluctuating magnetic fields produced by the electron spins can produce very efficient relaxation. Hence for solids where the nuclei are slowly relaxing and which will dissolve paramagnetic ions, small amounts (--~0.1 mol%) are added to aid relaxation. In materials containing conduction electrons these can also interact strongly with the nuclear spin via a contact interaction HI,: that produces both relaxation and a change in resonance position termed the Knight shift, which provide important information on the nature of the density of states at the Fermi surface. Nuclei with spin I > 1/2 are also affected by the electric quadrupole interaction (HQ), an interaction between the nuclear electric quadrupole moment (eQ) and the gradient in the electric field at the nucleus. Although this is an electrical interaction it depends on the magnetic quantum number and so affects the NMR spectrum. Of the NMR-active nuclei around three quarters have I >_ 1. The quadrupole interaction can cause a significant splitting relative to the Zeeman effect.
Multinuclear Solid-State NMR of lnorganic Materials
36
All these internal interactions that affect the NMR spectra can be represented by a general expression involving tensors Hm = kmliTmij.Amj
(2.45)
where Hm is one of the component Hamiltonians in Eq. (2.44). For each interaction there is a constant k, a 3 • 3 second-rank tensor T and another vector quantity, either a field or a spin with which the spin (I) interacts. The interactions are summarised in Table 2.2 along with the interactions with the external magnetic fields. Three numbers are needed to describe a 3 • 3 tensor relating two vectors and these are usually the isotropic value, the anisotropy and the asymmetry (Haeberlen 1976). Their exact definition can vary even though there are conventions that are normally adopted, but any paper should be examined carefully to see how the quantities are being defined. In a typical isotropic powder the random distribution of particle orientations means the principal axes systems (where the tensor only has diagonal elements) will be randomly distributed. In the presence of a large magnetic field, this random distribution gives rise to broadening of the NMR spectrum since the exact resonance frequency of each crystallite will depend on its orientation relative to the main magnetic field. Fortunately, to first-order, all these interactions have similar angular dependencies of (3cos20 - 1 + xlsin20cos2q~) where ~1 is the asymmetry parameter of the interaction tensor (xl = 0 for axial symmetry). Lineshapes can provide very important information constraining the local symmetry of the interaction which can often be related to some local structural symmetry.
Table 2.2. Summary of NMR Interactions.
Hm
Interaction
Tij
Ai
Typical Size (Hz)
Comments
Hz HRF HD
Zeeman RF Dipolar
Unitary Unitary D
Bo B~ I
107-109
Hcs
cr
Bo
102-105
Hj
Chemical Shielding Indirect Spin
J
I
1-103
He
Paramagnetic
P
S
102-105
HK
Knight Shielding Quadrupolar
K
S
102-105
eq
I
103-107
Interaction with main magnetic field Interaction with rf field Through space spin-spin interaction, axially symmetric traceless tensor Alteration of the magnetic field by the electrons Spin-spin interaction mediated via the bonding electrons through the contact interaction Interaction with isolated unpaired electrons Interaction with conduction electrons via the contact interaction Interaction of nuclear quadrupolar moment with the electric field gradient (q), effectively an 12 interaction
HQ
103-105 103-104
Physical Background
37
2.2.1. The dipolar interaction The dipole interaction arises from the coupling between two magnetic dipoles. Classically the energy of two interacting dipoles ~1 and tx2, a distance r apart, is given by
ED
~
r3
r5
(2.46)
The quantum mechanical Hamiltonian can be derived directly by substitution of ~1 = 3/lh/1 and ~2 = y2h/-2 which leads to Ho=/'to 4re
~/l~t2h2[ r3
1-1 "1"2-
3(/1" r)(I2"r) / r2-
(2.47)
or in Cartesian coordinates
Ii. r - I1xx + IlyY + Ilzz
(2.48)
This can be usefully rewritten in terms of polar coordinates as x = r sin0 cosq~,
y = r sin0sin q~, z = r cos0
(2.49)
This leads to the important "alphabet" expression for the dipolar Hamiltonian ]'to
H D = -~
~/1~/2h2 r3
(A + B + C + D + E + F)
(2.50)
The terms A - F in this Hamiltonian are
A - IlzI2z(3 COS20-1) B--1[I1+I2_+I1_I2+1(3cos20-1) 4 C - -3[I1+12z + 11zI2+](sin0 cos0) exp(-iq~) 2
(2.51)
D - --~[I1_I2z 3 + IlzI2_l(sinO cos0) exp(+iqg) E - - 3 [I1+I2+] sin20 exp(-2iq~) 4 F = - 3 [I1_I2_ ] sin20 exp(+2iq~) 4 Only terms A and B commute with Iz and this is the Hamiltonian HS~,~the secular part of HD which is independent of time.
Multinuclear Solid-State NMR of Inorganic Materials
38
__(A+B)=t.to 8iv ~/1~'2 r 3j~2 I 2Ilzi2z --21 [I1+12- + I1-I2+ ]1(3
H sec _ 1"/o ~/1~'2 h 2 D -3
47c
r
COS20-
1) (2.52)
Here 0 is the angle between the internuclear vector and the magnetic field Bo as the magnetic field lies along the z-axis. The terms C - F are called the non-secular part H 79~ sec of the dipolar Hamiltonian liD. These (C-F) are time-dependent at frequencies tOo and 2tOo. Since the frequency of the time dependency of H~ ~ sec is much higher than the frequency/energy of H~,C the time average of the non-secular part of the dipolar Hamiltonian is essentially zero and the non-secular part can thus be neglected for all spectroscopic purposes. The terms C-F still play a role however as they are responsible for relaxation of spins to Boltzmann equilibrium through the raising and lowering operators. For the secular term it is important to distinguish the effect of the B-term for homonuclear and heteronuclear cases. The spin part of term B is called the flip-flop operator because it can cause transitions between different spin states of two coupled spins. For two coupled spin-1/2 it can be envisaged that as one spin changes from m - 1/2 to m = -1/2 the coupled spin goes from m - -1/2 to m = 1/2. The probability of this flip-flop process occurring is highest when this process is energy conserving, i.e. no external energy needs to be supplied. Whether or not term B is energy conserving has implications as to whether it can be really included as a secular term, and hence for the effect of the dipolar interaction on the spectrum. The energy difference (AED) between the two states of the coupled spins is to a first approximation (2.53)
AED = - ( 7 , - ~ ' 2 ) h B o
So unless Yl - Y2 an energy difference between these states exists. If the coupled spins are different elements, the B-term in the dipolar Hamiltonian vanishes since the flipflop process is no longer energy-conserving. Hence the secular dipolar Hamiltonian will change for a set of like-spins Ii and Ij
H~ _ lao
E
l y'2h 2 2li_ij___~
I';+'; +'
cos2
-1)
(2.54)
whereas the secular heteronuclear dipolar Hamiltonian for spin I and spin S is given by
H ,oS = ~o ~~'I ~I S pl 2 21_S_(3cos 20is -1) 8Jr r/3 " "
(2.55)
Both the heteronuclear and the homonuclear dipolar coupling depend on the orientation of the inter-nuclear vector and the inverse cube of the distance, and often give rise to broad lines in powders. However the characteristic lineshape has features, particularly singularities. For an isolated pair of spin-l/2 nuclei a pair of strong singularities
Physical Background
39
are formed in a so-called Pake pattern (Figure 2.5A, Pake 1948). In single crystals relatively sharp lines are obtained but they could occur at positions covering a wide frequency range within the powder envelope. For a coupled spin-1/2 pair, a pair of lines is observed with the separation depending on the orientation and also the distance. The behaviour is similar for homonuclear and heteronuclear coupled spins but there is a scale factor of 1.5 for the homonuclear case compared to the heteronuclear one (this difference arises from the change from the nonsecular to the secular form of the B term in the dipolar H a m i l t o n i a n - in addition note the replacement of "~i for ~/s). The dipolar interaction is very important in NMR since by measuring the magnitude of the dipolar interaction between 2 nuclei the distance between these nuclei can be determined. An example of this dipolar coupling is the water molecule in NaA1SizO6.H20 (Figure 2.5B). The dipolar coupling is also central to many schemes for magnetisation transfer between nuclei using double resonance techniques such as cross-polarisation. The data become more difficult to unravel when a nucleus experiences numerous different dipolar interactions at the same time. Even an increase to just three spins causes the lineshape
D n - 3/~o72_____h h 32/r2r3
~ _ _ ~ _
zs _/ZoYzYsh - 16~.2r3
!
!
D H,Zs
,,
[
D n.Zs
D n aS
_D U,~'s
_
2
300
200
100
2
0
-100 -200
-300
1H shift in p p m
Figure 2.5. A. Schematic powder pattern of a spin-l/2 nucleus dipolar coupled to a second spin-l/2 nucleus. B. Example of a 1H Pake powder pattern from NaA1Si206.H20 from Yesinowski, Eckert and Rossman (1988) with permission of the American Chemical Society.
40
Multinuclear Solid-State NMR of Inorganic Materials
to become much more complex and to depend on the relative orientation of the spins. As the number of interacting spins increases, the lineshape rapidly loses distinctive features, and the observed distribution of intensity can be described by the method of moments. The nth moment for a lineshape L(v) is defined by oo
[. (V- Vo )" L(v)dv
M
Il
(2.56)
- - - -.-oo c~
I L(,v)dv The second moment is the most used description of the intensity distribution but this alone cannot unambiguously define the lineshape and higher order moments have to be examined. Often the lineshape in solids can be described by a Gaussian which has the distribution
l((V-Vo G ( v ) - 2 r ~ - A exp -
)2 )
(2.57)
A2
where A is the standard deviation of the line which is related to the more directly measurable full width half maximum (FWHM) by A = 2.355FWHM. The second moment can be related to the dipolar interaction. For the homonuclear interaction the second moment is given by
__ 3 (lUo ")2 t4];12i(i -]-1)Z
J
M211 -4 \ 41r J
(1-
3cos2Oij)2 ,)6
(2.58)
For a powder an average value of the factor (1 - 3cos20ij) 2 can be taken as 5/4, resulting in the expressions for M2
M2tt - -5 \ 4zc J ~'t h I(I + 1) 9
Mzls -
4.
)'1Ys2 h S(S + 1)
--T 1)j
(2.59)
r-6
(2.60)
If the lattice type is known then the lattice sum of rij -6 can be performed, and conversely from the second moment, information can be deduced about the distribution of nuclear spins.
2.2.2 Scalar coupling The scalar coupling (also called spin-spin, indirect or J-coupling) is a quantum mechanical-based indirect interaction of nuclear moments mediated via the electrons
Physical Background
41
in the chemical bonds between the nuclei. The first nucleus perturbs the surrounding electrons and this perturbation of the electrons produces an additional magnetic field at the second nucleus. In contrast to dipolar coupling, which is a through-space interaction, scalar coupling is a through-bond interaction. Hence, it is most important in covalent materials. The Hamiltonian for the scalar coupling is given by Hj - hJ12/l"/_2,
(2.61)
where J is the scalar coupling constant in Hz. The source of the scalar coupling can be spin-orbital, spin-dipolar and Fermi contact interactions. The Fermi contact interaction is probably the most important. The sources of these terms are discussed in detail by Jameson (1987) and Facelli (1996). From Eq. 2.61 there is no Bo dependence of this interaction. In reality the scalar coupling has to be represented by a tensor as it is orientation-dependent. However, since the tensor character is only apparent in solids where it is usually a relatively small perturbation compared to the other spin interactions it is generally ignored. The scalar coupling is mainly observed in liquids since, as it is usually small, it is often obscured in solids by other interactions. When there are several coupled spins, the total scalar Hamiltonian is Hj - Z hJijmimj i,j
(2.62)
Hence the pattern produced by the coupling gives important information about the local spin network and has proved very useful in liquids for identifying the local fragments of molecules, especially in large molecules such as proteins. If a spin-I is coupled to several spins S then including the Zeeman interaction the Hamiltonian is H = H z + H j = h VoI z + JlshlzSlz + JishlzS2z
(2.63)
With only the Zeeman interaction a single resonance is observed, but the scalar coupling now splits the resonance with the pattern depending on the number and type of coupled spins. The number and intensity ratios follow a binomial distribution. If the observed spin I is coupled to a single spin-1/2 nucleus then a pair of resonances split by the J-coupling value is observed, whereas if it is coupled to two identical spin-1/2 nuclei then the resonance is split in three with an intensity ratio of 1:2:1, again separated by the scalar coupling constant J (Figure 2.6A). The magnitude of the scalar coupling depends on the degree of covalence of the bonds, the number of bonds between the nuclei, the gyromagnetic ratio, and the degree of orbital overlap. Heavy nuclei have more extensive orbitals leading generally to larger scalar couplings. The value of J also depends directly on the product of the gyromagnetic ratios of the two nuclei involved, and often, reduced coupling constants normalised by this product are quoted to remove
42
Multinuclear Solid-State NMR of lnorganic Materials
A
B
I
C
J
'" "i
,
-640 .....
i
-680
-720
~19Sn shift (ppm) w.r.t. (CH3)4Sn Figure 2.6. Coupling multiplets of a spin which is coupled to A. one and B. two spin-l/2 nuclei, and C. example of l l9Sn in BaSnO3 showing the J-coupling with crystallographically equivalent but magnetically inequivalent ~17Sn, from Clayden, Dobson and Fern (1989) with permission of the Royal Society of Chemistry.
this factor (Jameson 1987, Sanders and Schrobilgen 1990). This reduced factor illustrates how other influences play a role, with one-bond coupling increasing for 1H from --- 40 to carbon to 940 for lead, and increasing for 19F from 195 for nitrogen up to 2100 for bismuth. There is no reason why this coupling should be purely isotropic and anisotropy in the J tensor is possible. The J-coupling is usually taken to be directed along the internuclear vector about which the tensor is probably axially symmetric. J is then described by an anisotropy AJ = J - J• The complete J-Hamiltonian is then ,, ,, Z~ r Hj - JI.S. +
[3]:j: (S. j) -
IzSz ]
(2.64)
where jz is a projection of the unique axis of the J tensor onto the z-direction. The anisotropic component interaction is identical in functional form to the direct throughspace dipolar coupling and hence it is very difficult to separate these contributions. Often a pseudo-dipolar Hamiltonian is written to describe the bilinear coupling between two spins I and S, which is the same as Eq. 2.64 except that the dipolar coupling is replaced by an effective dipolar coupling constant D ' = P~ 8Jr 2ri3
ALl 3
(2.65)
Then the coupling between the two spins I and S including these two effects gives a frequency shift for the (m, m - 1) transition of Av m - - m J + mD'(3cos 2 0 - 1 )
(2.66)
where 0 describes the direction of the internuclear vector in space. Reports of J-coupling are quite infrequent in solids, as the effects are often small compared to other effects
Physical Background
43
which mask the J-coupling. Figure 2.6B shows an example of a J-split spectrum of 119Sn in CaSnO3. Reports of anisostropy in J are even rarer.
2.2.3 Paramagnetic coupling If a material contains a localised unpaired electron (e.g. on an ion) this electron can cause an additional magnetic field at the nucleus. The interaction Hamiltonian between the nucleus (I) and the electron (S) can be generally written as H I, = ~ h I . P . S
(2.67)
The coupling tensor (P) to the paramagnetic ion produces two main sources of additional field. One is the Fermi contact or hyperfine interaction. The general form of the contact interaction produces a Hamiltonian Hcon = -12 - -o- 8re y , ye I. S a(r)
4re 3
(2.68)
where 8(r) is the Dirac delta function. Unpaired electron spin density, through spin polarisation, will induce unpaired spin polarisation on the s-orbitals of the nucleus, which has a finite value at the nucleus. This s-polarisation interacts directly with the nuclear magnetic moment to give a frequency shift of Avc - A y B ~ (Sz) h
(2.69)
where (A/h) is the electron-nucleus hyperfine coupling constant in Hz and (Sz) is the expectation value of Sz, the average component of the electron spin along the magnetic field. The rapid thermal flipping of the electron spin can greatly reduce this expectation from its full value, and is in general given by (Sz) _ _ ].tBJ(J + 1 ) g j ( g j -- 1)B o 3kT
(2.70)
where IXB is the Bohr magneton, J is the total angular momentum quantum number for the electron state and gj is the Land6 g factor given by gj =
3 J ( J + 1) - L ( L + 1) + S(S + 1)
(2.71)
2 J ( J + 1)
The mean electron moment given by Eq. 2.69 will only be parallel to Bo if g is isotropic. If the nucleus being observed is not the ion with the initial unpaired electron, spin s-orbital polarisation can still be induced through effects transferred via the chemical bond, in which case it is called the transferred hyperfine interaction. This effect can still be strong although it will decrease rapidly as the number of intermediate bonds
44
Multinuclear Solid-State N M R of Inorganic Materials
increases. For example in the series Y2-xRExSn207 each yttrium (ignoring oxygen) has a shell of tin, and further out a shell of yttrium. If a rare-earth (RE) substitutes for the central yttrium the shift effects on the tin shell are an order of magnitude greater than the next yttrium shell (Grey et al. 1989, 1990). In paramagnetic systems the Fermi contact term is expected to dominate. However in addition there is also the possibility of a through-space dipolar effect. The shift caused by the trace of this tensor interaction is often called the pseudocontact effect, which for a lanthanide ion is given by 2
2
AVpc = _ g j l J s J ( J + 1)(2J - 1)(2J + 3 ) F ) ' B o
1207r(kT) 2 r 3
(2.72)
where F is a function that depends on the angle between the principal magnetic axis and the lanthanide-resonating nucleus vector, the crystal field parameter of the lanthanide site and a numerical coefficient. In Y2-xRExSn207 for the tin sites the Fermi contact term dominates but for yttrium site contact and pseudocontact terms make comparable contributions, so it is important to take both effects into account (Grey et al. 1989, 1990). The Fermi contact interaction is isotropic but the pseudocontact interaction is a full anisotropic tensor interaction. The angular dependence of these terms is of the form A0 (3cos 20 - 1) + Azsin 20 cos2q~
(2.73)
2.2.4 C h e m i c a l s h i e l d i n g
The factor which turned NMR from simply an interesting physical observation of the nuclear magnetic moment into a widely useful technique in analytical sciences was the realisation that the exact resonance depends on the chemical environment of the nucleus (Proctor and Wu 1950, Dickinson 1950). This was termed the chemical shielding and means that the exact resonance frequency is an extremely sensitive discriminator of the chemical environment as it depends on the electron density and hence reflects the chemical bonding. The chemical shielding arises from the change in the actual magnetic field at the nucleus since the applied magnetic field induces changes in electrons about the nucleus with generate local fields. To a first-order approximation the chemical shielding is linear with the applied field strength (Grant 1996). This effect can either add to or subtract from the main applied magnetic field (termed paramagnetic or diamagnetic respectively). Hence the energy of interaction is E - -yhI
. B o - ~ It~
H ~~t~ ? ' h / ( l - o r - Z ) H o 27r )'h/~.(M_ + _ o ) - 2n: .__ .~
(2.74)
This equation splits the contributions related to chemical shielding and susceptibility effects. This distinction is somewhat arbitrary as they are both a result of the response
Physical Background
45
of the electrons to the applied magnetic field giving rise to additional internal magnetic fields. X is usually only significant if the susceptibility of the solvent in liquid state NMR is very different, or if the shape of a solid sample (including the grains of a powder) is very different, giving rise to very different demagnetising effects. The three-dimensional nature of the electron density in a material means that the chemical shielding is represented by a second-rank tensor (if) and the corresponding Hamiltonian is
Hcs
=
7~h I_.q. B_o
(2.75)
~
The orientational dependence is present in the chemical shielding tensor which in its principal axis system (PAS) is diagonal with principal components ~rxx, ~ryy and (rzz. In a crystalline powder sample all orientations of the PAS system of the shielding tensor are present and for most orientations the PAS does not coincide with the laboratory (LAB) frame where the z-axis is aligned with the direction of the applied magnetic field Bo. Hence the tensor representation gives in the PAS q P A S - - I ~ axx
0 ayy 0
00
I
(2.76)
r
which when transformed into the laboratory flame becomes O'xx
O'xy
(Txz
~= -- (Tyx
O'yy
(Tyz
O'zx
(Y zy
($ zz
(2.77)
The tensor components are distinguished by capital subscripts in the PAS and lower case subscripts in the laboratory frame. The two frames are related by rotation of the axes using matrices with the Euler angles defining the rotation to give G=Lab -- R ( a ~ ) q
(2.78)
PAS R -1 (aft'J/)
Then the field at the nucleus, including both the main field and the shielding, gives
1-a~ -a~y -a~z ~ I-a~z Bo B-(1-q).B o-
-Cryx -az~
1-Cryy -ay z -azy
1-azz
Bo
-ay z Bo ~,(1-azz)Bo
(2.79)
46
Multinuclear Solid-State N M R o f Inorganic Materials
It is only the zz-component that is of interest since after this transformation this is the component that is aligned with Bo, Oxz and (ryz make only second-order contributions. Hence the Hamiltonian representing the chemical shielding is now given by Hcs - ?'t hi_. o'__- B o
(2.80)
Crzzis obtained by performing the appropriate set of rotations from the PAS to the LAB frame and this tensor element is given by Ao" [(3cos=O-1)+ 77( sin=0 cos 20)]
(~zz -- l~iso + T
where Act - o'-: - ~riso is sometimes called the shielding anisotropy and r / -
(2.81) I~ XX -- l ~ y y
A~y
is the asymmetry. The angles 0 and + are the polar angles of the LAB frame in the PAS of the tensor. The variation of different components of the shielding tensor often reflects the local symmetry around the nucleus and is characterised by three parameters. Rather than the three principal components, combinations of these are quoted, namely the isotropic value, the anisotropy and the asymmetry. Chemical shielding (or) is normalised by looking at the difference of the resonance frequency between the sample (Psample) and a reference Larmor frequency (PLarmor)for that nucleus (i.e. the bare nuclear frequency). It is then normalised by (PLarmor) and quoted in parts per million (ppm)
O'sample =
VLarm~ -- Vsample (X 10 6 ).
(2.82)
VLarmor
This normalisation means that measurements at different magnetic fields can be directly compared. A lower resonance frequency indicates a higher shielding value. Rather than the chemical shielding the chemical shift is quoted, where the sample frequency (Psample) is referenced to the resonance frequency of a standard reference sample (Vref) instead of the Larmor frequency. It also is normalised, with units of ppm, and its symbol is 5. Chemical shifts are, in fact, used far more often than chemical shielding since this is the parameter experimentalists measure. 5sample -- Vsample -- Vref (• Vref
6)
(2.83)
For the chemical shift a higher resonance frequency means a higher chemical shift value. The chemical shielding and chemical shift are related by 5 -- l~ref -- l~Ysample 1 -- (~ref
(2.84)
Physical Background
47
which to a good approximation for IO'refl < < 1 gives ~sample -- O'ref -- O'sample
(2.85)
The difference in sign between g and cr arises because g is defined in terms of frequency whereas cr is defined in terms of the fractional change of the magnetic field. There are no inconsistencies with these approaches, which have arisen through the historical development of the technique in which sweeping the magnetic field has been largely superseded by pulsed Fourier transform methods. The different terminology can also be quite confusing when it is first encountered. A more positive value for indicates a higher frequency (and thus downfield) whereas for shielding this would mean a lower frequency. An interesting consequence of the definition of g has been pointed out by Neue (1996). If different references are used in different studies then to relate the two studies, the values are compared by making a simple additive correction of the two references. If the references are a long way apart then more correctly the shift on the second scale is I~2 -- (--I~R1,R2 nt- ~1 ) V~
(2.86)
Vo2
This uses the convention in Eq. 2.84 for the definition of the chemical shift where ~1 and ~2 are the shifts of a material with respect to references R1 and R2, and gm,r~2 is the shift of reference 1 measured against reference 2. The references 1 and 2 have absolute frequencies of Vo1,2 respectively. Hence it can be seen that for many compounds this correction factor will make very little difference, but for elements with big shift ranges or using different nuclei to obtain a reference means that this factor must be taken into account if high accuracy is required. The tensor elements of the chemical shift also need to be defined and in the literature there have been a number of approaches adopted to define parameters describing the tensor. Naturally the three principal components of the tensor and the spatial orientation of these components on the molecular frame will define the tensor. However, papers often do not define the individual tensor components and the parameters adopted by each paper to define the tensor need to be carefully considered. A recent suggestion (Mason 1993) for standardising the definitions is to have the three elements in the PAS designated ~11, ~22 and ~33, with ~11 corresponding to the highest frequency. Then the isotropic value is still the average of the sum of these three terms with the span defined as (2.87) which will always be -> 0, and the skew
48
Multinuclear Solid-State NMR of Inorganic Materials --0"22) = 3(~22 -- (~iso ) 0"33 --O'll (~11 --I~33
tr = 3(r
(2.88)
which will vary between - 1 and 1. The tensor elements are necessarily ordered for the above definitions as 033 -> 0"2= -> 0"33 and consequently 811 -> ~22 ~ ~33. Particular special cases are (Figure 2.7):
811=~'~2
833 811
822
833 811
822--q~33
Figure 2.7. Example of static CSA powder patterns showing different skews. spherically symmetric where 811 = ~22 = ~33, ~ -- K -- 0 axially symmetric (prolate) where 811 = ~22 5/:~33, ~'~ finite, K= 1. axially symmetric (oblate) where 81~ 4= ~22 = ~33, , [-~ finite, K = -- 1.
2.2.5 Knight shift In a conducting solid the nucleus will couple to electrons of the chemical bonds in the same manner as above for diamagnetic insulating solids, but there is the additional effect of the conduction electrons. Unlike paramagnetic and diamagnetic solids, the electrons are delocalised in conductors and a given nucleus will experience the magnetic field from a range of electrons. The effect of this collective influence must be described. The conduction electrons occupy a Fermi distribution within the electronic states of the material. These states are occupied in a pairwise fashion due to the Pauli exclusion principle (spin up and spin down) from the lowest energy up to the Fermi energy. At Bo = 0 the pairing will mean that there is effectively no net magnetic moment from these electrons. However, under a magnetic field there is a shift in energy of the spin-up and down states. The resulting imbalance in the number of spin-up and down states leads to a net magnetisation of the material and hence an associated susceptibility termed the Pauli susceptibility (Xp). Then, if the change in populations are n + 1/, M = / l e ( n ~ - n_,/2)
(2.89)
if the electron distribution is described by fiE) so that
M - lu2Bf nf(E)dE For a free electron gas, in the limit that Ef > > kT then
(2.90)
49
Physical Background M - IaBBon(E f ) ~ Zp - ~ t o l 2 B 2 n ( E f )
(2.91)
The shift in the field thus produced can be determined by calculating the field caused by the electrons producing the susceptibility. This can be expressed in terms of the hyperfine field produced by the conduction electrons, a contact type of interaction,
4re 3
F
This depends on the value of the wavefunction at the nucleus (r = 0) for electrons averaged over the Fermi surface, where 12 is the volume per electron. This shift of frequency in a metal was first observed in copper in 1949 by W.D. Knight and was subsequently termed the Knight shift (Townes, Herring and Knight 1950, Knight 1956), given by K - AB
2
(2.93)
Since K depends on the wavefunction density at the nucleus, the effect is dominated by s-electrons which is certainly true in metals with unpaired s-electrons. If the Pauli susceptibility and electron density can be independently measured then the Knight shift will give an independent measure of the s-component of the conduction electron spin density. These shifts are positive and are much larger than chemical shift effects, some typical values being L i - 0.025%, A g - 0.52% and H g - 2.5%. In other metals the situation is more complicated when the s-electrons are paired but there are other electrons (e.g. p but especially d). As only s-electrons have significant density at the nucleus the effects of these other electrons are much smaller. The hyperfine fields of these electrons induce polarisation in the s-electrons that subsequently produce a shift, termed core polarisation.
R-orb + (y
Total s h i f t - K + c r - K s + Kcp + "~d
(2.94)
A major problem is that it is the net value of the shift which is measured in the experiment. Two points emerge from this. The zero of the scale needs to be known so that the contribution of the chemical shielding has to be taken into account. Also, in more complicated metals the different terms have different signs, with Ks and Kd ~ positive whereas Kcp is negative. If the symmetry of the site is lower than cubic the full tensor form of the electronnucleus interaction needs to be used, so that in addition to an isotropic term there is an anisotropic contribution. If in the PAS of the Knight shift tensor the components of the tensor are Kx, Ky and Kz, then in the laboratory frame with its orientation in the frame defined by Bo described by the Euler angles 0 and +,
50
Multinuclear Solid-State NMR of lnorganic Materials B - B o { K z cos20+Kx sin2Ocos2~+Kr sin2Osin2~}
(2.95)
In many materials the tensor is axially symmetric so that Kx = Ky - - Kz/2 and the anisotropy is usually axially symmetric Kax. The frequency shift is then given by
AV-
YBo (Kis o + Kax(3 2re
COS 2
O-- 1))
(2.96)
2.2.6 Quadrupole interaction It can be shown that for nuclei with I > 1/2, the electrical charge distribution of the nucleus is non-spherical, giving rise to an electric quadrupole moment. The background to the quadrupole interaction is given in the classic article by Cohen and Reif (1957) and a comprehensive recent introduction has been given by Man (2000). The quadrupole moment is designated eQ and can be prolate (eQ > 0) or oblate (eQ < 0). The energy of a charge distribution in an electrostatic potential is V(r)
E - J
p(r). V ( r ) d r
(2.97)
In the volume surrounding the nucleus V(r) can be expanded as a Taylor series as
V(r)- V(O)+
Z
i--~t
i=x,y,z
+~
ij
r=0
" "
aiaj[
+
(2.98)
r=0
which can be substituted into Eq. (2.97) to give 1
i
(2.99)
t,y
where ~V
Vi - - ~ / . and
V~, -
~2V
Oi---0-]
(2.100)
Vii is a second rank symmetric tensor which is diagonal in its PAS so that Vq - ~ij . Vq
(2.101)
The zero-order term in Eq. 2.100 represents the electrostatic energy which is the same for all orientations. This term will have no influence on the spectrum and can be ignored. The first-order term represents the electric dipole moment and from the fact that the nuclear wavefunction is symmetric (i.e. r(r) 5 r (2 r)), the product rr(r) is antisymmetric and this term will be identically zero for all nuclei. The second-order term is the electric quadrupole moment and this is the most important in giving an
Physical Background
51
orientational dependence to the effect of this interaction on the NMR spectrum. The variation will be determined from the deviation of the charge distribution from spherical symmetry and is defined as
eQ - I P(r)(3z2 - r2 )
(2.102)
For the potential Laplace's equation states VZv = 0, from which it follows that for the electric gradient tensor, s
-0
(2.103)
i
This property means that to fully define V only two of its three components need to be known. It is usually chosen to define these as the largest component
Vzz = eq
(2.104)
and the asymmetry parameter
(Henceforth the term xI will be taken to refer exclusively to the asymmetry parameter in the quadrupole interaction). The energy equation for the quadrupole interaction can be transformed into a form that makes it compatible with the other Hamiltonians above by substituting spatial operators with spin operators using the Wigner-Eckart theorem (Slichter 1990) which after some manipulation gives the quadrupole Hamiltonian in the PAS of this interaction
)]-
3# -
+
(i+ +
where XQ is the quadrupole coupling constant defined as
ZQ=
eeqQ h
(2.107)
The electric field gradient (efg) is set up by the charge distribution outside the ion (e.g. A13+) but the initially spherical charge distribution of inner shells of electrons will become polarised to lower their energy in this efg. This polarisation of the inner electrons produces an additional efg at the nucleus so that eq, = eq (1 - ~/~) where ( 1 - ~/~) is the Sternheimer antishielding factor (Sternheimer 1954). This factor is a measure of the magnification of eq due to distortion of the inner electrons close to a nucleus. Full energy band structure calculations of efgs have improved markedly in recent years with developments of code such as WIEN97 (Blaha, Schwartz and
52
Multinuclear Solid-State NMR of Inorganic Materials
Dederichs 1988, Blaha, Schwartz and Luitz 1999). These show the importance of the contribution of the electrons on the ion itself compared to the lattice. These direct calculations remove the need to apply this correction factor but it is helpful to make it clear why some nuclei experience much larger quadrupole interactions than others. To obtain the quadrupole Hamiltonian of a spin in a magnetic field the Hamiltonian needs to be transformed from the PAS to the LAB frame, keeping only those terms that commute with Iz. This is called truncation of a Hamiltonian and is only valid when HQ < < Hz (the high field approximation). To perform the transformation it is much more convenient if second-rank irreducible spherical tensors are used. The Cartesian and spherical tensor elements (T) can be related (see Schmidt-Rohr and Spiess 1994 and Eq. 8, in Man 2000), with two of the more common elements being (2.108)
~o - i= and "~/-6/~2o- 3~2 _ 12 In these operators, the quadrupole Hamiltonian is
where Vi are the elements of the electric field gradient in the PAS so that
V0 = ,1~ eq, V+~ - 0 , ~2 -
V+_2=
eqJ~
(2.1 10)
2
In the high field limit, where the quadrupole interaction acts as a perturbation of the Zeeman states, the terms of this Hamiltonian which commute with Iz lead to the perturbation of first-order
4(1)_ o
eQ
4 1 ( ~ - 1)
~[3i2 i2]V0
(2.111)
and the second-order term is
(
H~2) _
2
/2
{VIV liz (412 -- 8 # - 1 ) +
eQ
V0 4 I ( 2 1 - I )
VzV2I~( 2 I
2 - 2 I 2 - 1)} ( 2 . 1 1 2 )
These perturbations then lead to a shift in the separation of the Zeeman states Em.m- 1 - E m - E m - 1 which to first-order is E(l) _
3eQ
- 41(21 - 1)
(1 - 2m)V0
(2.113)
PhysicalBackground
53
However second-order effects are also present and for the central (1/2, -- 1/2) transition as there is no first-order energy shift (m = 1/2), these become particularly important. This shift is E(2) =
2[
eQ ]2~V-~Vl(24m(m-1)-4I(I+l)+9)+} I V-2V2
v0 4 I ( 2 I - 1 )
(2.114)
(lZm(m-1)-41(I+l)+6)
The Vi then need to be transformed from the PAS to the Laboratory frame. If the Euler angles (or, [3, ~/) describe the orientation of Bo with respect to the PAS system the tensor elements can be transformed by Wigner rotation matrices Dij (n) (Haeberlen 1976) via
2 (2.115)
_ Z
j=-2
Substituting these values and operating on the states with the spin operators gives the perturbations and the orientation dependence
E(1)=
Zoh 8I(2I-1)
[ cos
0 -1+
0
-
+
The Zeeman splitting gives a set of equally-spaced energy levels, shown for a spin-5/2 system in Figure 2.8. To first-order, the quadrupole splitting gives a set of symmetric transitions with the magnitude of the effect depending on XQ and a characteristic shape determined by 0. The first-order term splits the spectrum into 2I components of which the single quantum intensity is detected. This intensity depends on I<mlIxlm+ 1>12 (proportional to I(I+ 1 ) - m ( m + 1 ) = oL) at frequency V(1)m, m -- 1 away. The intensity distribution between the different transitions has to be carefully accounted for when accurate quantification is required from NMR spectra. The splitting between different transitions also causes a change in the pulse response. If XQ is very small so that the energy levels remain effectively evenly spaced, the response to the if-pulse is as would be expected for a spin-l/2 nucleus with the same ~/(Schmidt 1971, Fenzke 1984, Man 1988). This is called non-selective excitation and the magnetisation precesses as sino~rft. When XQ becomes significant, the separation between adjacent levels is quite different and at each set of energy levels the nucleus behaves as an effective spin1/2 nucleus but with a different ~/. This limit is termed selective excitation. This ability to consider transitions in pairs has become known as the fictitious spin-b2 formalism with each different transition showing its own nutation rate, and its pulse response now being sinoLtOrft. This change of response to the rf pulse means that the effective 90 ~ pulse
et al.
et al.
54
Multinuclear Solid-State NMR of lnorganic Materials B0 only Zeeman interaction
-hv0m
First-order quadrupolar
Second-order quadrupolar
~0~(3cos20_1)
9hz~ 6400vo
5
5(2 sin 2 20+ sin 4 O)
/
-512
-3/2
_~
-1
.3(sin 4 8 - 2sin 228)
-1/2
N
-4
2(sin' O- 2sin' 20)
112
~
-4
/ 2(2sin' 20- sin' O)
3/2
_...
-1
/
3(2sin2 20-sin4 O) 5 5/2
-5(2sin 220+ sin4 O)
/
Figure 2.8. Energy level diagram of a spin-5/2 system showing the Zeeman interaction and the first- and second-order quadrupole perturbation of the energy levels.
Table 2.3. Changes in the 90 ~ pulse and the intensity distribution for different transitions for quadrupole nuclei. I 3/2 5/2
7/2
9/2
mz
1/2 3/2 1/2 3/2 5/2 1/2 3/2 5/2 7/2
1/2 3/2 5/2 7/2 9/2
90~ 0.500 0.578 0.333 0.354 0.447 0.250 0.258 0.389 0.378 0.200 0.204 0.218 0.250 0.333
Int (%) 40.0 60.0 25.7 45.6 28.6 19.0 35.8 28.6 16.6 15.2 29.0 25.4 19.4 11.0
t _ For large • comparedto the case where there is no quadrupole splitting.
55
Physical Background
changes, as shown in Table 2.3. The change of response to the rf pulse with XQ is the basis of the 2D quadrupole nutation technique (Samoson and Lippmaa 1983, Guerts et al. 1985). The differences in the pulse response and the observed intensity for the selective and non-selective cases need to be considered. For non-selective excitation,
2 lm+l,m _ am+l,m "nonsel -- I-1
sin mrf t
Z 2 am+l,m -I
(2.117)
and the equivalent expression for selective excitation is m+l,m i sel
_
am+l,m I-1
Z -I
sin a~o~ft
(2.118)
2
am+l,m
so that in the small pulse angle limit (sin0 --- 0) these two expressions become the same and intensity between sites with very different XQ can be compared directly (Schmidt 1971, Lippmaa, Samoson and Magi 1986). The frequency shift can cause the non-central transitions (i.e. m # 1/2) to be shifted sufficiently far from the Larmor frequency that these transitions become difficult to observe with conventional pulse techniques. Equation 2.116 also shows that there is an orientational dependence of the frequency so that very broad lines occur in a powder. This is important for spin-1 nuclei as there is no central transition and all transitions are broadened to first-order. For the satellite transitions the shape extent will depend on XQ and the shape will depend on ~ (Figure 2.9A). It should be noted that for m <--->- m transitions there is no first-order effect. For the central transition of non-integer quadrupolar nuclei, v(]~ _ i/ - 0 , and "2' 2 . the dominant perturbation is second-order. The rotation of the tensor elements in the second-order equation gives _
1//2
(--~
V1V_1 - - ~ - 3
COS2 20~ + 2 r / c o s 2 a - 3)cos 4 fl
e2q2 q-(~-3/72cos 2 2 0 : - 2 r / c o s 2 a -
1//2 --b3)cos 213
3
(2.119)
1//2 (1 - cos 2 20:) and 1
2
2
1
( - ~ r / cos 2 c t - - r / c o s 2 a 4
V2V_2 -
_3_.e2q2
+(--12//2 COS2
2
1.2
_
3
8 )cOS4fl
2a_ 1 2
77 - 3 )cos
1
3
+ - ,t cos 2 2 a + - 7/cos 2 a + 3 4 8
(2.120)
Multinuclear Solid-State NMR of Inorganic Materials
56 A
B
rt=l
.n=l
n =0.
n =0.9
n =0.
n =0.8 n = 0.7
n---0.4 r--J
~_~~--a
n--~
~ , ~
.... H
.... ,___ '--~ .....
I
-2
,__
n-0.3
....
n =0 I
n-0.4
n =0 I
-2VQ
I
l
-VQ
I
I
I
0
I
I
I
I
2VQ
VQ
I
I
1
I
0.5
0
I
-0.5
I
-1
I
-1.5
I
-2
A
Figure 2.9. The quadrupole perturbed powder patterns for an I = 3/2 nucleus. A. The outer satellite transitions to first-order. B. The second-order quadrupolar broadening of the central transition with A = (I(I+ 1) - 3/4)p~/I10. The second-order line can then be calculated for any transition so that for the (1/2, -- 1/2) transition ~,(2) _ 1 I 3ZQ ~-~ - - 6v----~ 2 I ( 2 I - 1)
[A(a,~)cosa fl+B(a,~)cosZ fl+C(a,~)]
I(I+1)-
(2.121) where 27 9 A ( a , rl) = - - - + - 0 c o s 2 a - 8 4
B(a, rl) =
30
/72
8
2
3 ~2
cos 2 2 a
(2.122)
8
277 cos 2 a +
3 -4~
2
cos2 2 a
(2.123)
Physical Background
57
2
C(a, 7/) - - ~3 + __f_0- - ~-r/cosZct--8 1 3 r/2 cos 2 2o~
(2.124)
To obtain the lineshape of the central transition the second-order perturbation of the quadrupolar interaction must be calculated on the (1/2,-1/2)transition. These give distinctive lineshapes for the centreband (Figure 2.9B) which provide information about the local symmetry of the electrical charge about the nuclear site. For the special axially symmetric case where rl = 0,
,
(2)
"~-~
_
1
3XQ
I ( I + 1)-
(1- cos 2 fl)(9 cos 2 f l - 1)
(2.125)
16v o 2 I ( 2 I - 1)
If the total width of the central transition is Av, an estimate of the quadrupolar interaction is
xQ~r/~ + 22r/Q + 25 - 8 I ( 2 I - 1)l
v~ 3 (I(I + 1 ) - ~)
(2.126)
2.2.7 Nature of interactions All these interactions are quite similar in form, containing an anisotropic part so that in powders the lines are usually significantly broadened. The first-order perturbation shows an angular variation of the form A((3cos213 - 1) + "q sin2[3cos2o0 which is similar for all interactions. Sometimes it is necessary to distinguish between homogeneous and inhomogeneous interactions. The distinction can affect the way their averaging must be approached and also the way the interaction responds to refocusing, e.g. in echo experiments. An inhomogeneous interaction means that a broad spectral line can be regarded as being made up of many individual contributions, with each spin, (e.g. depending on its orientation) contributing intensity to a specific part of the line. For a homogeneous line, a spin via its interactions with other spins (especially via spin diffusion caused by the flip-flop term of the dipolar Hamiltonian) contributes intensity effectively to the whole line. This distinction gives rise to the well known phenomenon in laser and optical spectroscopy of hole burning. If a resonance is strongly irradiated by narrow band radiation at a specific frequency within a broad spectral line, a homogeneous line will show a decrease in intensity across the whole line whereas an inhomogeneous line will show a decrease in intensity only in the vicinity of the irradiation. Most interactions are inhomogeneous e.g. shielding effects, the quadrupolar interaction and Bo inhomogeneity, whereas homonuclear dipolar coupling can be homogeneous. A more rigorous distinction between these two interactions
58
Multinuclear Solid-State NMR of lnorganic Materials
is that given by average Hamiltonian theory. If the Hamiltonian operator is taken as a function of time and the terms commute at different times, the interaction is inhomogeneous. There is also possible intermediate behaviour where irradiation at a specific point affects other parts of the line but not the complete line, for which the term heterogeneous has been coined, although this can be regarded as largely inhomogeneous. For these interactions there is also the concept of coherent and incoherent homogeneous interactions. T2 relaxation is incoherent due to the independently fluctuating local fields whereas coherent effects would be generated by multiparticle interactions such as the dipolar couplings between spins. This is discussed in some detail by SchmidtRohr and Spiess (1994). The inhomogeneous interactions such as shift anisotropy and dispersion, two-spin dipolar interactions and the quadrupolar interaction can be refocused by spin echoes (Hahn 1950) whereas coherent multispin interactions can only be refocused by sequences such as the solid echo (Powles and Strange 1965) and magic sandwich echo. Incoherent effects are essentially irreversible.
2.3. ONE DIMENSIONAL METHODS FOR IMPROVING RESOLUTION
The anisotropic part of the interaction can often provide insight into structure, but is usually regarded as the poor relation of the isotropic information. In materials with relatively few sites static NMR spectroscopy is often worth considering. Its disadvantage is that the anisotropic part causes broadening which can often be very significant so that there is strong overlap between different sites, meaning that different sites cannot really be resolved. In liquids the interactions responsible for line broadening are averaged by the continuous, random tumbling and translational motions of the molecules. This isotropic averaging of the second-rank tensor interactions produces high resolution spectra. However, for solids it is often necessary to improve resolution by deliberately averaging the anisotropic parts of the interactions, thereby obtaining line narrowing. The Hamiltonians representing these interactions have all been seen to contain a spatially-dependent part and a spin-dependent part. To average these interactions, either (or both) of these parts of the Hamiltonian must be manipulated which usually means making them time-dependent. By far the most common approach is to make the spatial part time-dependent, for example by applying magic angle spinning (MAS). Sometimes this is extended to other angles and is then called variable angle spinning (VAS). There are other more complex time dependencies imposed such as double angle rotation (DOR) and dynamic angle spinning (DAS). The spatial part can also be varied by diluting the spins (i.e. increasing r) which reduces the dipolar coupling since the magnitude of the dipolar coupling is proportional to the inverse cube of the distance between spins. By making that distance large, the dipolar coupling can be made small. This situation occurs naturally for 13C where the low natural abundance ensures that carbon-carbon dipolar couplings are not
Physical Background
59
observed in J3C spectra. For nuclei such as protons where the dipolar coupling between protons can be very strong, diluting the protons can be extremely helpful. This can be done by deuterating a sample, which is acceptable provided it does not change the chemical nature of the sample (which might not be true, for example, for hydrogen-bonding). For species that are naturally rare e.g. 13C, aSN, selective isotopic enrichment selectively introduces the dipolar coupling which can be used to determine bond lengths. Techniques for averaging the spin part include decoupling, both for heteronuclear and, by multiple pulse techniques, for homonuclear dipolar coupling cases. Averaging techniques can be used in combination with each other, leading to even higher resolution enhancement. Included in such approaches are the combination of multiple pulse homonuclear dipolar coupling averaging with MAS which is then termed CRAMPS (Combined Rotation and Multiple Pulse Sequence), and more recently for non-integer spin quadrupolar nuclei, multiple quantum excitation has been combined with MAS in MQMAS sequences. For multinuclear studies of solid materials all of these techniques should ideally be available so that high resolution solid-state NMR spectra can be obtained from a range of solids.
2.3.1 Magic angle spinning and first-order effects 2.3.1.1 Physical principles. Magic Angle Spinning is probably the most widely used technique to enhance spectral resolution in solid state NMR spectroscopy (Andrew 1971, 1981). The solid sample, usually a powder, is loaded into a container called a rotor which is inclined at a fixed angle to the magnetic field and rapidly spun about its symmetry axis. The Hamiltonian thus becomes time-dependent and to find its new form the tensor elements have to be rotated, now not directly from the PAS to the Laboratory (LAB) frame, as for a static powder, but first to the rotor frame (ROT). With the Euler angles describing the relative orientations of these frames shown in Figure 2.10 this can be readily achieved by using the Wigner rotation matrices to give in the Laboratory frame
H L~b(t) - CI~AjToo + CIA Z "mO r) (2) (0, O, (_Ort+ q~)Z D(2) m l m (CZ,i~, ~') m
(2.127)
m1
This then leads to a Hamiltonian that can be split up into modified static and timedependent parts
H Lab (t)
= H + H * (t)
(2.128)
Multinuclear Solid-State NMR of Inorganic Materials
60
ZL ZROT~~
io xL~
.....
n ""--~
x Yt~ YROT
X~S
Figure 2.10. The relative orientations of the principal axis system (PAS), rotor (ROT) and laboratory (LAB) frames.
The part which now appears static in the Laboratory frame is
H - Cl_AjToo + ~/-~CI:Aj (3 c~ "
0 - 1 ) A A [(3cos 2 / 3 - 1 ) + r/sin 2/3cos2y]
(2.129)
2
This Hamiltonian is very similar in form to the anisotropic parts of the Hamiltonians found above. Thus, in a powder where [3 and ~/are different for different crystallites, the same broadening effect would occur. However, every crystallite orientation has now acquired a modulation factor 1/2(3cos20 - 1) (= P2(cos0), the second-order Legendre polynomial). If 0 is set to 54 ~ 44' 8", the value of Pz(cos0) is zero and the anisotropic part disappears leaving only the isotropic part as would appear in solution; this is termed the magic angle. It should be emphasised that this is strictly true if only first-order broadening effects are present. An alternative way to envisage the effect of MAS on spatially-dependent Hamiltonians is to consider the dipolar Hamiltonian. For a fixed internuclear vector it is the orientation of this vector that determines the magnitude of the interaction. If the sample is rotated about an axis, for any given internuclear vector its "average" orientation will become the rotation axis (Figure 2.11) which can be set such that
(3C0S 2 0 - 1)- ~-1 (3C0S2 0 m --1)(3 cos 2 1 3 - 1 ) - 0
(2.130)
Physical Background
_
J
61
A>f
2o,.rp.,r
Figure 2.11. Schematic reorientation of a dipolar pair under MAS.
2.3.1.2 Formation of spinning sidebands. The above expression for the effect of MAS has neglected the H(t) part of Eq. 2.128. This is a good approximation if the spinning speed is very high compared to the magnitude of the interaction, or the Hamiltonian is sampled after complete cycles only (termed rotor synchronised acquisition). This is not usually the case so that the full signal produced by the modulation process is obtained. Without losing generality the phase factor + in Eq. 2.127 can be set to zero which gives
H(t)- ./27 CIzAiAA[-cos2~cos20cos(.Ort + sin 2 flsin 20cos2(-Ort ] ~32
L
(2.131)
This time modulation produces spinning sidebands which are signals separated from the isotropic line by integer multiples of the spinning frequency yr. They will extend out to cover a range comparable with the frequency extent of the static anisotropy. The spinning speed necessary to break a static powder pattern up into a set of spinning sidebands must be carefully considered. Sideband formation is discussed in a classic article by Maficq and Waugh (1979). For sample spinning to have any averaging effect, a causality argument can be invoked. If an interaction produces a line that spans a frequency range Av, in the time domain this magnetisation remains coherent for --~1/Av. Any imposed modulation must be occurring faster than this to have any effect. This would apparently limit the applicability of MAS since, even with speeds of 50 kHz becoming available, many lines encountered in solid state NMR are much broader. Fortunately a distinction can be drawn between homogeneous and inhomogeneous interactions. For homogeneous interactions (e.g. strong dipolar coupling between groups of nuclei such as protons) this condition does indeed have to be approached before MAS will have any significant effect since the spins lose complete coherence on the timescale 1lAy. However, for an inhomogeneous line, the transverse magnetisation dephases after a pulse, but this is because the individual spins lose coherence with one another. A 180 ~pulse would refocus this magnetisation and a spin echo would
62
Multinuclear Solid-State NMR of lnorganic Materials
form. This means that the lifetime associated with an individual spin in the transverse plane is quite long, i.e. the intrinsic linewidth associated with any particular spin is small. It is this intrinsic width associated with a given spin that the spinning speed must exceed. In terms of average Hamiltonian theory for an inhomogeneous interaction, the Hamiltonian commutes at different times, so precise averaging will occur at rotation rates small compared to Av. Following an rf pulse, because the tensor orientations of each crystallite are different, the resonant frequency for each crystallite is different and the magnetisation rapidly dephases. This can be envisaged pictorially from the chemical shift interaction. In the static powder pattern the frequency axis could be read as an orientation axis. Then in Figure 2.12 the two sets of spins starting off at A and B have different initial precession rates. The azimuthal phase angle picked up by each of these orientations after a time t is t
(2.132)
qgi(t)- OgoCrisot+ coo H i (t')dt" 0
The time domain signal (ignoring relaxation effects) is then t
gE (t) - - Z exp(i q~i(t)) - exp(i coo~Yisot )exp Z i coi ~ H;(t')dt" i
i
(2.133)
0
f~
"0
TR
Figure 2.12. Schematic variation of the frequency of two spin packets under a chemical shift interaction showing that the frequency varies during the MAS period (TR) and that the same range of frequencies are experienced by the two spin packets.
Physical Background
63
The purely oscillatory nature of H*(t) (Eq. 2.132) means that H*(t) will disappear at times Tr-- P/vr, where P is integer. Thus after one complete revolution the particles will have regained their phase coherence and an echo (a rotational spin-echo) will form. This can be seen since as the particles change orientation due to the spinning their instantaneous precession frequency, which depends on orientation, will change. However, over a complete cycle all spin packets will have experienced the same set of orientations so that at integer multiples of the spinning frequency all spins will have undergone the same average precession governed by the isotropic shielding. Hence in the time domain the signal will be formed by a series of these rotational echoes and it is the number of these rotational echoes, and hence how long the magnetisation lasts in the time domain, that governs the resolution of an MAS NMR spectrum. Spinning sidebands are formed through the accumulated phase with the FID represented by
g(t)- e i~r176176
(2.134)
which can be used to calculate the intensity of the spinning sidebands. The intensities of the spinning sidebands formed provide information about the anisotropic parts of the interactions. Detailed calculations have been made of the sidebands for different interactions, including the chemical shift interaction, by Maricq and Waugh (1979), and Herzfeld and Berger (1980). Nayeem and Yesinowski (1988) have carried out similar calculations for sideband formation in paramagnetic solids.
2.3.2 Magic angle spinning and higher-order effects In many cases it is not sufficient to consider only the first-order broadening terms. It has already been seen that the second-order quadrupole effects often dominate the lineshape of the static spectra of the central transition of non-integer quadrupolar nuclei. It can be anticipated from the more complex angular variation of this interaction that MAS is unlikely to achieve complete averaging of this term. Quadrupole effects often result in more complex spectra than from first-order effects alone. Also, in a system that experiences multiple interactions there is coupling between these interactions and the cross terms of the interactions (the second-order quadrupole interaction can be regarded as the cross term of the quadrupole interaction with itself). These terms can produce both additional isotropic effects and residual anisotropic effects under MAS. Stuart (1994) has dealt in detail with the case of magnetically dilute nuclei that can simultaneously experience the quadrupole effect, shielding anisotropy and heteronuclear spin coupling. There are two specific cases that are more commonly observed
64
Multinuclear Solid-State NMR of Inorganic Materials
than other combinations and will be examined here viz. (i) direct second-order quadrupole effects and (ii) the distorting effects of quadrupole mixing of spin states on spin coupling interactions.
2.3.2.1 MAS of second-order quadrupole effects. The same type of rotations using Wigner rotation matrices can be applied to the second-order effects as for the firstorder interaction which leads to a second-order quadrupole energy of interaction such that
2 Co (l,m)Fo (71)+ II3ZQIc2(I,m)Pz(cosO)F2([3,)',~)+ E~2)lm, m- 1 ) - V--o 21(21-1) C4(I,m)P4(COS 0 ) F 4 (/~, ~',/7)
(2.135)
1 P4(COS0) - ~ (35 cos 4 0 - 3 0 c o s 2 0 + 3)
(2.136)
where
772
F0(r/) - 1 + ~
(2.137)
3
1-y
(cos/3)7/sin2 t3 cos 27'
(2.138)
F4(/3,r, rt,)- 1+ i 8 P4(cos/3)+ 57/ 96 ( 7 c o s 4 f l - 4 c o s 2 f l - 3)cos 2)' + 35rl 2
~(cos4/3 1152
(2.139)
- 4 cos 2/3 + 3) cos 4~'
It is of central importance to realise that this second-order quadrupole broadening has an isotropic part as well as anisotropic terms that are proportional to the second-order and fourth-order Legendre polynomials. The variations of these two functions are sketched in Figure 2.13. It is apparent that there is no single angle which can simultaneously satisfy that P2(cos0) = 0 and P4(cos0) = 0. Spinning around a single axis can
Physical Background
65
T a b l e 2.4. The coefficients Cn(I,m) for second-order
quadrupole effects. I 3/2
M
Co(I,m)
C2(I,m)
C4(I,m)
1/2
2/5 6/5 -- 16/15 - 4/5 20/3 -- 30/15 - 54/15 30/15 294/15 - 48/15 - 108/15 - 60/15 168/15 648/15
- 8/7 0 - 64/21 - 40/7 40/21 - 120/21 - 96/7 - 240/21 168/21 - 192/21 - 168/7 - 600/21 - 336/21 432/21
54/35 - 6/5 144/35 228/35 - 60/7 270/35 606/35 330/35 - 966/35 432/35 1092/35 1140/35 168/35 - 2332/35
30.6 ~
54.7 ~
--
3/2 5/2
1/2
3/2 5/2 7/2
9/2
1/2
3/2 5/2 7/2 1/2 3/2 5/2 7/2 9/2
,... \
1
\ 0.5
X
9 ' x
""
X
I I "[
I"
P4(cosO)\\
I I I
.P2(cosO).
I I
\
70.1 ~
I
I
"'..
I / " I / I/ /
"'.
\1\
" "..
0
Ix I I i
".
/I /"'/ ...
\ \
'/ ~
"-
I
-0.5
I -I
'
0
10
20
30
40
50
60
70
" ""
80
90
0
Figure 2.13. The Legendre functions P2(cos0) and P4(cos0).
c o m p l e t e l y r e m o v e e i t h e r o n e or the o t h e r o f t h e s e t e r m s b u t w i l l t h e n o n l y p a r t i a l l y a v e r a g e the other. T h e a p p r o a c h o f v a r i a b l e a n g l e s p i n n i n g b a l a n c e s this p a r t i a l avera g e to a c h i e v e the b e s t r e s o l u t i o n (see b e l o w ) . B y far the m o s t c o m m o n a p p r o a c h is to spin at the c o n v e n t i o n a l m a g i c a n g l e o f 54.7 ~ as this e n s u r e s that
all
the first-order
effects are r e m o v e d a n d to live w i t h the c o n s e q u e n c e s o f o n l y p a r t i a l l y a v e r a g i n g the P4(cos0) t e r m . T h e n the f r e q u e n c y w i l l be g i v e n b y
Multinuclear Solid-State NMR of lnorganic Materials
66
3[xQ 3[xQ
Q(2), Vm,m+ l -- _
(I(I + 1 ) - 9m(m + 1 ) - 3) 1 +
40v o 1(21-1)
2 (61(1 + 1)- 34m(m + 1)- 13)
80v o 1(21-1)
(2.140)
X [F4 (0~, F])cos 4 ~-k- F2 (a, ~)cos 2 ~--I- Fo (a, F])] where 105 16
35 35,2 8 r / c o s 2 a +~48 ,i cos 2 2 a
(2.141)
F2(a, r/) - - ~45 + 57/2 + 57/cos 2 a - 35r/2 cos 2 2 a 8 12
(2.142)
F4 (a, rl)
9
~+ F~(a' r/) - 16
7/2
5 r/cos 2 a -k-~ 35 7/2 COS2 2 a
.... 3
8
48
(2.143)
For the normally observed central transition then, this frequency in the fast spinning limit is (2)MAS
~'-~
1 [
I(I + 1)-
3XQ
6v o 21(21-1)
F~(a' r/) c~
fl +
(2.144)
F2 (a, q) cos 213+ F4 (a, r/)
where F0 (a, r/) -
~__m21 7 16
8
7
r/cos 2 a + ~ (r/cos2o~) 2 48
F2 ( a, tl ) -- ----9 + ~712 7/COS2 a -- ~(7/COS 7 20;) 2 8
12
(2.145)
(2.146)
24
5 1 7 F4 (a, r / ) - 1---~- ~ r / c o s Z a +-4--~(rlcosZa) 2
(2.147)
The centrebands produced under this limit will also have characteristic lineshapes even though the shapes will be very different from the static ones. The scale of the
67
Physical Background
narrowing can be gauged from the change in ~M2 under MAS which varies from 3.6 ('q = 0) to 2.4 (~q - 1) (Behrens and Schnabel 1981). The ratio of the overall linewidth static to MAS is generally given by the factor 7(1/2 + 2277 + 25) 2(6+//) 2
(2.148)
Again, if the singularities can be observed the quadrupole parameters can be obtained from spectral simulation and the lineshape is a strong function of xl (Figure 2.14). These lineshapes were seen in early calculations of the effect of MAS on the central transition (Kundla et al. 1981, Samoson et al. 1982, Muller 1982). In the case where 11 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 .......
,,
4t)
,
.
20
.
.
.
0
,
-2t) kl-l[z
.2 4;0.... '
20'
I) . . . .
,
,
-40
,
-r
\ -20
'-4'0
-60
8~o.~ ~ Figure 2.14. The MAS centreband of a second-order quadrupole perturbed powder pattern as a function of the asymmetry parameter xl for an I = 3/2 nucleus with XQ = 3.2 MHz at Uo = 95 MHz.
Multinuclear Solid-State NMR of Inorganic Materials
68
the spinning speed is not in the "infinite" speed limit, spinning sidebands appear. There will then be a dependence on the third Euler angle ~/. An interesting and complicating consequence of this is that the lineshape, even from the same transition, will vary according to the order of the spinning sideband. Hence a complex set of spinning sidebands will result which can make interpretation difficult, especially if a sample contains multiple sites with similar chemical shifts. Another consequence of the residual second-order term is that the centre of gravity of the lineshape does not coincide with the isotropic chemical shift. There is an isotropic second-order quadrupolar shift which is given generally by v~2) ( i , m ) _ -
Q,iso
3Z~
40Voi~i~i_ l)2
[i(i+l)_9m(m_l)_3]ll+~_l(2.149)
When only a featureless, usually asymmetric line is observed the peak position is often quoted and this can be significantly different from the isotropic chemical shift. This effect can be so large that a peak is shifted into an isotropic chemical shift range usually associated with another structural unit. There are numerous examples in the literature of mis-assignments based on this effect. Often when a material is not sufficiently crystalline for the singularities to be observed the resulting line is not just simply a featureless symmetric peak but is asymmetric. This asymmetry results from structural disorder producing a range of electric field gradients. Hence, if the sites had very similar isotropic chemical shifts (as expected if the sites are chemically very similar) then the lineshapes start from approximately the same position but the differing values of • stretch the lineshapes to different extents towards lower frequency (Eq. 2.149 is negative for m = 1/2). Hence the range of • stretches the overall lineshape to negative frequency giving the distinctive tail observed in the MAS powder spectra of disordered solids for nuclei such as 23Na and 27A1 (Figure 2.15). The inverse dependence of this effect on the applied magnetic field means that the lines become more symmetric as the magnetic field is increased and the actual peak position becomes closer to the true isotropic chemical shift. It is emphasised that most papers concerning such quadrupole nuclei when a featureless lie is observed quote the peak position. This position is, of course, a characteristic of the peak but must be quoted in the context of the applied magnetic field used since it is not a universal characteristic. Hence for the (1/2, - 1/2) transition the effect of the residual second-order isotropic shift is to produce a shift of the centre of gravity of ~cg, which means the overall position is given by 2'Q ~cg-~c~iso-3F(1)~2 1+ ~' 40 vo
(2.150)
69
Physical Background
A
B
D
i
i
1
'
'i
i
0
'i
i
-1
i
i
-2
i
-3
A Figure 2.15. Effect of a distribution of interactions on the second-order quadrupole powder lineshape of the central transition in detail with the mean interaction XQ = 2 MHz and ~q -- 0 at a Larmor frequency of 80 MHz showing A. no distribution, B. a Gaussian distribution of isotropic chemical shifts with FWHM = 0.17A, C. a Gaussian distribution of the quadrupole interaction of 340A and D. both the chemical shift and quadrupole interactions distributed, (A = 2344Hz). where F(I) is a spin-dependent factor given by Table 2.5. Spin dependent factors of the central transition.
I F(I)
3/2 1/3
5/2 2/25
7/2 5/147
9/2 1/54
The residual second-order effect produces a linewidth which is a function of I and m and then 2
Av(eZ~(i,m)~
ZQ [6I(I+l)-34m(m-1)-13] Voi2(2I-1) 2
For the central transition m = 1/2 a width can be associated with
(2.151)
Multinuclear Solid-State NMR of Inorganic Materials
70
2
2
Av~2) - f (l) ZQ(6 + ~) 224v o
(2.152)
This represents the speed that must be exceeded for MAS to cause even partial averaging of the second-order quadrupolar interaction, but fulfilling this condition does not mean that the fast spinning limit has been achieved. Although it is the central transition that is usually observed, the non-central or satellite transitions (i.e. different values of m) can also be observed. The same situation where MAS removes the Pz(cos0) term, but only partially averages the P4(cos0) term, remains. However it can be noted from Eq. 2.140 that the coefficient is a function of I and m (Samoson 1985). This means that most importantly the residual second-order quadrupole effects under MAS will be different for each transition and each spin. The factors for the relative isotropic chemical shift and residual widths normalised to the central transition are given in Table 2.6. The removal of the second-order quadrupolar width to improve resolution is the most important consideration. The best combinations of I and m for reducing these residual effects are the
(
+ 3 2 9 transitions of I = 2
tranitionsof I
n
~
5 2
nuclei with
and
/ +-
m
nuclei with only 5.6% of the width compared to the central
transition. This dependence was first noted by Samoson (1985) and was then developed extensively by the groups of Jager (see Jiiger 1994 for a summary) and Skibsted (1991), with the technique becoming known as satellite transition spectroscopy (SATRAS). For the above transitions not only are second-order broadening effects reduced, so that resolution is better, but the second-order shift away from the isotropic chemical shift position is smaller. Satellite transition spectroscopy has been most widely reported from I - 5/2 spins because of its applicability to nuclei such as 27A1 and 1 7 0 . The different factors for the different transitions have effects similar to determining the spectrum of thecentral transition at different applied magnetic fields. The m-dependence of the shift for a particular value of I also produces a different net isotropic shift, and this allows the isotropic chemical shift to be determined from a single spectrum. Hence simulation of all transitions provides an internal check on the NMR parameters. For amorphous solids the extent of the sideband manifold can be used to estimate the Table 2.6. The second-order isotropic shifts 8 (Q)isoand residual w i d t h s AV(2)Qin MAS spectra for satellite transitions (I, m) normalised to the central transition for the same spin. x/2, y/2 ~(2)Oiso
Avr
3,3 5,3 5,5 7,3 7,5 7,7 9,3 9,5 9,7 9,9 2.000 - 0.125 - 3.500 0.400 - 1.400 - 4.400 0.625 - 0.500 - 2.375 - 5.000 -0.889 0.292 - 1.833 0.622 -0.511 -2.400 0.764 0.056 - 1.125 -2.778
--
Physical Background
71
AV77
o ,~ I 1 ' 1
+
,~l++ltlN~itlllllill]l~l!lll[[tll)!lll I II ,,t t'1
-
,
,
c
(*10Snz) 8
,
,
4
.....
1'
,
9
0
,
20
.-
0
(*10Snz)
Figure 2.16. Effect of a changing distribution in the main component of the electric field gradientVzz comparing the effect on A. the satellite transition and B. the centreband with a mean quadrupole frequency of 1 MHz and then a distribution of quadrupole frequencies as shown. Only one half of the satellite transition is shown. The Larmor frequency is 104.26 MHz and a spinning speed of 11.5 kHz has been taken. From J~iger et al. 1993 with permission of the copyright owner.
average value of • and the shape of the envelope can be used to estimate the distribution. 27A1 SATRAS has been applied to a range of compounds, including ordered crystalline materials, atomically disordered solids and amorphous solids. A general problem with 27A1 NMR spectra from highly disordered solids is that the centreband peaks tend to be broad and asymmetric, often with extensive overlap of resonances that make the complete lineshape difficult to simulate. However, the inner satellite transition sidebands for I = 5/2 are better resolved and, just as importantly, are significantly more symmetric since the second-order quadrupolar broadening is reduced relative to the chemical shift dispersion broadening, allowing more straightforward deconvolution of the spectra. The effects of distributions on the shape of the centreband and the envelope of the spinning sideband from the inner transition have been calculated (J~iger et al. 1993). The sidebands take on a more monotonic decrease as the distribution becomes broader relative to the mean interaction (Figure 2.16). Even for nuclei where the satellite transitions are broader than the central transition (Table 2.6), if their second-order quadrupolar structure can be observed, they act as an independent check of quadrupolar parameters deduced from the central transition. The larger shift of the satellite transitions for I = 3/2 has been extremely useful in identifying different boron species from 11B NMR of borate glasses.
2.3.2.2 Residual coupling effects due to quadrupolar nuclei in M A S spectra. It was seen that for both dipolar and spin-spin coupling between nuclei the first-order anisotropic effects are proportional to Pz(cos0) so that under MAS if a nucleus experiences both these effects, only the isotropic effects of J remain. However, the
72
Multinuclear Solid-State NMR of lnorganic Materials
case is complicated if one of the coupled spins is quadrupolar. Suppose a spin-l/2 nucleus (I) is being observed which is coupled to a quadrupole nucleus (S). If the quadrupole interaction becomes strong enough, the eigenstates m of the quadrupole spin are not pure Zeeman states and mixing of spin states occurs, leading again to second-order perturbation terms of the Hamiltonian (Bohm et al. 1983, Harris and Olivieri 1992). This leads to a second-order frequency shift of
E
AV~Q2)- 3D'zo_ S ( S + I ) - 3 m 8Vos S ( 2 S - 1)
21
sin20sin/3Icos/3cos~O(3-r/cos2a) ]-r/sinOsin2a
]
(2.153)
This term is anisotropic and produces a powder pattern. It has been derived under the assumptions that first-order perturbation of the S-states is sufficient, that the J tensor is axially symmetric and that the unique axis of J is aligned with the internuclear vector. Under MAS this term will be scaled but, as it is not proportional to P2(cos0), it cannot be completely removed. Hence the MAS spectrum will still have some residual width, but the most profound effect is to leave an isotropic term which can be calculated by averaging the powder lineshape. Hence for a J-coupled system with an axially symmetric quadrupole interaction, the spectrum is shifted from the isotropic chemical shift by: Av- -mJ+
(2.154)
10Vos
S(2S - 1)
Hence the basic multiplet structure remains, although at low field the additional perturbation can strongly disrupt the coupling pattern. The implications of this frequency shift are that for non-integer spin-S the innermost and outermost lines will be shifted in opposite directions. Also the centre of gravity of the entire multiplet pattern is invariant under second-order effects, so that the chemical shift can be estimated from the centre of gravity of the spectrum. For a spin-l/2 nucleus coupled to a non-integer spin quadrupolar nucleus the spacings of the different transitions have been given (see Tables 2 and 3 in Harris and Olivieri 1992). The variation of the spectrum as the parameter ZD' varies is shown schematically in Figure 2.17A. Vos This effect was first reported for 13C-14N(Opella et al. 1979, Frey and Opella 1980, Groombridge et al. 1980) and was apparent as an asymmetric 1:2 doublet. Since that time these residual coupling effects have been observed for a wide range of spin pairs including 119Sn-35'37C1, 1H-14N, 13C-2H, 13C-59C0, 13C-75As, 295i-14N, 29Si-27A1, 3~p_55Mn ' 63,65Cu' 95,97M0" An example of its application is given by compounds of the family [Cu(PPh3)2(4-RC6HaNCN)]2 which have a spin fragment of the type P2CuN2 with the copper in tetrahedral coordination. This provides a cluster of three coupled spins of P2Cu in which the two phosphorus atoms have slightly different
PhysicalBackground
73 Magnetic field (T) total
A
11.7
--~&~-~-,~'~" ,~-#"".~-"~-r- ~sc u ......... ! ..... . ...........
~..!
2000
0
-2000
(Hz)
f
total
w
9.4
-%.
___s- / 1~_s ,~,-u~ , - ~ _ 6SCu 1 ""20~)0....... 6 ...... -2o0o
total ~
~
~
~
~ 2000
~
6
,
S
~
C
0
7.05
63Ci 1
u
-2000
2.11
~
,
total 63Cu
6SCu
"'20'r ...... 6...... -20'00 (nz)
Figure 2,17, A. The effect on a multiplet due to a spin-S/2 nucleus of increasing parameter
(XQD'/voS) with B. an example of 31p multiplets from coupling to copper. Note that there are two inequivalent phosphorus nuclei and that there are 2 copper isotopes (63'65Cu) both with spin-S/2 but with different moments and quadrupole effects that cause distortion. From Hanna et al. (1992) with permission of the American Chemical Society.
chemical shifts. The coupling pattern will be 2 sets of multiplets for the chemically distinct phosphorus (1,2) which are each split into a quartet by the J-coupling to the S = 3/2 copper. An additional complication is that copper has 2 isotopes both with a spin of 3/2 but with different nuclear properties. The scalar couplings will differ by a factor of 0.934 and the quadrupolar perturbation is proportional to the quadrupole moment so the asymmetry will differ by a factor 1.07. The coupling patterns are then weighted by the natural abundances of 69.09% for 63Cu and 30.91% for 65Cu. An example of this is shown in Figure 2.17B for [Cu(PPh3)z(4-PhNCN)]2. Data were recorded at 4 magnetic fields from 2.11 to 11.7 T. From observation of the multiplet coupling the asymmetry can be deduced, which depends on the both the parameter
ZQD"and the relative VoS
orientation of the quadrupolar and coupling tensors.
74
Multinuclear Solid-State NMR of lnorganic Materials
Thus, to obtain definitive information from these measurements requires other input such as a determination of the orientation of the quadrupole tensor relative to the internuclear vector, a knowledge of the internuclear vector or a value of XQ from other determinations. Given these constraints these coupling patterns have provided very useful information about the local structure around metal centres. To calculate these effects average Hamiltonian theory has been applied. There have also been full calculations of these effects, (Menger and Veeman 1982) which supersede average Hamiltonian theory when the Hamiltonian cannot be averaged by MAS. These authors applied an adiabatic variation of the eigenstates. The calculation by Menger and Veeman also showed that the isotropic J-coupling will be affected by the quadrupole interaction but this is a higher order effect and most experiments are carried out at sufficiently high magnetic field that J can be regarded as an invariant. These effects are not common in MAS spectra but are very interesting spin effects that can provide additional structural information. 2.3.2.3 Nonequivalent homonuclear spins. If two homonuclear spins that are not equivalent are coupled, then when other interactions are present the MAS lineshape will depend on the MAS rate (Wu and Wasylishen 1993). In the case of ~SN in cisazobenzene dioxide large changes in the lineshape were observed and simulations showed that the lineshape was dependent on the relative orientation of two chemical shift tensors and their orientation with respect to the internuclear vector.
2.3.3 Variable angle spinning Spinning a sample rapidly around any angle will introduce a scaling of the NMR inhomogeneous lineshape for the central transition. If the quadrupolar interaction is dominant so that effectively only second-order quadrupolar broadening is present in the central transition, there are angles that are more efficient at reducing second-order effects than 54.7 ~ This approach has been termed variable angle spinning (VAS) (Ganapathy et al. 1982). When dipolar and chemical shielding effects can be neglected, VAS can be effective, but as these interactions are not normally negligible, VAS is useful only in a limited range of applications. However, it should be noted that when the second-order perturbation is modulated there is no single angle that can remove both the P2(cos0) and P4(cos0) terms simultaneously. The effect on the lineshape when second-order effects dominate is shown in Figure 2.18. Spinning at 0 ~ produces the same lineshape as the static case. Spinning at the conventional magic angle produces the familiar lineshape, narrowed by comparison with the static case. The total width of the centreband of the central transition has its minimum in the range 60 ~ - 70 ~ depending on xI, the line being approximately twice as narrow as under MAS (Lefebvre et al. 1987, Amoureux et al. 1990). At 43.5 ~ the linewidth is independent of rl. For xl = 0,
75
Physical Background
q -->
0.0
0.3
0.6
N~ 4,
1.0
90
A____A__
___3_2___
- - - - - - - - ~ ----~------ -----.,~
~
70
__h__A___A_ A_ _A___A._ ~ - - - ~
__k._ __/.A_ l,
10
I
I
~
i--
_~
_
9
I
I
,
so
~
A____A
a _ ,
i_
80
40
L_
0
0 -1010 0 -10 10 0 -10 10 0 -10
(v- v,) (kHz) Figure 2.18. Variable angle spinning spectra for different angles as a function of different ~q, from Ganapathy, Schramm and Oldfield (1982) with permission of the copyright owner.
spinning at 36 ~ and 75 ~ gives a line consisting of an extremely intense narrow component with a broader component, so that although the total linewidth is not at its narrowest at this point, in a complex material the resolution of the intense component may be an advantage. This advantage becomes increasingly less evident for higher ~q. Calculations have also been performed for the sidebands of the central transition formed by spinning off the magic angle (Ganapathy et al. 1990).
2.3.4 Double angle spinning Although the quadrupolar information is itself useful, often it is desirable to produce better resolution by complete removal of second-order quadrupolar effects. As spinning about a single axis is unable to remove the P2(cos0) and P4(cos0) effects simultaneously, a more complex time dependence needs to be imposed upon the sample. The most direct solution to this problem is to make the spinning axis a continually varying function of time (Llor and Viflet 1988, Samoson et al. 1988, Zwanziger and Chmelka 1994). A scheme that achieves this uses a spinning rotor (termed the inner rotor) which moves bodily as a function of time by being enclosed in a spinning outer rotor so that the axis of the inner rotor describes a complicated but continuous trajectory as a function of time (Figure 2.19A). This is termed double angle rotation (DOR) and the second-order quadrupolar frequency of the central transition experiences a double modulation in the laboratory flame (similar to the single modulation of MAS). The effect of this modulation can be calculated by using Wigner rotation matrices, but now there are three
Multinuclear Solid-State NMR of Inorganic Materials
76
B ~ z ~ Bo
~ z,o~.~
~_~.~ ~~01 ~
/
Static
-"z,o,
_54,7o 02=30.6~
L
I ....
1 , , ~ , 1 , , , , I , ~ , , I
1 O0
....
0
I,,,,I
....
!
- 1 O0
Frequency(ppm)
Figure 2.19. A. The relative orientations and the motion of the rotors in a DOR experiment. B. The increase in resolution possible for 170 in wollastonite (CaSiO3) comparing static, MAS and DOR spectra from Wu, Sun and Pines (1990) with permission of the copyright owner. consecutive transformations, from the PAS of a given crystallite to the inner rotor axes which are defined by the Euler angles [3 and o~, then from the inner rotor to the outer rotor and, finally, from the outer rotor to the laboratory frame. The angle between the outer rotor axis and the applied magnetic field is 0~ and the angle between the two rotor axes is 02. If the angular velocity of the inner rotor is o~ and +1 describes the azimuthal orientation of the outer rotor in the laboratory, then in the laboratory frame the secondorder quadrupolar Hamiltonian becomes
~ (I,m)Fo(~)
I
+A 2 (I,m)P: (cosP 1)P2 (c~ 02 )F2 (fl, a , 77)
(2.155)
H ~ ) ( t ) - 2 V o ( 2 1 ( 2 1 _ 1)) 2 +A4 (l,m)P4 (c~ 01 )P4 (c~ 02 )/74(fl, a, 77) +terms o~ cos(t~ + ~1 )
where 01 and 02 c a n be chosen so that P2(cos0]) = 0 (01 -- 54.74 ~ and P4(COS02) -- 0 (02 = 30.56 ~ or 70.15~ DOR works well if the quadrupolar interaction is dominant and the sample is highly crystalline, in which case extremely impressive gains in resolution are achieved.
Physical Background
77
Simulation of the complete DOR spectrum (centreband plus the spinning sidebands) will yield the NMR interaction parameters (Sun et al. 1992, Cochon and Amoureux 1993, Amoureux and Cochon 1993). However, it is most usual to perform the experiment to give improved resolution and simply quote the measured peak position which appears at the sum of the isotropic chemical and second-order quadrupole shifts. DOR experiments at more than one applied magnetic field will allow these different contributions to be separated and hence provide an estimate of the quadrupole interaction via the combined quadrupole effect parameter PQ
(2.156)
/72
Po - Z O (1+ T )
This approach is similar to the use of the field variation of the centre of gravity of the MAS centreband, but has the advantage that the narrower, more symmetric line makes determination of the correct position of the centre of gravity more precise. For experiments carried out at two magnetic fields where the Larmor frequencies are vol and Vo2 for the measured DOR peak positions (in ppm) at the two magnetic fields of ~dorl,2 then
(~iso,cs =
(2.157)
V21(~dorl __ V022(~dor2 2 2 V01 -- V02
and I 3(I(I + 1 ) - 3 ) 1 772 4012(21 1)2 Z~(1 + - T ) --
2
2
Vo1Vo2((~d~ --(~dor2)2
(2.158)
V01 -- V02
This means that measurements at multiple magnetic fields will constrain the interactions. Detailed examples given in Chapter 6.
2.3.5 Multiple quantum transitions It has been pointed out that the central (1/2,-- 1/2) transition does not experience any first-order quadrupole interaction. The absence of first-order broadening effects is a general property of symmetric ( m , - m) transitions. There are cases where this can be a distinct advantage, the most direct instance being for integer spin nuclei (e.g. 2D and 14N, both I = 1) where there is no (1/2,- 1/2) transition. The main problem is to excite and detect such higher-order transitions, for which there are two separate approaches. The sample may either be irradiated and detected at the multiple quantum frequency (called overtone spectroscopy) or the MQ transition can be excited arid a 2D sequence used to detect the effect on the observable magnetisation.
78
Multinuclear Solid-State NMR of Inorganic Materials
Overtone spectroscopy developed for 14N irradiates the sample at approximately twice the Larmor frequency (Tycko and Opella 1987). If the quadrupole interaction is sufficiently large that second-order quadrupole effects are significant, the ( - 1 <--->1) transition becomes weakly allowed. In powders the spectrum is still structured, allowing the interactions to be deduced, but is narrowed by a factor of 8Vo/XQ. Symmetric transitions remove first-order quadrupole effects and the coefficients of the anisotropic terms of the second-order broadening depend on I and m. Amoureux (1993) pointed out that for the 3Q transition of I = 3/2 spins the coefficient of the P2(cos0) term is zero. Hence spinning at either 30.56 ~ or 70.12 ~ will remove the P4(cos0) term. The other first-order interactions still have the P2(cos0) variation so that spinning away from 54.7 ~ will only partially scale these. Comparing the widths of these interactions at the roots of P4(cos0) for the MQ transition with the static width of the (1/2, - 1/2) transition shows a scaling of 1.84 at 30.56 ~ and 0.98 at 70.12~ hence the latter is usually preferred. The other combination where there may be an advantage is the m = 7/2 transition for I = 9/2. Although calculations have been carried out there are no experimental data using this approach in the literature to date.
2.3.6 Ultrasonically-induced narrowing Narrowing of NMR lines has been observed in colloidal suspensions of ultrafine particles, presumably by Brownian motion. A suggestion related to this principle is to take small solid particles into a liquid medium and induce sufficient reorientational motion of these particles to produce narrowing (Satoh and Kimura 1990). Ultrasound induces translational motion and collisions of the particles produce rotation. This sonically induced narrowing has been observed for 27A1 in aluminium sulphate, with the advantage that no spinning sidebands were observed (Homer et al. 1991). The experiment depends on the frequency and the power of the ultrasound and the liquid medium. Although the first reports of this approach appeared in 1989 there have been few developments in this field.
2.4. DIPOLAR DECOUPLING
2.4.1 Heteronuclear dipolar decoupling Heteronuclear dipolar coupling can be a major broadening mechanism, especially as in a number of materials protons are present which often have dipolar couplings to other nuclei in excess of 50 kHz. This means that MAS is usually not sufficient to remove the effect of the dipolar coupling in the spectra. ~3C NMR spectra are, for instance, often obscured by dipolar coupling to the protons. In order to remove the carbon-proton coupling the protons are irradiated with an rf field while, at the same time, the carbon spectrum is measured. Consider the CH system where the heteronuclear dipolar
Physical Background
79
interaction is proportional to the factor Sz(carbon)Iz(proton). Initially, the proton magnetisation will lie parallel to the z-axis (Iz). The rf field rotates the proton magnetisation to the xy-plane and then to t h e - z-axis so that the proton magnetisation is then represented b y - Iz. Hence the spin part of the Hamiltonian is manipulated. Representing this schematically, the proton spin will sequentially vary as Iz --a -Iz --~ Iz ~ -Iz ~ Iz ~ -Iz. If this rotation is rapid compared to the timescale of the experiment, the time-average of Iz will tend to zero. Hence the heteronuclear dipolar Hamiltonian oscillates between + IzSz so that the average of the spin operator will be zero. This is an example of averaging interactions by manipulating the spin part of the Hamiltonian. In order for this to work, the rf field strength has to be greater than the strength of the dipolar interaction. This technique is often used in combination with MAS, resulting in almost liquid-like resolution for solids. In recent years there has been a significant amount of theoretical and experimental work on schemes to improve on standard CW decoupling, and some of these are discussed in Chapter 3.
2.4.2 Homonuclear dipolar decoupling Solid-state spectra of many materials containing protons (and fluorine) are usually dominated by the homonuclear proton-proton coupling (or fluorine-fluorine, and in cases where the phosphorus density is high, phosphorus-phosphorus). This is especially true when the nuclei are physically close together so that r is small, making the dipolar coupling strong. The proton spectra are usually featureless due to the fact that protons have a high natural abundance and a high gyromagnetic ratio causing even distant protons (--~10.~) to contribute to the broadening of the spectrum. To remove homonuclear dipolar coupling the lifetime of the magnetisation must be prolonged in the transverse plane (Powles and Mansfield 1962). Several multiple pulse schemes (Mansfield 1971) have been developed to accomplish this, including WHH4 (Waugh et al. 1968), MREV-8 (Mansfield et al. 1973, Rhim et al. 1974), BR24 (Burum and Rhim 1979) and BLEW-12 (Caravatti et al. 1982, 1983). These sequences involve long trains of pulses that cause coherent motion of the magnetisation. A distinction must be drawn as to whether the sequence is being used simply for decoupling and the heteronucleus is being observed, or whether the same nucleus is being observed. In cases where observations of the same nucleus are being made, a so called "windowed" sequence is necessary so that the magnetisation can be recorded. These sequences are quite unlike all those encountered to date as the multiple pulse sequences sample the magnetisation stroboscopically. This means that there are certain times within the pulse sequence, which often involve repeated short cycles where the magnetisation is in the correct state. At each point a single data point is recorded. Just as using the rotating frame oftensimplifies the understanding of many NMR experiments, to understand these multiple pulse sequences it
80
Multinuclear Solid-State NMR of Inorganic Materials
is easier to observe the magnetisation in a flame in which it remains stationary so that the flame's motion compensates for the effect of the rf pulses. In the literature this is termed the toggling flame. Note that to properly understand such pulse sequences, second-averaging and Magnus expansion techniques should be applied (Gerstein and Dybowski 1985). Consider the simplest of these sequences, WHH4, where the cycle has four pulses and five time-intervals as shown in Figure 2.20. If the dipolar coupling is dominant then, in the toggling flame, the magnetisation is rotated successively to different directions, the average dipolar interaction being "" tl
)-
(3Ilxl2x
--/-1" ~/2 )'k-
(3Ilyl2y - 11" [2 )+ (311zl2z - 1-1"1-2)
(2.159)
The factor of 1/3 arises from the fact that the spin spends an equal amount of time along each of the Cartesian directions. It is clear from this expression that the sequence successfully removes the homonuclear dipolar coupling. However, it is important to realise that such manipulation by pulses has the effect of scaling other operators and not completely removing them. The most important is Iz, its average value for the above sequence being
(1 eft) -- 89 x + Iy + lz) ]
(2.160)
This means that there is an effective field along the unit vector 1L/3(1,1,1) direction in the Cartesian basis, producing an effective scaling of the frequency of 1/~/3. This basic sequence was developed into longer sequences such as MREV-8 and BR-24 (Table 2.7).
90~
Magnetisation along
Z
90~
Y
90%
x
90~
Y
z
Figure 2.20. The periods of a WHH4 multipulse experiment for narrowing homonuclear dipolar coupling.
Physical Background
81
Table 2.7. Comparison of some of the more commonly used homonuclear decoupling sequences with a normal one pulse experiment. Sequence
Pulse cycles (phase)
Cycle time
Scale factor for Izeff
One pulse
x
"r
1
WHH-4
x- yy- x
6'r
MREV-8
x - yy - x - x - yyx
12'r
BR-24
xy-y-x -xy-yx yx-x-y -yx-xy xy-x-y -yx-xy
36'r
45
2
As the cycles become longer they have better self-compensation and second-averaging of the interactions which improves the resolution of the spectrum, and they are more robust to frequency offset effects. The downside is that the cycle time increases markedly. The causality argument applied to MAS can also be invoked here in that the cycle time which is doing the averaging must be fast compared with the timescale of the modulation of the magnetisation caused by the interaction to be averaged. Hence this condition is much more easily met when the sequence has a shorter cycle time. When the number of pulses increases, the only way to decrease the cycle time is to decrease the n- spacing, which is governed by the rate of recovery of the receiver system after a pulse and the speed of the electronics. The 90 ~ pulse time can also be reduced which is a stringent test of the power handling capabilities of the probe. When it is simply decoupling that is required (as opposed to observation as well) an off-resonance proton field can be applied such that the proton magnetisation is spin-locked at the magic angle. This is termed Lee-Goldburg decoupling (Lee and Goldburg 1965). A variant of this sequence currently favoured is Frequency Switched Lee-Goldburg decoupling (FSLG) (Bielecki et al. 1990). The ability of NMR hardware to abruptly change the frequency while keeping the phase has meant that this approach is now possible. This approach does not require the use of hard, resonant 90 ~ pulses and is therefore relatively more efficient at decoupling I-spins over a greater frequency. Such considerations have become more important even for 1H as very high magnetic fields are applied. The basic principle of the Lee-Goldburg decoupling concept is that it causes coherent precession of the magnetisation about an effective field inclined to Bo. With B1 (say) along the x-axis, this effective field will be inclined at 0 to Bo, defined by
82
Multinuclear Solid-State NMR of Inorganic Materials
tanO =
BI Bo
~o,
(2.161)
If the effective field is directed along the magic angle (i.e. directed along the direction (111)), 0 = 54.7 ~ This will remove the dipolar coupling since the effective spin direction is inclined at the magic angle to Bo. The offset frequency ALG = tOo - tO1.Rather than continuously irradiate at this single frequency, it has been found to be much more efficient to flip the frequency by _+ ALG with precession for a time "rm corresponding to 2-rr. Between each period the phase is also successfully changed by ~r. The FSLG decoupling has been shown to be one of the most effective decoupling schemes. It will also scale other spin operators, with a factor of (~/3)-1 for the chemical shift. FSLG has been used for the observation of abundant spins as an alternative to the multiple pulse sequences based on hard 90 ~ pulses (Levitt et al. 1993). These sequences have the benefit of short cycle times and high scaling factors but are sensitive to rf homogeneity. These pulse sequences completely remove only the homonuclear dipolar coupling, which means that interactions such as shielding are scaled but still present. The anisotropy can, of course, provide useful information but to achieve the best resolution this should also be removed. This is readily achieved by MAS and the combination of multiple pulse and MAS averaging is termed CRAMPS (Combined Rotation And Multiple Pulse Sequence) (Ryan et al. 1980, Jackson and Harris 1988, Maciel et al. 1990). When such multiple pulse sequences and MAS are combined care must be taken to ensure that the independent averaging processes do not interfere with each another. Hence it has been assumed that there should be at least five multiple pulse cycles per rotor period for this to be approximately true, so that modest spinning speeds (i.e. --~2 kHz) were typically used. In strongly dipolar coupled proton systems CRAMPS gives much better resolution than MAS (Figure 2.21). For Lee-Goldburg decoupling, similar time constraints are necessary so that 2"rLC should be short compared to the period of sample rotation. More recently, consideration has been given to combining multiple pulse sequences with fast MAS. The timing of the fast MAS with a sequence means that symmetry of the sequences allows their combination. The sequences proposed have largely been windowless so that one data point is accumulated at the end of the complete sequence and the sequence is then incremented (Hafner and Spiess 1998, Filip et al. 1993). If indirect detection is being used, as in many 2D sequences, this is not a drawback. The technique has been developed still further to use semiwindowless decoupling so that at points in the sequence quasi-static conditions apply, even under fast MAS. These approaches may offer new impetus to high resolution proton spectroscopy even in systems with a high proton density.
83
Physical Background
\
15
10
5
0
-5
\
PPM
/ \
/ \
r \
/ \
/ \
/ ~
/ /
-~~
~
A
10.7 kHz
j
~
9.2 kI-Iz
5.0 kHz
40
20
0
-20
-40
kHz
Figure 2.21. Example of ~H spectrum comparing static, MAS and CRAMPS from citric acid from Dec et al. (1989) with permission of the copyright owner.
2.5. SPIN-LOCKING A useful concept is the creation of spin-locked transverse magnetisation after a pulse, with the phase of the rf then flipped by 90 ~ This means that in the rotating frame after the phase shift of the rf field, the transverse magnetisation and the B1 field are aligned. The transverse magnetisation will not then dephase under interactions such as chemical shift and dipolar coupling, as would normally be the case. The transverse magnetisation will decay by genuine relaxation processes in the rotating frame, characterised by the relaxation time TI~. In this situation the magnetisation is said to be spin-locked, since the transverse magnetisation lasts much longer than under normal T2 dephasing. For quadrupole nuclei the situation is more complex and it is well known that
84
Multinuclear Solid-State NMR of Inorganic Materials
rotationally induced spin-locked magnetisation can appear when an if-field is applied to a quadrupolar nucleus in conjunction with MAS. The spin-locking behaviour of quadrupolar nuclei on resonance can be described by considering the first-order quadrupolar interaction only, so the Hamiltonian in the rotating frame will be given by the first-order quadrupolar interaction (Eq. 2.111) plus the rf contribution VlIx. Diagonalising this Hamiltonian gives the eigenfunctions (Vega 1981, 1992), which, in the case of the quadrupolar interaction being much larger than the if-field for an I = 3/2 spin, are the single (c+) and triple (t_) coherences, defined as
ic+) = [ ~ ) + - I - ~ ) and It+)= 13/2)+1-3/2) -
(2.162)
-
The energy level diagram for these eigenstates is shown in Figure 2.22 which demonstrates that the eigenvalues change smoothly as the magnitude of the quadrupole interaction changes relative to the rf field. When IVQI > > Vl the states are spin-locked. However, when VQ is comparable to v~, c+ and t+ are no longer the eigenstates but linear combinations are (Vega 1992). In a spinning sample the orientation of the quadrupole nucleus changes continuously. Hence, the splitting between the energy levels which depends on the crystallite orientation through the anisotropic term will vary, and level crossings between the different spin levels will occur (Vega 1992a, Grey et al. 1993, Grey and Vega 1995). During one rotor period, spin density is transferred from the outermost spin levels to a spin-locked state and back again, continuing as the rotation continues. Depending on the orientation of the quadrupole tensor with respect to the spinning axis, either two or four level crossings occur per rotor period. There are three spin-lock regimes depending on the so-called adiabaticity parameter = v~2/VQVr (Vega 1992). For e~ > > 1 the level crossings are adiabatic and spin density will be transferred from one level to another. For e~ < < 1 the change is sudden and no transfer of population occurs, and in this case the normal spin-lock behaviour is expected. In the intermediate regime the transfer of spin density is not efficient but still occurs, giving rise to very short effective spin-lock times. The situation is more complex when there are additional levels for higher spins, but the same reasoning can be used. For 27A1 when • -- 2 MHz, spinning at 5 kHz and an rf field-strength of 60 kHz gives oL = 2.4, which means it is in the adiabatic regime. So for effective spin-locking the system should be well into the adiabatic or sudden regime. The rotationallyinduced level crossings adversely affect the performance of various NMR experiments such as cross-polarisation (Sec. 2.6) and nutation NMR (Sec. 3.6.1). It also lies behind the TRAPDOR effect (Sec. 3.8.3). A more detailed and thorough discussion of MAS and spin-locking of quadrupole nuclei has been provided by Vega (1992a).
Physical Background
85
Spin I=3/2 _
~Eigenvalue (v0 c+
4
t+
Spin I=5/2
Figure 2.22. The variation in the energy level eigenvalues of a quadrupole nucleus as a function of the ratio VQ/V~which is changed by sample rotation since VQ is a function of orientation. The c+ and t+ states are as given in Eq. 2.162 and the diagrams follow ideas from Vega (1992a), and Grey and Vega (1995).
2.6. C R O S S - P O L A R I S A T I O N
Even with the line-narrowing techniques described earlier, NMR experiments on solids with dilute spin-1/2 nuclei are still relatively unattractive on two principal counts. One is the lack of sensitivity due to their low net polarisation and the other is the relatively long spin-lattice relaxation time that is often encountered. In solids where both abundant (I) and dilute (S) nuclei coexist, polarisation transfer techniques can usually be used to overcome both these problems. There are many schemes to effect such a transfer but the most common technique is to create and then spin-lock transverse I-magnetisation. This experiment is best understood using ideas from spin thermodynamics. The magnetisation is given by Curie's Law (Eq. 2.21) and the temperature in
86
Multinuclear Solid-State NMR of Inorganic Materials IH Channel 90, ~ i |
(Spin-lock)y Decoupling
,, i
X Channel F1D
CP
H-spins
X-spins
Lab
Rotating Frame
Bo ~ ~
Bm Blx Hartmann-Hahn matched
Lab Be
C
,
0
~
5
....
, ....
, ....
, ....
10 15 20 25 30 35
msec
Figure 2.23. A. Schematic representation of the CP sequence for ~H --~ X. B. the changes of the energy levels from the laboratory frame and in the rotating frame showing the Hartmann-Hahn match (separations are not to scale). C. The build-up of magnetisation for the 13C signals in the 2 carbons of glycine as a function of contact times with an inset of short contact times showing the effect of different Tis for each carbon.
this equation is the thermodynamic lattice temperature. The pulse sequence has three main steps (see Figure 2.23A), which in terms of the spin thermodynamics are" 1. Cooling down the abundant spin system, 2. Contact between the I and S spins to allow polarisation transfer, 3. Observation of the dilute spins.
Physical Background
87
The transverse I-magnetisation is created and then spin-locked. In the spin-locking frame (which is equivalent to examining the magnetisation in the rotating frame as in Eqs. 2.36 and 2.37) the effective field is only B1. In the spin-locking frame, immediately after the 90 ~ pulse, the I-magnetisation is still MI with the same degree of order, but the energy levels are now much closer (i.e. ~lhB1 as opposed to ~/h Bo). With the new field B1 the system can now be assigned with an effective spin temperature Tpl defined by M1=
NlyZh2I(I + 1)Bo
-
N1YZh2I(I + 1)BlI
(2.163)
As B li < < Bo and the degree of order amongst the spins remains the same (i.e. M~ is constant) it follows that Bli
Tpl - -~o Tc
(2.164)
Hence the I-spins are effectively very cold. The S-spins, in contrast, start off with no transverse magnetisation so, in terms of thermodynamics, are very hot. There is a thermodynamic driving force for the transfer of magnetisation. However, the spin systems have to be allowed to communicate efficiently and this is achieved by applying a second B1 field, this time to the S-spins. If the two spins to be brought in contact are spin-1/2 then the condition the two fields must meet are the Hartmann-Hahn condition (Hartmann and Hahn 1962) given by (Figure 2.23B). ~/IBII -
(2.165)
YsBls
When this term is present the dipole flip-flop terms (e.g. I+S_) are energy conserving so that order can be transferred between the I and the S spins. Thermodynamics means that the transfer of order occurs, tending to give the two systems a common spin temperature. Magnetic energy is conserved, given by
E - N?2h2I(I + 1)B2 3kT
(2.166)
Conservation of total energy amongst the spin system from these flip-flop terms can be invoked on a timescale that is less than the spin-lattice relaxation times so that
N, yZh:I(I + 1)B2I
NiY~h2I(I + 1)B2I + Nsy2h2S(S + 1)B2s
3k 1
3k 2
(2.167)
Using the Hartmann-Hahn condition and rearranging leads to a new temperature of
Multinuclear Solid-State NMR of lnorganic Materials
88
(2.168)
The implication of this equation is that the spin temperature for the I-spin system changes very little, so that I-magnetisation essentially remains unchanged as a result of the contact, and the S-magnetisation created is
Ms=
NsT2h2S(S + l) T1 1 NsT~h2S(S + I)B o = 3kTp2 ~"s 1 + A 3kTL
(2.169)
where A
n
NsS(S+I) NII(I+I)
(2.170) ~I I
Hence in a single contact experiment the gain in signal intensity is approximately
7s compared to a single 90 ~ pulse on the S-spins (which for I - 1H and S - 29Si produces a factor of--~ 5). This produces transverse S-magnetisation. Although this describes the driving mechanism for the transfer of the magnetisation between I and S, it gives no clue as to the dynamics of the process. Mehring has carried out an extensive analysis of the CP process (Mehring 1983), leading to
M ce(t) - )'i Mo exp -~/~ 3 - e x p Ts (P+-P-)
(2.171)
Tis )]
where
,+ [l+I/1 ]
(2.172)
with
x--
1/ 1+a+~7-+~-3--
2
T~p
T~p
andy-
1+
T~p~,
+
s
T~So) Tip
(2.173)
where ot is a factor depending on the number of nuclei of each type in contact with one another. It has been shown that for many CP experiments where there are many more I spins than S and when Tis < < TS~p, the following dynamics approximately hold
Physical Background
exp -
89
-exp -
M ce (t) - MCoe
(2.174) 1
Tip
In this equation T~s determines the rate of polarisation transfer and hence the build up of signal, while Tlo is the relaxation time of the spin-locked I-magnetisation in the rotating frame determining the time scale of the decay of the reservoir of magnetisation. The rate of transfer of the magnetisation is found to depend on the second moments (M2) of the I-S and I-I spin systems as
1
M ,s
Tzs = Czs ~/MzII
(2.175)
C~s is a constant. In cases where the CP occurs between relatively isolated spins (e.g. pairs) the spin thermodynamic description cannot be applied. Rather than a smooth, continuous build-up of magnetisation, the CP curve shows structure, termed dipolar oscillations (Muller et al. 1974). The magnetisation oscillates between the proton and the X-nucleus. In the case where the system is exactly Hartmann-Hahn matched, the magnetisation of the S-spin is given by (Levitt et al. 1986)
M s ( t ) - Mso ~I [I_cos(_P2(cosO)t)],
(2.176)
where Mso is the equilibrium Zeeman magnetisation of the S-spin system. An example is shown in Figure 6.7 for 1H-170 from Mg(OH)x(OCH3)z-x allowing the distance between these nuclei to be determined. As the contact time increases, destructive interference occurs and the homonuclear proton dipolar coupling begins to dominate the CP curve. There is also an interesting contrast between Mg(OH)x(OCH3)z-x and Mg(OH)2 where the more dilute proton bath in the former leads to weaker proton-proton coupling and hence more pronounced oscillations. The above analysis is usually applied to spin-1/2 systems but there is no reason why magnetisation transfer cannot be applied to quadrupole systems. If XQ is small the theory is identical to that given above, but if • becomes significant the HartmannHahn condition is modified to
aiTiBli = aS~tsB1s
(2.177)
90
Multinuclear Solid-State N M R o f Inorganic Materials
where oL- 1 for spin-l/2 systems and a - ~ / [ S ( S + 1 ) - m ( m - 1 ) ] for the ( m , m - 1 ) transition of a spin-S quadrupole nucleus. The enhancement of the signal depends on the heat capacities of the two spin systems, which depend in turn on the gyromagnetic ratios of the two spins and the number of spins in contact. Thus, the CP magnetisation (Mcr,) is related to the normal Zeeman magnetisation (Woessner 1987, Walter et al. 1988) Mcp = M o
7t N s [S(S + 1)- m ( m - 1)]1 NI
I(I + 1)
J
(2.178)
All of the above remains only strictly true for a static sample, but CP is usually combined with MAS. If hvr < < HII, the effect of sample spinning on the cross-polarisation process may be ignored. Once MAS begins efficiently to average the I-I dipolar coupling, CP may be weakened as magnetisation can no longer diffuse freely through the I-spin reservoir. This often leads to modulation in the Hartmann-Hahn-match condition across the spectrum causing distortions in the spectrum. This effect has become more noticeable as faster-spinning probes have been used to overcome problems caused by spinning sidebands arising from CSA as ever higher applied magnetic fields become available. Quantification is often required, and, in CP, signal generation depends on the heteronuclear dipolar coupling which determines the CP rate via Tis. At slow MAS rates a broad matching condition is usually obtained around the central Hartmann-Hahn condition and any mismatch is relatively unimportant since the strong homonuclear dipolar coupling compensates. As the spinning speed increases, the matching profile breaks up into a series of narrowing matching bands separated by yr. The CP rate at the central matching condition is slower than at some of the sidebands (Metz et al. 1994, 1996). The ability to maintain efficient matching under fast MAS has attracted much attention. Sequences have been developed based on either phase and/or amplitude modulation. Ramping the amplitude across one of the matching sidebands with the ramp centred on one of the matching sidebands has been found to improve greatly the CP efficiency and broaden the matching condition. This approach is straightforward to implement on modern spectrometers. An example of the improvements brought by this approach is that of N-t-Boc-alanine (Metz et al. 1996), where there are several different carbons with different dipolar couplings, together with the influence of different match conditions (Figure 2.24).
2.7. TWO-DIMENSIONAL METHODS
Most of the methods described to this point have simply excited single quantum magnetisation and manipulated the Hamiltonian by making it time-dependent. They are all
Physical Background
91
90~ CP
L
Decoupling
,,...f
SACP Centreband
B
SACP Sideband
RAMP-CP Sideband
i~l
e ~
0
1---
-
9
0
-
-
9
-
9
9
9
9
- ,
. . . .
,
~
10
1
,
0
.
.
.
.
.
.
.
,
9
-
9
9
9
9
9
9
9
,
~
1
10
0
10
Contact Time (ms) Figure 2.24. A. One of the pulse sequences for modulating the Hartmann-Hahn match condition by ramping the X-nucleus transmitter amplitude. B. Comparison of 1H --+ 13C CP for conventional CP (SACP) matched on the centreband and a sideband with RAMP-CP on a sideband of N-t-Boc-alanine showing the very much improved quantitative data for the six equally populated carbon sites. Taken from Metz, Ziliox and Smith (1996) with permission of the copyright owners. essentially one-dimensional techniques, the only time domain associated with the experiment being that of the normal FID. A whole range of further experiments becomes available when a second time domain is introduced. There are many experiments that use this concept of two-dimensional (2D) NMR; some of those more commonly used are described in Section 3.2. The concept of the experiment is that the Hamiltonian is manipulated in some way so that the system is prepared in a particular state. This state is then allowed to evolve for a time (tl) and the system is interrogated by applying a pulse that creates observable magnetisation. The FID signal accumulated in this time is designated t2 (Figure 2.25, Ernst et al. 1988). On the basis of one experiment nothing can really be said about the effect of the evolution in tl. However, a whole series of FIDs are accumulated in which the time tl is varied. Fourier transforming this set of FIDs will produce spectra which are modulated in some way by the evolution of the system during the preceding tl-period, and a second FT will produce a second frequency dimension containing information about this modulation, and hence about the interaction causing it. This 2D concept is important in some of the approaches to be described next.
92
Multinuclear Solid-State NMR of Inorganic Materials tvl
t2 v
Evolution Interrogationi
Figure 2.25. Time intervals in a basic two-dimensional pulse sequence.
2.7.1 Dynamic angle spinning Dynamic Angle Spinning (DAS) is a 2D experiment in which the sample is spun about different axes sequentially during different periods (Eastman et al. 1992, Zwanziger and Chmelka 1994). During the first evolution time tl the sample is spun at an angle of 01 degrees. The magnetisation is then stored along the z-axis and the angle of the spinning axis is changed to 02. After the rotor has stabilised (--~ tens of milliseconds) the magnetisation is then brought into the xy-plane again and a signal is acquired. The secondorder quadrupole frequency of an individual crystallite depends on the angle of the spinning axis, so during tl the quadrupole interaction will produce a frequency in the crystallite of v~, and 1)2 during t2. If v2 is of opposite sign to vl the signal from the crystallite will be at its starting position again at some time later during t2. The angles can be chosen in such a way that the signals from each individual crystallite will be at the starting position at exactly the same time so that an echo forms with the secondorder quadrupolar broadening removed at the peak of the echo. The two angles should fulfil the following equations simultaneously P2(cos 01 ) = -kP 2(cos 02 ) and P4 (cos 01 ) --
-ke 4(cos 0 2 )
(2.179)
where k is the scaling factor. There is a continuous set of solutions for 01 and 02, the so-called DAS complementary angles, and each set has a different scaling factor. For these solutions, the second-order quadrupole powder pattern at 01 is exactly the scaled mirror image of the pattern at 02, and an echo will form at t2 = ktl. For the combination 01 = 30.56 ~ 02 = 70.12 ~ the P4(cos0) terms are zero and the scaling factor k = 1.87. For the combination 01 = 37.38 ~ 02 = 79.19 ~ the scaling factor k = 1, the spectra are exact mirror images and an echo will form at t l = t2. Finally, the combination 01 = 0 ~ 02 = 63.43 ~ (k = 5) is special because it allows efficient CP for signal enhancement prior to the DAS sequence. There are several ways in which the DAS spectra can be acquired; one can acquire the entire echo, which means that the resulting 2D spectrum will be sheared. Some additional processing is then required to obtain an isotropic spectrum in F1. The advantage of acquiring the entire echo is that the second-order information is retained. Hence the 2D data set has improved the resolution whilst providing secondorder quadrupolar information as well. Alternatively, the acquisition could also start at
Physical Background
93
the position of the echo so that an isotropic spectrum in F1 is obtained directly. This has the disadvantage that the S/N ratio is less than if the complete echo is acquired. A third possibility is to carry out the experiment as a pseudo-1D experiment where only the top of the echo is acquired as a function of t l. In this case the isotropic spectrum is acquired directly but there is no shortening of the duration of the experiment. It is, however, the best way to combine DAS with 2D heteronuclear correlation. The DAS sidebands can be analysed to deduce interactions (Sun et al. 1992).
2.7.2 2D MQMAS From the DAS experiment it is known that in different time intervals the sample is spun at different angles so there is a point (where the echo forms) where anisotropic effects are refocused. Also it is known that there are no first-order quadrupole effects in symmetric transitions. Using these ideas, in 1995 a new experiment emerged from the group of Frydman that has had an enormous impact on solid state NMR spectroscopy of noninteger quadrupolar nuclei (Frydman and Harwood 1995, Medek et al. 1995). The 2D Multiple Quantum Magic Angle Spinning (2D MQMAS) experiment greatly enhances resolution of the spectra of half-integer spin quadrupolar nuclei. Under MAS for a symmetric transition, as the P2(cos0) term is removed the frequency evolves under the quadrupole interaction as
I
(2.180)
2I(2I-1)
+ v o [C4(i,m)P4(54.7O)F4(fl,)/,r D
where F4(~,'y,'q) is defined in Eq. 2.139. The experiment correlates ( m , - m) (the multiple quantum ( p - 2m) transition) to the central (~/2,-1/2) transition. The resolution enhancement stems from the fact that the quadrupole frequencies for both transitions are correlated. It can be seen from the coefficients in Table 2.8 that both the isotropic and anisotropic parts show different time evolution for the different coherences. The experiment is shown schematically in Figure 2.4, together with the coherence level diagram. The 3Q experiment has so far been the most common since, of the higher coherence transitions, 3Q is the easiest to generate. Initially the 3Q transition is generated and the Hamiltonian evolves for a period h. Another pulse converts the 3Q signal to an observable ( - 1) single quantum coherence (in Figure 2.4 a three pulse sequence where the coherence changes is 0 ~ +_ 3 --~ 0 ~ - 1) so that at t2 = ktl the anisotropic F2 term in Eq. 2.180 is refocused when
k - C2(I'3/2) C2(I,1/2)
(2.181)
94
Multinuclear Solid-State NMR of Inorganic Materials Table 2.8. Coefficients for the second-order quadrupolar interaction and the ratio for the echo periods. I
3/2
m
p
Co
C2
C4
k
1/2
1 3 1 3 5 1 3 5 7 1 3 5 7 9
3 - 9 8 6 - 50 15 27 - 15 - 147 24 54 30 - 84 - 324
- 12 0 - 32 - 60 20 - 60 - 144 - 120 168 -- 96 - 252 - 300 - 168 216
- 27 21 - 72 - 114 150 - 135 - 303 - 165 483 -- 216 - 546 - 570 - 84 116
NA - 7/9 NA 19/12 - 25/12 NA 101/45 11/9 - 161/45 NA 91/36 95/36 7/18 - 31/6
3/2 5/2
7/2
9/2
1/2 3/2 5/2 l/2 3/2 5/2 7/2
1/2 3/2 5/2 7/2 9/2
NA - not applicable.
At this point an e c h o forms but the coefficients of the isotropic terms are different and so are not r e f o c u s e d at this point. This has i m p o r t a n t c o n s e q u e n c e s for the e x p e r i m e n t . T h e isotropic part VoQ of the q u a d r u p o l e f r e q u e n c y is
vQ - - _
(2.182)
,~'~(3 if-/7 2 )
4012 (21 - 1)2V0 and the anisotropic part vaQ([3,~/) is g i v e n by
7
4
i - ~ ( 3 - ~ c o s 2 ~ ' ) 2 sin fl 9Z~ 44812 (21 - 1)2Vo
/7 2
+ 2 ( r / c o s 2 y - 2 - --~-) sin 2 fl
(2.183)
2//2
4 +-45 5
As the 2D e x p e r i m e n t is carried out at different e v o l u t i o n times the echo m a x i m u m will always be free from anisotropy but will s h o w m o d u l a t i o n due to isotropic effects. A double F T will then contain both isotropic and anisotropic information. After a 2D F T the signals s h o w up as ridges lying along the Q u a d r u p o l e A n i s o t r o p y (QA) axis w h i c h has a gradient d e p e n d i n g on k (Figure 2.26). T h e isotropic s p e c t r u m can be obtained by projection of the entire 2D s p e c t r u m on a line t h r o u g h the origin p e r p e n d i c u l a r to
95
Physical Background Site
1
2
3
vl ~ p
~ F1
Co(P) Co(l) -'"
ganiso--"
C4(P )
C2(1---~ F2
Figure 2.26. A schematic 2D MQ MAS data set of a sample containing 3 sites- 1 (large XQ, no distribution), 2 (moderate XQbut with some distribution) and 3 (very small XQ and no distribution). 3 lies on line Vl = PP2, 2 and 3 displaced from this line along a line with gradient giso,Qand 3 showing a lineshape along a line with gradient ganiso.
the QA-axis (vide infra). Cross-sections along the ridge will retain the second-order quadrupolar lineshape. The sign of k is varied for different coherence orders and has important consequences for the way the experiment will be carried out. If the value is negative, anisotropic broadening is refocused at a positive value of t2 and the correlation must be between - p and - 1 (the observable coherence). However, for positive k, to get refocusing at positive t2 the correlation has to be between the + p and - 1 coherences. A scheme that gives refocusing at positive t2 values is termed an echo pathway whereas refocusing at negative values is an antiecho pathway. Under MAS the terms involving P2(cos0) (for both the quadrupole and chemical shift) and P4(cos0) are refocused along the ridge but there are isotropic terms. The isotropic parts of both the chemical shift Aviso and the isotropic second-order quadrupole effects v Q, are scaled. These scale factors (SF) are for the chemical shift
SFcs(i,m)_
p+ k
l+lkl
(2.184)
96
Multinuclear Solid-State NMR of lnorganic Materials
and for the quadrupolar term
C~
-k
SFQ(I,m) - CO(I,- 1/2)
(2.185)
l+lkl The MQ experiment involves excitation of higher order coherences and selection of particular pathways by phase cycling. These effects lead to considerable reduction in intensity compared to normal single quantum experiments. Hence, much attention must be paid to efficient generation of the MQ coherences and the efficient reconversion of the MQ signal to observable single quantum magnetisation. If the quadrupole interaction is significant, single pulse excitation is a comparatively efficient way of generating MQ coherences. For samples where there is no quadrupole interaction there can be no MQ signal generated, and these sites are missing from the MQ spectra. There have been a number of investigations to optimise these experiments. Generation of MQ transitions depends upon both the ratio r -- VQ/Vl and the flip angle of the pulse. The optimum pulse tip angles are almost independent of the ratio r but depend on the coherence order to be maximised. However, the efficiency of the generation of the MQ transition is a peaked function (Figure. 2.27), having an optimum value of r. The key is that with typical • values there is usually a gain from using higher rf fields. This is increasingly true as the order of the coherence to be generated increases. This has meant that most work has concentrated on the lower coherence orders. Most of the calculations have assumed a static sample but MAS imposes a time dependence on the Hamiltonian. As spinning speeds increase the generation of the MQ coherence decreases, but even at the highest speed available (--~45 kHz) the intensity is reduced by a factor of only --~3 and the optimum pulse angle is not much changed. Numerical calculations have been carried out on the effect of the angle on the conversion efficiency. The work of Amoureux and Fernandez (1998) is summarised in Table 2.9. One of the key observations is that the efficiency of the flip angles shows no real dependence on r. However, the conversion is quite an inefficient process so that the overall sensitivity of the experiment is poor relative to one-pulse experiments and much effort has gone into improving the excitation and reconversion efficiency. As I increases, the optimum flip angle for maximum excitation decreases. Since both excitation and conversion are dependent on r, MQ spectra are normally not directly quantitative, so great care has to be taken to interpret the spectra. The situation will generally improve as the applied rf field increases. Since the 2D data set contains the anisotropic information, lineshapes can be analysed and the quadrupolar parameters deduced. The lineshape will show distortion as there will be crystallites whose orientation is such that their quadrupolar frequency is zero and will not contribute. Apart from the excitation and the conversion, the actual coherence pathway
97
Physical Background 60./'~3a i\ '
-
40
20"
3Q
/=\
$
/ /i i '............ . ' ~ '"~'" '~\" ''\
/;. ,o7~ \:\
0"~
'
I
'
I
,
! ~
,I
'
I
30-
20
""
5Q
i~,- ,t2
.. ,,..
~aS!s
/
20
il\i
"~'~
10-
fl
._n
-4
-2
0
2
4
6
VQffS(S+I)-3/~4/V1 =2" F i g u r e 2.27. The overall efficiency of a 2-pulse MQ pulse sequence for different excitation (3Q, 5Q, 7Q, 9Q) for different spins with the quadrupole frequency scaled by the spin factor to allow direct comparison of the different spins with the optimum pulse angle as given in Table 2.9, after Amoureux and Fernandez (1998).
Table 2.9. Numerically calculated optimum flip angles and the relative efficiency of the sequences after Amoureux and Fernandez (1998) for VQ/Vl = 1.25. I
3/2 5/2 7/2 9/2
Pulse
1 2 1 2 1 2 1 2
Ipl = 3
Ipl = 5
Ipl = 7
Ipl = 9
0~
Eft
0~
Eft
0~
Eft
0~
Eft
240 55 180 60 120 45 90 35
1.50 0.30 1.50 0.30 1.50 0.30 1.50 0.26
170 72 120 65 100 50
0.32 0.05 0.49 0.05 0.64 0.07
220 80 150 72
0.21 0.03 0.29 0.03
250 86
0.14 0.02
The relative efficiency (Eft) is one for the central transition observedafter a perfectly selective90~pulse.
98
Multinuclear Solid-State N M R of Inorganic Materials
will affect the lineshape, so, for example, the coherence pathway 0 --~ p --~ - 1 will produce an echo but there will be absorptive and dispersive components. Ideally the sequence 0 --~ _+ p --> - 1, (especially if both pathways are equal) will markedly improve the lineshape. For I = 3/2, echo and anti-echo pathways can be equalised but this is not possible for higher spins. Triple quantum experiments are the most popular because they are most readily generated, but with higher spins there is the possibility of generating higher order coherences. The sensitivity of such experiments decreases rapidly for the higher order coherences. There has been much work to improve the efficiency of such experiments. Instead of single pulses both composite pulses (Marinelli et al. 1998) and shaped pulses (Ding and McDowell 1997) have been investigated. The group of Kentgens has used amplitude modulation of the rf field which amounts to a sweep of the frequency (DFS) and causes an adiabatic transfer between coherences. Through DFS good enhancement of the signal and excitation of an improved range of XQ were obtained (Kentgens and Verhagen 1999, Iuga et al. 2000, Schiifer et al. 2001, Iuga and Kentgens 2001). A more readily implemented scheme is the fast amplitude-modulated conversion pulses of Madhu et al. (1999, 2000) (FAM) which again produce coherence transfer through adiabatic passages. Wu et al. (1996) showed that as for spin-locking, rotation of the samples produces interconversion between 1Q and 3Q coherences, this approach being termed RIACT. The conversion of the single quantum to triple quantum coherence by the first long pulse and its reconversion by the second pulse depends on two main factors. The situation is most closely described for I = 3/2. High power rf and relatively slow MAS are required to ensure the conditions are as close as possible to adiabatic. Under adiabatic conditions, for spins to contribute, the system must undergo an odd number of passages or zero-crossings during the period of the spin-locking pulse. An adiabatic zero-crossing will the convert central-transition coherence (1Q) to 3Q, but after either two or four passages, the system will return to its initial state. The RIACT(II) sequence in which the excitation and conversion of the coherences is achieved by a spin-lock of duration Tr/4 instead of a hard pulse, has the advantage of more uniform excitation and conversion, which is essential in obtaining quantitative information. Schemes for sensitivity enhancement based on the selective inversion of the satellite transitions have been proposed (Haase et al. 1994). This can be combined into the MQ sequence as has been successfully demonstrated for 23Na and 87Rb (Gan 2000, Yao et al. 2000).
2.8. NMR RELAXATION
2.8.1 Introduction to relaxation
The spectroscopy described above is effectively the static part of the interaction, but there are dynamical processes that determine how rapidly the spins lose coherence and
99
Physical Background
how rapidly thermodynamic equilibrium is achieved between the spin states. This aspect of N M R is termed relaxation. Relaxation can provide information on the processes that cause transitions. First, a description of the process is necessary and a classical approach is to adopt the phenomenological B loch equations for an ensemble of spins in an external magnetic field. This model describes the behaviour of the magnetisation through the torque as in Eq. 2.3. Parallel to the main magnetic field the time dependence of the magnetisation is given by (2.186)
dMz = M~ - Mz dt
T1
where T1 is the longitudinal or spin-lattice relaxation time. If the sample is placed in the Bo field and initially has no magnetisation, the magnetisation along the direction of the magnetic field builds up as
exp/;//
(2.187)
The transverse components of the magnetisation decay as dM x = -M x dt
(2.188)
dMy _ -My
T2
dt
T2
where T2 is the transverse or spin-spin relaxation time. Then, in the laboratory frame the overall equation of motion of the magnetisation in an applied magnetic field B will be given by dM
---=-- = ?'M_x B dt
-
M x i + Myj
- -
M~ - M o k
T2
T1
(2.189)
-
which can be rewritten in the rotating frame as
dM
_ ) ' M x B_eff
MX'i + My,j
(2.190)
Mz' - M~ k
dt
T1 CO
In the rotating frame at v - Vo if there is no rf field present then Beff= B + -o = 0 and the B loch equations can be written as -o ), dM~, =
dt
M X,
r2'
dMy______.~=, _ My________[, dMz._____~,= M o - M~,
dt
dt
The solutions for the transverse (x',y') magnetisation are
T,
(2.191)
100
Multinuclear Solid-State NMR of Inorganic Materials M,(t)- M:,,(O)e-t/7"2 and My,(t)- My,(O)e-t/T2
(2.192)
Hence any magnetisation in the x'y'-plane (and xy) decays exponentially to zero. The transverse relaxation time is a result of the dephasing of the magnetisation in the xy-plane of the magnetic moments precessing about Bo. In principle, if each magnetic moment of the same nuclear species precessed with the same Larmor frequency about Bo no dephasing would occur. However, the moments do not experience the same magnetic field due to interactions and inhomogeneities in the static magnetic field Bo. The result of these effects is that each magnetic moment precesses with a different frequency which will result in the dephasing of the magnetisation in the xy-plane. These processes can be coherent or incoherent so that the T2* seen through the decay of the magnetisation in the FID after a pulse can be regarded as being made up of different contributions and written as 1
1
1
---r = - - - t - ~
(2.193)
where T2 is the decay of the magnetisation due to the interactions, and T'2 is due to the the effect of inhomogeneities in the field. The real physical interest lies in T2, since it can provide information about the interactions and the nature of the motion that modulates the interactions and causes relaxation. Spin echo techniques allow T2 to be measured rather than T2*. In this technique a sequence of two rf pulses is used as shown in Figure 2.28. The first pulse is a 90 ~ pulse and following this the individual nuclear moments spread out around the transverse plane due to their different precession rates. After a time "r a second pulse of 180 ~ is applied. All the spins are thus flipped about B1 so that those moments which had been precessing faster than average prior to --~2
~
3 ----~ 4
J
i ' v
AA~
'
5
L,
I I" 5 ,~..___m
Figure 2.28. The basic spin-echo pulse sequence and effect on spin packets showing the formation of a spin-echo.
Physical Background
101
application of the second pulse, and had thus got ahead of the other moments now find themselves, after the second pulse, behind the others. They are, however, still precessing faster than average and so, after a further time "r, they catch up with the slower moments. The same argument applies in reverse to the slower moments, the nett effect being to restore phase coherence to the precession at a time 2"r after the first pulse, thus forming a spin echo. The echo shape itself decays away with the characteristic time Y2* as the moments continue to precess at different rates. During the time 2T irreversible T2 processes will have been going on so that the re-phased magnetisation is less than Mo. Consequently the echo amplitude is reduced and varies with 2"r according to the B loch equation (Mx,(0) = Mz,(O) = O)
dMy, _
My,
My, (2z')- My,(O)e -2"c/T2
dt
(2.194)
By measuring the amplitude of the signal as a function of the echo time 2-r, T2 may be determined. This method assumes that each nucleus remains in the same field for the duration 2"r of the experiment. If, however, the nuclei move into a different field e.g. through diffusion, the echo amplitude will be further reduced because the precession frequency of the individual nuclei will have changed during the echo as they move from a region with one particular value of the magnetic field to another region. The signal amplitude at t = 2-r is now
2"c Mxy(2"c) = M ~ ( 0 ) e x p ( - T 2
2y2G2D'c 3 ) 3
(2.195)
where G is the magnetic field gradient in the z-direction and D the diffusion coefficient. To measure T2 in the presence of diffusion in an inhomogeneous field the Carr-Purcell spin-echo technique (a train of 180 ~pulses) can be applied. Carrying out the experiment with the echo spacing constant means that field gradient effects are much less significant than in a experiment with varying "r delays, but the disadvantage is that each 180 ~ pulse has to be very accurate. Diffusion measurements can also be made by deliberately applying a pulsed field gradient during both -r periods of a spin echo, and measuring the signal as a function of the gradient strength.
2.8.2 Mechanism for relaxation processes Transitions between Zeeman energy levels, and hence nuclear magnetic relaxation, are caused by fluctuations (time variations) in the local interactions at the nucleus that can cause transitions. The transition rates between these energy levels that cause nuclear spin relaxation depend on two factors: Rate of relaxation - (Strength of interaction) • (Number of fluctuations at Vo)
(2.196)
Multinuclear Solid-State NMR of lnorganic Materials
102
The fluctuations are often caused by atomic motion e.g. Brownian motion in liquids, ionic hopping, molecular rotations, librations and atomic vibrations. These motions are often complex and it is the range of frequencies that are present in the motions that determine relaxation. The spectral density function describes the relative intensities of different frequencies in the motions and can be used to calculate relaxation rates. Let y(t) be some function of time, such as the orientation of the internuclear vector and some other function f (e.g. the dipolar interaction) that depends on y. It is possible to define a probability function p(y,t) which is the probability that at a time t, the internuclear vector has some orientation y. Then (2.197)
f (t) - ~ p(y,t)f (y)dy
This assumes that y(t) is a stochastic function (i.e. y(t) varies completely randomly) but often y(t) shows some correlation. It is useful to define the probability function P(y~,t~;y2,t2) which means that if y = y~ at t = t~ this value is the probability p of y = Y2 at t -- t2. Then the probability that y = Y2 at t -- t2 and y = y~ at t = t~ and y = y~ at t -- t~ is p(y~,t~)P(y~,t~;y2,t2). The correlation function G(t~,t2) is
G(tl ,t2 ) - f (tl ) f * (t2) - ~ P(Yl ,tl )P(Yl ,tl ;Y2,t2 ) f (Yl ) f * (Y2)dYldY2
(2.198)
Since G is the correlation of the function with itself it is termed the auto-correlation function of f(y). If the function y(t) is stationary then it is invariant to changes of the time origin and G depends only on the difference, a" -- t2 - t~, so that G(r)
~ P(Y~ )P(Y~,Y2, v)f(y~ ) f * (Y2)dyldy2
(2.199)
The spectral density function is defined to be the temporal Fourier transform of G('r) so that ~x~
oo
--~
0
J(m) represents the "strength" (or energy) of fluctuations in f(y) at the frequency m and, as an example, the dipole interaction can be considered using the alphabet expression of Eq. 2.52. In Eq. 2.52 the terms A, B, C, D, E and F can be considered as having the form (Geometric term i.e. depends on the polar angles (0,~) fly)) • (Spin-operator term). The spectral density functions are
J(P)(m)- f f where
Y2p(Oa, ~a ) 3
J~,
Y2p(Ob'g~ 1~,3
,r b , m) dr_ dr b
(2.201)
Physical B a c k g r o u n d
103
oo
P(r~a ' ~ b , co) - FT[P(r~a ' r_~, r)] - I P(ra'~ rb , oo)e -iarcdr
(2.202)
--oo
with Y2p(Oi, (~i) the spherical harmonic functions. As an example for homonuclear dipolar coupling
1
-~1
l
3
3,2
-
-~
= -~'4h2I(I
(//0) 2
[jO) (CO~ )+ j(2) (2r ~)],
(2.203)
[J(~ (2(.01) + 10J(1) ((.0o) + J(2) (2(.0o)],
(2.204)
h 2 I ( I + 1) G
+ 1)
(/./O ~2 \4~J
and
l = 3 /4}~2i(I -t-1)(1"/o ~2[J(~ T2 8 \4,cJ
- 10j(1)((.Oo)--kJ(2)(2(.0o)].
(2.205)
The functional form of J(m) in turn depends on the form of P(G, G, ~) which will depend on the details of the atomic motion. The function P can often either be derived analytically or can be calculated numerically. The key is to understand that these expressions show that the different relaxation times probe different frequencies. The spectral density function is a measure of the distribution of fluctuations. For relaxation to be efficient there needs to be significant spectral density at the frequencies in Eqs. 2.203-2.205. Often a simplified functional form can be assumed, of which the most common is the BPP approximation (Bloembergen, Purcell and Pound 1948). This model assumes that the autocorrelation function is exponential
exp( /
206,
where % is a correlation time and is related to the jump rate. Then J(P) ((.o) - 2 G (p) (0)
vc
1 + (o)~"C)2
(2.207)
The relaxation rates are dominated by the behaviour of the spectral density function at frequencies related to the spectral densities at ~o (T1), o~1 (T~p) and 0 (T2). The correlation time % follows an Arrhenius-type activation so that r c - r o exp I Ea ]
(2.208)
104
Multinuclear Solid-State NMR of lnorganic Materials
where Ea is an activation energy for the motion and % is the inverse of the jump attempt frequency. If an experiment is carried out at constant Bo (and hence too) while the temperature (T) is changed then the form of the spectral density will change. This is shown schematically in Figure 2.29 so that as the temperature is increased the spectral density is spread out over a wider frequency range. There is a temperature where there will be a maximum amount of spectral density at COo. The correlation time can be represented by
Ea
ln(r C) - ln(~') + ~ kT
(2.209)
Therefore, a plot of In(relaxation time) against T - 1 gives a characteristic curve (Figure 2.30) with the position of T1 being minimum at ~Oo'rc - 1 since this is the maximum of j(1) (COo). Similarly, the minimum of Tlo occurs at tol"rc = 1/2. The slopes at high and low temperatures for T1 can be used to estimate Ea of the motion since
j(~)' Tx '
Tv
COO
O~
Figure 2.29. Schematic representation of the spectral density functions J(~o) at three temperatures where Tx < Tu < Tz.
\
/B01
Rigid Lattice Limit -T2
1/T Figure 2.30. Plots of the relaxation times In T1, T~p and T2 as a function of inverse temperature T-1
Physical Background
105
/
ln(T1) ~ in 1 + 0) 2 2 Tc
(2.210)
then
T --->,,o :cor << 1 : ln(T1) ~ : - I n ( z ) ~:
-E a
(2.211)
kT
and T ---->0"coy >> 1" ln(T1) ~: -ln(CO2T) ,~ ln(CO2) + E. kT
(2.212)
This symmetry is lost if there is a range of activation energies, as in a glassy material (Brinkman 1992). Hence relaxation measurements can provide significant insight into atomic scale motion. T2 has the same slope at high temperature as T1, but at low temperature the J(0) term leads to a constant value termed the rigid lattice limit. The use of relaxation time measurements can have important effects on constraining local motion in materials (Strange 1987, Brinkmann 1992).
REFERENCES Amoureux, J.P., Fernandez, C. & Lefebvre, F. (1990) Mag. Resort. Chem., 28, 5. Amoureux, J.P. (1993) Solid State Nucl. Mag. Reson., 2 83. Amoureux, J.P. & Cochon, E. (1993) Solid State Nucl. Mag. Reson., 2, 223. Amoureux, J.P. & Fernandez, C. (1998) Solid State Nucl. Mag. Reson., 10, 211. Andrew, E.R. (1971) Progress Nucl. Mag. Reson. Spectrosc., 8, 1. Andrew, E.R. (1981) Int. Rev. Phys. Chem., 1, 195. Behrens, H.S. & Schnabel, B. (1981) Physica B, 114, 185. Bielecki, A., Kolbert, A.C., de Groot, H.J.M., Griffin, R.G. & Levitt, M.H. (1990) Advances in Mag. Reson., 14, 111. Blaha, P., Schwartz, K. & Dederichs, P.H. (1988) Phys. Rev. B, 37, 2792. Blaha, P., Schwartz, K. & Luitz, J. (1999) Technical University of Vienna, Austria, ISBN 3-95010031-0-4. Bloembergen, N., Purcell, E.M. & Pound, R.V. (1948) Phys. Rev., 73, 679. Brinkmann, D. (1992) Progress Nucl. Mag. Reson. Spectrosc., 24, 527. Bohm, J., Fenkze, D. & Pfeifer, H. (1983) J. Mag. Reson., 55, 197. Burum, D.P. & Rhim, W.-K. (1979) J. Chem. Phys., 71,944. Caravatti, P., Bodenhausen, G. & Ernst, R.R. (1982) Chem. Phys. Lett., 89, 363. Caravatti, P., Braunschweiler., L. & Ernst, R.R. (1983) Chem. Phys. Lett., 100, 305. Clayden, N.J., Dobson, C.M. & Fern, A. (1989) J. Chem. Soc., Dalton Trans., 843.
106
Multinuclear Solid-State NMR of Inorganic Materials
Cochon, E. & Amoureux, J.P. (1993) Solid State Nucl. Mag. Reson., 2, 205. Cohen, M.H. & Reif, F. (1957) Solid State Phys., 5, 321. Dec, S.F., Bronnimann, C.E., Wind, R.A. & Maciel, G.E. (1989) J. Mag. Reson., 82, 454. Dickinson, W.C. (1950) Phys. Rev., 77, 736. Ding, S. & McDowell, C.A. (1997) Chem. Phys. Lett., 270, 81. Eastman, M.A., Grandinetti, P.J., Lee, Y.K. & Pines, A. (1992) J. Mag. Reson., 98, 333. Ernst, R.R., Bodenhausen, G. & Wokaun, A. (1988) Principles of Nuclear Magnetic Resonance in One and Two Dimensions, Oxford University Press, Oxford. Facelli, J.C. (1996) Encyclopedia ofNMR, Eds. Grant D.M. & Harris, R.K., John Wiley and Sons, Chichester, p. 2576. Farrar, T.C. (1990) Concepts in Mag. Reson., 2, 1. Farrar, T.C. (1990a) Concepts in Mag. Reson., 2, 55. Fenkze, D., Freude, D., Frohlich, T. & Haase, J. (1984) Chem. Phys. Lett., 111, 171. Fernandez, C. & Amoureux, J.P. (1998) Solid State Nucl. Mag. Reson., 10, 211. Filip, C., Hafner, S., Schnell, I., Demco, D.E. & Spiess, H.W. (1999) J. Chem. Phys., 110,423. Frey, M.H. & Opella, S.J.J. Chem. Soc., Chem. Commun., 474. Frydman, L. & Harwood, J.S. (1995) J. Amer. Chem. Soc., 117, 5367. Gan, Z. (2000) J. Amer. Chem. Soc., 122, 3242. Ganapathy, S., Schramm, S. & Oldfield, E. (1982) J. Chem. Phys., 77, 4360. Ganapathy, S., Shore, J. & Oldfield, E. (1990) Chem. Phys. Lett., 169, 301 Geurts, F.M.M., Kentgens, A.P.M. & Veeman, W.S. (1985) Chem. Phys. Lett., 120, 206. Grant, D.M. (1996) Encyclopedia ofNMR, Eds Grant D.M. & Harris, R.K., John Wiley and Sons, Chichester, p. 1298. Grey, C.P., Dobson, C.M., Cheetham, A.K. & Jakeman, R.J.B. (1989) J. Amer. Chem. Soc., 111,505. Grey, C.P., Smith, M.E., Cheetham, A.K., Dobson, C.M. & Dupree, R. (1990) J. Amer. Chem. Soc., 112, 4670. Grey, C.P., Veeman, W.S. & Vega, A.J. (1993)J. Chem. Phys., 98, 7711. Grey, C.P. & Vega, A.J. (1995) J. Amer. Chem. Soc., 117, 8232. Groombridge, C.L., Harris, R.K., Packer, K.J., Say, B.J. & Tanner, S.F. (1980) J. Chem. Soc., Chem. Commun., 174. Haase, J., Conradi, M.S., Grey, C.P. & Vega, A.J. (1994) J. Mag. Resort. A, 109, 90. Haeberlen, U. (1976) High Resolution NMR in Solids: Selective Averaging, Advances in Magnetic Resonance, Academic Press, New York. Hafner, S. & Spiess, H.W. (1998) Concepts in Mag. Reson., 10, 99. Hahn, E.L. (1950) Phys. Rev., 80, 580. Hanna, J.V., Smith, M.E., Stuart, S.N. & Healy, P.C. (1992) J. Phys. Chem., 96, 7560. Harris, R.K. (1983) Nuclear Magnetic Resonance Spectroscopy, Pitman Books Limited, London. Harris, R.K. & Olivieri, A.C. (1992) Progress Nucl. Mag. Reson. Spectrosc., 24, 435. Hartmann, S.R. & Hahn, E.L. (1962) Phys. Rev., 128, 1042. Herzfeld, J. & Berger, A.E. (1980)J. Chem. Phys., 73, 6021. Homer, J., McKeown, P., McWhinnie, W.R., Patel, S.U. & Tilstone, G.J. (1991) J. Chem. Soc., Faraday Trans., 87, 2253.
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Iuga, D., Sch~ifer, H., Verhagen, R. & Kentgens, A.P.M. (2000) J. Mag. Reson., 147, 192. Iuga, D. & Kentgens, A.P.M. (2001) Chem. Phys. Lett., 343, 556. Jackson, P. & Harris, R.K. (1988) Mag. Reson. in Chem., 26, 1003. J~iger, C., Kunath, G., Losso, P. & Scheler, G. (1993) Solid State Nucl. Mag. Reson., 2, 73. J~iger, C. (1994) inNMR Basic Principles and Progress, vol. 31, Eds. Blumich, B. & Kosfeld, R., Springer-Verlag, Berlin, p. 135. Jameson, C.J. (1989) in Multinuclear NMR, Ed. Mason, J., Plenum Press, New York, p. 89. Keeler, J. (1990) in Multinuclear Magnetic Resonance in Liquids and Solids - Chemical Applications, vol. 322, Eds. Granger, P. & Harris, R.K., NATO ASI, p. 103. Kentgens, A.P.M. & Verhagen, R. (1999) Chem. Phys. Lett., 300, 435. Knight, W.D. (1956) Solid State Phys., 2, 93. Kundla, E., Samoson, A. & Lippmaa, E. (1981) Chem. Phys. Lett., 83, 229. Lee, M. & Goldburg, W.I. (1965) Phys. Rev. A, 140, 1261. Lefebvre, F., Amoureux, J.P., Fernandez, C. & Derouane, E.G. (1987) J. Chem. Phys., 86, 6070. Levitt, M.H., Suter, D. & Ernst, R.R. (1986) J. Chem. Phys., 84, 4243. Levitt, M.H., Kolbert, A.C., Bielecki, A. & Ruben, D.J. (1993) Solid State Nucl. Mag. Reson., 2, 151. Levitt, M.H. (1997) J. Mag. Reson., 126, 164. Llor, A. & Virlet, J. (1988) Chem. Phys. Lett., 152, 248. Maciel, G.E., Bronnimann, C.E. & Hawkins, B.L. (1990) Advances in Mag.Reson., 14, 125 Madhu, P.K., Goldburt, A., Frydman, L. & Vega, S. (1999) Chem. Phys. Lett., 307, 41. Madhu, P.K., Goldburt, A., Frydman, L. & Vega, S. (2000) J. Chem. Phys., 112, 2377. Man, P.P. (2000) in Encyclopedia of Analytical Chemistry, Ed. Meyers, R.A., John Wiley and Sons, Chichester, p. 12224. Man, P.P., Klinowski, Trokiner, A., Zanni, H. & Papon, P. (1988) Chem. Phys. Lett., 151, 143. Mansfield, P. (1971) Progress Nucl. Mag. Reson. Spectrosc., 8, 41. Mansfield, P., Orchard, M.J., Stalker, D.C. & Richards, K.H.B. (1973) Phys. Rev. B, 7, 90. Maricq, M.M. & Waugh, J.S. (1979) J. Chem. Phys., 70, 3300. Marinelli, L., Medek, A. & Frydman, L. (1998) J. Mag. Reson. A, 132, 88. Mason, J. (1993) Solid State Nucl. Mag. Reson., 2, 285. Medek, A., Harwood, J.S. & Frydman, L. (1995) J. Amer. Chem. Soc., 117, 12779. Mehring, M. (1983) Principles of High Resolution NMR in Solids, Springer-Verlag, Berlin. Menger, E.M. & Veeman, W.S. (1982) J. Mag. Reson., 46, 257. Metz, G., Wu, X. & Smith, S.O. (1994)J. Mag. Reson. A, 110, 219. Metz, G., Ziliox, M. & Smith, S.O. (1996) Solid State Nucl. Mag. Resort., 7, 155. Mtiller, D. (1982) Annales der Physike, 39, 451. Mfiller, L., Kumar, A., Baumann, T. & Ernst, R.R. (1974) Phys. Rev. Lett., 32, 1402 Munowitz, M. (1988) Coherence and NMR, John Wiley and Sons, New York. Nayeem, A. & Yesinowski, J.P. (1988) J. Chem. Phys., 89, 4600. Neue, G., Dybowski, C., Smith, M.L., Hepp, M.A. & Perry, D.L. (1996) Solid State Nucl. Mag. Reson., 6, 241, Opella, S.J., Frey, M.H. & Cross, T.A. (1979) J. Amer. Chem. Soc., 101, 5856. Pake, G.E. (1948) J. Chem. Phys., 16, 327. Powles, J.G. & Mansfield, P. (1962) Phys. Lett., 2, 58
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Powles, J.G. & Strange, J.H. (1963) Proc. Phys. Soc., 82, 6. Proctor, W.G. & Yu, F.C. (1950) Phys. Rev., 77, 717. Rhim, W.-K., Elleman, D.D. & Vaughan, R.W. (1973) J. Chem. Phys., 58, 1772. Ryan, L.M., Taylor, R.E., Paff, A.J. & Gerstein, B.C. (1980) J. Chem. Phys., 72, 508. Samoson, A., Kundla, E. & Lippmaa, E. (1982) J. Mag. Reson., 49, 350. Samoson, A. & Lippmaa, E. (1983) Phys. Rev. B, 28, 6567. Samoson, A. (1985) Chem. Phys. Lett., 119, 29. Samoson, A., Lippmaa E. & Pines, A. (1988) Molecular Phys., 65, 1013. Sanders, J.C.P. & Schrobilgen, G.J. (1990) in Multinuclear Magnetic Resonance in Liquids and Solids - Chemical Applications, vol. 322, Eds. Granger, P. & Harris, R.K., NATO ASI, p. 157. Sato, N. & Kimura, K. (1990) J. Amer. Chem. Soc., 112, 4688. Sch~ifer, H., Iuga, D., Verhagen, R. & Kentgens, A.P.M. (2001) J. Chem. Phys., 114, 3073. Schmidt, V.H. (1971) Proceedings of the Ampere International Summer School//, p. 75. Schmidt-Rohr, K. & Spiess, H.W. (1994) Multidimensional Solid State NMR and Polymers, Academic Press, London. Skibsted, J., Nielsen, N.C., BildsCe, H. & Jakobsen, H.J. (1991) J. Mag. Reson., 95, 88. Slichter, C.P. (1990) Principles of Magnetic Resonance, Springer-Verlag, Berlin. Sternheimer, R.M. (1954) Phys. Rev., 95, 738. Strange, J.H. (1987) Cryst. Lattice Defects and Amorphous Mater., 14, 183. Stuart, S.N. (1994) Solid State Nucl. Mag. Reson., 3, 199. Sun, B.Q., Baltisberger, J.H., Wu, Y., Samoson, A. & Pines, A. (1992) Solid State Nucl. Mag. Reson. 1,267. Townes, C.H., Herring, C. & Knight, W.D. (1950) Phys. Rev., 77, 852 Tycko, R. & Opella, S.J. (1987) J. Chem. Phys., 86, 1761. Vega, A.J. (1992) J. Mag. Reson., 96, 50. Vega, A.J. (1992a) Solid State Nucl. Mag. Reson., 1, 17. Vega, S. (1981) Phys. Rev. A, 23, 3152. Walter, T.H., Turner, G.L. & Oldfield, E. (1988) J. Mag. Reson., 76, 106. Waugh, J.S., Huber, L.M. & Haeberlen, U. (1968) Phys. Rev. Lett., 20, 180. Woessner, D.E. (1987) Z. Phys. Chem. Neue Folge, 152, 309. Wu, G. & Wasylishen R.E. (1993) J. Mag. Reson. A, 102, 183. Wu, G., Rovnyak, D., Sun, B. & Griffin, R.G. (1995) Chem. Phys. Lett., 249, 210. Wu, G., Rovnyak, D. & Griffin, R.G. (1996) J. Amer. Chem. Soc., 118, 9326. Wu, Y., Sun, B.Q. & Pines (1990) J. Mag. Reson., 89, 297. Yao, Z., Kwak, H.-T., Sakellariou, D., Emsley, L. & Grandinetti, P. (2000) Chem. Phys. Lett., 327, 85. Yesinowski, J.P., Eckert, H. & Rossman, G.R. (1988) J. Amer. Chem. Soc., 110, 1367. Zwanziger, J.W. & Chmelka, B.F. (1994) in NMR Basic Principles and Progress, vol. 31, Eds. Blumich, B. & Kosfeld, R., Springer-Verlag, Berlin, p. 202.
Chapter 3
Experimental Approaches Basic Experimental Principles of FT NMR Instrumentation 3.2.1 Overview of a Pulsed FT NMR Spectrometer 3.2.2 Magnets 3.2.3 Shimming 3.2.4 Transmitters 3.2.5 Probes 3.2.6 Connection of the Probe 3.2.7 Signal Detection 3.2.8 Additional Equipment 3.3. Practical Acquisition of NMR Spectra 3.3.1 Processing the FID to Produce a Spectrum 3.3.1.1 Window Functions 3.3.1.2 Shifting of the Time Origin and Linear BackPrediction 3.3.1.3 Zero Filling 3.3.1.4 Phase Correction 3.3.1.5 Baseline Correction 3.3.2 Complications in Recording Spectra 3.4. Static Broad Line Experiments 3.4.1 Pulsed Echo Experiments 3.4.2 Stepped Experiments 3.5. One-Dimensional High Resolution Techniques 3.5.1 Magic Angle Spinning (MAS) 3.5.2 Extraction of Parameters from MAS NMR Spectra 3.5.3 Suppression of Spinning Sidebands 3.5.4 Special Considerations for MAS of Quadrupolar Nuclei 3.5.5 Magic Angle Spinning Observation of Satellite Transitions 3.5.6 Double Angle Rotation of Quadrupolar Nuclei 3.5.7 Practical Implementation of CRAMPS 3.6. Two-Dimensional Experiments 3.6.1 Nutation NMR 3.6.2 Off-Resonance Nutation 3.6.3 Order-Resolved Sideband Spectra 3.6.4 Dynamic Angle Spinning (DAS)
3.1. 3.2.
111 112 112 113 115 116 120 122 124 127 127 128 128 129 129 130 130 130 133 133 136 138 138 143 143 144 149 150 152 153 153 154 155 156
3.6.5 3.6.6 3.6.7
Two-Dimensional Sequences Developed from Solution NMR Multiple Quantum Experiments in Dipolar Coupled Systems Multiple Quantum NMR Experiments of Non-Integer Spin Quadrupolar Nuclei 3.6.8 2D XY Correlation Methods 3.6.9 Correlation of Tensor Information- Separated Local Field Experiments 3.7. Summary of Approaches for Examining Quadrupole Nuclei 3.8. Multiple Resonance 3.8.1 Cross-Polarisation (CP) 3.8.2 SEDOR, REDOR and TEDOR 3.8.3 TRAPDOR and REAPDOR 3.9. Techniques for Determining Relaxation Times and Motional Parameters 3.9.1 Measurement of T~ 3.9.2 Other Spin-Lattice Relaxation Times (Tip, T1D) 3.9.3 Transverse Relaxation Times (T2) 3.9.4 Molecular Motion 3.9.5 Diffusion Measurements 3.10. NMR Under Varying Physical Conditions 3.10.1 Variable Temperature NMR 3.10.2 High Pressure Experiments References
157 160 161 168 170 172 172 173 178 182 183 183 184 185 186 187 187 187 189 190
Chapter 3
Experimental Approaches 3.1. BASIC EXPERIMENTAL PRINCIPLES OF FT NMR
In principle the NMR experiment is straightforward in that it has relatively few requirements. The equilibrium magnetisation must be produced by the application of a high static magnetic field which creates the Zeeman states lifting the degeneracy of the nuclear spin energy levels. The central task of an NMR spectroscopy experiment is to determine the separation of these energy levels. This measurement is carried out by the application of a usually much smaller magnetic field oscillating at a frequency that can induce transitions between the energy levels. This condition dictates that with magnetic fields in the range 4.7 to 18.8 T typically available today, the corresponding resonance frequencies are in the range 10-800 MHz, and it will not be long before 1H NMR is carried out at 1 GHz. The original method employed to detect a signal was to scan either the frequency of the exciting oscillator or the applied magnetic field until resonant absorption occurred. However, compared to simultaneous excitation of a wide range of frequencies by an rf pulse, scanning either the frequency or Bo is a very time-inefficient way of recording the spectrum. In the linear approximation there is a direct Fourier relationship between the time domain signal after a pulse (the FID) and the spectrum so that the spectrum is produced by Fourier transformation (FT) of the FID. Hence, with the advent of computers that can be dedicated to spectrometers, and efficient Fourier transform algorithms pulsed FT has become the normal mode of operation in NMR experiments (Ernst and Anderson 1966). Operating at constant field and frequency also produces big advantages in terms of the stability of the whole experiment. With the development of persistent superconducting magnets very stable applied magnetic fields are now used. In an FT NMR experiment a pulse of duration Tp is applied close to resonance. The discussion of the rotating frame in Chapter 2 shows that the pulse causes a coherent motion of the magnetisation, tipping the magnetisation through an angle 0p. The signal observed in the NMR coil by electromagnetic induction is termed the FID. To improve the signal/noise ratio (S/N), a number of FIDs (n) are usually coherently added, increasing the signal, with the S/N improving as ~n. It is a tacit assumption that everything behaves in a linear fashion, for example, that the excitation (or effective rf field) is uniform across the entire spectrum. In many cases this situation is closely approximated but distortions can occur for the broad lines that may be encountered. The frequency spectrum A(v) of a pulse of duration Tp applied at Vo is given by a sinc function sin ~Tp (V o - V) A(v) -
rC(Vo - v ) T p
111
(3. l)
Multinuclear Solid-State NMR of Inorganic Materials
112
A(v) rf
/'~
T Tp Fouriertransform,. field[~ ] ]Envelopeof ' ~
sincoet Vo 1
Frequene3 1
Figure 3.1. Fourier relationship between an rf pulse of duration Tpand the amplitude distribution A(v) of the frequency components present.
The sinc function is simply the FT of a rectangular pulse. Hence, from the bandwidth theorem, as the duration of the pulse decreases the frequency range covered increases. In Figure 3.1 the central lobe of this sinc function increases its frequency coverage as the pulselength decreases. To obtain an undistorted spectrum it should ideally be confined to the central portion of this lobe so that the irradiation is then uniform across the spectrum. Unfortunately there are examples in the solid state NMR literature of wide lines where the resonances are clearly much broader than the central lobe so that effectively only the irradiation envelope is reported and not the true spectral lineshape. The sinc 2 function describes the best possible case, with often a much stronger frequency dependence of power output delivered at the probehead. (It should be noted here that alternative excitation schemes to pulses are possible such as adiabatic passage (Kentgens 1991) and stochastic excitation (Yang et al. 1998) but to date these are only very infrequently applied).
3.2. INSTRUMENTATION 3.2.1 Overview of a pulsed F T N M R spectrometer The basic components of a pulse FT NMR spectrometer are shown schematically in Figure 3.2. There is a high field magnet which these days is nearly always a superconducting solenoid magnet. The probe circuit containing the sample is placed at the centre of this magnetic field. The probe is connected to the transmitter. All parts of the spectrometer are coupled together via transmission lines whereby the rf power/signal are transferred. Typically the coaxial cable which acts as the transmission line has a characteristic impedance determined by its capacitance and inductance per unit length. In NMR experiments, the convention is to standardise on an impedance of 50 l). The transmitter consists of a synthesiser to produce the central frequency, which is gated to form the pulses. These pulses must be amplified so they can be made sufficiently short to cause broad enough frequency excitation for the frequency width to be observed.
Experimental Approaches
ii ii
@
I
113
Magnet
Synthesiser r
j Ampl~'~r
Figure 3.2. Basic components of FT NMR spectrometer.
The probe is also connected to the receiver, requiring careful design to ensure that the receiver that is sensitive to IxV does not see any of the large excitation voltages produced by the transmitter. The induced voltage detected in the receiver must be digitised for storage in a computer where it can be manipulated and converted into a spectrum. The relatively simple concept of both the experiment and the instrument belies the extensive research and development effort that has gone into NMR spectrometers.
3.2.2 Magnets NMR demands ever higher magnetic fields, the highest currently commercially available for solid state NMR being 18.8 T. Standard instruments are now considered to be 4.7-9.4 T. The drive for higher fields is based on the increased chemical shift dispersion (in Hz) and the increase in sensitivity via both the Boltzmann factor and higher frequency of operation. Superconducting solenoids dominate the magnets sold and are usually based on Nb3Sn or NbTi multifilament wire maintained at liquid helium temperatures. However, fields and current densities now used are close to the critical limits of these materials, demanding improved materials technology. The demand for ever higher magnetic fields has seen the extension of the superconductor performance by operating at lower temperature. This is achieved by pumping on part of the helium cryostat which reduces the boiling point of helium. The principle of operation is very simple; a high current is circulated through a long (several km) coil of wire at a typical current of at least 50 A. This means that the magnet stores significant amounts of energy (up to 10 MJ) in its field (= 1/2LI2, where L is the solenoid inductance and I is the current flowing). A superconducting magnet consists of a cryostat, main coil, superconducting shim set and a means for attaching the current supply to the main coil (Figure 3.3). The
114
Multinuclear Solid-State NMR of lnorganic Materials
Longitudinal cut through a vertical cryostat ~ ] A - liquid helium (-269~ B - superconducting coil C - l i q u i d nitrogen (-196~
D - vacuum space E - room temperature bore
Figure 3.3. Schematic view of a high field superconducting magnet.
cryostat consists of two vessels for the liquid cryogens, the inner one for helium and the outer one for nitrogen. These are insulated by several vacuum jackets with a radiation shield. The aim is to reduce heat leakage to the inner chamber to conserve helium. NMR superconducting magnets are usually operated in persistent mode, which means that after a current is introduced, the two ends of the main coil are effectively connected so that the current follows a continuous path within the superconductor, allowing the power supply then to be disconnected. To attach and detach the coils (main and shim) to the power supply, the circuits within the cryostat have a superconducting switch. The coil circuits are also designed to cope with a sudden irreversible loss of superconductivity, termed a quench. There are resistors present (called dump resistors) to disperse the heating effect and prevent damage to the main coil when a quench occurs (Laukien and Tschopp 1993).
Experimental Approaches
115
Magnet technology is continually advancing, even for "standard" 7.05 T solid-state NMR magnets which have been available for --~20 years. In early versions the field was typically generated by a current of 50A, whereas modem versions run at nearer 90A since the inductance of the main coil in newer magnets is much lower. This brings the advantage that modem magnets can be charged much more rapidly. The complexity and size of the associated cryostat is greater than in the more conventional magnets. Although higher magnetic fields are available than those used in NMR, the NMR experiment imposes additional constraints on the field. An NMR spectroscopy experiment also demands homogeneity and stability of the magnetic field. Long term stability is aided by persistent-mode operation and the drift should be < 2 X 10 - 7 per day. Homogeneity requirements for solid state NMR experiments are typically 2 X 10 - 9 over a volume of--~ 1 cm s. The main coil alone is unable to produce this level of homogeneity so in the main cryostat there is a set of smaller superconducting coils called cry0shims. The number of these cryoshims depends on the design and also the purpose of the magnet (e.g. solid state NMR, high resolution NMR, imaging), but typically varies between three and eight. For many solid state NMR experiments the homogeneity produced by the cryoshims is sufficient. However, most commercial spectrometers also have a room-temperature shim set which further improves the homogeneity of the magnetic field. A final consideration for the magnet is the accessible room-temperature bore size of the magnet. A standard magnet has a bore of 52 mm diameter, but most solid state NMR spectroscopists prefer an 89 mm bore as this gives much more room for the probe, allowing the use of larger, more robust high-power electrical components, and accommodating some of the more specialised probe designs (e.g. double angle rotation, dynamic angle spinning etc.). Superwide bore magnets also exist, with an accessible diameter of 150 mm. For imaging experiments, even wider-bore magnets are made, and often rather than the bore being in a vertical orientation, as is favoured in spectroscopy, the bore is horizontal.
3.2.3 Shimming For most solid-state NMR spectroscopists, shimming is a relatively minor consideration. For many of the studies discussed later in this book the resolution provided by a well cryoshimmed magnet is more than sufficient. Nevertheless, there will be occasions when shimming is important. The resolution required for high-resolution solution experiments is typically --~0.1 Hz. At a resonance frequency of 400 MHz this corresponds to a magnetic field homogeneity of one part in 2.5 X 10 ~~ this resolution ideally achieved over a cylindrical volume typically 20 mm high and 10 mm in diameter. Most commercial spectrometers are bought with a room temperature shim set. Shimming to this level of precision can be difficult, as the process amounts to trying to find a global minimum in the linewidth determined by the inhomogeneity in the
116
Multinuclear Solid-State NMR of Inorganic Materials
magnetic field which is a multidimensional function. For an ideal shim set all shims would be orthogonal with each gradient having no interaction with other gradients. No shim set is perfect but often local minima are only relatively shallow, although this is not always so. Each shim gradient (the direction and shape of gradient that results when a current is applied to that specific coil) has a shim centre. In a cryomagnet there will be quite a strong field dependence (i.e. a gradient) along the z-axis. If a shim current is used to apply an additional field to cancel out this gradient there should be a change in the value of the field at all points except one; this unchanged point is termed the shim centre for that gradient. The determination of this point for the z direction is made in the factory and is recorded in the magnet handbook as a distance from the magnet top and bottom flanges. Different magnet manufacturers supply different cryoshims, some making only x, y and z shims available but others offering xy, xZ--y2, XZ and yz as well. Cryoshimming is carried out during installation so that most operators never come into contact with this procedure, which needs not be repeated unless the magnet is moved to a significantly changed environment (e.g. another magnet is located close by), or a cryoshim quenches. The authors' experience is that even after a full quench the old cryoshim currents are usually satisfactory. Cryoshimming should ideally be carried out on the largest sample volume likely to be used. Most NMR spectrometers have 12 to 18 shim controls (Churmny and Hoult 1990). Each user will adopt their own procedure but the aim is to produce the minimum linewidth consistent with a good lineshape. In practice, some shims are much more significant than others and for particular probes different shims will be important. For solid-state operation, shimming usually needs to be carried out relatively infrequently. One possible procedure for probes tuned to ~H is to crudely shim on H20. If there is no proton channel most multinuclear probes will tune to 2D, so D20 can be used. For CPMAS probes that tune to 13C, adamantane is a useful compound which should be shimmed under spinning and 1H decoupling conditions. A typical resolution for 13C in admantane of 3-4 Hz at 7.05 T and --~10 Hz at 11.7 T should be achieveable. In any shimming procedure the gradients should be varied systematically (Churmny and Hoult 1990) and it should be borne in mind that even with very good shim sets the gradients will be interdependent. The speed of the Fourier transformation in modem spectrometers allows the spectrum itself to be used for shimming, rather than the FID. There will always be a compromise between the very narrowest lines that can be produced and a good lineshape (free of shoulders and humps).
3.2.4 Transmitters The frequency required to observe a particular nucleus is the first consideration. This frequency was formerly synthesised by analogue methods. Frequencies below 200 MHz
ExperimentalApproaches
117
were synthesised directly, whereas higher frequencies were synthesised via frequency mixing. Synthesising the frequency in a stable and accurate way is very important. The technology of synthesisers has advanced, with digital synthesisers now available. In some experiments the frequency of the synthesiser needs to be switched rapidly. There are two alternative schemes for switching the frequency. One possibility is that it is phase-continuous, whereby, when the frequency changes, the phase continues instantaneously with the same value or phase coherently (i.e. the phase of the second frequency is as if it had started at the same point in time in phase with the other frequency). This frequency is normally continuously synthesised and then gated to form the pulse. Solid-state NMR experiments often call for short pulses and rapid phase switching, and in earlier spectrometers the most commonly-used quadrature phases (0 ~ 90 ~ 180~ 270 ~ or x, y , - x , - y) were simultaneously present in a hardwired modulator. Each of the four channels had separate amplitude and phase controls, and the rf was generated simply by taking the appropriate output. This set-up requires maintenance to correct for changing conditions and the frequency dependence of the operation means that different settings will be necessary for the different channels at different frequencies. Such a set-up produces the maximum switching speed for the phases, but in recent years digital synthesis has become more common. Here, the computer memory holds a sine wave in digital form. The required frequency is then produced by the speed at which the computer memory is read out, the rf being generated by a digital-to-analog converter. The phase shifts are generated by simply varying the memory address from which the signal read-out commences. This means that pulse phases can be generated continuously with typical settability currently of--~0.1 ~ Hence, non-90 ~ phase shifts are readily generated, an important consideration since an increasing number of experiments routinely demand such phase differences (e.g. MQ-MAS of quadrupole nuclei). The pulse from the modulator, which is typically at the --~V level, is then amplified to a level that depends on the type of experiment. The power of a transmitter can be determined by examining the pulsed voltage on an oscilloscope (but NEVER plug a high power transmitter directly into an oscilloscope). Attenuation is measured in decibels (dB) and in terms of voltage:
10
(3.2)
Hence 3 dB represents a factor of 1.4 in voltage while 20 dB is a factor of 10. If Vpp is the measured peak-to-peak voltage after taking into account the attenuation, the power is
P(watts) = 200-,~
(3.3)
118
Multinuclear Solid-State NMR of lnorganic Materials
Amplification is achieved via solid-state transistors or vacuum tube amplifiers. Transistors have always been used for power of less than 100 W, but for amplification up to the 1 kW level the philosophy has changed in recent years. Originally, tube amplifiers were the only real choice for high power rf applications. However, solid-state transistor technology has advanced so that up to 200 MHz 1 kW transistor amplifiers are now a serious option. At higher frequencies, tube or cavity amplifiers are still the only real possibility. For more demanding high-power applications, tube amplifiers are preferred since, if the duty cycle increases, producing a significant heating effect, transistors are susceptible to changes in their characteristics, whereas tube amplifiers already run hot. Tube amplifiers are usually part of an active tuned circuit so that as the frequency of operation is changed the amplifier circuit has to be retuned. By contrast, a transistor amplifier is broadbanded, requiting only the new frequency to be set. This makes the transistor design increasingly popular and the spectrometer easier to use. The behaviour of rf amplifiers falls into one of several classes (normally termed A, AB, B or C) depending on the relationship between the input and the output voltage. Figure 3.4 gives a diagrammatic representation of this relationship. Class A is termed linear, the output being directly proportional to the input. This relationship can make resetting of the CP condition at different power settings relatively straightforward and is necessary for the application of high-power shaped pulses in solid-state NMR. The disadvantage of linear amplifiers is that without careful circuit design the output can be less stable (e.g. due to heating effects). Also, as the circuit operates in the active region it generates significant noise even in the quiescent state, and so needs to be actively blanked by a gating circuit. For some applications (e.g. CRAMPS), the speed of the Anode current
Anode current
Class A
Class C
. . . .
Output
0 ,---i .....
o Grid voltage
i
i i
Grid voltage
i
'Input Input Figure 3.4. Schematic representation of the input-output relations of Class A and C amplifiers with
O the operating point of the grid voltage.
Experimental Approaches
119
Gate
TR i-,,
. v~
I -~-...................... i................................. j ............ [ ,,/~ "--,, i]~Droop i," ~ A-""-^-.....~--~- ....... ~.........................
i/'
.
.
.
.
.
.
.
Ripple
Figure 3.5. Schematic representation of pulse imperfections with TR the rise time and TF the fall time. blanking circuit becomes an important consideration. Class C operation is highly nonlinear but has several advantages. In its quiescent state, the amplifier is biased hard off so that there is very little noise generated by the transmitter between pulses. It then turns on at a fixed level with relatively little change in the output with input level, making the transmitter very stable. Often class AB operation is preferred which, at the extremes, shows the advantages of class C behaviour while nevertheless having a linear region. The pulses from the transmitter are often far from ideal. To check their non-ideality they should be observed on an oscilloscope via a broadband 50 ~ impedance, termed a dummy load. Typical characteristics are the rise and fall times, the on-off ratio and the amounts of tipple and droop. These are shown schematically in Figure 3.5. The turn-on time is usually defined as the time to go from 10 to 90% of the maximum power. The on-off ratio is especially important for determining the noise generated by the pulse during the FID. The effect of droop becomes more noticeable when longer pulses are applied, as for spin-locking. All these effects which alter the shapes of pulses mean that the irradiation profile is often much more complicated than the sinc function of Eq. 3.1. Also, for a pulse incident on a narrowed-banded mismatched load (e.g. a probe), the effects and the irradiation are more complicated still. Even if the bandwidth of the pulse is high, any narrow bandwidth components in the spectrometer sequence will cut this down and will also give rise to transient oscillations.
120
Multinuclear Solid-State NMR of lnorganic Materials
3.2.5 Probes
The probe is the heart of the NMR experiment. It is essentially a tuned resonant circuit with the sample contained within the main inductance (the NMR coil). The inductance of the coil is proportional to p~l,Zon21A where n is the number of turns per unit length on the coil (i.e. N (total turns)/1 (coil length), IX~o is the permeability of the sample and A is the cross-sectional area of the coil. The details of the circuits are often complex, and a typical circuit is shown (Figure 3.6). The circuits often use inductive coupling so that different parts of the circuit need not be physically linked (Kuhns et al. 1988). Both series and parallel circuit designs are used (Fukushima and Roeder 1981, Stejskal and Memory 1994) but they all consist of capacitances and inductances which have frequency-dependent reactive impedances. Usually a parallel-tuned circuit is preferred, and in simplified form the inductance (L) and capacitance (C) can be related to the required resonant frequency (Vo) via Vo - 1/(2-rrL ~ ) . The resonance frequency is the most important parameter, but the input impedance, which should be 50 12, also has to be satisfied. Furthermore, the quality factor Q, (a measure of the sharpness of the resonance of the probe circuit, one definition being the resonance frequency/half width of the resonance response of the circuit, also = tooL/R, R is the coil resistance), must also be considered. The probe must satisfy competing requirements which can be characterised in terms of Q, as summarised in Table 3.1. It can be immediately seen from Table 3.1 that a probe cannot be designed with all the properties optimised because of their differing Q-dependencies. This means that most probes are a compromise, focussing on the most important aspects for a specific application. Several probe designs exist which have advantages for different applications. 74 nH
10 nH 22 pf . . . 1 _
1-10 pf 13C <-",
.,
22pf + '
-r-
/l't-
'lgpf I
13 nH 1.5 pf
1-10 pf
----) I
'P
t"~ /
11. l i Tt
J'
400 MHz 100 MHz
_ J 1-10 pf
:~
1-10 pf
l r -
7nH~ ,
10.,n _
,,
~
1 ~r
- :
~
"
33 nH 1
Figure 3.6. Typical circuit of a doubly resonant probe from Doty et al. (1988) with permission of the copyright owner.
121
Experimental Approaches
Table 3.1. Summary of the dependence on various properties of the NMR probe on the quality factor (Q). Property
Q-dependence
Length of a 90~pulse at constant transmitter power Probe deadtime Voltage developed across the circuit Frequency response bandwidth Sensitivity per nucleus
1/]Q Q Q I!Q
Various choices of coil are also possible (Hoult and Richards 1976), the optimum design again depending on the specific application. The traditional coil design, particularly applicable at lower frequencies and for solids, is the conventional solenoid. For large samples loaded externally to the magnet, Helmholtz or saddle coils are used widely in high resolution liquid state NMR studies, but in MRI birdcage coils are finding increasing use. The resistance of the coil (wire) increases with the frequency of the experiment through an electromagnetic phenomenon known as the skin effect. This effect produces an ac current in a layer determined by the skin depth d d - ~[
1
(3.4)
rCfloOV o
As frequency increases, d decreases, causing the wire to present a smaller cross-sectional area. This increases the ac resistance and consequently lowers the circuit Q. NMR coils are often made of flat wire rather than normal round wire, the former also having the advantage of better rf homogeneity. The magnitude of the induced B1 field is related to the power applied, Q, and the coil volume, V, by B 1 - J t't~ 7Wv o
(3.5)
Coils operate better if they do not have any sharp comers, since there is less tendency for the magnetic field to bunch, and also voltage breakdown is more likely to occur at sharp points. The capacitors are also important in probe design since they need to be able to withstand high voltages. The maximum voltage developed in a probe circuit is given approximately by Winput X Q Vmax --
5O
(3.6)
For a 1 kW transmitter, the peak-to-peak input voltage is --- 600 V, and with a Q of 250, which is common in commercial CP-MAS probes, 3 kV can be generated. It is thus important for the capacitors to be able to withstand such voltages and for their dielectric
122
Multinuclear Solid-State NMR of Inorganic Materials
insulation to have a high breakdown voltage. High power rf components must be handled with great care so that grease and dirt are not introduced as these will severely degrade performance; once the performance has begun to deteriorate it is very difficult to improve the situation without complete probe overhaul. The leads between the coil and the probe tuning, and matching capacitors and inductors also play a central role in the overall performance of the probe. A simple consideration is the length of the leads from the coil; the longer the leads, the farther other probe components can be removed from the active area of interest. Removing components minimises their possible disturbance of the magnetic field, although this is often not a significant consideration for solid-state NMR. The downside is that the leads add to the inductance and capacitance of the probe circuit. Stray capacitance can be a significant problem especially if one is interested in higher frequencies, since the main tuning capacitor may only be a small part of the total circuit capacitance, giving a small tuning range and limiting the minimum capacitance that can be achieved. The leads also add inductance to the circuit and since they do not contain sample, they reduce the filling factor of the circuit. Maximum sensitivity occurs when all the circuit inductance is filled with sample. One way to minimise the effect of the leads is to get as much of the tuning capacitance near the active part of the coil as possible. Linear analysis is often applied to determine the electrical response of a probe circuit, but in probes designed for solids (and therefore for high-power operation), nonlinear effects can occur (Doty 1996). In multiple-resonant probes, for decoupling and cross-polarisation, even small voltages generated at the second resonant frequency are unacceptable since these would swamp NMR signal generated in the coil which is part of more than one resonant circuit. A detailed consideration of the design criteria for doubly-tuned CP-MAS probes has been given (Doty et al. 1988). All probes have background signals. This is more of a problem for static wideline work, since the background signals are usually themselves broad. Wide background signals would not have much effect on the narrower spectral windows used in high resolution work. Some common background signals are given in Table 3.2. To check that a signal comes from a sample, a dataset should be acquired under identical conditions but without the sample. Some of the interfering nuclei listed in Table 3.2 are very important for solid-state NMR and probes can always be constructed to minimise the background from a particular nucleus, to allow the observation of that nucleus in the sample.
3.2.6 Connection of the probe It is usual to have the probe physically connected at all times to both the transmitter and the receiver. The electrical circuit acts effectively to couple the probe to the transmitter and receiver when the pulse is respectively on and off (termed duplexing). In solid-state NMR operation the duplexing circuit should withstand high power rf and
123
Experimental Approaches Table 3.2. Nuclei observed as background signals. Source of background
Nucleus 1H
~B 17 0
19F 23Na 27A1 29Si 35,37C1 63,65Cu lO7,1O9Ag
Moisture, polymer parts Ceramic and glass parts As the background is unenriched not normally a problem but for weak MAS signals ZrO2rotors can be seen at "-~ 383 ppm Ceramic and polymer parts, PTFE tape, Teflon of the coaxial cable Glass and ceramic parts, be careful with salt off the fingers Ceramic parts, capacitors and from aluminium metal itself which is widely used in sample manufacture, A1N impurities in Si3N4rotors Ceramic and glass parts, Si3N4rotors Some fluorinated polymers give a strong chlorine signal Coil wire Coil wire
Crosseddiodes
[ Tx L
(~l
Z/4 ___~ Probe [
~4 Crosseddiodes< >1
] Preamp ,l
TX on /
/
Open
Closed .
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
ti
Preamp ,I
Figure 3.7. Lowe-Tarr circuit design for connecting the NMR probe to both the transmitter and the receiver with crossed diodes and M4 cables. remain stable throughout the sequence of pulses. A common approach to this is the Lowe-Tarr circuit (Lowe and Tarr 1968) which contains a series of diodes and path lengths related to the wavelength of the rf radiation (Figure 3.7). Pairs of diodes in parallel but with their conducting direction oppositely oriented (termed crossed) act as active switches. When the pulse is on, the two switches are closed. Hence the probe is
124
Multinuclear Solid-State NMR of Inorganic Materials
connected to the transmitter so that with the impedance transformation of the h/4 cable, the far side of the probe appears earthed and the receiver is protected, allowing the maximum voltage to be delivered to the probe. Then when the pulse is off the transmitter is effectively disconnected from the probe which is itself connected to the preamplifier. The path lengths may be physically produced by cables of the correct lengths (the simplest approach). The drawback is that the h/4 length of cable is only efficient over a relatively narrow bandwidth of frequencies. Reactive components provide alternatives to cables; several circuit designs from the literature are extensively discussed by Fukushima and Roeder (1981). More sophisticated active networks have been suggested, such as pin-diode switches.
3.2.7 Signal detection The microvolt NMR signal generated in the coil requires amplification prior to detection and digitisation. The first stage is typically 20-30 dB amplification, using a preamplifier whose most important characteristic is the noise figure (NF). This is essentially a measure of the noise added to the signal by the amplifier, and is defined by,
I V2nRs NF - 10 loglo Vth2R~ ) - G
(3.7)
where W n is the output noise voltage, R~ is the load resistance, Rs is the input resistance of the preamplifier, Vth is the thermal or Johnson noise and G is the power gain of the preamp. Careful consideration is necessary for the production of a low noise figure and rapid recovery from saturation. Another way of regarding the noise figure is the ratio of the minimum detectable signal plus the thermal noise to the minimum detectable signal. The nearer this ratio is to one, the less noise is added in the preamplifier. Here again some compromise is necessary, with recovery of < 2 Ixs normally sufficient. The preamplifier should also have extremely good linearity and rapid recovery from saturation. The initially amplified signal is then usually detected in the receiver using a threeport device acting as a double-balanced mixer. Three port devices, which can be used as gates, mixers or phase-sensitive detectors, consist of a series of diodes and transformer coils. The phase-sensitive detector has two input signals, the NMR signal (COo) and the frequency of the synthesiser ((Oref). The output is proportional to COS((O)o- O)ref) -- (~) -[- COS(((.Oo -'['- (.Oref) Jr- (~).
(3.8)
The sum frequency is much larger than the total bandwidth of the spectrometer and so is lost, leaving the difference frequency, which is the frequency of the observed FID. This lower frequency is usually further amplified by audio-amplifiers. It can be seen that phase-sensitive detection is equivalent to examining the NMR signal in a reference
ExperimentalApproaches
125
flame rotating at (.Oref. If the frequencies too and O)re f are equal and relaxation effects are ignored, a constant output is obtained that depends on the phase difference (+), hence the term phase-sensitive detector. Most modern spectrometers employ two phase-sensitive detectors having reference signals that differ in phase by 90 ~, hence termed quadrature phase-sensitive detection. Quadrature detection (Traficante 1990) has several advantages compared to single channel detection. The most important is that quadrature detection can distinguish whether a signal is above or below the reference frequency. The positioning of the transmitter frequency in the spectral window being examined is then less critical. This ability to place the transmitter frequency near the centre of the range of interest improves both the pulse power efficiency and the signal-to-noise ratio. However, the system needs to be set to remove imbalance in the gains of the two channels and to correct for any deviation from 90 ~ of the phase shift between the two channels. The two channels are termed the real and imaginary channels since, if the signal is Fourier-transformed, the channels will give the absorptive and dispersive spectral components. If these factors are not corrected for, spurious signals termed quadrature images will appear. Any net DC offset, either between the channels or overall, will also cause spectral artefacts, most obviously as a zero-frequency spike. Careful set-up of the spectrometer can effectively remove such effects. However, phase cycling will also efficiently cancel quadrature images. The most common sequences include the use of four phases 90 ~ apart. The CYCLOPS sequence is widely used to remove quadrature artefacts (Hoult and Richards 1975). This pulse sequence has four steps in the phase cycle. The phase cycling and routing of the signal, shown in Figure 3.8, eliminate DC offset, amplitude imbalance and any deviation from 90 ~ of the phase between the two channels. It should be emphasised that this phase cycle only removes artefacts by cancellation not reconstruction so that S/N is poorer than if the system is set up perfectly. To improve S/N, the high frequency noise from outside the spectral range of interest should be cut down. As the signal is digitised, the Nyquist sampling theorem for a discretely sampled time domain signal (with a spacing of DW = the dwell time) shows that the highest frequency that can be unambiguously determined is 2/DW. Higher frequencies thus appear at lower frequencies, folding higher-frequency noise into the spectral range. Audio filters are used to cut out this high frequency noise. An optimum value is 1.2-1.3 of the spectral width of interest and the spectrometer manufacturers usually build this automatically into their software as the default setting. There are various designs of analogue filter available. The simplest are RC circuits, where the time constants determine the upper frequency response. A number of more complex filter circuits exist, two commonly used in NMR being the Bessel and Butterworth designs (Fukushima and Roeder 1981). Filters do not have infinitely sharp cut-offs and these designs show differing fall-off characteristics. Older designs of NMR spectrometer used Bessel or Butterworth filters for spectral ranges below 100-120 kHz, whereas
126
Multinuclear Solid-State NMR of Inorganic Materials Reference Frequency
Cyclops Computer memory
r
] Filter+l 1~
PS
"[ ADC
Real
Tx Phase X Y -X -Y
Signal sent to Real 1 2 -1 -2
Imaginary 2 -1 -2 1
7-"
Signal I
I
900 Phase shift
-'9 PSD2 [i "-i
Imaginary
] Filter + 2_ l .
7 ADC
Figure 3.8. Quadrature phase sensitive detection along with the phase cycling and signal routing used in CYCLOPS for eliminating quadrature artefacts.
above this range RC filters were used. A compromise must be made in improving the S/N by filtering, since the filter introduces distortion (e.g. deadtime, overshoot) to the start of the FID that can distort the lineshape of broad resonances. In determining the optimum filtering, the philosophy normally adopted is to use a filter value slightly larger than the spectral range of interest, but in certain cases this is not necessarily the best approach. If a broad line is of interest, the signal can be digitised very rapidly so that the FID is recorded accurately but the filter is set significantly smaller than spectral range. This improves the S/N, but the noise will show a variation in the spectral range. This is not a problem as long as it does not coincide with the spectral position. The signal is then digitised for storage in the computer memory. There are two ways in which the data can be captured when the two quadrature channels are being used. A data point for each channel can be captured at the same time (called simultaneous acquisition) or, alternatively, one data point is taken at each dwell time but routed to the different channels of the phase-sensitive detector (called sequential acquisition). The relationship of the spectral width and DW is slightly different for the two approaches: Simultaneous acquisition.'DW =
1
SW
2
,and sequential acquisition: DW = ~ SW
(3.9)
The number of time domain data points (TD) means that the time over which data are collected (termed the acquisition time (AQ)) is given by AQ - TD • DW. Digitisers are characterised by the number of bits (determining the dynamic range with which the
Experimental Approaches
127
voltage is recorded), the rate of digitisation (determining the dwell time) and the size of memory capable of storing the data points. Until very recently the ability to record narrow spectral objects over a broad range of frequencies was limited, usually by the on-board computer memory, but commercial spectrometers have now addressed this problem. Another possible approach to filtering that is becoming increasingly popular is to filter the digitised data (termed digital filtering).
3.2.8 Additional equipment Although spectrometers are self-contained, it is often very useful to have some additional hardware for optimising the operation of a spectrometer, especially for those who want to build components (e.g. probes) themselves and develop new approaches. A good oscilloscope is essential with a high rf bandwidth and sweep rates allowing very high frequencies to be detected. The triggering and calibration should be reliable. For normal phosphor screen instruments, intense displays are necessary to observe shortlived events. More recently oscilloscopes with liquid crystal displays have become available, allowing operation much closer to the magnetic field. Attenuators are essential so that pulses can be observed either on oscilloscopes or with the spectrometer receiver without causing damage. Although NMR pulses can be of high power, they are usually on for a relatively short period of time so the CW power rating need not be too high. The attenuator should have a broad bandwidth. An oscilloscope and attenuators are probably the minimum additional requirement for a properly equipped NMR laboratory. Directional couplers are also useful since the power reflected from the probe can be continuously monitored, thereby allowing tuning and monitoring for breakdown, which is sometimes important in high-power experiments. For probe tuning, a magic T with a sweep generator is a worthwhile asset, especially for development and modification of probes. There is increasing sophistication in this area and self-contained instruments that simply monitor circuit response are now available. A vector impedance meter is the top of the range instrument for examining probe circuits. 3.3. PRACTICAL ACQUISITION OF NMR SPECTRA
In single pulse acquisition the following simple steps need to be followed. (1) Choose an appropriate frequency for observation of the nucleus of interest and tune the probe for the experiment. (2) Start with an appropriate set-up compound that allows the 90 ~ pulse to be determined for given settings of the transmitter and the chosen probe. This set-up compound should also allow the spectral reference to be determined. (3) Make sure that the irradiation bandwidth and frequency range being digitised
128
(4)
(5) (6) (7)
Multinuclear Solid-State NMR of lnorganic Materials (usually termed the spectral or sweep range), are appropriate for the width of the resonance being observed. Choose a proper dead time delay between the detection pulse and the beginning of the acquisition. An optimum choice is to set this duration to approximately 0.75DW. This represents the very best that can be done and has to be set to avoid deadtime and tinging effects. Set AQ sufficiently long time that the signal is not truncated. Choose an appropriate gain in the receiver (pre-amplifier) so that the signal does not overload the receiver. Use a recycle time that means all the sites are fully relaxed in the spectrum.
3.3.1 Processing the FID to produce a spectrum The FID contains all the information of the NMR signal from the sample. All the spins have been excited simultaneously, and the FID is a record of all the information about the nuclei (e.g. the distribution of Larmor frequencies, interactions etc.) Spectroscopists find it somewhat difficult to interpret the time domain data but Fourier transformation to a spectrum greatly aids understanding. Usually a full discrete complex Fourier Transform is carried out using a numerical algorithm. The Cooley-Tukey (1954) (or the FFT (fast FT)) algorithm carried this field forward. The efficiency of this algorithm results from a repetitive pairwise sorting of data points, for data sets with a power of 2 number of data points. For direct transformation, the consumed time increases with N 2 of data points while it increases with 2NlogN for the FFT algorithm. Much development has occurred in numerical FT algorithms but they are largely based on the original algorithm. T2 relaxation processes mean that the information is not equally distributed throughout the time domain data. The T2 decay of the signal means that the contribution from the signal relative to the noise decreases as the time increases. Therefore weighting functions can be applied to the time domain data points in order to emphasise the parts of the FID where the signal preferentially resides. 3.3.1.1 Window functions. The experimental signal S(t) is multiplied by the window function in a process termed apodisation to produce a new weighted signal A(t) which is Fourier transformed. Useful window functions commonly used are: (i) Exponential multiplication A(t) = S(t) X e x p ( - 7rLBt)
(3.10)
This function signal improves S/N as it cuts down the high frequency noise contribution. It causes Lorentzian linebroadening, LB being the broadening (in Hz) produced by the function, and it reduces effects caused by truncation of the FID.
Experimental Approaches
129
(ii) Gaussian multiplication A(t) - S(t) x exp - ~ L B ( t - (2GB.AQ) )
(3.1~)
This can be used to improve the S/N by using Gaussian linebroadening. If the constant is set to a negative value, S/N will be degraded but the line is narrower (i.e. resolution enhancement). Such a process is often used in solution since it requires good S/N to be effective.
(iii) Trapezoidal multiplication S(t) can be multiplied by a trapezoidal shape. If a FID ends well before the end of AQ this can be used to cut down the high frequency noise. It can also be used to define a region around an echo to cut out deadtime effects.
(iv) Squared sine bell multiplication
[E AI
A(t) - S(t) x sin ~: -
AQ
A
(3.12)
The multiplication here is a good approximation to exponential multiplication (for A = 2) and to Gaussian multiplication (for A ~> 3). For zero shift, the first data point will be nulled and hence dispersive contributions to the FID are eliminated. An advantage of this function over the commonly used real EM or GM is the fact that the sine functions decay to zero, eliminating truncation oscillations.
3.3.1.2 Shifting of the time origin and linear back prediction. Often because of various deadtime effects (Sec. 3.3.2) the data cannot be recorded accurately and sometimes result in the appearance of large spurious induced voltages in the FT as spectrum-like features. These features can be removed by moving the time domain zero point in a process called left shifting. This is done by simply specifying the number of data points to be removed. The opposite process can move the data set back towards the real zero time domain point. However, this raises the question of what to insert into this region to avoid sinc oscillations. Linear back prediction has become standard in commercial spectrometer software, but because of the complex lineshapes that often occur, such an approach should be used with caution in the NMR of solids. The process works by looking at the data points and then predicting data points at earlier times using a relatively simple function.
3.3.1.3 Zero filling. To increase the digital resolution of the spectrum, the number of points can be increased before FT. The FID then consists of the acquired number
130
Multinuclear Solid-State NMR of lnorganic Materials
of data points plus additional data points containing zeroes. This simulates longer acquisition of the FID, but if the FID has already decayed, no noise is accumulated. However, there is no advantage in zero-filling more than three times (i.e. 2 3 = 8 times more data points).
3.3.1.4 Phase correction. Phase-sensitive acquisition of the FID with quadrature detection means that the pure absorption lineshape can be formed after FT but it may require some manipulation of the data to achieve this. If the real and the imaginary parts of the signal are AR and A~, the actual absorption spectrum A(oJ) can be formed as A(co) - pA R cos(to) + ~/1 - p2 AI sin(og)
(3.13)
A phase difference between the carrier frequency and the pulse leads to a phase shift which is almost the same for all resonance frequencies (o~). This effect is compensated for by the so-called zero-order phase correction, which produces a linear combination of the real and imaginary parts in the above equation with p - Po. The finite length of the excitation pulse and the unavoidable delay before the start of the acquisition (dead time delay) leads to a phase error varying linearly with frequency. This effect can be compensated for by the frequency-dependent, first-order phase correction p - Po + p~(~o - COo),where the factor p is frequency dependent. Electronic filters may also lead to phase errors which are also almost linearly frequency-dependent. 3.3.1.5 Baseline correction. The baseline of a spectrum may be distorted for a number of reasons, but often for reliable evaluation of the spectral information it is important that the baseline be flat. Commercial spectrometers are equipped with software packages containing a number of useful correction algorithms. Different algorithms can be used, depending on the nature of the baseline distortion. These include automatic linear baseline correction where a straight line is subtracted from the spectrum, which is calculated from a fit to the first and last N data points of the spectrum. Similarly, there are higher-order automatic polynomial corrections, a sinc curve or a set of data points can be defined manually and written into a file. These data points define the baseline of the spectrum and are used with a function fit between them. The availability of similar automatic routines is especially important for baseline correction of 2D NMR spectra when a large number of rows or columns need to be corrected.
3.3.2 Complications in recording spectra The best possible case for recording an undistorted spectrum when the pulse is incident on the probe circuit is given by the sinc function for the power distribution of the pulse. Complications beyond this include the frequency response of the probe in detecting the
Experimental Approaches
131
signal intensity. Thus, a broad line will not only experience non-uniform irradiation, but the intensity detected per spin at different frequency offsets will depend on this probe response, which depends on Q. The width of the frequency response decreases as Q increases, so typically for a Q of 100, the halfwidth of the frequency response is about 1.5 MHz. If sensitivity can be sacrificed, broad lines can be recorded by deliberately degrading or "spoiling" the Q by putting extra resistance in series with coil. Direct FT-pulse observation of broad spectral lines becomes impractical with pulse techniques for linewidths greater than --~200 kHz. Broader spectral lines can be reproduced provided corrections are made for the rf-irradiation and probe responses. Other potential pitfalls are experimental effects which corrupt the start of the time domain signal; these can be usefully split into deadtime and probe ringing. Deadtime is a general term used to describe effects causing the spectrometer system to be unable to record the NMR signal. Deadtime comes from several sources, the two principal ones being (i) the probe and (ii) other electronic spectrometer components including filters, preamplifier, receiver and even the cables. The pulses applied to the probe, which is essentially a tuned rf circuit, are usually rectangular (i.e. with very rapid rise and fall times compared to the duration of the pulse). However, the time at which the voltage in the circuit actually falls to zero depends on a time constant which is determined by the circuit (Hoult et al. 1983). The deadtime is the time constant of the exponential decay at the probehead and is given by
tdead =
Q (3.14) /~'V o
For a Q of 100 at 20 MHz the minimum value of tdead is 1.6 tXS. Sometimes a more practical definition of deadtime is the time for any induced voltage to drop to --~ 3 times the thermal noise. Hence the voltage will be too large in the coil for the sensitive receiver to record the tiny NMR signal for --~10 txs, and this represents the best possible case. Again the situation could be improved by reducing Q. Decreasing Q will also reduce the rf field at the probehead (for a given power), increasing the 90 ~pulse length. To obtain the most uniform irradiation over a broad range when observing broad lines, a short 90 ~ pulse length is necessary. The effect of the loss of the start of the FID (f(t)) can be considered most easily by writing an idealised time domain representation of the FID: D(t) • f(t), where D(t) = 0 when 0 --< t --< tdead, and D(t) = 1 when t >- tdead (3.15) Fourier transforming this gives the FT of the FID (i.e. the expected spectrum) convoluted with sinc(o~- tOo)t~ead, the latter manifested as a rolling of the baseline. An example is shown in Figure 3.9A. The effect of this deadtime-induced baseline roll is not a significant problem if the spectral lines are narrow compared to the frequency of the
132
Multinuclear Solid-State NMR of lnorganic Materials
A
Baseline = A
6000
2000
4000
0
sink(v-I/o) k(v- Vo)
-2000 -4000 -6000
Shift ppm B line
....
I
I
-800
I
I
I
-1000 -1200 -1400 -1600
sgy chemical shift in ppm Figure 3.9. A. The spectrum over a 1.66 MHz spectral range at 130.32 MHz for 27A1 with the initial deadtime producing a sinc roll of the baseline (taken from Alemany et al. 1991 with permission of the copyright owner). B. At 17.64 MHz the more complex baseline produced by ringing effects at the low frequency with the 89y just visible (taken from Smith and van Eck (1999) with permission of the copyright owner). baseline roll. The practical situation is often more complex than this analytic function, and an example is shown in Figure 3.9B. If the resonance was much broader, the lineshape would be significantly distorted by the baseline roll and it would be difficult to know what the true lineshape was. Working at higher frequency decreases tdead. However, practice often shows that the advantage of removing distortion by working at higher frequency is not as marked as might be anticipated, usually because the start of the FID is corrupted by ringing. Ringing is an all-embracing term to describe effects that induce FID-like voltages in the
Experimental Approaches
133
coil. Typical sources of this effect are the electromagnetic effects of the pulse interacting with the surroundings (e.g. the probe body) so that either mechanical and/or re-radiation effects occur. Acousto-mechanical effects can also occur in the coil and can continue after the pulse is turned off, producing a short-lived pseudo-FID. The overall efficiency (Ec) of this process is given by
klB~
EC =
(3.16)
d v s 1+
4
Vs
where kl and k2 are constants, e is the coil resistivity, d is the bulk density and Vs is the acoustic wave velocity (Fukushima and Roeder 1979). Eq. 3.16 shows that for a constant Bo the situation is worse at lower frequency, and at a constant frequency it is worse at higher Bo. These short-lived voltages can look like broad resonances or baseline distortion. Careful probe design can reduce ringing effects and the considerable work put into reducing these effects has been extensively reviewed by Geronthanassis (1987). In addition to the effects from the probe there is the electronic deadtime, including pulse ringdown (100 ns), preamplifier recovery (800 ns), filter overdrive recovery (1 txs) and ADC conversion droop (200 ns) (Hoult 1979). Magnetoacoustic ringing (up to 200 ~s) can be very significant if careful probe design (e.g. coil wire) is not considered. Samples that exhibit peizoelectric behaviour can lead to very long response times of up to 10 ms.
3.4. STATIC BROAD LINE EXPERIMENTS
3.4.1 Pulsed echo experiments Static powder patterns offer one way of characterising a material, and if spectral features can be observed and the line simulated, accurate determination of the NMR interaction parameters is possible. A simple consideration not to be overlooked is that the theoretical powder lineshape assumes the powder sample to be randomly oriented. This means that the powder should be finely ground. Coarse powders show structure in the lineshape which can complicate the lineshape analysis. Despite the effects outlined above, one-pulse acquisition experiments allow relatively narrow spectral features such as singularities to be recorded over a broad spectral range (Figure 11.3A). Detailed analysis of the singularity positions of the different transitions has allowed the different interactions (e.g. quadrupolar, shift) to be estimated (Baugher et al. 1969). Spectral intensity between the singularities is mostly lost so that true lineshape is not observed.
134
Multinuclear Solid-State NMR of Inorganic Materials
A common way of overcoming deadtime problems is to form a signal with an effective time-zero point outside the deadtime, i.e. an echo. There are many methods for forming such echoes. Most involve two-pulse sequences with the classic spin-echo (Sec. 2.8). In general, the loss of transverse magnetisation is incoherent, but under some circumstances this phase loss can be reversed and echoes observed in solids. If this dephasing is due to effects such as inhomogeneity of the applied magnetic field, chemical shift effects or heteronuclear dipolar effects, all these interactions are proportional to the operator iz. A 180 ~ pulse reverses the direction of iz and hence the magnetisation refocuses. In the other important cases of homonuclear dipolar and quadrupolar interactions echo phenomena cannot really be described classically and the loss of phase coherence is properly described by quantum mechanics (Solomon 1958) by which a complete density matrix description of the system is developed. The lineshape will only be accurately reproduced with the spin-echo if iz is accurately refocused. This will certainly be true at short refocusing times. Since the dipolar coupling and chemical shift do not commute, under some circumstances if there are dipolar couplings, the echo will not necessarily be correctly refocused. The spin-echo for an I spin will also not refocus well if the S-spin undergoes spin fluctuations on the timescale of the experiment. Such interactions will occur for strong homonuclear dipolar coupling or if S nuclei show short T~. Hence if there is strong homonuclear dipolar coupling or short S T1, the I spin is not fully refocused by a spin echo via the heteronuclear dipolar coupling. The efficiency of signal recovery in the echo is greatly reduced as a- is increased and the optimum echo signal amplitude will be observed when -r is equal to the spectrometer dead-time following the 90 ~ rf pulse. The echo decay shape is a good replica of the original FID and its observation can be used to obtain more reliable and quantitative information about solids. Echo methods can be used to observe just the central transition of quadrupolar nuclei, allowing the determination of sites or nuclei with larger quadrupole interactions, or the whole satellite transition manifold can be determined. In forming echoes it is convenient to consider two cases depending on the spacing a- relative to the length of the FID (Tf). A so-called Solomon echo forms if a- < < Tf, whereas a Hahn type of echo forms if a- -> Tf. Early calculations of this behaviour assumed a "hard" pulse regime in which the rf nutation frequency is much greater than the effective frequency of the interaction, so evolution during the pulse is solely governed by the if-pulse and the effect of the interaction can be ignored. With the advent of computers, numerical calculations of echo formation have become possible. These effects have been most widely calculated for the quadrupole interaction (initially the second-order quadrupole interaction) and can be readily extended to the regime where the first-order quadrupole interaction is significant, so evolution during the pulse occurs under both the rf and first-order quadrupole interactions. Calculations have been made for a variety of two-pulse sequences and for varying if-field strengths (Haase and Oldfield 1993, 1993a, Man 1993, 1995). These calculations have been
Experimental Approaches
135
extended to include other interactions as well (Bodart et al. 2000, Man 2000). The general conclusion of this work is that echoes will generally be formed but that to obtain quantitatively reliable information great caution has to be exercised. For quadrupole nuclei it is useful to note that VQ can be deduced from the intensity variation as a function of the length of the second pulse (Haase and Oldfield 1993a, Man 1995). In practice, hard if-pulses are used for uniform excitation of broad lines. Our own work has tended to use an echo sequence with the phase cycling first proposed by Kunwar, Turner and Oldfield (1986) which combines quadrature phase cycling with further cycling designed to cancel direct magnetisation (the remaining FID) and ringing effects: Phase pulse 1: Phase pulse 2: Receiver phase:
xxxxyyyy - x- x- x- x- y- y - y- y xy- x- yxy - x- yxy- x- yxy- x- y - yy - yy - xx - xxy - yy - yx- xx- x
Other phase cycling to prevent distortion due to mis-set pulse lengths (Rance and Byrd 1983) has pointed out that response of a spin system covering a wide frequency range is not simply a reflection of the power distribution in the pulse, but that there are spectral distortion effects with some spins remaining in the same state before and after the pulse. The rotation produced by the second pulse in the two-pulse echo experiment is not critical. In practice, the best choice is to make the second pulse twice the length of the first, the actual length being a trade-off between sensitivity and uniformity of the irradiation. The uniformity can be checked by decreasing the pulselength and seeing if the lineshape changes. The excitation has only sufficient bandwidth to allow detection of the first two singularities of the satellite transitions, but compared to the one-pulse experiment, it provides a better record of the satellite transitions between the singularities, although still below the theoretical expectation. An important practical consideration in recording echoes is that the reason for applying the echo is to move the effective t -- 0 position for the FID outside the region where the signal is corrupted. However, so that phasing problems do not re-emerge, the data sampling rate should be sufficient to allow this point to be accurately defined. If T2 is sufficiently short that an echo time can be used that allows the whole echo (both before and after the maximum) to be accurately recorded without an unacceptably large loss of intensity, there is no need accurately to define the new t = 0 position. Fourier transformation of the whole echo (which effectively amounts to integration between +_w), followed by magnitude calculation removes phasing errors and produces a pure absorption lineshape with a signal-to-noise ~/2 larger than that obtained by transforming from the echo maximum. Rather than using a single echo it has recently been suggested that a train of refocusing w-pulses (selective on the central transition), similar in form to the Carr-PurcellMeiboom-Gill (CPMG) sequence, could have significant advantages. Early work,
136
Multinuclear Solid-State NMR of Inorganic Materials
termed quadrupolar CPMG or spikelet spectroscopy, was developed in Aarhus (Larsen et al. 1997, 1998, 2000). The w-pulses refocus the quadrupolar-induced dephasing. At the start of the sequence an optimised quadrupole echo is used. Hence, the refocused magnetisation decays relatively slowly compared to the timescale of the decay of an individual echo. A string of echoes is recorded and the complete echo train is Fouriertransformed to give a series of sharp bands that follow closely the static lineshape. The inhomogeneous interaction is refocused at the echo maxima. FT of the full echo contains both the homogeneous and inhomogeneous interactions. The homogeneous interaction affects the lineshape of the individual bands and the envelope provides information about the inhomogeneous part of the interaction. An extensive phase cycle is applied for the 90~ - 180~ -- [ 180~ ]n -REC+rec sequence (Larsen et al. 1997).
+2
+3 +rec
xy-x-yxy-x-yxy-x-yxy-x-y yxyxy- x- y- xyxyxy- x- y- x yxyx - y- x - y- x - y- x- y- xyxyx - x- yxy - x- yxy- x- yxy- x- yxy
This sequence provides significant signal enhancement, since the intensity from the powder lineshape is concentrated into a series of sharp bands. This gain is especially true if T~ is long compared to the timescale of the echo train. For example, if T~ required a recycle time of 16 s but 60 echoes could be collected without significant magnetisation decay, an experiment using single echoes would take --~30 minutes compared to a few ms using an echo train. The interpulse delay ('ra) determines the frequency spacing of the sharp lines (1/a'a) that make up the spectrum. The delays must be adjusted so that baseline artefacts are minimised, to give significant improvement in S/N but also to provide enough lines to define accurately the powder lineshape allowing its simulation. Splitting the spectra into a series of narrow bands also has the advantage that low-intensity second sites can be observed, and even if baseline artefacts are not completely removed, functions can be used to correct any residual problems (Section 3.3.2) because the baseline can be observed.
3.4.2 Stepped experiments Several alternative approaches have been developed for recording broad spectral lines, based on the philosophy that although a line is broad it can be recorded stepwise using a series of narrow-banded experiments to overcome the distortions introduced into a broad spectrum. One of these approaches is to carry out a spin-echo experiment using relatively weak rf pulses, recording only the intensity of the on-resonance magnetisation and repeating the experiment at many frequencies to map out the lineshape. The system has to be retuned at each new frequency so that the lineshape is mapped out
Experimental Approaches
137
point by point. This is an extremely laborious process, but the lineshapes are accurately recorded, as for high temperature superconductors and 91Zr in ZrO2 (Bastow and Smith 1992). The titanium signal from YzTi207, with a lineshape 250 kHz wide recorded by this method is shown in Figure 3.10, which also shows a pulse echo experiment for comparison. In the direct experiment, the singularities of the powder lineshape can be readily seen but bandwidth effects (excitation and response) result in the centre of the spectrum having much enhanced intensity compared to the edges, distorting the lineshape. Although these static experiments which step the frequency produce accurate lineshapes, they are relatively time inefficient. The time efficiency can be improved by slowly rotating the sample (--~ 1 rpm) to bring different crystallites into resonance. This approach, termed ROTISSERIE, has been shown to give large savings in the time necessary to produce 14N spectra from ceramics (Yesinowski and Hill 1999). The modulation of the echo amplitude produced by the rotation itself contains
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138
Multinuclear Solid-State NMR of lnorganic Materials
information about the interaction and has been used in an experiment termed STEAMER (Yesinwoski and Hill 1999). Another approach is to use low power, frequency selective pulses (Sindorf and Bartuska 1989) with phase-coherent sampling. A long (1 ms) pulse at --~10 mW acquires a single data point, with each point shifted by a frequency of 1 kHz. The spectrum is obtained directly without the need for Fourier transformation but this approach has not been widely applied. An alternative is to sweep the main magnetic field. There are several examples of solid spectra obtained in this way dating from the earliest days of NMR, but only a limited number of reports have used superconducting magnets. Much of the work to date has used relatively low-resolution magnets. Examples of studies made using superconducting magnets include work on model alumina-containing compounds (Xu et al. 1994), studies of the aluminium sites in catalyst-related materials and on high temperature ceramic superconductors. There is no doubt that this approach will work in all cases. It is possible for a single NMR spectrometer to be capable of both conventional high resolution spectroscopy and also field-sweep operation at relatively little extra cost. The field sweep in the 7.05 T instrument described by Poplett and Smith (1998) was limited to + 0.5 T, sufficient to cover many broad lines. Control of the field is completely automated and integrated with the pulse programme. As with the stepped-frequency experiment, relatively soft pulses are applied, and although strictly the on-resonance part of the magnetisation should be used, direct use of the spin-echo intensity accurately reproduces the lineshape. 27A1 spectra of ot-A1203 obtained by one pulse, spin-echo and field-sweep methods are shown in Figure 3.11. The intensity between the singularities in the field sweep spectrum is much higher and much more closely matches the theoretical expectation. There is also essentially no bandwidth limit on the field sweep experiment so that the outer singularities are easily recorded. The total frequency width of this lineshape is --~1.4 MHz but the approach can be applied to much broader lines, as demonstrated for 27A1 in A13Zr where the frequency width is well beyond 3 MHz (Figure 11.4A). A field-sweep approach could be extremely useful as an alternative method for examining low-y nuclei, where narrowing techniques do not yet offer widespread opportunities for improved resolution.
3.5. ONE-DIMENSIONAL HIGH RESOLUTION TECHNIQUES
3.5.1 Magic angle spinning (MAS) The most widely used technique for solid state NMR observation is MAS. The early design of MAS rotors by Andrew (1981) had a single bearing/drive surface over which compressed gas was forced (Figure 3.12A). This provided a very low friction gas bearing allowing rapid rotation. These rotors were typically --~ 1 cm diameter, and, using compressed air, could be spun to 3-5 kHz. On spinning at 5 kHz the peripheral
ExperimentalApproaches
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acceleration is - 5 • 105g, so only very strong materials are employed. For inorganic solids these rotors were typically made of plastics such as Delrin and Torlon. Commercially available systems for rapid spinning have tended towards variations of the double bearing (DB) design (Doty and Ellis 1981, Figure 3.12B). A large range of diameters of rotor is available commercially (2.5 to 14 mm) with 1.8 mm diameter rotors currently providing MAS of - 50 kHz. The driving force is provided by compressed gas, usually air or nitrogen, forced through a series of nozzles on to vanes, usually on the rotor cap. Very careful design of the optimum nozzle diameter and angle of incidence on to the vanes is needed. The very tight clearance also requires accurate
140
Multinuclear Solid-State NMR of lnorganic Materials
Figure 3.12. MAS rotor design showing A. A single beating Andrew-Beams (Andrew 1981a) and B. a double bearing (Doty and Ellis 1981) with permission of the copyright owner.
machining of tough ceramics. For example, the optimum gap to the beating in a 12 mm rotor is 0.027 mm (Doty 1994). The nature of the gas supply is important, the minimum requirement being that it is dry and free from oil. Nitrogen boil-off is undoubtedly the best drive gas but is much more expensive than using compressed air for this purpose. The very high spinning rates create considerable pressures across the wall from the resulting centripetal force, and material strength is often the limiting factor for safe spinning speed. Careful note needs to be taken of the sample density as this will determine the fastest speed that can safely be used. Polymeric materials have now largely given way to advanced engineering ceramics such as Si3N4 and stabilised ZrO2. For many sequences it is often better to have a uniform rf field over the sample. This can be achieved by decreasing the volume occupied by the sample within the rotor, and spacers can be used. In CRAMPS experiments spacers with spherical cavities often give
Experimental Approaches
141
the best results. Other more specialised designs have been suggested, some for spinning air-sensitive samples (Gay 1984, Lee et al. 1984, Carpenter 1986, Merwin et al. 1989). Even though air bearings are used, allowing cooling by the gas stream to occur, there has been considerable discussion of the magnitude of the frictional heating effects in such systems. In addition to frictional heating, rapid expansion of the gas through the nozzles in the stator brings about the Joule-Thompson effect. There is no doubt that for spectroscopy at accurately known temperature, careful calibration is required. Elements with large shift ranges (such as 2~ or paramagnetic compounds with a large Curie temperature dependence of shift such as 31p in (VO2)2P207(Pan and Gerstein 1991) and ~3C in samarium acetate (Haw 1988) are ideal for this purpose. Lead in PbNO3 has been widely used to determine the temperature in MAS experiments (Bielecki and Burum 1995, Mildner et al. 1995). The 119Sn signal from Sm2Sn207 is also very sensitive to temperature changes (van Moorsel et al. 1995, Grimmer et al. 1997, Langer et al. 1999). It is apparent that the temperature is a strong function of Yr. For 7 mm rotor the temperature rises by --~ 4 K as the spinning rate is increased from 2-5 kHz (Bielecki and Burum 1995) and for a 2.5 mm rotor spinning at 35 kHz the temperature rise is 30-40 K (Langer et al. 1999). Mildner et al. (1995) have modelled the temperature difference as 2
Tsample - Tinle t
--
3~2FlsR2 Vr
(3.17)
where "qs is the viscosity of the gas, R is the radius of the spinner and X is the thermal conductivity of the gas. In addition to the average rise there will also be temperature gradients which must be well characterised. The temperature rise can change the observed chemical shift tensor and the gradient can limit resolution (Brus 2000). To achieve efficient narrowing by MAS, the magic angle has to be accurately set and fine adjustments can be achieved by the quadrupole satellite technique (Frye and Maciel 1982). This technique is based on materials which contain quadrupole nuclei in a nominally cubic local environment, some of which are distorted, causing first-order quadrupole broadening of the non-central transitions at some sites. As the narrowing factor is ~(3COS2[~ -- 1) for a deviation of d[3, the residual broadening factor is 3cos[3sin~3d[3. Hence, the first-order quadrupolar broadening in a compound to be used to set the magic angle should not be too severe, to enable the spectrum to be narrowed into a visible set of sidebands over a limited but not too restrictive a range of angles about the magic value. Far away from the angle, only the sidebands from second-order broadening of the central transition are observable. Then, as the magic angle is approached, the sidebands from the satellite transitions are observed, and become progressively narrower. The bromine resonance of KBr is typically used for this purpose, but with increasing demand for more accurate setting of the magic angle, other compounds are now often used. Other commonly used angle references are 23Na in
142
Multinuclear Solid-State NMR of Inorganic Materials
NaNO2, and a favourite of the authors is 27A1 in Y3A15012, in which the A106 site with a small XQ value can be used for initial setting of the angle (as good as KBr and the carbonyl carbon in glycine). Fine adjustments are made using the A104 site with a much larger XQ. Figure 3.13A shows the changes in the 27A1 FID as the magic angle is approached. Initially, away from the magic angle, only a few sharp rotational echoes are seen. As the magic angle is approached these become sharper, and more importantly they can be seen at longer times. Typically with 24 scans, using a 4 mm rotor, these should be visible out to --~ 5 ms if the angle is well set. In the corresponding spectra (Figure 3.13B) as the correct angle is approached, the satellite transition sidebands become much sharper. For lower frequency probes that will not tune to the region of
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Experimental Approaches
143
23Na, 27A1 and 79'81Br, 85Rb in RbC1 is ideal. Alternative nuclei with small magnetic moments that can be used to set the magic angle include 14N in compounds where the resonance can be narrowed into a set of sidebands over a range that is not too limited. The molybdenum resonance of Mo(CO)6 has also been suggested as a candidate for angle setting since it provides a strong signal with sidebands whose widths are sensitive to angle but can nevertheless be observed a long way off the magic angle. A number of factors result in residual broadening of MAS NMR spectra (Andrew 1972): (a) Instrumental- Bo, B1 inhomogeneity, inaccurate setting of the magic angle, too slow spinning, bulk susceptibility effects, temperature gradients. (b) Residual interactions- residual dipolar coupling, chemical shift distribution, antisymmetric tensor-terms, quadrupole effects. (c) Motional and relaxation effects- spin-lattice and cross relaxation, slow internal motions.
3.5.2 Extraction of parameters from MAS NMR spectra Once the NMR spectrum has been obtained, the NMR interaction parameters must be extracted. The use of the sideband intensity to extract CSA parameters has always been popular. The relative sideband intensities depend on the asymmetry and the anisotropy; Herzfeld and Berger (1980) produced extensive graphical plots allowing the CSA parameters to be deduced from this relative intensity. Now a variety of software packages are available such as WINFIT (Massiot et al. 1994) and SIMPSON (Bak et al. 2000) to simulate solid-state NMR spectra. Some packages assume that when centrebands (e.g. of quadmpole nuclei) are being simulated, one is in the "infinite" spinning speed limit. Alternatively, the low-order moments of spectra can be used to deduce the interactions (Maricq and Waugh 1979, Herreros et al. 2000).
3.5.3 Suppression of spinning sidebands Where there is a combination of different sites, each of which produces a significant number of sidebands, simplification of the spectrum by removal of the spinning sidebands would be extremely helpful. Dixon (1982) introduced the concept of breaking up the rotation period into intervals separated by 180~ pulses. The phase of each spin is inverted by the pulses and the total phase accumulated by each spin can be calculated. Then for all the sidebands to be removed, the sum of the phase for spins contributing to the spinning sidebands needs to vanish, independent of the orientation of the particular crystallite. The basic TOSS sequence has been developed to compensate for imperfections by extension of the applied phase cycles and by paying careful
144
Multinuclear Solid-State NMR of lnorganic Materials
attention to the timings in the pulse sequence. The sequence is sensitive to the accuracy of the 180~ pulses and works by effectively cancelling the magnetisation in the sidebands, although it does not regain the magnetisation in the centreband. Hence, the recorded intensities from a sample containing sites with differing anisotropies will not reflect the site distribution. In addition, when the anisotropy becomes large compared to both the spinning speed and the irradiation width of the 180~ pulses, complete cancellation of the sidebands cannot be achieved. Careful selection of the applied magnetic field can often greatly aid in minimising the sidebands, for example, making it advantageous to accumulate l l9Sn and 2~ spectra at lower applied magnetic fields. For 13C, TOSS has been combined with the sequences for suppression of protonated species (Carduner 1987) described in Section 3.7.1. The TOSS sequence has also been demonstrated for 27A1 (Carduner 1989). This greatly clarified the spectrum when spinning speeds of only a few kHz were available, but has largely been superseded by the use of very much faster spinning speeds. Since sidebands form an important part of MAS spectra, there has been continued development of TOSS. More recently, new shorter sequences have been proposed to optimise TOSS sequences, including one employing 5 almost evenly spaced 180 ~ pulses (Samoson and Tegenfeldt 1995). Antzutkin (1994) has extensively reviewed sequences for manipulating sidebands. An alternative scheme for sideband suppression is to interrupt the evolution of the magnetisation for fractions of the rotor period by using pairs of rf pulses with reversed phases. The interruption means that destructive interference occurs for the magnetisation producing the sidebands (Hong and Harbison 1993). The sequence, termed SELTICS, prevents the sideband magnetisation from refocusing, rather than manipulating the magnetisation to cause cancellation.
3.5.4 Special considerations for MAS of quadrupolar nuclei The technique to be applied depends on the magnitude of VQ (= 3XQ/(2I(2I-1))). For XQ < 0.7 MHz, one pulse acquisition of the entire spectrum is possible (e.g. 27A1 in LaA103). If features of the centreband lineshape can be observed, the quadrupole parameters can be deduced by simulation from the MAS centreband. As the spinning speed and applied magnetic field are increased, sites with larger • values will narrow. Accurate 27A1 lineshapes have thus been recorded for kyanite (XQ up to 10 MHz), andalusite (• up to 15 MHz) and zoisite (XQ up to 10 MHz) (Alemany et al. 1991, 1999, 2000). However, the spectra show distortions from the expected powder pattern until the spinning speed well exceeds the residual second-order quadrupole width. This is shown in Figure 3.14 for the 23Na MAS spectrum of crystalline albite acquired at speeds from 0-10 kHz. The true "infinite" spinning speed lineshape of this material is not obtained until --- 15 kHz at 7.05 T. The spin-echo sequence developed for improving the sensitivity of the static lineshapes from low-~/nuclei (Larsen 1998, 1998a) has
Experimental Approaches
145
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Figure 3.14. 23Na spectra at 7.05 T from crystalline albite (NaA1Si3Os) as the spinning speed is increased from 0 to 10 kHz showing the distortion at low spinning speed from Smith and van Eck (1999) with permission of the copyright owner.
also been used to increase the S/N ratio of the MAS centrebands from sites with large XQ values. In this case, the sequence must be set up so that the spin-echo and rotationalecho effects form coincidentally. When no distinct second-order quadrupole lineshape can be observed, the quadrupole product Po - (Xo +~/2/3)1/2 and the ~cs,isovalue can be estimated by recording the peak position as a function of the applied magnetic field. Eq. 2.150 shows that the field-dependence will give a linear plot (Figure 3.15). Good spectra can also be obtained, often at relatively low spinning speeds, from amorphous materials with a distribution of interactions, provided care is taken to ensure that narrowing of all the sites in the sample occurs and that the spectrum is representative of the whole sample. Eq. 2.152 shows there is a spinning speed that must be exceeded for narrowing to occur. In glassy samples, more sites will narrow as the spinning speed increases, and since these sites have different second-order quadrupole shifts, the composite line will apparently change so that at higher spinning speeds the peak becomes less positive. This shift is accompanied by broadening (Figure 3.16) as Vr increases, since the residual secondorder quadrupole width of the narrowed sites is greater. Quantification of MAS NMR spectra of quadrupolar nuclei posed many problems 10-15 years ago, with a number of 27A1 NMR studies on both crystalline and glassy powders noting that the integrated signal intensity failed to agree both with the known
146
Multinuclear Solid-State NMR of Inorganic Materials Shift of Centre of Gravity (ppm)
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aluminium distribution in different sites, and with the total aluminium content. However, the understanding and available experimental technology have improved markedly over the last few years. Use of the correct experimental conditions (e.g. small tip angles) with higher magnetic fields and faster MAS rates has removed many of the difficulties that gave rise to the idea of "NMR invisible" aluminium. By using small flip angle pulses so that work is performed in the linear regime, sites with widely differing XQ values can produce the same intensity response (a minor problem with this is that typical values of B1 of 75 kHz for 27A1 would require only a 0.4 Ixs pulse to be used!). This is illustrated by the two sites in Y3AlsOl2, whose response is the same at small angles (Figure 3.17). Even after this precaution is taken, quantitative interpretation of quadrupole-perturbed NMR spectra needs to be approached cautiously. One of the sources of error is that it is often only the prominent centreband that can be recorded accurately. The intensity will be distributed between the centreband and the spinning sidebands; although these could be integrated as well, with more than one transition for quadrupole nuclei, partitioning this magnetisation can be problematic. Full-scale simulation of the complete manifold can be time consuming and requires a good S/N ratio of the spinning sidebands. A more pragmatic approach is to record the centreband and
Experimental Approaches -15
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20
spinning speed (kHz) Figure 3.16. The effect of spinning speed on the peak position and the linewidth of 23Na from albite glass at 7.05 T from (Kohn et al. 1998 with permission of the copyright owner).
to correct the observed intensity by a factor corresponding to the amount of magnetisation that appears in the centreband. For the (1/2,-1/2) transition, this depends on the parameter v~/VoVr, while for the satellite transitions it is a function of 11-2 m IvQ/Vr, both being only weak functions of rlQ (Figure 3.18A,B, Massiot et al. 1990). Once these factors are taken into account, the measured integrated intensity can be corrected to give the actual distribution in compounds to a high degree of accuracy, even for sites with widely differing XQ values. An interesting case study is provided by Y3A15012, which has A104 and A106 sites in the structure in the ratio of 3:2. Integration of the
148
Multinuclear Solid-State NMR of Inorganic Materials (1/2,-1/2) intensity per 27A1 nucleus (Arbitrary units) 25
20
15
AI(H20)6] 3+ in aqueous
/
/
\
1
--~o, YAG vo = 900 kHz
I \.. -5
Tp (~ts)
regime
Figure 3.17. The pulse response of 27A1 comparing a solution where XQ = 0 and the two resonances in Y3A15012 with, for the AIO6, XQ = 0.6 MHz and, for the AIO4, XQ = 6 MHz.
1.0
1.0
0.75
0.75
0.50
o.so
B
C
]
Cat a
L @ ~ ~ i a t i o
JL
n
AIO,0/Z,1/2) AIO6(:~3/2, •
0.2:5
0.25
11=0 _
AIO6(:~-5/2,:t:3/2)
rl 0
1
2
3
VQ2/VOVr
4
0
]"-A, 20
40
60
ll-2mlvQ/Vr
80
200
0
AIO4(1/2,-1/2) -200
Z7Al c h e m i c a l shift in p p m
Figure 3.18. The dependence of the intensity of the centreband of an MAS spectrum for the A. central and B. satellite transition as a function of the spinning speed Vr, the quadrupole frequency VQ and the Larmor frequency Vo along with C. the 27A1 MAS NMR spectrum of Y3A15012 at 7.05 T at a spinning speed of 7 kHz. From Massiot et al. (1990) with permission of copyright owner.
149
Experimental Approaches
Table 3.3. Correction of integrated centreband intensities to derive the actual aluminium site distribution in Y3A15012 derived from the MAS spectrum accumulated at 7.05 T spinning at 7 kHz shown in Figure 3.18C.
Factor
A104 signal
A106signal
])Q (kHz) Experimentally integrated intensities of the centreband ( m , - 1/2) transition
900 1
90 1
vQZ/voVr
1.48 1 0.78 0.78
0.015 1 1 1
257 1.78 0 0
25.7 1.78 0.045 0.08
514
51.4
1.11 0 0 0.78 1.28 1.44
1.11 0.304 0.039 1.12 0.89 1
Inherent relative intensity of the transition* Fraction of this transition contributing to the centreband Intensity this transition contributes to the centreband (_+ 3/2, + 1/2)transition [1-2 m[vQ/vr Inherent relative intensity of the transition Fraction of this transition contributing to the centreband Intensity this transition contributes to the centreband (_+ 5/2, + 3/2)transition I1- 2 mlvQ/vr Inherent relative intensity of the transition Fraction of this transition contributing to the centreband Intensity this transition contributes to the centreband Total relative intensity contributed to the centreband from all transitions Corrected centreband intensity to give the actual site distribution Actual intensity normalised to A106 resonance
* See Table 2.3 where the intensity column has been used and the contributions are normalised to that from the (1/2,-l/z) transition.
centrebands of the 7.05 T, 7 kHz 27A1NMR spectrum gives a ratio of 1:1 (Figure 3.18C). In this case, the centreband of the A104 site effectively contains only intensity from the central transition, with 78% of this central transition intensity present and no contribution from the outer transitions (Table 3.3). However, for the A106 site, all the central transition intensity is in the centreband, which also contains a significant contribution from the outer transitions (Figure 3.18C, Table 3.3). Once these factors are corrected for, the centreband intensity turns out to be 1.44:1, close to the value of 1.5:1 expected from the crystal structure. The advantages of fast spinning and high fields are also apparent in other materials. The observed signal of 27A1 MAS N M R spectra of certain zeolites increased from 65 to 95% of that expected from the aluminium content when the magnetic field was increased from 9.4 to 18.8 T; it should also be noted that these were samples in which the maximum • was only 6.3 MHz (Fyfe et al. 2001).
3.5.5 Magic angle spinning observation of satellite transitions Although the majority of N M R studies of non-integer spin quadrupolar nuclei have concentrated on the central transition, observation of the satellite transitions under
150
Multinuclear Solid-State NMR of Inorganic Materials
MAS has distinct advantages. If the quadrupolar interaction is small but non-zero, the second-order quadrupolar structure of the central transition can be obscured by other broadening mechanisms. However, if the powder pattern of the outer transitions can be recorded, XQ and -q can be deduced. As detailed above, this can be carried out in static experiments, but with fast, accurate (--- 0.005 ~ MAS this pattern splits up into a set of sharp sidebands that closely follow the contour of the powder pattern envelope. The MAS approach is less susceptible to initial deadtime problems (Section 3.3.2) and provides sensitivity advantages. The sidebands can be simulated to deduce • and xI, and using conventional pulse spectrometers for solids, this method is applicable to materials with values of XQ up to 2 MHz and 3.3 MHz for spins of 3/2 and 5/2 respectively (Skibsted et al. 1991). In amorphous solids, the change in the extent of the sideband manifold and the shape of the envelope can be used to estimate the average XQ value and distribution for each site. The main practical requirements are for fast, stable spinning precisely at the magic angle and accurate probe tuning. Any variation of the spinning speed or mis-setting of the angle will degrade the resolution of the sidebands. In accumulating the data, the corrections made necessary by if-irradiation and the probe can be achieved by semi-theoretical modification (for samples with smaller values of XQ), or by measuring the spectrum at several frequencies and combining these after applying correction factors (for samples with larger values of XQ). Since single-pulse excitation is used there are also often baseline distortions present, but provided the spinning sidebands are separated so that intermediate baseline can be observed, normal spline fitting using the spectrometer software is adequate. There is no reason why the field sweep method outlined above for accumulating broad static lines cannot be used for such MAS satellite spectra, conveniently making the step size in the field equal to Yr.
3.5.6 Double angle rotation o f quadrupolar nuclei Spinning a sample about two axes simultaneously can be achieved in specialised probes (Figure 3.19, Samoson et al. 1988). From a mechanical point of view, DOR is intriguing since the angular momentum of the inner rotor will tend to inhibit rotation of the outer rotor. However the torque due to the inner rotor may be eliminated (Wu et al. 1990) if the rotation rates of the two rotors satisfy
v,
V2
-- COS ~ l
(,,r / la.,.
(3.~8)
where [31 is determined from the NMR conditions. Since Itr and Iax are the transverse and axial moments of inertia of the inner rotor, the ratio of spinning rates is typically around 5. The stability of this system with respect to changes in rotation rate is best when Vl/V2 is close to, but slightly higher than the value given by the above equation,
Experimental Approaches
151
10' a'. .
9 ;.
13'
2'
4 14
--.-.
Figure 3.19. An example of a DOR rotor from Wu et al. (1990) with permission of the copyright owner. since the system will then self-adjust for small variations of the inner rotor. The magic angle must be set as for MAS experiments by using Na2C204 with a pulse that selectively excites the central transition, the sidebands narrowing as the magic angle is approached. The intensities and phases of the sidebands depend on the relative phases of the two rotors and the ratio of the spinning speeds of the two rotors. The dependence on the phase angle Y2, the instantaneous orientation of the outer rotor, means that by triggering the signal acquisition synchronously at ~2 = 0 and ~2 = q ' r , the inversion symmetry of the odd-order sidebands eliminates the odd-order sidebands upon co-addition of the signals, effectively doubling the outer rotor speed (Samoson and Lippmaa 1989). If the quadrupolar interaction is dominant and the sample is highly crystalline, extremely impressive gains in resolution are observed. One of the major limitations is the relatively slow rotation speed of the large outer rotor. The maximum actual spinning speed that can be routinely obtained from the latest system with active computer control of the gas pressures has been reported in the literature to be --~1800 Hz, and
152
Multinuclear Solid-State NMR of lnorganic Materials
undoubtedly the technology associated with the technique will continue to improve, leading to increased spinning speeds and expanding the application of the technique. Higher spinning speeds not only mean fewer spinning sidebands but will broaden the applicability of the technique to amorphous materials, since the outer rotor speed must exceed the residual width, produced, for example, by a dispersion of parameters such as the chemical shift. An example of DOR narrowing from an amorphous solid has been reported (MacKenzie et al. 2001). The other main problem with DOR is that the rf coil encloses the whole system, giving a small filling factor which leads to low sensitivity. The large coil size also means that the rf generated is quite low. Double tuning for cross-polarisation is also difficult, although such an experiment has been performed (Wu et al. 1992). Simulation of the complete DOR spectrum (centreband plus the spinning sidebands) yields the NMR interaction parameters. However, it is most usual to perform the experiment to give improved resolution and simply quote the measured peak position which appears as the sum of the isotropic chemical and second-order quadrupole shifts. DOR experiments at more than one applied magnetic field allow these different contributions to be separated and hence provide an estimate of PQ (see Section 6.5.1.2). This approach is similar to that using the field variation of the centre-of-gravity of the MAS centreband, but has the advantage that the narrower, more symmetric resonance makes determination of the correct position of the centre of gravity more precise.
3.5.7 Practical implementation o f C R A M P S
The drawback to CRAMPS is that the long pulse trains must be set very accurately with stable pulse lengths and phases. This has discouraged the wider use of the approach, but improved reproducibility of digitally-held information in the most modem spectrometers (rather than analog control) should result in CRAMPS being more routinely used. Accurate control of the phase and amplitude of each channel is necessary. Practical considerations have been detailed (e.g. Jackson and Harris 1988, Maciel et al. 1990, Burum 1990) but a key for success is short (1-2 Ixs) homogeneous 90 ~pulses, so that the sample is often constrained to a spherical volume at the centre of the coil. The smaller volume and the reduction of susceptibility by using a spherical sample results in more homogeneous rf across the sample. The recovery of the probe from the pulse should be rapid, so that the sampling windows can be short, keeping the cycle time down. Since recovery is important, CRAMPS probes tend to have lower Q than conventional CP-MAS probes. The multiple pulse sequence also scales the chemical shift by effectively scaling i: (Section 2.4.2), and this must be carefully calibrated, depending on the pulselength and the cycle time (Haeberlen 1976, Gerstein and Dybowski 1985). The averaging efficiency and frequency range covered is improved if quadrature rather than single-phase detection is employed (Burum et al. 1993). This also
Experimental Approaches
153
means that 19F CRAMPS becomes more amenable, since its much wider chemical shift range often resulted in folding in singly-detected CRAMPS experiments. This is not a problem in CRAMPS with quadrature detection.
3.6. TWO-DIMENSIONAL EXPERIMENTS
3.6.1 Nutation NMR Quadrupole parameters can often be obtained by measurement of the static or MAS spectra of powdered samples as outlined above. Unfortunately, the distinct features of a quadrupolar lineshape are often obscured by the existence of a spread in chemical shifts, residual dipolar broadening and/or the quadrupole interaction. The idea behind nutation NMR is to study the quadrupole interaction in an if-field in the rotating frame, whereby the influence of the chemical shifts is dramatically reduced since they scale linearly with the applied field. At the same time, there is the sensitivity benefit of working in a high static magnetic field. The 2D nutation experiment consists of monitoring the magnetisation as a function of the nutation pulse (h). During tl the spin system is governed in the rotating frame by the Hamiltonian.
H1
__VIIx + Vo(O,c~)(3iZz
_ I2)
(3.19)
with VQ(O,~))- VQ(3COS2 0 - 1 + r/sin 2 0 cos 2q~)
(3.20)
To calculate the nutation spectrum the density matrix formalism is used, in which the Hamiltonian is diagonalised either numerically (Kentgens et al. 1987) or analytically (Pandey et al. 1986, Janssen and Veeman 1988). In Section 2.2.6, the nutation frequency of a quadrupolar nucleus was seen to vary from (I + l/2)v 1 when vl < < 1)Q to vl when 1)1 > > I)Q. The nutation spectra display distinct features in the region where vl --~VQ. Simulation of nutation behaviour has shown that the useful range of rf field-strengths lies with VQ/Vl in the range 0.05-1. The distinct features of nutation spectra allow the quadrupole parameters to be obtained. This means that only quadrupole frequencies up to a certain limit are accessible with currently available rf-fields of 500 kHz in specially dedicated probeheads, and in fields of around 100 kHz in most commercially available systems. This constraint especially limits the investigation of spin I = 3/2 nuclei such as 23Na. In cases where more than one resonance could be observed, one would like to increase the resolution in the F2-dimension by applying MAS. However, as was discussed previously, rotationally-induced spin-locked magnetisation can appear when an
154
Multinuclear Solid-State NMR of lnorganic Materials
rf-field is applied in conjunction with spinning. This process severely distorts the nutation spectrum as it gives rise to strong dispersive components in the 2D spectrum and a large signal at zero frequency. The spinning speed should therefore be kept low (2-4 kHz) so that the nutation signal has decayed within a quarter rotor period at most. It is possible however to simulate the nutation behaviour of quadrupole nuclei under the influence of MAS, from which the quadrupole parameters can be extracted (Nielsen et al. 1992). A 27A1 quadrupole nutation experiment on A1PO4-8 under fast MAS revealed higher resolution in the cross-section at 3v~f compared to that at Vrf. This was attributed to a smaller distribution of orientations contributing to the 3Vrf cross-section (Rocha et al. 1992).
3.6.2 Off-resonance nutation Off-resonance nutation has been developed (Kentgens 1993) to increase the upper limit of quadrupole couplings that are accessible by nutation NMR. In this experiment the nutation behaviour of the spins is monitored in the effective field rather than in the rffield, the effective field being the vector sum of the if-field and the resonance offset. This greatly increases the upper limit of accessible • values since the resonance offsets can be freely chosen and are limited only by the Q of the probe. For increased sensitivity and for the reduction of the zero frequency signal it is beneficial for the magnetisation to nutate perpendicularly to the effective field. Therefore, one can either apply a frequency-stepped adiabatic half passage (Veenedaal et al. 1998) or a soft 90 ~ pulse to bring the magnetisation into the xy-plane; the former option gives the best results. The if-phase is then shifted 90 ~ and the frequency is switched to the required resonance offset. The useful range of resonance offsets is between one to eight times v~, depending on the ratio VQ/V~. After a time t~ the signal is detected on resonance. The phase-coherent frequency switching (switching times of around 200 ns) of the if-field required in this experiment is within the capabilities of most modem spectrometers. An extensive review has dealt with many of the details of off-resonance nutation spectroscopy (Kentgens 1998). Whereas the on-resonance nutation spectra are amplitude-modulated and can be properly phased in the F1 dimension, the off-resonance nutation spectra are phasemodulated and can no longer be phased properly. Therefore, a magnitude calculation is performed after Fourier transformation. Furthermore, because the magnetisation evolves during t~ around the effective field, the sine and cosine components of the modulation are different and the amplitudes of positive and negative F1 signals are unequal. Figure 3.20 shows an example of the strength of the off-resonance nutation technique applied to various samples of ~-alumina impregnated with phosphorus and/or molybdenum. The nutation spectrum of the tetrahedral resonance is very similar to the nutation spectrum of "7-A1203. The octahedral resonance however shows a nutation
155
Experimental Approaches
A
B
-400
C
0
400
Nutationfrequency(kHz) j
~~~~~
|
-400
|
i
'l
!
0
j
u
i
!
400
Nutationfrequency(kHz) Figure3.20. Off-resonance nutation experiments on Mo(12)p(2)/~/-A1203 with an rf field strength of 41 kHz. The normal 1D MAS spectrum A. is shown and the nutation spectra of the tetrahedral B. and octahedral region C. with the tetrahedral region made up of only ~/-A1203 but the octahedral region requiring a superposition of ~/-A1203 and Alz(MoO4)3 from Krauss et. al. (1996) with permission of the American Chemical Society.
spectrum that can only be simulated using two subspectra, one the nutation spectrum of ~/-A1203 and the second with quadrupole parameters similar to those of A12(MoO4)3. This led to the conclusion that impregnation of ~/-A1203 with molybdenum results in an interaction with the 7-A1203 surface to form A12(MoO4)3 (Kraus et al. 1996).
3.6.3 Order-resolved sideband spectra Techniques have been developed for modulating the phase of sidebands according to their order through a series of pulses. These 2D experiments separate the centreband and sidebands in the second dimension even if they strongly overlap in the normal 1D spectrum. In these sequences, sidebands can often be summed by using either hopping
156
Multinuclear Solid-State NMR of Inorganic Materials
or synchronised acquisition. Combining the individual slices then produces in the isotropic dimension effectively an infinite spinning speed spectrum. Such approaches are especially useful for systems where the fastest spinning speeds cannot really narrow the spectra. Examples include chemical shift distribution of heavy elements such as l l9Sn and 2~ in glasses. There are also nuclei which show considerable second-order quadrupolar broadening of the central transition (e.g. 69'71Ga). In the case of spin-l/2 nuclei, approaches to separate the sidebands include magic angle turning or MAT (Gan 1992, Hu et al. 1994), and phase-adjusted spinning sidebands or PASS (Antzutkin et al. 1995). In these sequences the rotor period is split up by a series of pulses. The timing of the pulses is varied and the different sideband components pick up differing phases depending on their order. MAT forms a tl-modulation of information which contains the infinite spinning speed spectrum. PASS has 6 periods during the rotation time, creating a pitch (i.e. phase) of the magnetisation that scales with sideband order. PASS has the advantage over MAT in that it minimises the number of t~ experiments necessary to fully describe and hence separate the sidebands. Modification of PASS with a shifted-echo (Grandinetti et al. 1993) produces an isotropicanisotropic correlation experiment yielding an isotropic spectrum free from spinning sidebands (Fayon et al. 1999). For quadrupolar nuclei a sequence has been developed called QPASS (Massiot et al. 1997) with nine 180 ~ pulses. Usually the magnetisation of the central transition is manipulated by this sequence. One of the most important practical considerations of this approach is the requirement for highly stable spinning. In practice, the solution for sorting the sidebands is unaffected by an increased delay prior to forming the echo by integer multiples of the spinning speed, commonly done to free the echo formation from deadtime effects. The pulses can constitute a significant fraction of the rotor period, demanding the use of quite high power. This becomes even more of a problem with the use of higher spinning speeds with this sequence. Recent work has shown that the order sorting can be carried out over more than one rotor period, and experiments using two and four periods have been published (Vogt et al. 2000). This alleviates the more stringent power requirements and allows the irradiation profile of the pulses to be improved by the use of composite pulses (Vogt et al. 2000).
3.6.4 Dynamic angle spinning (DAS) In DAS the spinner axis is changed sequentially using a stepper motor and a pulley system (Figure 3.21) to produce the more complex time variation of the spinner axis and remove second-order quadrupole effects. The angle switching must be as fast as possible and reproducible. The complete stator is moved by a stepper motor between 37.38 ~ and 79.19 ~ in 36 ms, triggered by pulses from the pulse programmer. The procedure is to set the conventional magic angle, and then, knowing the number of steps
Experimental Approaches
157
Figure 3.21. Design of a DAS probehead from Eastman et al. (1992) with permission of the copyright owner. required by the stepper motor the other angles can be reached, with motor resolution of--~ 0.16 ~ It should be noted that the 90 ~ pulses will depend on the angle from the quadrupolar influences and the orientation of the coil in the magnetic field. The rf coil can be fixed to the rotor, thus changing its orientation as the angle is switched. This gives a good filling factor, but the rf field will vary with the angle and the rf leads must be flexible and resistant to metal fatigue. The coil may also surround the whole assembly and remain static. In this case the filling factor will be poorer but pulse lengths will remain relatively constant, and most importantly, cross-polarisation at an angle of 0 ~ with the static field is now possible, by contrast with the other design. Appropriate phase cycling and processing allows a pure absorption spectrum to be obtained. A limitation of the DAS technique is that it cannot be used for compounds with a short T a because of the time needed to reorient the spinning axis. Furthermore, the experiment requires a dedicated probe of sufficient reliability to be able to mechanically flip accurately between the pair of angles for thousands of transitions. Detailed calculations of the spinning sidebands in DAS spectra have been carried out using average Hamiltonian and irreducible tensor approaches (Sun et al. 1992). In DAS spectra the sideband intensities and their moments depend on the relative rotor phase between the two evolution periods. The sideband intensities additionally depend on the ratio of the time spent at each angle. The 2D 170 DAS spectrum of zeolite Sil-Y (Figure 3.22) shows three lines in the ratio 2:1:1 (Bull et al. 1998). Simulation of the anisotropic slices from the 170 2D DAS spectrum for each peak allows extraction of XQ and xl for each resonance.
3.6.5 Two-dimensional sequences developed from solution NMR Two dimensional sequences are commonly used in solution state NMR spectroscopy to elucidate connectivity of atoms within molecular structures. Sequences are available for heteronuclear shift correlation, homonuclear correlation (COSY and INADEQUATE) and other sequences for longer-range connectivities. In general the basis of these
158
Multinuclear Solid-State NMR of Inorganic Materials
Figure 3.22. Full 2D 170 DAS spectrum of siliceous-Y zeolite (Bull et al. 1998) with the isotropic projection showing the increased resolution and the individual slices providing the anisotropic interaction, with permission of the American Chemical Society. 2D techniques is that transverse magnetisation is created, and evolution is permitted under some of the coupling(s) present. The phase of the magnetisation at the end of the evolution period will depend on the coupling and the time. Thus, if spectra are accumulated for different evolution times, an FT gives the connectivity information in the F1 dimension. In principle, solution techniques can be carried over directly to solids. The criterion for their success is that the magnetisation persists for sufficiently long times that the weak interactions will cause modulation of the magnetisation. Standard 2D solution sequences can be combined with line-narrowing methods such as MAS and decoupling to lengthen T2. The transverse magnetisation can also be created by CP rather than directly from a 90 ~ pulse. COSY has proved very useful in examining the silicon connectivity in aluminosilicate frameworks such as zeolites and minerals, and has been used with 3~p to study organometallic complexes. A COSY experiment on a 29Si-enriched sample of Na2Si409 glass showed small off-diagonal cross-peaks revealing the connectivity of the Q3_Q4 units present (Knight et al. 1990). Heteronuclear correlation is now being used more widely in 13C-~H work on solids since the development of more robust sequences. The great advantage of this sequence is that 1H information is much more spread out by the ~3C shift dispersion than in its own shift dimension. The presence of anisotropic interactions in solids leads to a greater variety of 2D sequences. One of the simplest is the isotropic/anisotropic correlation, where high resolution is achieved in one dimension while preserving anisotropic information in the second dimension. The dipshift and J-resolved experiments correlate isotropic chemical shift with dipolar and J-coupling information respectively. In these sequences the
159
Experimental Approaches
evolution period remains fixed and an integer number of rotor periods is used with decoupling divided between multiple-pulse and high power decoupling. The latter removes all coupling while the former eliminates only the coupling between the protons, allowing the direct X-1H coupling to remain. The time of multiple-pulse coupling is then incremented at the expense of high power decoupling. The F1 dimension can be simulated to extract coupling information to distinguish CH, CH2 and CH3 groups. The multiple-pulse decoupling is as described in Section 2.4.2, but since the sequence does not require observation to be made, more efficient sequences such as semi-windowless MREV-8 and completely windowless BLEW-12 can be used. 2D sequences can also be used to study various exchange processes. The magnetisation produced is labelled by chemical shift and stored so that exchange can occur before it is sampled. Where exchange occurs and a nucleus moves to a chemically distinct site, the nuclear magnetisation is now labelled with a different shift value (i.e. frequency) and appears as a cross-peak in the 2D plot. The same is true for spin-diffusion and has been used with 1H to study domain sizing in polymers. These sequences are largely based on the ideas of Goldman and Shen (1966) in which domains are labelled according to T2 prior to the start of spin diffusion. Spiess, Schmidt-Rohr and co-workers have developed a series of pulse sequences for 2D and more recently 13C, to study slow motions by the modulation of anisotropic contributions of the NMR interactions since these depend on orientation. As the molecular orientation changes, the resonant frequency also changes, resulting in off-diagonal intensity. For simple motions, the off-diagonal intensity is in the form of ellipses whose axial ratio (b/a) equals Itan oLI where oLis the angle of the jump motion (Schmidt-Rohr and Spiess 1994). These are just some examples of the burgeoning field of 2D applications to solids. Other references are given in Table 3.4. In order to provide an even greater spread of information, three-dimensional sequences have now been developed which are essentially combinations of 2D sequences. Their rapid development has arisen through
Table 3.4. Some 2D pulse sequences that have been applied to solids. Sequence COSY INADEQUATE Chemical exchange Spin diffusion HETCOR DIPSHIFT
Purpose
Reference
Fyfe et al. (1989), Kohn et al. (1991), Hanna et al. (1992) Benn et al. (1988) Homo/heteronuclear connectivity when coupling is weak Szeverenyi et al. (1982) Physical exchange of groups Homonuclear connectivity
Domain sizing e.g. polymers, connectivity Heteronuclear correlation (e.g. 13C-1H) Separated local fields to determine dipolar/J-coupling interaction at sites.
Caravatti et al. (1983), (1982) Burum & Bielecki (1991) e.g. Webb & Zilm (1989), Jakobsen et al. (1982)
160
Multinuclear Solid-State N M R o f Inorganic Materials
Table 3.4. (Continued) Sequence NOESY SUPERCOSY VACSY J-resolved J-scaled COSY INEPT TOCSY
Reference
Purpose Local spin exchange (physical and diffusion) Longer-range connectivity Correlation of anisotropy with isotropic shift with fast sample spinning. Correlate isotropic shift with 1H-X coupling Connectivity Connectivity through J coupling Through-bond connectivity
Szeverenyi et al. (1982), Twyman & Dobson (1990) Kolodziejski & Klinowski (1992) Bax et al. (1983), Yang et al. (1990), Lee et al. (1994) Kolodziejski & Klinowski (1992) Kolodziejski et al. (1991) Fyfe et al. 1995, Kao & Grey (1998) Hartmann et al. (2000)
their application to biological solids in which an understanding of connectivity information is important.
3.6.6 Multiple quantum experiments in dipolar coupled systems Although single quantum coherence is the only magnetisation that can be directly observed, for a collection of N strongly dipolar coupled spins, it is possible to induce multiple quantum coherences. A number of pulse sequences have been developed for this purpose (Gleason 1993), for which one of the most common generators is 1
< H'a >= - - 2-
"~'~'i<jDij(l+il+j + l - i l - j ) '
(3.21)
where Dij is the dipolar coupling between spins i and j. This is achieved by applying an eight-pulse sequence of four x-pulses followed by f o u r - x pulses with spacings of A and A', where 3,' = 2A + Tp. Multiple quantum experiments are 2D in nature, the preparation time creating the multiple quantum coherence by an integer number of eight pulse cycles. Evolution can then be permitted, but this is usually set to zero if the interest is only in the number of spins in a cluster. The mixing period is the same number of cycles but time-reversed (effectively with y and - y pulses) so that longitudinal magnetisation is recreated, with the change of intensity reflecting the creation of a multiple-quantum coherence. This is sampled by applying a 90 ~ pulse after a short delay to allow transients to decay. The preparation time can be incremented, but the experiment is commonly performed by keeping the time constant and changing the phase of the preparation sequence using the TPPI principle (Ernst et al. 1988). The different coherences are then distinguished by the fact that for a phase increment A+ an n-quantum coherence will pick up a phase change nA+. Hence, by performing the experiment as
ExperimentalApproaches
161
a function of +, the signal intensity will be S(+) (if'r is a constant at an optimum value). Fourier transforming with respect to + then gives an intensity I(n), i.e. a function of the coherence order, which from the Nyquist criterion means that Inl < ~r/~+. As in multiple-pulse line-narrowing experiments, these experiments require high stability over long pulse trains with the pulses accurately set for both intensity and phase. It is also necessary to be able to set small phase increments A+ < 2.8 ~ Experiments have been developed to create double quantum (DQ) dipolar coherences under MAS. One of the most useful applications is the elucidation of cluster sizes of protons in materials such as the semiconductor a-Si:H. The nature of the experiment requires strong homonuclear coupling and high sensitivity. The intermediate range order in materials can be very difficult to characterise accurately. In silicate and phosphate glasses, 1D MAS NMR gives an accurate picture of the number of different SiO4 and PO4 tetrahedra which are distinguished by their connectivity (i.e. Qn-type discussed below in Chapters 4 and 7). However, these data do not reveal how the units are joined together to produce the intermediate-range order, neither do they show the pair connectivity of the structure. Phosphates are ideal for study since 31p is 100 percent naturally abundant and has a substantial dipolar coupling which provides the mutual interaction to convey the information. In the 2D DQ data set there will be cross-peaks if there is connectivity. The DQ effectively reintroduces dipolar coupling even under MAS. In the 2D data set, the peaks in the DQ (F1) dimension appear at the sum frequency of the two peaks which are coupled. This type of experiment can readily be extended to glassy silicates and phosphates with greater intrinsic linewidths.
3.6.7 Multiple quantum NMR experiments of non-integer spin quadrupolar nuclei Numerous schemes exist to obtain 2D MQMAS spectra. The simplest form of the experiment (Figure 3.23A) is the excitation of the MQ transition by a single, high power if-pulse, after which the MQ-cohercnce is allowed to evolve for a time h. After the evolution time, a second pulse is applied which converts the MQ coherence into a p = - 1 coherence, observed during t2. The signal is then acquired immediately after the second pulse and the echo will form at a time t2 -- IQAI.h, where QA (Quadrupolc Anisotropy) - C 4 ( p ) / C 4 ( 1 ) , with the Cs given in Table 2.8. The information about the MQ coherence is conveyed by the way it modulates the observed echo, either via the amplitude or phase. The modulation depends on the coherence pathway employed. In amplitude modulation, pathways are present such that the signal formed will always be in a particular mode, usually chosen to be absorption. Then, as t~ is varied the lineshape remains constant, changing only in intensity (amplitude modulation). Alternatively, the selected pathways result in the weighting between the different components of the magnetisation varying with t~ and changing the lineshapc between absorption and
162
Multinuclear Solid-State NMR of Inorganic Materials
3
po-! -2
Acquisition
-3
n
3 1 .2
Acquisition
-3
C '~
3
p0
~ ~j
-t -2
Acquisition
-3
D
3 p0
Acquisition
-1 -2 -3
Figure 3.23. Examples of MQ-MAS pulse sequences that are commonly used A. two pulse, B. z-filter, C. split-t~ and D. RIACT(II). dispersion (phase modulation). Both pulses are non-selective and will excite all coherences to a varying degree. After a 2D Fourier transformation the resonances will appear up as ridges lying along the QA-axis. The isotropic spectrum can be obtained by projection of the entire 2D spectrum on a line through the origin (Vl = v2 = 0) perpendicular to the QA-axis. Figure 3.23B shows one of the MQ pulse sequences commonly used, which includes a z-filter pulse (selective, low if-power) and has the advantage of a symmetrical coherence transfer pathway (Amoureux et al. 1996). Other sequences have used 180 ~ pulses to obtain pure absorption lineshapes (Vosegaard et al. 2000). Figure 3.23C shows a split-t~ experiment (Brown and Wimperis 1997, 1997a) in which the isotropic spectrum is directly obtained without shearing of the data (see below). The disadvantage of the split-t~ experiment is that the echo formation must be dominated by the quadrupole interaction. In practice split-t~ experiments give good lineshapes. Figure 3.23D shows the RIACT(II) sequence (Wu et al. 1996) in which excitation and conversion of the coherences is achieved by a spin-lock of duration Tr/4 instead of a hard pulse, having the advantage of more uniform excitation and conversion which is essential to obtain quantitative information. Significant analytical and
Experimental Approaches
163
experimental work comparing the efficiency of various schemes has been carried out (e.g. Amoureux et al. 1996, Amoureux and Fernandez 1998, Pruski et al. 2000). In an MQ experiment the overall efficiency depends on the initial excitation of the MQ coherence and its conversion back to observable magnetisation. Using density matrix methods, Goldbourt et al. (2000) calculated what they termed the echo efficiency parameter, and also examined the effect of the different schemes on the anisotropic lineshape. This lineshape can itself impose very important constraints on the quadrupole interaction parameters. Direct excitation, RIACT, double frequency sweeps and fast amplitude modulation were compared. This work concluded that fast amplitude modulation gave superior conversion efficiency. Although it is not the most sensitive approach and causes more lineshape distortion than some other methods, RIACT has a very weak dependence on XQ and so produces more reliable site distribution information (Lim and Grey 1998). Generation of 3Q and 5Q for I = 5/2 nuclei by RIACT has been compared (Mildner et al. 1999, 1999a) and showed a broader response to XQ than normal pulsed experiments. On balance, the majority of MQ NMR reported in the literature is obtained using hard pulses. This has implications for the choice of probe. For typical XQ values, the operating conditions are shown to the right of the maximum in Figure 2.27. Hence better sensitivity can be achieved by using higher if-fields. This means that smaller coils can be used which have the advantage of providing faster MAS rates, while their high Vr reduces the effect of sidebands in the MQ direction (Amoureux et al. 1998). However, the reduction of sample eventually outweighs these advantages, making a compromise necessary. Depending on the sensitivity of the nucleus being examined, probes of 2.5 to 4 mm diameter are ideal for MQ MAS. There is no doubt that higher order coherences become more difficult to generate, decreasing the sensitivity of the experiment and making it a much more important consideration. In principle, MQ could be combined with spinning at other angles (Duer and Stourton 1997). The two methods usually employed for the phase sensitive detection in the F1 dimension both depend on amplitude modulation. A straightforward TPPI can be incorporated in the pulse sequence where the phase of the first pulse is incremented by 360/4p degrees for each tl value. This means that each tl value contains alternately real (in phase) and imaginary (out of phase) signals (Marian and Wutrich 1983). The effective spectral width in the F1 dimension must be halved in this case. The other method is the hypercomplex States TPPI (States et al. 1982) where for each t~ value a real and imaginary data point is obtained. In this case the effective spectral width in the F1 dimension is still the inverse of the tl dwell time. The difference between the two methods does not seem large. However, if rotor-synchronisation in the F1 dimension is required, the difference in effective dwell time reduces the maximum spectral width. For example, with a rotor speed of say 20 kHz, rotor synchronisation (Massiot 1996) demands a dwell time of tl = Tr requiring a spectral width of 2/Tr, (10 kHz) for TPPI
164
Multinuclear Solid-State NMR of Inorganic Materials
but 1]Tr (20 kHz) for the States TPPI method. As the former might be too small, the latter is clearly preferred. The first report of MQ from non-integer spin quadrupole nuclei appeared in 1995, generating much subsequent research activity and development of a number of schemes for processing and presenting the data. The relationship between the measured peak positions and the NMR interaction parameters depends crucially on the processing and referencing conventions adopted. There are two main approaches. In one, the MQ evolution is regarded as having taken place only in the evolution time (tl). This is the convention adopted for example by Medek et al. (1995) and Hanaya and Harris (1997). In the other approach, the period up to the echo is regarded as also being part of the evolution time which is then (1 + QA)tl, as adopted by Massiot et al. (1996) and Wang et al. (1997). A detailed critique of these two approaches and the consequences of each has recently been given by Man (1998). The isotropic shift and the quadrupole induced shift (QIS = Co(p)/Co(1), with Cs defined in Table 2.4) can readily be obtained from the data. Firstly, the 2D spectrum must be referenced correctly. To obtain the correct ppm scale in the F1 dimension one must set the spectrometer frequency in the F1 dimension to pro. The shift reference is most readily set at the carrier frequency, i.e. the shift in ppm at the carrier frequency in the multiple quantum dimension F1 is the same as the shift in ppm at the carrier frequency in the single quantum dimension F2. This is valid for data obtained with or without delayed acquisition and processed with or without shearing. The isotropic chemical shift is the same for both the single quantum and the multiple quantum dimensions. The QIS is different in both dimensions, however, and is given by vQ x 106 (in ppm) (3.22) p. v o The position of the centre of gravity ~ g with p = 1 for the single quantum dimension and p = Am for the multiple quantum dimension, is given by (~qPs -- C~
(3.23)
(~Pcg= (~cs,iso + (~qPs
Hence the isotropic shift can readily be retrieved using 6cs,iso - C~
- P'C~ Co(p)-p'Co(1)
(in ppm)
(3.24)
and the isotropic quadrupolar shift is
v0 =
' v 0 (in Hz) Co(1)-Co(p) / p
(3.25)
165
Experimental Approaches
A graphical method to obtain the isotropic shift and QIS from unsheared 2D spectra can give a quick assessment of these parameters. A line is drawn through the estimated centre of gravity with a slope equal to the QIS axis crossing the isotropic chemical shift line, g(F1) - g(F2). The intersection of these two lines gives the isotropic shift, and the difference in the shift between the centre of gravity and the isotropic chemical shift gives QIS for both transitions. Note that the slope of the QIS axis is in Hz per Hz and must be adjusted to the ppm scale, i.e. it is less steep by a factor of 1/p. The key to determining the quadrupole parameters is the accurate location of the position of the centre of gravity. Both the excitation efficiency as well as the presence of significant intensity in the spinning sidebands can adversely affect this accuracy. It is therefore advisable to obtain the quadrupole parameters by simulation of the 1D MAS spectrum, using the constraints on • and ~q dictated by QIS, and the isotropic chemical shift as starting parameters. The inhomogeneously-broadened line in the unsheared data will be directed along v l = kv2. The isotropic shifts due to chemical shift and second-order quadrupolar effects are directed along different directions, each different from the inhomogeneous broadening direction. Pike et al. (2000) have considered the resolution of the MQ spectra and have shown that the chemical shift-like terms (i.e. isotropic chemical shift and offset frequency) are scaled by a factor SFcs where P+k Sfcs(i,p ) - - ~
(3.26)
l+lkl
The isotropic second-order quadrupolar shift SFQs(I,p) can be derived directly from SFcs using a factor of -(10/17) (Equation 14 in Pike et al. 2000). The chemical shift scale factors are given in Table 3.5. Table 3.5 implies that for I = 5/2, the 5Q transition should give better resolution than the 3Q transition. Some samples show better resolution at higher coherence order Table 3.5. Chemical shift scale factors for different spins and coherence orders and the relative factor compared to 3Q for the spin. I 3/2 5/2 5/2 7/2 7/2 7/2 9/2 9/2 9/2 9/2
P
SFcs(I,p)
ISFcsl relative to 3Q
- 3 3 - 5 3 5 - 7 3 5 7 - 9
17/8 - 17/31 85/37 - 17/73 - 17/10 238/103 - 17/127 - 85/131 - 119/25 85/37
1 1 155/37 1 73/10 1022/103 1 635/131 889/25 635/37
166
Multinuclear Solid-State N M R of Inorganic Materials F2
E1
4 0
300
200
Hz
F1
C
kHz
L
Q
F1
-
i
6
!
4
'
'
-i
!"
3
2
kHz
kHz Figure 3.24. Improved resolution achieved comparing A. 3Q, B. 5Q and C. 7Q experiments of 45Sc in Sc2(SO4)3 from Pike et al. (2000) with permission of the copyright owner.
(Figure 3.24, Pike et al. 2000), with exceptional gain found for some samples (Rocha et al. 1996). The gain in resolution depends on both the dataset and also the relative contribution of homogeneous and inhomogeneous interactions. A key point made by Pike et al. (2000) is that the inherent resolution is often not the problem, but poor S/N which can truncate the data in the t~ dimension, causing broadening (Ernst et al. 1998, Callaghan 1991). Decoupling has also been applied in conjunction with MQ MAS (Hanaya and Harris 1997, Lacassagne et al. 1998). In the MQ direction the dipole interaction becomes magnified and provides distance information (Duer 1997). Shearing of the data is carried out to obtain isotropic spectra in the F1 dimension and to facilitate easy extraction of the 1D slices for different peaks. Shearing is a projection of points lying on a line of slope equal to the anisotropy axis, on to a line parallel to the F2 axis. The lines intersect at the F2 zero frequency (i.e. the carrier frequency)
167
Experimental Approaches
line. In effect, this means that a point that had a frequency of (1)1,1)2) will now lie at frequency (1)1 - kv2, 1)2), with k equal to QA (see C4(p), C4(1) from Table 2.4). If the absolute frequencies are taken relative to the carrier frequency in the F1 and F2 dimension: 7
V1 -- Co(P)V Q - --~C4 (p)vQ (O,d/)) + p(Viso + Voff )
(3.27)
and v : - Co(1)VoQ -
7
c4(1)vQ4 (o,0)+ Veto+ VoH
(3.28)
then the sheared frequency Vl' = Vl - kv2 in F1 becomes V; -- [C 0 ( p ) - kC 0 ( l ) ] v Q -k- [p - k](Vis o nt- Voff )
(in Hz)
(3.29)
The scale of the Fl-axis remains unaltered and the frequency of the isotropic peaks can readily be obtained. Converting the Fl-axis from frequency units to ppm, such that 1 ppm equals [p - k]vo Hz, will give the peak position C O(p) - kC o (1) vQ (in ppm)
- 'Lo +
(3.30)
[ p - k]Vo
when referenced correctly (i.e. ~ at the carrier frequency in F1 is ~ at the carrier frequency in F2). Shearing can be achieved in two ways. After the 2D Fourier transformation, the position of the data points is recalculated to obtain the sheared spectrum. The disadvantage of this method is the need for interpolation. Alternatively, prior to Fourier transformation with respect to t l, first-order phasing is applied to the data in the t l dimension by multiplying all points by e x p ( - 27rivztlk), where v2 is the frequency from the carrier frequency in F2. Shearing essentially achieves the same as the split-t1 experiment or delayed acquisition of the echo. Although sheared spectra may look more attractive, they do not add extra information and are not necessary for the extraction of the isotropic quadrupole and chemical shifts. Moreover, shearing introduces an extra processing step, which may introduce artefacts. The centre of gravity of the lineshapes in the two dimensions F1 and F2, designated ~1 and ~2, can be related to the isotropic chemical shift (~cs,iso) and second-order quadrupole isotropic shifts (~Q,iso(2)). 9 e(2)
~1,2 -- a(~cs,iso + D~
(3.31)
168
Multinuclear Solid-State NMR of lnorganic Materials
Table 3.6. The relation of the positions in the F1 and F2 dimensions for unsheared and sheared data sets for different MQ transitions. P
3/2
3
5/2
3 5
gl,2
g~ g2 gl ~2 ~ g2
Unsheared data Sheared and split-tl data a
b
a
b
3 1 3
6/5 -2/5 -4/5
17/8 1 17/31
1
-16/15
1
5 1
20/3 -16/15
85/37 1
1/2 -2/5 32]93 -16/15 160/111 -16/15
The coefficients a and b will differ for unsheared and sheared data (Table 3.6)
3.6.8 2D X Y correlation methods Heteronuclear XY correlation experiments are very useful for determining connectivities between different nuclei when many resonances are present. To measure a 2D heteronuclear correlation (2D HETCOR) spectrum, magnetisation must be transferred between the heteronuclei. This can be achieved by CP (Section 3.8.1) or TEDOR (Section 3.8.2). A variable time t~ is inserted prior to the magnetisation transfer from, say, spin I to spin S. By measuring the FID of spin S as a function of the time tl the S-spin FID is modulated by the signal intensity of spin I at tl. A 2D FT will give the IS 2D correlation spectrum. Phase sensitive detection in F1 can be achieved, for instance, by TPPI. The main difference between 2D HETCOR (which involves only spin-l/2 nuclei) and cases involving quadrupolar nuclei, is the attention that must be given to the transfer of the magnetisation. Overlapping lines in the 1D spectrum of the quadrupolar nucleus can be resolved in the 2D correlation spectrum. However, this does not always occur because of the second-order quadrupole broadening which can hinder the assignment of the correlation peaks. One would therefore like to incorporate a line-narrowing method such as DOR, DAS or MQMAS into the 2D HETCOR experiment. DAS has been used to obtain a high resolution 2D HETCOR spectrum of sodium trimetaphosphate (Jarvie et al. 1995). This experiment involved spinning at two different angles (79.19 ~ and 37.38 ~ during tl such that the isotropic echo is formed at the end of the t~ evolution period. The magnetisation was stored along the z-axis and then transferred from 23Na to 31p at an angle of 0 ~ the 31p signal being detected at the magic angle. Three reorientations of the spinner axis were used during the experiment, which is not suitable for compounds with short T~ values. However, the number of reorientations during the experiment can be reduced to only two by selecting a different set of DAS complementary angles, in which case 01 = 63.43 ~ and 02--0 ~ eliminating the need for an extra reorientation for the CP sequence.
169
Experimental Approaches
Another possibility is to incorporate the MQMAS experiment into the 2D HETCOR sequence (Wang et al. 1997). In this experiment one performs a split-t1 MQMAS experiment during the tl evolution period, such that at the start of the magnetisation transfer the isotropic echo is formed. Figure 3.25 shows the results of a MQMAS/ HETCOR experiment on sodium trimetaphosphate, in which the resolution in the 23Na dimension was greatly increased compared to the straightforward HETCOR experiment. This sequence has been improved by changing the position in the sequence at which CP occurs (Steuernagel 1998). It should be noted that in both types of experiment the results are not quantitative. A
-12.5 ~ . . . . . . . . . . . . .
o
I
-lO -2o ppm from solid NaCI
-17.5
- 2 0 ~ . ~ ~ -22.5
~'~
31p
0 -10 -20 ppm from solid NaCI Figure 3.25. 23Na-31p2D HETCOR spectra of Na3P309A. Obtained with the normal HETCOR pulse sequence and B. with MQ-MAS incorporated in the pulse sequence to obtain higher resolution from Wang, De Paul and Bull (1997) with permission of the copyright owner.
170
Multinuclear Solid-State NMR of lnorganic Materials
3.6.9 Correlation o f t e n s o r i n f o r m a t i o n - s e p a r a t e d local field experiments
Valuable structural information is contained in the orientation and magnitude of tonsorial interactions that can be measured in NMR spectra. Determination of the principal components of such interaction tensors by simulation of the normal 1D NMR spectra is reasonably straightforward. However, it is much more difficult to relate the principal components of these interaction tensors to the molecular frame. By examining two interaction tcnsors (e.g. the dipolar tensor and the chemical shift tensor), separated local field experiments (Hestcr et al. 1976, Linder, H6hener and Ernst 1980) have the potential to relate the tensor orientation to the molecular frame. It is necessary for the orientation of one of the tensor interactions to be known in the molecular frame. Since the dipolar tensor is axially symmetric and usually collinear with the internuclear vector, measuring the relative orientation of the CSA tensor provides the possibility of relating the CSA tensor to the molecular frame. Various experiments have been developed for dipole and CSA tensor correlation in samples under either static conditions (Wu et al. 1994) or MAS (Munowitz et al. 1981, Munowitz and Griffin 1982). The success of this approach is illustrated by the numerous publications using these separated local field experiments to obtain the CSA tensor orientation for the structural characterisation of various materials (Nakai et al. 1988, Schmidt-Rohr et al. 1993). Most separated local field experiments concentrate on the CSA. In spectra of halfinteger nuclei with I > 1/2, the CSA is often much less important than the secondorder quadrupole interaction. Static or MAS spectra display a typical quadrupolc powder pattern from which the quadrupolc coupling constant and the asymmetry parameter can be determined. These quadrupolc parameters can then be used to determine the principal components of the electric field gradient tensor, which arc related to the local environment of the quadrupole nucleus. Knowledge of the local geometry is extremely valuable, and correlation of the quadrupole and dipole information of OH groups provides useful insights into structure. Lindcr et al. (1980) have indicated the possibility of correlating the quadrupole and dipole tensor by quadrupolc separated local field experiments on powder samples. This has been applied to the 1H-170 system in Mg(OH)2 and Mg(OH)• (van Eck and Smith 1998). Lce-Goldburg dccoupling during the dipolar evolution provides better resolution in the dipolar dimension. The normal chemical shift dimension produces the second-order quadrupolar lincshape and the dipolar dimension allows the dipolar interaction to be calculated. The 2D data contain more information since the intensity distribution depends critically on the relative orientation of the two tcnsors. Since the dipolar interaction is axially symmetric, only the azimuthal and polar angles are necessary to describe the relative orientation, and the intensity distribution changes markedly with this orientation (Figure 3.26). MQ can be used in correlation experiments, allowing the orientation of the quadrupole and dipolar tcnsors to be deduced (Duer and Painter 1999). A modified version of an MQ-MAS experiment has been used to cross-correlate quadrupolc interactions at different sites (Dowellet al. 2001).
171
Experimental Approaches
20 .
.
.
......
.
I I
12
o
0
r
800
.
.
.
.
.
.
.
,
400 0 -400 -800 C h e m i c a l shift ( p p m )
20 16 12
1 0
800
460
6
-400
-800
C h e m i c a l shift ( p p m )
Figure 3.26. A quadrupole-dipole separated local field experiment for the OH group in Mg(OH)x(OCH3)z-x with A. the experimental data compared with B. the simulation of the intensity assuming the tensors are collinear from van Eck and Smith (1998) with permission of the copyright owner.
When anisotropic interactions are present, the angle can be changed during an experiment such as DAS to scale or recouple these interactions without removing them, providing a two-dimensional data set (Frydman et al. 1992). Such an approach for 29Si in a silicate glass has provided an isotropic/anisotropic correlation giving a large improvement in the quantification of the different Qn species (Zhang et al. 1996). An experiment has also been developed to enable the correlation of the quadrupole and CSA tensors by using switched angle spinning (Shore et al. 1996). A 2D correlation experiment using MAS and off-MAS has been applied to a study of phosphates, 23Na in crystalline NazSO3 and 11B in a potassium borate glass (Hartmann et al. 1999). In a glass with
172
Multinuclear Solid-State NMR of lnorganic Materials
a high potassium content, the borate network was sufficiently broken up that there was effectively little correlation between the orientation of the BO3 units present. This type of experiment has been used to correlate the quadrupole interaction (Joo et al. 2000).
3.7. SUMMARY OF APPROACHES FOR EXAMINING QUADRUPOLE NUCLEI
There now exists a large range of techniques for observing quadrupole nuclei. A combination of techniques must be used to deduce the NMR interactions (specifically the quadrupole interaction) and the distribution of sites. The situation is evolving rapidly (cf. Smith 1993), the current possibilities being: (i) (ii) (iii) (iv) (v)
One pulse static, One pulse MAS, DOR, MQ or DAS, Nutation.
The improved resolution afforded by DOR, MQ and DAS by removing second-order effects can be used to reveal the number of sites. The field dependence of the isotropic position of the high resolution data allows 8iso,cs and PQ to be deduced. To constrain the data, DOR and MQ can be combined (Anup~ld et al. 1998). Very extensive data sets have been collected using MQ and DOR on single compounds, and the combination of this with one dimensional data provides a powerful approach, as demonstrated for 23Na (Engelhardt et al. 1999), 27A1 (Dupree and Smith 2001) and 170 (Section 6.2.2, Figure 6.7). The quality of MAS spectra has continued to improve with the availability of higher magnetic fields. When nuclei such as 23Na, 27A1 and 170 can be observed at 1 GHz, direct high resolution MAS of such quadrupole nuclei will be possible.
3.8. MULTIPLE RESONANCE
Many multiple resonance methods such as cross-polarisation (CP), Rotational Echo Double Resonance (REDOR), and Transferred Echo Double Resonance (TEDOR) are available. Their application can be differentiated between systems involving exclusively spin-l/2 nuclei and those containing quadrupolar nuclei. The presence of quadrupolar nuclei may prohibit straightforward application of these experiments and can affect the interpretation of the results. There are, however, benefits arising from the presence of quadrupolar nuclei, as illustrated by the Transfer of Populations by Double Resonance (TRAPDOR) experiment which will only work for quadrupolar nuclei.
ExperimentalApproaches
173
3.8.1 Cross-polarisation (CP) Cross-polarisation has played a very important role in the development of solid state NMR, giving improvements in sensitivity compared to one pulse operation allowing ~3C spectroscopy in polymers and other organic solids to become feasible. Normally the abundant spin is 1H or 19F, for which a general procedure can be followed to set up the CP experiment. It is helpful if a well-defined standard set-up compound can be identified for each particular pair of nuclei. Steps to setting up a CP-MAS experiment: 1. 2. 3. 4. 5. 6. 7.
Spin the well-defined set up sample relatively slowly (1-3 kHz). Tune both the X and 1H/19F channels. Observe the 1H/19F, set on resonance by shifting the transmitter frequency and set the power to produce the desired 90 ~ pulse. Observe the X-nucleus and if a signal can be seen in decoupled one pulse operation, set X close to resonance also. Switch to CP mode and increment the transmitter power of the X nucleus until the maximum signal is observed (i.e. the optimum match is obtained). Adjust the offset frequency of the 1H/19F channel until the optimum CP and decoupling is observed. The system may then need to be rematched. Change to the compound to be studied and ensure that the system is accurately retuned as for the set-up compound.
Not all samples will provide a signal readily in one pulse operation. However for ~H-~3C there are a number of compounds which are helpful for optimising CP operation. The authors always use a combination of adamantane and glycine. Adamantane can be observed with only decoupling and it is sensitive to the shimming of the system. Glycine is sensitive to the angle setting through the CSA of the carbonyl and to the decoupling efficiency (offset and power) through the oL-carbon. Both these compounds can also be used as secondary references for the chemical shift scale. CP between spin-~/2 is most straightforward and well-known set-up compounds are available. There are many considerations that determine optimum compounds. The relaxation characteristics are very important; Eqs 2.171-174 show that the best compounds for CP have short T~s values and long ~H T~0 values. For the best results, the ~H T~ should preferably be short enough for the S/N to be improved rapidly by keeping the recycle time short. The lack of a CP signal from a particular compound may be due to its unfavourable relaxation characteristics. CP to ~H has been extensively applied to more unusual combinations such as heavy metals and low-~/spin-~/2 nuclei (Sebald 1992). Each of these groups has particular difficulties associated with CP. The heavy metals have large chemical shift ranges, so the offset for each compound may be very different. Such nuclei also tend to have large
174
Multinuclear Solid-State N M R o f lnorganic Materials
CSA, splitting the signal into many sidebands and significantly lowering the intensity of the centreband. Low-~/nuclei present inherent sensitivity difficulties, and to satisfy the Hartman-Hahn condition for low-~/nuclei, considerable power must be applied on the X-side of the probe to secure a reasonable ~H 90 ~ pulse. Care must be taken in using high power that too much strain is not placed on the preamplifier and probe. Much of this work was done during the mid-eighties, before high power automated linear amplifiers became widely available, making the determination of the set-up conditions an extremely intensive process. For low-~/spin-1/2 nuclei the optimum contact times tend to be long (10--100 ms).
Table 3.7. Some set-up compounds for CP between Spin-1/2 X-1H. X
Set-up compounds
Shift (ppm)
CT (ms)
Rec (s)
13C
Adamantane Glycine
5 1
10 5
~5N
5
l0
298i
Enriched glycine Kaolinite QsM8
38.4 176.03 (a-carbon) -345 -91.7 12.1 (t), - 180(o)
2 5
10 10
31p 778e
(NH4)zHzPO4 (NH4)SeO4
-1.7 -- 1040.2
1 3
4 4
10
10
89y l~
Y(NO3)3.6H20 - 53.2 Silver lactate
345.9
50
l0
ll3Cd
Cd(NO3)z.4H20
- 100
15
8
Jl9Sn
Sn(cyclohex)4
-97.35
5
10
Xe in 222 hydroquinone clathrate 171yb Yb(PPh2)2(thf)4 440.6
30
5
6
183W
(NH4)2WS4
3639.6
100
10
195pt
K2Pt(OH)6
8024.5
1
199Hg
Hg(Oac)2
- 2487 -2493
5
129Xe
10
Comments
Reference
Good for shimming Good for checking the angle and decoupling efficiency Natural abundance is visible in one scan Cheap Large signal, good Engelhardt & secondary reference Michel (1978) but expensive Many compounds will do A narrow line Collins et al. (1987) Visible after one scan Merwin & Sebald (1990) Four signals most Merwin & positive shift given Sebald (1992) Charles et al. (1983) A large signal Harris & Sebald (1987) Lee et al. (1988) Needs many 1000 Rabe & Sebald transients (1996) 400 transients were acquired Merwin & Sebald (1992) Harris & Sebald (1987) 16 scans necessary, Harris & Sebald has a large CSA but is (1987) poisonous
175
Experimental Approaches
Table 3.7. (Continued) Shift (ppm)
CT (ms)
Rec (s)
199Hg (NEt4)Na[Hg(CN)4]
-434
15
3
(NBu4)z[Hg(SCN)4]
-615
20
2.5
Pb(C7H7)4
-148.8
5
12
X
2~
Set-up compounds
Comments
Reference
Signalobservable Eicheleet al. in one scan (1995) Eichele et al. Signal observable (1995) in one scan P o i s o n o u s Harris& Sebald (1987)
CT is a typical optimum contact time and Rec is a typical recycle time.
There are many variants of the basic CP sequence. These days the most common variants include switching the power between the matching and decoupling. The probe is much more likely to break down when both rf fields are on so the 1H power can be lower during the match period and increased for the decoupling. Other procedures for adjusting the power requirements include CP-TAPF (Takegoshi and McDowell 1986), in which the decoupler phase is varied between _+ y with + y for tl a n d - y for t2, scaling the power requirement by ( t l - t2/t~ + t2). Another such sequence is TPPM, in which the decoupling is split into two and the phase of the rf is switched between _+ +, giving considerably improved decoupling. When the phase modulation frequency matches the rate at which the magnetisation nutates in the rf field, additional decoupling results (Bennett et al. 1995). Other variants have been suggested, including frequency modulation and combinations of amplitude and phase modulation (Gan and Ernst 1997). Ramped CP is now used to broaden the match condition and reduce the effects of fast MAS (Section 2.6). If the T~p of the ~H is long enough and T2 short enough, the residual transverse proton magnetisation after the contact can be flipped back to the z-direction (Pines et al. 1973). This shortens the recycle time, and although the sequence may not provide much gain it cannot do any harm and so can be used routinely. CP can also be used for spectral editing. The relative spatial position of nuclei can be deduced simply through the effectiveness of the CP. Dipolar dephasing is a specific sequence for discriminating species (Opella and Frey 1979) in which the 1H channel is interrupted for 30-100 p~s during the decoupling period. Often a 180 ~ pulse is introduced in the centre of the interruption period for the X nucleus to refocus the chemical shift effects (Murphy 1983). The sequence depends on the reappearance of the dipolar coupling when the decoupler is turned off, so any X nuclei strongly coupled to ~H will dephase rapidly and be lost from the spectrum. Signal remains from those nuclei which are not directly coupled, which for 13C are the quaternary carbons, giving this sequence the name non-quaternary suppression (NQS). In ~3C spectroscopy, other
176
Multinuclear Solid-State NMR of lnorganic Materials
mobile species, especially the methyl groups, will also survive due to their rapid rotation which averages the dipolar coupling. Other spectral editing schemes for discriminating CHx fragments have been proposed (Wu and Zilm 1993). There is also a number of sequences that allow the dynamics to be probed. The CP curve itself can be mapped out by using variable contact time, and fitted to determine TIs and Tlw Other sequences for determining the various relaxation times (X T1 and TI~, 1H T~ and T~o) are shown in Figure 3.27. The IH T~ can be detected via the carbon which sometimes allows the T~ of the different protons to be distinguished. Even if T1x is long, it can be determined via the proton magnetisation, provided T1x > > T1H (Torchia 1978)
900
J
CP 90*
900
CP
I~.
r 900
cP
L v
v
~
90*
180 ~
Dee
L
Dee
L
90 ~
CP
~ v r . ~-
Figure 3.27. Pulse sequences (upper channel 1H lower channel X-nucleus in each pair) for determining relaxation times in CP-related experiments A. T~ of the X-nucleus, B. Tlo of the X-nucleus, C. Tip of IH and D. T1 of 1H.
177
Experimental Approaches
Although CP experiments are predominantly between 1H----~X,other combinations of nuclei exist. In particular, 19F can be used as the source of magnetisation (Sebald et al. 1992). 19F can be more difficult to set-up since its chemical shift range is that much greater than in 1H, so the decoupler offset must be recalibrated. It is also recommended that the 19F is observed directly to check where the decoupler frequency is set relative to the chemical shift for that compound. Appropriate set-up compounds are necessary for optimisation of the CP condition. NazSiF6 is a good set-up compound for 298i. CP to quadrupolar nuclei often produces only weak signals because of the difficulty of spin-locking the magnetisation discussed in Section 2.5. This is especially important under MAS, which rapidly destroys the spin-locked magnetisation. However, since some of these nuclei (e.g. 23Na, 27A1) produce strong signals, CP can be used to distinguish sites. Optimum compounds for setting up CP and the effect of the conditions have been extensively discussed (Table 3.8). Because of the complex changes occurring in the eigenstates during rotation for a spin-locked quadrupolar nucleus there can be significant distortion of the observed lineshape in the CP of such nuclei (Barrie 1993, Hayashi 1994). 19F can be cross polarised to quadrupolar nuclei, and has been used in 19F----~ZVA1studies of fluorine-bearing aluminosilicate glasses (Kohn et al. 199 l a) and a ~9F---~23Nastudy of NasW309F5 (Duet al. 2000). There are now experiments to generate MQ from CP coherences (Pruski et al. 1997, Ashbrook et al. 1998, Lim and Grey 1999, 2000, Ashbrook and Wimperis 2000, 2000a). A two-dimensional experiment has combined CP and MQ MAS (Ashbrook and Wimperis 1999). The CP experiment can be extended to double CP e.g. ~H---~X--~Y. The initial CP is used to enhance the sensitivity of the experiment and the second stage investigates the connectivity of the X and Y nuclei. This approach has become particularly well developed for 13C and ~SN to probe the connectivity of biomolecular solids (Schaefer et al. 1984). Spectrometers now often come equipped with at least two channels (designated X and Y) and often three lower frequency channels. This allows polarisation transfer experiments between X and Y nuclei. Examples of X---~Ymagnetisation transfer are rapidly increasing and some examples are given in Table 3.9.
Table 3.8. Suggestedset-up compounds for CP 1H-quadrupolenuclei. Nucleus 11B 170 23Na 27A1
43Ca 95M0
Set-up compound
References
Woessner (1987) kernite Smith (unpublished) NaB(OH)4.2H20 Mg(OH)2 Walter et al. (1988) Harris & Nesbitt (1988) NaBH4,NazB407.19H20 boehmite(A10(OH)) Blackwell & Patton 1984, Morris et al. (1989), (1990), Kellberg et al. 1991, Rocha et al. (1991), Mortuza et al. (1993) Ca(OH)2 Bryant et al. (1987) Edward & Ellis (1990). (NH4)6M07024
178
Multinuclear Solid-State N M R o f Inorganic Materials
Table 3.9. Some examples of X---~Ymagnetisation transfer used in NMR investigations of inorganic solids. X---~Yand application
Reference
27A1,23Na---->29Sienhancing 295i signal in aluminosilicates 27Al<---->31Pin molecular sieve VPI-5 31p-->~Hspectral editing 23Na<--->11B,27A1<___>11B,7Li<--->ZVA1 3~P-->~3Cd, 3~P-->29Siconnectivity in semiconductors 11B<-->27AIconnectivity in aluminoborate glasses 31P----)TVSein [3-P4Se3 DQ CP ~1B4--~Z3Na,~B+--~2VA1to examine connectivity in aluminoborate glass
De Paul et al. (1997) Fyfe et al. (1992) Crosbie et al. (1988) Eastman (1999) Franke et al. (1992) Van Wullen et al. (1996) Pietrass et al. (1997) Chan et al. (1998)
3.8.2 SEDOR, REDOR and TEDOR
The Spin Echo Double Resonance (SEDOR), REDOR and TEDOR experiments are all multiple resonance experiments that exploit heteronuclear dipolar coupling. Qualitative information regarding the proximity of spins I and S, and even quantitative information regarding the I-S distance can be derived. Examples of these approaches applied to materials will be given in later Chapters. Their application to specific areas such as aluminosilicates has been reviewed (Ba et al. 2000). The SEDOR experiment (Ehmswiller et al. 1960, Wang et al. 1984, van Eck and Veeman 1992) uses a static sample, in which an echo pulse sequence is applied to one nucleus, e.g. spin I (Figure 3.28A). The echo refocuses the heteronuclear dipolar coupling and the CSA. During the echo period a single 180 ~ pulse is applied to the other nucleus (spin S). This inverts the sign of the dipolar coupling which perturbs the dipolar refocusing process, and consequently the echo intensity is diminished. The amount of signal intensity lost (the SEDOR fraction) depends on the magnitude of the I-S dipolar coupling, the echo time, and the position of the perturbing S-spin 180 ~ pulse. By measuring a SEDOR curve as a function of echo time or pulse position one can determine the IS distance through the dipolar coupling constant D-1
y; ys h
2n"
1"/3
~to (in Hz)
(3.32)
4n
The magnitude of the dipolar coupling that can be readily determined by this method is limited by T2. MAS will readily average away the heteronuclear dipolar coupling but the REDOR sequence (Gullion 1998, Gullion and Schaefer 1989, 1989a, van Eck and Veeman 1993) is designed to reintroduce this coupling in an MAS experiment, making it the MAS equivalent of SEDOR. Spinning the sample greatly enhances the sensitivity and
Experimental Approaches
,H
179
F] !
0
....
i
2
Aq
.
.
.
.
|
r
2r
3
s
~
Nc
i
o
-
~ -
.... F ] _ F L F L .
i
.
,R~FI...~ M~__FI Nc,
-
o
~
0
'
.
~
r.
i
.
2
!
l
.
.
.
.
.
j
.
.
R_ FL_ I;1 _ F! .. H~Q
.
.
.
.
.
J
LI-
~/ ............. Nc
.
.
,i
L
i
J
2
3
,
4
Figure 3.28. Double resonance experiments for providing connectivity/proximity information
A. SEDOR, B. REDOR, C. TEDOR and D. TRAPDOR.
resolution, and allows the echo to be measured for longer times. There are various pulse schemes to implement REDOR. In one, a rotor-synchronised echo sequence is applied to spin I which is detected after a time 2t equal to an even number of rotor periods (Figure 3.28B). Two dephasing 180 ~ pulses per rotor period are applied to spin S, ensuring that the sign of the dipolar coupling at the start of each rotor period is the same so that the dipolar dephasing adds up for each rotor period. In contrast to the static SEDOR experiment, where the echo was necessary to refocus the CSA and dipolar dephasing, the REDOR experiment employs MAS to induce rotational echoes which refocus the CSA and the dipolar interaction. The refocusing 180 ~ pulse is still required as it ensures the detection of an in-phase signal. The dipolar coupling can be obtained by measuring the intensity loss as a function of the number of rotor periods (echo time),
Multinuclear Solid-State NMR of Inorganic Materials
180
or as a function of the position of the dephasing 180~ pulses on spin S. Dipolar couplings as small as 25 Hz have been measured by the REDOR experiment (Merrit et al. 1998). REDOR, like SEDOR, is a difference experiment where the attenuated signal is subtracted from the full echo to obtain the REDOR fraction. The magnitude of the dipolar coupling (D) is characterised by the difference of the signal detected with (S) and without (So) in the presence of the 180 ~ pulse in the sequence (Figure 3.28B) with AS = S o - S. AS depends on D and the time for which evolution is permitted, an experimentally adjustable parameter. For an isolated spin-l/2 pair,
AS=l_ 1 SO
2re 1r/2
2re ; ~ c~ 0
(3.33)
0
where ot and 13 are the polar angles that define the orientation of the PAS of the dipolar tensor in the laboratory frame (Pan et al. 1990). Fitting the REDOR curve then gives D. There has been considerable work in solving this equation to deduce the distance numerically (Mueller 1995) and to use a transform for the two-spin case (Mueller et al. 1996). Quantitative distance information is relatively easy to extract for isolated two-spin systems, which often requires specific labelling of samples. The interpretation of the results becomes much more complex when more spins are involved, e.g. IS2 IS3 or IS4 spin systems. It is possible to obtain distance information for these more complex systems but to do so it is necessary to assume a structural model (van Eck and Veeman 1992). Analysis can also be applied only to the start of the REDOR curve (Bertmer and Eckert 1999) to simplify the problem. A modified REDOR sequence that overcomes the effects of imperfections at the start has been used (Chan and Eckert 2000). To circumvent this problem, theta-REDOR, an elegant variation of the REDOR experiment has been developed (Gullion and Pennington 1998) in which a pulse with a small flip angle is applied instead of a dephasing 180 ~ pulse. In a multispin system, for example IS4, this means that the probability of all 4 S spins flipping and hence affecting the dipolar dephasing of spin I is very small. By contrast, the probability of only one spin undergoing a transition is highest (for small flip angles). Thus, it is not necessary to consider a complicated 5-spin system but to analyse four isolated 2-spin systems. For a given order of spin system (e.g. ISx for a particular x) at short evolution times, the geometry dependence of the S-spins is negligible. For quadrupolar nuclei it is assumed that the analysis in the limit/4~ << H,f << H(I~ is relatively straightforward. In this case only the central transition is efficiently observed, but it must be uniformly excited, and the 180 ~ pulses affect this transition only. If the relaxation time is relatively long (compared to the REDOR experiment), only I nuclei in the central transition produce dephasing of the S-signal. However the analysis may not be as straightforward as this (Chopin et al. 1998). In practice, to compensate for complications in interpretation, the experiment is
Experimental Approaches
181
calibrated by using well-defined crystalline materials (Bertmer et al. 2000). REDOR with additional recoupling has been proposed for use under very fast MAS (Chan 2001). REDOR and MQ-MAS have been combined with recoupling, applied during the 3Q period to improve the sensitivity for weak dipolar couplings (Pruski et al. 1999). The TEDOR experiment (Hinton et al. 1992, 1993, van Eck and Veeman 1992) is capable of transferring magnetisation between heteronuclear spins, and consists of two consecutive REDOR experiments, first on spin I and then on spin S which is observed (Figure 3.28C). Using the spin density matrix formalism one can easily show that the first REDOR sequence creates 2IySz coherence which is transferred to 2IzSy coherence by the application of two 90 ~ pulses. The subsequent REDOR sequence on spin S transfers this coherence to an observable S• coherence. Therefore, instead of determining the amount of signal lost on spin I, as in SEDOR and REDOR, one measures the signal that is gained on spin S. The dipolar coupling constant between I and S can be obtained by measuring the signal intensity as a function of the number of rotor cycles (from either the first or second REDOR period) or as a function of the position of the dephasing pulses. Each of these experiments has pros and cons. The SEDOR experiment is easiest to set up but lacks the resolution since it is made on a static sample, and is often limited in the range of accessible dipolar couplings by T2. The REDOR experiment is more elaborate to set up and requires stable spinning, but its increased resolution enables multisite systems to be explored, while the longer T2 enables longer distances to be determined. TEDOR is less sensitive than REDOR as it involves a transfer step with a theoretical maximum efficiency of 50 percent. However, it is not a difference experiment, and is therefore less prone to experimental errors. An important application of TEDOR is its use in 2D-correlation spectroscopy. All these experiments perform well for spin-l/2 systems and when an isolated spin pair is present they allow the accurate determination of the IS distance. They all rely on the ability to accurately invert the spins by applying 180 ~ pulses. With quadrupolar nuclei it is usually not possible to invert the spin population with a 180 ~ pulse. Therefore, in using REDOR and SEDOR in a system with a quadrupolar spin and a spin-l/z, the echo sequence should be applied to the quadrupolar nucleus and the dephasing pulses to the spin-I/2. In this way, accurate distance information can still be obtained, otherwise it can only be qualitative. A further advantage of performing SEDOR and REDOR in this manner is that the T1 of the quadrupolar nucleus will usually be shorter than that of the spin-I/z, thereby reducing the experimental time. If it is possible to apply an accurate 180 ~ pulse to the quadrupolar nucleus, this will mainly invert the population of the central transition and will leave the other spin levels relatively unperturbed, affecting only a fraction of the quadrupolar nuclei (shown in Table 2.9). This can provide a convenient means of creating isolated spin pairs, as in the theta-REDOR experiment. When using TEDOR to gain qualitative information, the best choice might
182
Multinuclear Solid-State NMR of Inorganic Materials
be to start with the nucleus with the shortest T1, usually the quadrupole. To obtain distance information, the number of rotor periods of either REDOR section of the TEDOR experiment can be varied. It would be preferable to vary the REDOR section in which the dephasing 180 ~ pulses are applied to the spin-l/2 system. 3.8.3 TRAPDOR and REAPDOR The development of the double resonance experiments described in Section 3.8.2 was targeted at spin-1/2 systems. TRAPDOR and Rotational-Echo Adiabatic Passage Double Resonance (REAPDOR) experiments are designed specifically for quadrupolar nuclei and do not work on systems containing only spin-l/2 nuclei. These MAS experiments still rely on the modulation of the heteronuclear dipolar coupling (as in the REDOR experiment) to prevent an echo from refocusing, but the modulation is no longer by 180~pulses. In the TRAPDOR experiment (Grey and Veeman 1992, Grey et al. 1993) the observed nucleus can be either spin-1/2 o r quadrupolar, but the dephasing spin must be a quadrupolar nucleus. During the rotor synchronised echo of the observed nucleus the dephasing quadrupolar spin is continuously irradiated (Figure 3.28C). Continuous irradiation of a quadrupolar nucleus in combination with MAS leads to rotationallyinduced level transitions (Section 2.5). Thus, an individual spin will sample different spin states (m) in each rotor period, each of which is affected differently by the dipolar coupling (2mD). Transitions between levels occur either two or four times per rotor period, causing dipolar dephasing which is additive for each rotor period. Furthermore, by contrast with REDOR which mainly affects the 1/2 and - 1/2 levels, the spin-locking affects all transitions, so the dephasing in a TRAPDOR experiment will be greater than in the REDOR experiment (van Eck et al. 1995). As it is difficult to know the spin states of the dephasing quadrupole nucleus at each instant of the rotor period, and because the efficiency of the spin-locking strongly depends on the adiabaticity parameter e~ (Section 2.5), it is difficult to calculate exactly the dephasing effect on the other nucleus. For very small values of or, no TRAPDOR effect will be visible, while the dephasing effect will be greatest for completely adiabatic spin-locking. Therefore, accurate values of the dipolar coupling cannot be obtained using the TRAPDOR experiment, which should only be used qualitatively. Another interesting feature of the TRAPDOR experiment is its use to estimate XQ values. When the irradiation frequency of the quadrupole nucleus falls outside the first-order quadrupole spectrum, the TRAPDOR effect is zero. Hence, the position of the cut-off frequencies for the TRAPDOR effect gives a direct estimate of v o from which the VQ can be calculated. For a spin I = 5/2 nucleus the cut-off frequencies will be at _+ 2VQ. This has allowed XQ values of up to 15 MHz to be estimated (Grey and Vega 1995, Kao and Grey 1996). The REAPDOR experiment (Gullion 1995, 1995a) can be considered as a variation of the TRAPDOR experiment, in which a train of rotor-synchronised 180 ~ pulses is applied to the spin- 1/2nucleus, spaced half a rotor period apart. The experiment uses an
Experimental Approaches
183
even number of rotor periods. At the centre of the train of 180 ~ pulses (the dipolar evolution period), the 180 ~ pulse is omitted. In the first half of the dipolar evolution period the spins will dephase due to CSA and the heteronuclear dipolar coupling. The second half will refocus the magnetisation and an echo is formed at the end of the dipolar evolution period. A short rf-pulse, the adiabatic passage pulse, is applied for a duration -r to the quadrupolar nucleus at the centre of the dipolar evolution period. The spin states of the quadrupolar nucleus will change (Section 2.5) and hence the value of the dipolar coupling between the quadrupolar nucleus and the spin-1/2 will change. The second half of the dipolar evolution period will still refocus the chemical shift anisotropy but no longer the dipolar dephasing, just as in the REDOR experiment. The optimum duration of the adiabatic passage pulse has been determined to be 1/3 of a rotor period for 14N (Ba et al. 1998). As in the TRAPDOR experiment, attention has to be paid to the adiabaticity of this pulse. An advantage of the REAPDOR experiment over the experimentally less demanding TRAPDOR experiment is that continuous irradiation of the quadrupole nucleus is not required, reducing the chance of probe breakdown. Furthermore, it should be easier to calculate the dephasing effect of the REAPDOR pulse sequence as one only needs to simulate the behaviour of the quadrupole spins with MAS. REAPDOR has been used to examine the location of cations in molecular sieves (Ganapathy and Vega 1998). It should be noted that in both the TRAPDOR and REAPDOR experiments, the rf field on the dephasing quadrupolar nucleus remains on for a considerable time, which can result in a Bloch-Siegert shift (Bloch and Siegert 1940) appearing as a misset in the zero-order phasing of the reduced echo compared to the full echo spectrum.
3.9. TECHNIQUES FOR DETERMINING RELAXATION TIMES AND MOTIONAL PARAMETERS
Since the NMR experiment is essentially tuned to one type of nucleus it is usuaffy a relatively easy matter to distinguish atomic motion of the different constituents of a material. Each will have its own relaxation parameters which will depend on the local environment of the nucleus and which can therefore be investigated separately. The power of NMR for microdynamics studies lies in the wide range of frequencies that can be investigated non-destructively and under many environmental variations, including temperatures from sub-Kelvin to > 2000K.
3.9.1 Measurement of T1 There are several ways of determining T1, the most reliable method probably being saturation-recovery, in which two 90 ~ pulses are applied separated by a time "r of the order of T1. The signal height is then proportional to Mz('r), given by
184
MultinuclearSolid-StateNMR of InorganicMaterials Mz(T)- Mo(1-e-~l I
(3.34)
Repeated measurement of Mz(a') as a function of a" allows T1 to be determined. Mo is the fully relaxed, equilibrium magnetisation, and is normally taken as the measured signal after a time greater than 5TI. The experimental repetition interval can be considerably less than T1 for this method, since the first pulse will set any magnetisation present to zero provided no transverse (e.g. spin-locked) magnetisation causes interference. A safer way to ensure zero Mz is to apply a saturating comb of several 90 ~ pulses (with separation times much less than T1 but longer than T2 to prevent spinlocking), followed by the measurement pulse at time a"after the last 'comb' pulse. This is particularly useful for solids with long T1. The dynamic range of the experiment can be doubled by the inversion-recovery method, in which a 180 ~ inversion pulse is followed by an inspection pulse of 90 ~. The signal is given by
Mz('C)- Moll-2e-~' I
(3.35)
The experiment is repeated with different "r values. The repetition time of the experiment must now allow full recovery of Mz (i.e. "--5T1) so that the experimental time for a full data set is greatly increased. This often more than negates the increased accuracy obtainable from the greater dynamic range produced by inversion. This method is also particularly sensitive to the accuracy of the 180 ~ pulse setting. Both methods normally require a good measurement of Mo; although a fitting procedure (Fukushima and Roeder 1981) using Mangelsdorf plots (Mangelsdorf 1959, Livesey 1979) can make allowance for inadequate precision of the Mo measurement. This is only of use if the system is simple and can be described by a single T1. Multiple exponentials are often observed in complex systems and the saturation recovery method is probably the best approach in this case. Even in relatively simple systems of identical small molecules, non-exponential relaxation is to be expected if more than one type of magnetic nuclide is present. A good approximation to exponential behaviour is often observed over the initial part of the recovery, which depends less on cross-correlation effects. This simplification is valuable in the experimental interpretation of molecular motion.
3.9.2 Otherspin-latticerelaxationtimes (Ttp TtD) Measurement of T~p requires the equilibrium magnetisation to be spin-locked, usually achieved by a 90• ~ pulse followed immediately by a long pulse of duration to along the y-axis provided that H~ > > Hloca~(quite rigorously = HD, HQ etc.). The magnetisation is effectively locked along the rotating B1 field (rotating y-axis) and its decay is
Experimental Approaches
185
observed by inspecting the magnetisation remaining at the end of the spin-lock pulse. The residual magnetisation produces a FID that can be measured as a function of to with My ("C)
-
Moe->~"
(3.36)
If the condition B1 > > Bloc is not obeyed, quantisation is no longer correctly described along this B1 direction. However, it is possible to perform an experiment in which the B 1-field is reduced to zero following the spin-locking sequence. This is adiabatic demagnetisation in the rotating frame (ADRF). The sample can be remagnetised by restoring B1 provided this is done before relaxation occurs. Relaxation during the 'demagnetised' state occurs now in the dipolar field and can be calculated theoretically using a spin-temperature approach. This relaxation time, T1D, may be thought of as the ultimate value of T10 when eo~ -+ 0. An alternative method for T1D measurement employs a Jeener-Broekart pulse sequence which consists of 90x~ 45y ~ 'r 45y ~ pulses (Jeener and Broekart 1967), but the signal strength is at best only about 50 percent of that achieved by ADRF.
3.9.3 Transverse relaxation times (1"2) The transverse relaxation time was discussed in Section 2.8.1. As atoms in a solid acquire greater mobility with, for example, increased temperature, an averaging of the local field seen by any one nucleus occurs, resulting in the phenomenon of motional line-narrowing. The onset of narrowing occurs when there is significant modulation (due for example to molecular reorientation or atomic diffusion) of the rigid lattice local field. This occurs typically at 104 s-1. Below this rate of motion, T2 is constant at its rigid lattice value, but as motion increases and T2 lengthens, the relaxation can be described in terms of the spectral density function J(o~) (Section 2.8) with to = 0. In the well-narrowed region T2 can be written 1
2
- Bmrc
(3.37)
Again, T2 can be used to measure motion. Measurement of T2 in solids is usually made directly from the FID, assuming magnet inhomogeneity effects can be neglected (Eq. 2.192). The solid echo can be used to advantage to produce effectively zero time resolution. Since T2 in solids is rarely exponential, the FID shape should be recorded. The working definition often adopted for T2 is the time for the FID to drop to 1/e of its initial value. Following the onset of motional narrowing, the decay usually becomes exponential and Y2 is more easily defined. A
186
Multinuclear Solid-State NMR of lnorganic Materials
spin-echo sequence (90~ - 180~ ,r echo) is used to remove magnet inhomogeneity decay. If translational diffusion is also present, the echo can suffer additional attenuation at long "r. This can be reduced using the Carr-Purcell (1954) echo sequence (90 ~ -r 180 ~ [a" echo "r 180~ by making n" sufficiently short, since the echo height at t = 2n'r is given by -2 n/r@~ M,,, (2nr) - Moe
-
(3.38)
The Meiboom and Gill (1958) modification (CPMG) to the above sequence prevents accumulation of errors due to mis-setting of the sequence. It simply requires a 90 ~ if-phase shift between the 90 ~ and the 180 ~ pulses. In solids, where motion is usually relatively slow, care must be taken to know whether this sequence is measuring T2 or Tip in an 'average' B1 field, since the CPMG experiment effectively spin-locks the magnetisation. Alternate 180 ~ pulses can be phase-reversed to avoid this effect, but such a strategy also spoils the corrective aspect of the sequence. Phase reversal after every second 180 ~ is much more satisfactory (Suh et al. 1994). 3.9.4 M o l e c u l a r motion
The two distinct types of motion common in materials are reorientation and translation (diffusion). Lattice vibrations are usually too high in frequency and produce too small a field fluctuation to be effective in relaxation unless the nucleus in question has a large electric quadrupole interaction. Reorientational motion is easily distinguished from translational diffusion by its effect on the linewidth (or T2). Since reorientation about one or more molecular axes will only partially average nuclear dipole-dipole interactions, the resulting line-narrowing is also partial, the rigid-lattice value changing by a factor of typically three or four. Translation usually produces extreme narrowing, and, provided the motion is sufficiently rapid, the change in linewidth (or T2) can be several orders of magnitude. The application of relaxation time measurements to study segmental motion (in polymers) as well as diffusional chain motion is very well documented but is still a subject of study, particularly using the frequency dependence of relaxation times to test the detailed predictions of models (McBriety and Packer 1993). The anisotropy of reorientation can also be studied conveniently, and recent interest in motion of molecules on surfaces (e.g. water on porous silica) has been investigated with great success (Gladden 1993). Since the dipolar interaction is usually both intermolecular and intramolecular, the relaxation of spin-l/2 nuclei (e.g. 1H) in the same molecule as a quadrupolar nucleus (e.g. 2H) can permit a complete study of reorientation and translation at a microscopic level (Schmidt-Rohr and Spiess 1994).
Experimental Approaches
187
3.9.5 Diffusion measurements Whereas relaxation measurements are sensitive to microscopic aspects of molecular motion in that % is usually the quantity determined, translational diffusion can be measured on a macroscopic scale by NMR using field gradient methods (Callaghan 1991). When nuclei are placed in a uniform magnetic field they lose phase coherence which can be recovered by a spin-echo (Section 2.8). If, however, diffusion carries nuclei to a different position (Bo value) during the refocussing period, recovery will be incomplete (Eq. 2.194). Pulsed gradients switched on during the time intervals between the 90 ~ and 180 ~ pulses and the 180 ~ pulse-echo period, allow larger gradients to be used and thus enable the study of slower diffusion rates (Stejskal and Tanner 1965). The range of diffusion constant, D, available is typically 10 -6 to 10-12 mZs - 1, but has been extended to even slower diffusion and applied to the study of polymeric systems by Kimmich and co-workers by using the strong static peripheral gradients at the edge of a superconducting solenoid magnet. D values down to 10-14 mZs - 1 have recently been measured by this method, which has a predicted practical limit of 10-16 mZs - 1 ( K i m m i c h
1997).
3.10. NMR UNDER VARYING P H Y S I C A L C O N D I T I O N S
It is usual to vary the conditions in any physical experiment. Since the earliest days of NMR spectroscopy, the sample temperature has been varied in order to gain information about the activation energy of motional processes and for the in situ study of materials processing. The effect of varying the sample pressure is now also being more widely studied by NMR.
3.10.1 Variable temperature NMR Most commercial spectrometers provide variable temperature facilities that operate from --~120 to 400 K using a cooled/heated gas flow passing over the sample and rf coil assembly, and contained in a thermally insulated environment. A more efficient and stable low temperature probe assembly uses heat conduction to a cooled heat bath or direct injection of liquid cryogen. Stable temperatures down to - 80 K can be achieved using liquid nitrogen. Lower temperatures require liquid helium which has a relatively low latent heat and is expensive, with the additional problem that it has a relatively low breakdown voltage. A variety of designs are available for different experimental applications. NMR equipment for use in the range 1 to 120 K has been reviewed by Conradi (1993). A gas-flow technique is usually favoured but must be combined with elaborate dewar vessel provision, radiation shielding and careful attention to heat leaks
188
Multinuclear Solid-State NMR of Inorganic Materials
via the electrical connection to rf coils etc. Samples are usually remote, probes long, and sample exchange is relatively difficult. Very useful practical limits on cryostat design to overcome effects such as Taconis oscillations and acoustic ringing are also discussed by Conradi (1993). While low temperatures increase the signal strength (due to Boltzmann factor) and reduce thermal noise, the opposite is true for elevated temperatures. The construction of probes for high temperatures, (i.e. 500 to 2000 K) requires special attention to the maintenance of a reasonable Q value for the tuned NMR sample coil, and to the thermal problems of protecting the magnet and enabling it to provide a stable and homogeneous magnetic field protected from the high temperatures within it. High temperature probes are usually contained in a heated furnace rather than using a gas-flow system, which becomes very inefficient above about 500~ but has been used successfully with an MAS probe to about 700~ Examples of various methods have been discussed in a comprehensive review giving examples of applications to systems such as coals, oxides, ionic conductors, phase transitions, molten salts, metals and semi-metals (Stebbins 1991). The probe structure is constrained by the requirement to fit within a magnet, to transfer negligible heat to the magnet, and to provide sufficient internal space around the coil to avoid undue rf damping of the resonant circuit. Designs usually employ resistive heating with the heater surrounding the chamber holding the NMR coil. Care has to be taken that the heater coil does not introduce too much noise nor produce an additional magnetic field. The heater coil must be wound non-inductively. Other designs have used a compact water-cooled furnace that fits in the NMR coil (Adler et al. 1990). Variable temperature ~70 NMR studies have included motion in ionic conductors (Adler et al. 1990), in situ observation of sol-gel processing (Poplett et al. 2000) and dehydroxylation (Figure 3.29). Some probe designs have replaced resistive and gas flow heating by optical heating. A probe using bulbs shone on to the sample capable of reaching 1300K has been described (Poplett et al. 2000). Even higher temperature designs using laser heating now appear feasible. Two alternative designs for use in different temperature ranges are being examined. A slightly modified MAS probehead with laser heating can be used up to 600~ (Coutures et al. 1990). For extreme temperatures where ceramics such as alumina could be melted (> 2000~ the system used is based on containerless levitation, with the sample in a split-gap resonator rather than a conventional coil (Florian et al. 1995). Probe design for heating samples is becoming ever more sophisticated, with the advent of specialised equipment such as a probe for the in situ observation of catalysis reactions in flowing reactants, gases, etc., held at temperature for long periods. MAS probes capable of carrying out such flow experiments at temperature now exist (see Carlson et al. 2000, and references therein). In all cases, accurate temperature calibration is best carried out directly on an NMR sample which shows a sharp phase transition and can readily be monitored (Table 3.10).
189
Experimental Approaches
500
400
300
250
110
Room
i,,,,I,,,,l,,,,l,,,ll,,,~l',,I
3000
2000
1000
0
-1000
I
- 2 0 0 0 -3000
ppm Figure 3.29. In situ dehydroxylation of Mg(OH)2 from room temperature to 500~ followed by 170 NMR. Table 3.10. Solid-solid phase transitions useful for calibrating NMR probes. System
Transitiontemperature (K)
Reference
13C in squaric acid 13Cin d-camphor 13Cin CBr4 23Na in LiNaSO4 31p in P453 87Rb in RbNO3
373.2 238 320 791 314 437
Klymachov & Dalal (1996) Haw et al. (1986) Van Moorsel et al. (1995) Massiot et al. (1990a) Van Moorsel et al. (1995) Van Moorsel et al. (1995)
3.10.2 High pressure experiments The pioneering work of Benedek and Purcell (1954) on the observation and measurement of NMR parameters as a function of pressure has opened up the possibility of obtaining structural, dynamic and kinetic information about materials by NMR. Two reviews have reported extensively on the experimental techniques available for NMR
190
Multinuclear Solid-State NMR of Inorganic Materials
at pressure (Jonas 1972, Horvath and Miller 1991). Much less attention has been paid to the solid state, although NMR has successfully been used to measure activation volumes for self-diffusion (Ross and Strange 1978), studies of structure (Bertani et al. 1992) and rotational phase changes (Mackowak and Brown 1983). The diamond anvil method has been used for very high pressures, up to 100 kbar (Lee et al. 1987, Bertani et al. 1992). This specialised technique can only be used with very small samples, a considerable disadvantage for NMR which is inherently insensitive.
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MacKenzie, K.J.D., Smith, M.E., Schmficker, M., Schneider, H., Angerer, P., Gan, Z., Anup~ld, T., Reinhold, A. & Samoson, A. (2001) Phys. Chem. Chem. Phys., 3, 2137. Mackowak, M. & Brown, R.J.C. (1983) J. Mag. Reson., 52, 71. Man, P.P. (1993) Appl. Mag. Res., 4, 65. Man, P.P. (1995) Phys. Rev. B, 52, 9418. Man, P.P. (1998) Phys. Rev. B, 58, 2764. Man, P.P. (2000) in Encyclopedia of Analytical Chemistry, Ed. Meyers, R.A., John Wiley & Sons, Chichester, p. 12224. Mangelsdorf, P.C. (1959) J. Appl. Phys., 30, 442. Marian, D. & Wutrich, K. (1983) Biochem. Biophys. Commun., 113, 967. Maricq, M.M. & Waugh, J.S. (1979) J. Chem. Phys., 70, 3300. Massiot, D., Bessada, C., Coutures, J.P. & Taulelle, F. (1990) J. Mag. Resort., 90, 231. Massiot, D., Bessada, C., Echegut, P., Coutures, J.P. & Taulelle, F. (1990a) Solid State Ionics, 37, 223. Massiot, D., Thiele, H. & Germanus, A. (1994) Bruker Report, 140, 43. Massiot, D., Touzo, B., Trumeau, D., Coutures, J.P., Virlet, J., Florian, P. & Grandinetti, P. (1996) Solid State Nucl. Mag. Reson., 6, 73. Massiot, D. (1996) J. Mag. Reson. A, 122, 240. Massiot, D., Trumeau, D., Touzo, B., Farnan, I., Rifflet, J.C., Douy, A. & Coutures, J.P. (1995) J. Phys. Chem., 99, 16455. Massiot, D., Montouillout, V., Fayon, F., Florian, P. & Bessada, C. (1997) Chem. Phys. Lett., 272, 295. McBriety, V.J. & Packer, K.J. (1993) NMR in Solid Polymers, Cambridge University Press, Cambridge. Meiboom, S. & Gill, D. (1958) Rev. Sci. Instrum., 29, 688. Merrit, M.E., Goetz, J.M., Whitney, D., Chang, C.C.P., Heux, L., Halary, J.L. & Schaefer, J. (1998) Macromol., 31, 1214. Merwin, L.H., Sebald, A., Espidel, J.E. & Harris, R.K. (1989) J. Mag. Reson., 84, 367. Merwin, L.H. & Sebald, A. (1990) J. Mag. Reson., 88, 167. Merwin, L.H. & Sebald, A. (1992) J. Mag. Reson., 97, 628. Merwin, L.H. & Sebald, A. (1992a) Solid State Nucl. Mag. Reson., 1, 45. Mueller, K.T. (1995) J. Mag. Reson. A, 113, 81. Mueller, K.T., Jarvie, T.P., Aurentz, D.J. & Roberts, B.W. (1996) Chem. Phys. Lett., 254, 281. Munowitz, M., Griffin, R.G., Bodenhausen, G. & Huang, T.H. (1981) J. Amer. Chem. Soc., 103, 2529. Munowitz, M. & Griffin, R.G. (1982) J. Chem. Phys., 76, 2848. Murphy, P.D. (1983) J. Mag. Reson., 52, 343. Mildner, T., Ernst, H. & Freude, D. (1995) Solid State Nucl. Mag. Reson., g, 269. Mildner, T., Smith, M.E. & Dupree, R. (1999) Chem. Phys. Lett., 301,389. Mildner, T., Smith, M.E. & Dupree, R. (1999a) Chem. Phys. Lett., 306, 297. Morris, H.D., Bank, S. & Ellis, P.D. (1990) J. Phys. Chem., 88, 6135. Mortuza, M.G., Dupree, R. & Kohn, S.C. (1993) Appl. Mag. Reson., 4, 89. Nakai, T., Terao, T. & Shirakawa, H. (1988) Chem. Phys. Lett., 145, 90. Nielsen, N.C., Bilds0e, H. & Jakobsen, H.J. (1992) J. Mag. Reson., 97, 149. Opella, S.J. & Frey, D.M.H. (1979) J. Amer. Chem. Soc., 101, 5855.
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Multinuclear Solid-State NMR of Inorganic Materials
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This Page Intentionally Left Blank
Chapter 4
zgsi NMR 4.1.
4.2.
4.3. 4.4. 4.5. 4.6.
4.7. 4.8.
4.9.
4.10. 4.11.
General Considerations 4.1.1 Broadening Effects in 298i Spectra 4.1.2 Relaxation Effects in 298i Spectra 4.1.3 Effect of Structure on 298i Spectra Si-O Compounds 4.2.1 Relationships between 298i NMR Spectra and Structure/Bonding 4.2.2 Four-Coordinated Si-O-Compounds 4.2.3 Tetrahedral 298i Chemical Shifts in Silicates 4.2.4 298i Chemical Shifts in Aluminosilicates 4.2.5 Effects of Other Nearest Neighbours on the 29Si Shift Order-Disorder Effects in Minerals Identification of Silicate Minerals Thermal Decomposition of Silicate Minerals Relationships between 298i Chemical Shift (d) and Structure 4.6.1 Relationships between 6 and the Si-O Bond Length 4.6.2 Relationships between 6 and the Si-O-Si Bond Angle 4.6.3 More Complex Relationships between 6 and the Structure Five and Six-Coordinated Si-O Compounds Cross-Polarisation (CPMAS) Experiments 4.8.1 Cross-Polarisation between 1H and 298i 4.8.2 Cross-Polarisation between 19F and 298i 4.8.3 Other Cross-Polarisation Experiments with 29Si Glasses, Gels and Other Amorphous Materials 4.9.1 Silicate Glasses 4.9.2 Deconvolution of 298i NMR Spectra 4.9.3 Connectivities in Glass 4.9.4 Chalcogenide Glasses 4.9.5 Gels 4.9.6 Other Amorphous Materials S i-N and S i-N-O Compounds Si-A1-O-N Compounds 4.11.1 ]3-Sialon, Si6_zAlzOzNg_z 4.11.2 O-Sialon, Si2_xAlxOl+xNz_x
201 201 202 204 205 205 205 205 206 208 208 212 214 217 218 219 223 225 227 227 229 229 230 231 235 236 238 240 242 244 247 247 250
4.11.3 X-Sialon, nominally Si12Al18039N8 4.11.4 Polytypoid Sialons, (Si,A1)m(O,N)m+l 4.11.5 oL-Sialons, MxSil2_(m+n)Alm+nOnN16-n 4.12. Other Metal Silicon Nitrides and Oxynitrides 4.13. Si-C, Si-C-O and Si-C-N Compounds 4.13.1 Silicon Oxycarbide Species 4.13.2 Silicon Carbonitride Species 4.14. Other Materials 4.14.1 Biologically Compatible Glasses 4.14.2 Cements 4.14.3 Inorganic Polymers References
251 253 253 253 255 256 257 257 257 257 259 260
Chapter 4
29Si NMR 4.1. GENERAL CONSIDERATIONS
Silicon is the most abundant element in the earth's crust excluding oxygen (at 26% it is about 3.5 times as plentiful as the next most abundant element, aluminium). It is therefore fortunate for experimental mineralogy, geochemistry, ceramics and inorganic materials generally that 29Si is a nuclide from which useful NMR spectra can readily be obtained. The growth in the development and application of solid state NMR spectroscopy in materials science owes much to the success of the technique with this ubiquitous element.
4.1.1 Broadening effects in 29Si spectra 29Si has a spin 1=1/2, which means it is not subject to quadrupolar peak broadening and distortion. Despite its relatively low natural abundance (4.7%), the spectral resolution of 29Si is high due to its relatively narrow resonance lines. It is, however, subject to the two principal sources of broadening in a spin I= 1/2 nucleus (Chapter 2). These are: (1) Dipole-dipole interactions, in which the magnetic dipoles of the neighbouring nuclei interact with the nucleus under investigation. Since this effect is orientationdependent, and since a powder sample contains all possible orientations, this can be a source of significant broadening. The dipole-dipole interaction is described by an expression containing a term in 3cos20 - 1 which is averaged to zero by magicangle spinning (MAS). The MAS speed must be greater than or equal to the static (unspun) linewidth in Hertz but, because in 29Si compounds this linewidth is narrow, useful spectra can be obtained without having to resort to high magnetic fields and spinning speeds. Typically a spinning speed of about 4 kHz in a field of 4.7 T is adequate for the acquisition of useful 29Si MAS NMR spectra. (2) Chemical shift anisotropy. The nucleus is shielded by the surrounding electrons which give rise to the chemical shift. Since this shielding and its resulting chemical shift is orientation-dependent it causes broadening in a powder sample. The chemical shift anisotropy can be described in terms of the shielding along the three symmetry axes at the nucleus in question. Magic angle spinning averages the chemical shift to a single isotropic value, significantly narrowing the resulting lines even when the spinning speed is less than the static linewidth. 201
202
Multinuclear Solid-State NMR of lnorganic Materials
4.1.2 Relaxation effects in 29Si spectra The relaxation time of e9si can vary enormously; in natural materials containing paramagnetic impurities such as Fe, recycle delays of only a few seconds can be used, whereas some SiC polytypes may take several hours to relax (Hartman et al. 1994), making the acquisition of reasonable SiC spectra a very lengthy process. The 295i relaxation time of pure compounds can be shortened by the deliberate addition of small amounts of paramagnetic ions such as the rare earths. The most effective additive for relaxing both 295i and 89y in yttrium silicon oxynitride compounds was found to be Eu 3+ added at concentrations < 360 txmol g-1 (Meinhold and MacKenzie 1995). Higher concentrations of paramagnetic ions, such as the Fe often present in natural material, caused an increasing amount of the signal intensity to be transferred to the spinning side bands, which grow at the expense of the central peaks, making the spectra unusable in the worst cases. Figure 4.1 illustrates the effect of increasing Fe content on the spectra of two natural muscovite mica samples, both of which contained sufficient Fe to permit a complementary study to be made by 57Fe Mossbauer spectroscopy (MacKenzie et al. 1987). The limiting Fe concentration at which 295i NMR spectroscopy becomes impossible varies somewhat with the structure of the material since this determines the proximity and disposition of the Fe and the S i atoms. As a rule-of-thumb, Fe concentrations greater than about 5% (expressed as FeeO3) are likely to make 295i spectroscopy marginal, since in addition to decreasing the relaxation time, the presence of paramagnetic species also
A
B
4.6% FezO3.
t
100
t
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I
-100
,
1
2.4% Fe203
,
t
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I
-300 100
,
I
-86
|
-lOO
|
|
,
t
-3o0
zgSi shift (ppm) w.r.t. T M S
Figure 4.1. 295i MAS NMR spectra of two natural muscovite micas illustrating the effect of the Fe203 content on the broadness of the central transition and the size of the spinning side bands (indicated by asterisks). A. Muscovite from Stewart Island, New Zealand, B. Muscovite from North Carolina, U.S.A. Adapted from MacKenzie et al. (1987).
29Si NMR
203
125 Of
100
" 14
cD
7s
9
0
~
~
12 ,~
50
25 J e~ 0
0
0.2
0.4
0.6
-~10 0.8
MnO content (mol%) Figure 4.2. Opposing effects on the 298i relaxation time and peak width of adding paramagnetic MnO to sodium disilicate glass, from the data of Mortuza (1989).
broadens the 29Si spectra. This is illustrated by the addition of Mn 2+ to Na2Si206 glass (Figure 4.2), in which the addition of up to 0.8 mol % MnO decreases the relaxation time from 112 to 1.8 s, but increases the resonance width from 10.5 to 15 ppm (Mortuza 1989). Thus, the beneficial effect of deliberately added paramagnetic species on the relaxation rate must be traded off against spectral broadening. The enhancement of the relaxation process in natural hydrated silicate minerals by paramagnetic species such as Fe may be due to their facilitating the interaction between the 29Si nucleus and the electron spin as a result of the relatively high mobility of the hydrated paramagnetic species. A similar effect has been reported in zeolites (Cookson and Smith 1985, Klinowski et al. 1986), in which the relaxation rate is significantly shortened by the presence of paramagnetic oxygen molecules. The increase in intensity of the spinning sidebands with increasing Fe content (Figure 4.1) can be due to chemical shift anisotropy, but in the case of some natural minerals, it appears to be due instead to magnetic susceptibility broadening arising from the presence of magnetic impurities such as Fe304 (Oldfield et al. 1983). An important practical implication of the wide differences in 298i relaxation rates arises from the fact that in the same sample, various Si units may have quite widely differing relaxation rates. If the recycle delay time chosen for the experiment is not sufficient to allow full relaxation of all the Si species, those with the longer relaxation times will be under-represented in the resulting spectra. These possibilities must be kept in mind, especially when using 29Si NMR spectroscopy to make estimates of the relative abundance of the various Si species present in a sample.
204
Multinuclear Solid-State NMR of lnorganic Materials
4.1.3 E f f e c t o f structure on 29Si spectra 29Si NMR spectroscopy provides direct information about the structure of materials
from measurements of the isotropic chemical shifts. This parameter is influenced most significantly by the coordination number of the Si, which in compounds coordinated by oxygen varies by about 50 ppm in going from Si(IV) to Si(V) to Si(VI) (Table 4.1). The 29Si isotropic chemical shift can also undergo changes of similar magnitude when the element in the first coordination sphere is changed, for example from Si-O to Si-N to Si-C (Table 4.1). Changes in the next-nearest neighbour produce smaller but readily measurable changes in the 29Si chemical shift, typically of the order of l0 ppm when the connectivity changes from Q2 to Q3 to Q4 units in Na2Si206 glass (Table 4.1). A change of similar magnitude is brought about when the next-nearest neighbour element is changed; the magnitude of this effect depends on the chemical nature of the substituted element. Even smaller but still measurable changes (typically ---2 ppm) in the 29Si chemical shifts result from the presence of crystallographic distortions in Si environments which are otherwise similar, for example, the Q4(3A1) site in the mineral natrolite ( - 87.7 ppm) which beomes - 86.3 and - 89.1 in the distorted sites of scolecite (Table 4.1). Thus, the 295i chemical shifts can provide information about a considerable range of perturbations in the Si environment.
Table 4.1. Influence of structure on the isotropic chemical shift of 29Si Effect
Typical magnitude
Example
Coordination number SiOx
----50 ppm for Ax -- 1
SiO4 - 110ppm
---) SiO5 ---) SiO6 - 150 ppm -200 ppm
Nearest neighbour
Depends on element
SiO4 - 110ppm
----) SiN4 ~ SiC4 - 49 ppm - 18 ppm
Next-nearest neighbour (nnn) (i) Connectivity (Q")
--~10 ppm for An = 1
Q2 - 78 ppm
Na2Si206 glass* ~ Q3 ___.) Q4 - 88 ppm - 100 ppm
(ii) Element
Depends on element
thompsonite ~ natrolite** nnn nnn nan 4AI --->3A1 + Si 2AI + 2Si -83.5 ppm -87.7 ppm -95.4 ppm [Q4(4A1) ~ QZ(3A1) + Q4(2A1)]
Crystallographic distortion
Depends on distortion, typically ---2 ppm
Qa(3A1) site in: natrolite ~ scolecite -87.7 ppm -86.3 and -89.1 ppm
* Dupreeet al. (1984) **Lippmaaet al. (1981)
29Si NMR
205
4.2. Si-O COMPOUNDS
4.2.1 Relationships between 29Si NMR spectra and structure~bonding Si in inorganic materials is most commonly bonded to oxygen. Although it prefers fourfold coordination, five-fold and six-fold coordination is not unknown in some glasses and materials produced at high pressures (Stebbins and Poe 1999). The 29Si chemical shifts of Si-O units are sensitive to the coordination number, those of Si(IV) units occurring in the range - 6 0 to - 1 2 0 ppm (with respect to TMS). The shifts of Si(V) and Si(VI) occur at about - 1 5 0 and - 1 8 0 to - 1 9 0 ppm respectively. This is illustrated in Figure 4.3 by an unusual spectrum of a high-pressure triclinic modification of crystalline CaSi2Os, in which all three coordination states are present (Stebbins and Poe 1999).
4.2.2 Four-coordinated Si-O compounds Small variations in the local environment of silicon in tetrahedral Si-O sites of a sample can give rise to broad, somewhat featureless spectra, as in glassy or amorphous SiO2 (Oestrike et al. 1987) (Figure 4.4A). On the other hand, in some circumstances where the various tetrahedral Si sites are crystallographically well-defined, they can be resolved by 29Si NMR spectroscopy, as in the spectrum of the siliceous zeolite ZSM-5 (Figure 4.4B), in which 21 of the 24 crystallographically distinct sites can be resolved (Fyfe et al. 1987). However, most solid-state 29Si spectra fall between these two extremes.
4.2.3 Tetrahedra129Si chemical shifts in silicates The chemical shifts of 29Si spectra are commonly quoted with respect to tetramethylsilane (TMS). Systematic variations of tetrahedral Si chemical shifts with structure are well documented, and can be used for "fingerprinting" (identification of a silicate species ~
,
a
.
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.
.
.
.
r
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!
,
.
.
J
!
,
a
9
-50 -100 -150 -200 29Si shift (ppm) w.r.t. TMS Figure 4.3. 295iMAS NMR spectrum of the high-pressure crystalline triclinic phase of CaSi2Os, showing resonances corresponding to 4, 5 and 6-coordinated Si-O. From Stebbins and Poe (1999), by permission of the American Geophysical Union.
206
Multinuclear Solid-State NMR of Inorganic Materials A
IP
-60
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~
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I
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'
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29Si shift (ppm) w.r.t. T M S
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-1 10' ' '
-1 14 '
'
' -118 '
29Si shift (ppm) w.r.t. T M S
Figure 4.4. Extremes in the form of 298i MAS NMR spectra of tetrahedral Si-O compounds. A. SiO2 glass, showing a single broad composite resonance envelope, B. Highly siliceous zeolite ZSM-5, showing resolution of 21 of the tetrahedral Si sites, from Fyfe et al. (1987), by permission of MacMillan Magazines Ltd. Note the difference in the chemical shift scale between the two spectra. or, more usually, the class of structural unit present). Silicate structures may be regarded as being built up of tetrahedral units with varying degrees of polymerisation. These can be described in terms of a "Q" notation, where Q denotes a silicon bonded to four oxygen atoms. A superscript n, where n = 0 to 4 is used to indicate the number of other Q units attached to the unit in question. Thus, QO denotes a silicon bonded through oxygen to no other network-forming elements, whereas Q4 denotes a silicon bonded through oxygen to four other silicons. The 298i chemical shift becomes increasingly negative with each additional Si-O-Si linkage, due to increased electronic shielding of the central Si. QO units in monosilicates have typical shifts of about - 6 5 ppm, changing in steps of about 10 ppm for each additional bonded Si tetrahedron, up to about - 110 ppm for the Q4 units of fully polymerised silica polymorphs (as in quartz or cristobalite). This is shown schematically in Figure 4.5, which also illustrates the degree of overlap occurring between the chemical shifts of each of these groups. This overlap can introduce a degree of ambiguity into the assignments made on this basis alone. The influence of the next-nearest neighbour atoms must also be taken into account, as discussed below for the aluminosilicates, a large group of compounds in which some of the next-nearest neighbours are A1.
4.2.4 29Si chemical shifts in aluminosilicates The aluminosilicates constitute an important class of inorganic compounds. The occurrence of aluminium atoms in the second coordination sphere of the silicon, to which they are bonded through oxygen atoms, produce systematic changes in the 295i chemical shift in a similar way to the changes which are associated with differences in the
207
29Si NMR -
Si ( 4 Z n )
= Si (3Zn)
- - - si (2Zn) Si ( 1 Z n ) -------
Si ( 0 Z n )
Si ( 4 T i )
-
si (3Ti) -- Si (2Ti) ---
Si (1Ti) Si (0Ti)
si (4Al) ----
Si (3A1)
---- Si (2A1) ----
Si (1AI)
.......
Si ( 0 A l )
QO Q1 Q2
Q~ Q4 SiWO
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= Si ~
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-160
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-180
,,
t
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-200
29Si shift (ppm) w.r.t. TMS Figure 4.5. Schematic representation of the range of 298i chemical shifts in Si-O compounds of differing coordination and with different nearest-neighbour atoms.
SiO4 polymerisation. In general, the substitution by A1 of each of the four silicons surrounding the central Si of a Q4 unit results in a change in the 298i chemical shift of about 5 ppm towards less negative values. Thus, the 29Si chemical shift range of a Q4 unit with no bonded A1 atoms (denoted Q4(0A1)) is about - 102 to - 116 ppm; this becomes about - 97 to - 107 ppm for Q4(1A1), - 9 2 to - 100 ppm for Q4(2A1), - 85 to - 94 ppm for Q4(3A1) to about - 82 to - 92 ppm for Q4(4A1). As can be seen from the range of these values, shown schematically in Figure 4.5, the shifts overlap somewhat but can be used as a guide to the degree of Al-for-Si substitution, which in turn can provide information about the disordering of an aluminosilicate framework.
208
Multinuclear Solid-State NMR of lnorganic Materials
4.2.5 Effects of other nearest neighbours o n t h e 29Si shift The effect of other nearest neighbour atoms on the 29Si shifts of silicates has been systematically studied in a few systems. Balmer et al. (1997) have reported the effect in titanosilicates, finding that the shift increases with increasing oxygen formal charge in a similar manner to the aluminosilicates. The results, based on observations for a number of crystalline titanosilicates of known structure, are shown in Figure 4.5. The 29Si chemical shifts of titanosilicates were also reported by Labouriau et al. (1998). Caution should however be exercised in making comparisons with other Ti-containing systems, as the effect of the Ti depends on whether it is playing a network-forming or network-modifying role. This is illustrated by the ab initio calculations of Ricchiardi and Sauer (1999) for the substitution of Si by Ti in silicalite indicating such substitutions result in a very small effect of only about 1 ppm on the 29Si chemical shift. A chemical shift relationship has been established for the effect of zinc nearest neighbours in a series of tectozincosilicates (Camblor and Davies 1994). These results are also plotted in Figure 4.5. Studies on crystalline gallosilicate molecular sieves with the beta structure (Occelli et al. 1999) have led to the suggestion that substitution of A1 by Ga in these structures leads to a change of 3 ppm towards higher frequencies, but a systematic determination of the range of shifts in gallosilicate compounds has yet to be made. A series of lead silicate glasses and their crystalline equivalents has revealed a linear relationship between the number of Pb atoms in the second coordination sphere of the structural Si tetrahedra (p) and the isotropic 29Si chemical shift (Bessada et al. 1994), defined by: 6iso --
--
106.005 + 5.537p
(4.1)
In a 2 9 S i NMR study of microporous niobium silicate catalysts, Rocha et al. (1998) tentatively identified resonances at - 95.6 ppm as Si(2Si,2Nb) or Si(3Si, lNb), at - 105.5, - 1 0 7 and - 1 0 8 ppm as Si(3Si, lNb) or Si(4Si,0Nb) and at - 1 1 1 as Si(4Si,0Nb) groups. These assignments were made by analogy with aluminosilicates, and not based on measurements of crystalline niobium silicates of known structure.
4.3. ORDER-DISORDER EFFECTS IN MINERALS
If the various aluminosilicate units present in a mineral can be resolved sufficiently for their relative populations to be simulated, the Al-for-Si disorder can in principle be determined. In practice, the resonances from the various structural units often overlap, making it necessary to deconvolute the broad spectral envelope by curve-fitting procedures. Gaussian peaks are most commonly fitted, since they allow good visual fits to
29Si N M R
209
be achieved, are readily calculated and can be justified in terms of a random distribution of parameters. The curve-fitting process must, however, be carried out with care, bearing in mind the possibility that the various structural parameters may not necessarily combine to give a Gaussian distribution of NMR lineshapes (see section 4.9.2). All five of the Q4(nA1) aluminosilicate units can be identified in the 298i NMR spectrum of the zeolite mineral ultramarine (Na7.sSi6A1602484.5) (Klinowski et al. 1987) (Figure 4.6). In this and similar studies, the information on the Si site occupation is extracted from the spectrum by computer-fitting the component peaks followed by integration of their area. The ultramarine spectrum was simulated by fitting five Gaussian peaks which provided an estimate of the relative numbers of each type of unit. This A1 distribution was then compared with the distributions calculated for a number of structures of varying Al-for-Si disorder. The results show this synthetic ultramarine to be completely disordered, thus apparently disobeying Loewenstein's aluminium avoidance principle that in aluminosilicates, links between A104 tetrahedra are rare or absent, and A1-O-A1 bonds should not be found (Lowenstein 1953). These NMR results have been taken to suggest that Lowenstein' s Rule may only apply to crystalline compounds formed under equilibrium conditions (Klinowski et al. 1987). Determination of the distribution of 298i over the various aluminosilicate units by curve fitting provides an accurate and convenient method of obtaining the Si/A1 ratio of the aluminosilicate framework by applying Loewenstein's Rule. This procedure has been used in conjunction with 27A1 NMR in a structural study of NaY zeolite stabilised
-92.5 -97.6 observed
/ V ~/~
s.,4 yi i! ! ate, ..........J,/
componen--ts ...........
:
;il '~
. . . . . .
"
-80 -100 -1 0 29Si shift (ppm) w.r.t. TMS Figure 4.6. Observed and simulated 298i spectra of the synthetic aluminosilicate mineral pink ultramarine, including fitted peaks, showing resolution of all possible Si(nA1) units. From Klinowski et al. (1987), by permission of MacMillan Magazines Ltd.
210
Multinuclear Solid-State N M R o f Inorganic Materials
by lanthanum (van Bokhoven et al. 2000), allowing the framework and non-framework components to be quantitatively distinguished. This procedure has also been used to study the effect of annealing time on the ordering of synthetic cordierite (Putnis et al. 1985). Eight distinguishable tetrahedral sites were observed in the most disordered form, but only two in the most ordered form, allowing changes in the site environments to be determined as a function of annealing time. The ordering data thus deduced from the 298i NMR were also used to provide information on the energetics of the Si-A1 interchange in these cordierites (Putnis and Angel 1985). The 298i NMR spectrum of ordered [3-eucryptite (LiA1SiO4) contains one resonance, but samples crystallised from a glass have been found to contain additional 298i peaks indicating a significant level of short-range Si,A1 disorder which decreases exponentially with annealing time at 1173K (Phillips et al. 2000). 298i has also been used to study ordering effects in a number of other minerals, including albite and oligoclase (Yang et al. 1986), Mg-Si garnets (Phillips et al. 1992), solid solutions of pyrope and grossular garnets (Bosenick et al. 1999), [3-eucryptite (Phillips et al. 1999), gallium fluor-amphiboles (Sherriff et al. 1999), fibrolitic sillimanite (Stebbins et al. 1993), calcic and sodic-calcic amphiboles (Welch et al. 1998), a synthetic leucite analogue (Kohn et al. 1991, Kohn et al. 1995) and in a series of cation-exchanged analcite and leucite minerals (Kohn et al. 1997). The 2-dimensional 298i COSY spectrum of a K-Mg leucite was obtained in an attempt to determine the connectivities between the various Si sites, but this system was too complex for an unambiguous assignment of sites in the structure (Dupree 1991). 298i measurements have been used to establish the Si distribution over the tetrahedral sites in the mica minerals muscovite, phlogopite, vermiculite and margarite. These results indicate that the Si-A1 configuration of the tetrahedral sheets of these minerals is governed by (a) Loewenstein's Rule, which excludes neighbouring A1 tetrahedra, and (b) by the need for local charge compensation, which requires adequate dispersion of A1 over the structure (Herrero et al. 1985). Where the 298i spectra are poorly resolved, as in the case of a series of natural illite-smectite clays (Lausen et al. 1999), a number of equally good computer fits to the spectrum are possible, but strategies have been devised for identifying the most likely fit on the basis of correlations established from well-resolved phyllosilicate spectra between the 298i shift and the A1 substitution (Lausen et al. 1999). An elegant example of the use of advanced curve-fitting procedures to extract structural data from 298i spectra is provided by a study of the various crystal modifications of the silica polymorph tridymite (Kitchin et al. 1996). The room-temperature monoclinic form has 12 Si sites, and the spectrum can be simulated by fitting 12 peaks of equal area (Figure 4.7A). At higher temperatures, the mineral progressively transforms through three orthorhombic forms to a high-temperature hexagonal form. The broad 298i spectrum of the lowest-temperature orthorhombic form (Figure 4.7B) contains the
29Si NMR
211
observed
simulated
A
_ .
..
,
|
-108
|
-
J
.
-112
9
i
_J
-108
. . . . .
!
-112
,
I
,
, ,
-106
|
_L__..a_.__s~
-112
-118
-116
~
_
.
.
.
.
.
.
.
.
.
.
.
.
; ' . . - ' . . . ' - - , . , " . . . . . . . . e.-,,,--..:..-. . . . . . . .
t
-116
-110
-114
-118
A _ .
-108
__
~
,
-112
. . . . . . . .
-116
29Sishift(ppm)w.r.t. TMS
i52-"-"
-112
-" i . . . . . . "
-113
" , "",..
-114
-115
29Si shift (ppm) w.r.t. TMS
Figure 4.7. Observed and curve-fitted 298i spectra of three crystalline forms of the silica polymorph tridymite. A. Room-temperature ordered monoclinic form, showing resolution of nine of the twelve Si sites, fitted to 12 pseudo-Voight lines of equal area with a Gaussian:Lorentzian ratio of 0.3. B. Orthorhombic form at 142~ fitted to six lines broadened by an incommensurate plane-wave modulation. C. Orthorhombic form at 202~ fitted to a single line simulated with a non-linear incommensurate modulation. Adapted from Kitchin et al. (1996), by permission of the Mineralogical Society of America. overlapping resonances from at least six sites, which, however, can only be satisfactorily fitted by a model which takes into account the effect of incommensurate structural modulation. A similar fitting procedure is necessary to accommodate the shape of the next orthorhombic phase to be formed, but the two highest-temperature phases can be fitted with a single narrow resonance, consistent with their expected single Si site (Kitchin et al. 1996). The availability of NMR probes capable of operating at elevated temperatures up to 600-700~ has opened up the possibility of studying temperature-induced processes in minerals. The thermal changes in analcime have been studied in situ by multinuclear NMR including 298i (Kim and Kirkpatrick 1998). Pre-melting processes in lithium and sodium metasilicates have also been studied by 29Si NMR at temperature (George et al. 1998).
Multinuclear Solid-State NMR of lnorganic Materials
212
4.4. IDENTIFICATION OF SILICATE MINERALS By their nature, silicate minerals contain Si in a huge range of environments and variety of structural units. C o m p o u n d s containing structural units corresponding to those found in various minerals are frequently encountered in inorganic materials, either in their own right or as products or intermediates of inorganic syntheses. Because of the high resolution of 29Si N M R spectroscopy and the sensitivity of its isotropic chemical shift to local variations in the atomic environment,
29Si M A S
N M R can be correlated
with the structural units occurring in the various classes of silicate mineral structures. Thus, the orthosilicates (containing the silicate tetrahedra in isolated m o n o m e r i c units or anions) have shifts ranging from about - 6 0 to - 8 6 ppm, the sorosilicates (tetrahedra in linearly connected trimeric structures) have shifts ranging from about - 7 2 to - 9 5 ppm, the inosilicates (in which the tetrahedra are connected in infinite chains) have shifts from about - 8 2
to - 9 2
ppm, the phyllosilicates (containing the
tetrahedra connected to form sheets of two-dimensional single layers) have shifts from about - 7 6
to - 9 7 p p m and the tectosilicates (containing the tetrahedra linked in
infinite three-dimensional frameworks) have shifts of about - 8 3
29Si M A S
to - 114 ppm. The
N M R spectra of a large n u m b e r of silicate minerals have now been reported,
for which a selection of chemical shifts are given in Table 4.2. Since natural minerals have variable compositions and impurity contents, the actual values can vary somewhat; the chemical shift values of Table 4.2 have therefore been rounded, and should be regarded as a guide only.
Table 4.2. Typical 29Si chemical shifts reported for a selection of silicate minerals. Shifts in ppm, quoted with respect to TMS, rounded to nearest whole number. Mineral
29Si shift
Orthosilicates andalusite - 80 datolite - 83 kyanite - 82,- 83 mullite - 86,- 90,- 94 phenacite - 84 sillimanite - 86 topaz - 86
Ref.
Mineral
a b c e a c a
chondrite grandidierite monticellite olivine phenacite sphene zircon
29Si shift -
60 80 66 62 84 80 82
Ref.
a r d a a f a
Sorosilicates akermanite lawsonite rankinite
- 74 - 81 - 75,- 76
d b a,d
gehlinite piemontite thorveiteite
- 73 - 82,- 86,- 90 - 95
d c a
Inosilicates clinoenstatite jadeite orthoenstatite
- 82,- 84 -92 - 82
a,d d e
diopside omphacite pectolite
- 85 - 85 - 86
c c c,d
29Si N M R T a b l e 4.2.
213
(Continued)
Mineral
298i shift
Ref.
Mineral
298i shift
Ref.
spodumene wollastonite
- 91 - 8 6 , - 92
a,c,d c
tremolite
- 8 7 , - 91
a,c,d
Phyllosilicates a
apophyllite - 92 beidellite - 8 8 , - 93 danburite - 89 hectorite - 94 lepidolite - 90 montmorillonite - 94 palygorskite - 9 2 , - 9 4 , - 98 phlogopite - 86 saponite - 9 6 , - 9 1 , - 85 serpentine - 94 vermiculite - 88
g b g a,g g j,s a,i g a
endellite - 93 chlorite - 91,- 87,- 83,- 79 fluorophlogopite - 93,- 89,- 96 kaolinite - 92 margarite - 76 muscovite - 86,- 90 paragonite - 8 9 , - 8 4 , - 81 pyrophyllite - 94 sepiolite - 92,- 94,- 98 - 97 talc
a g g
a,c,h a,i a,Li g a,c j,s a,c
i
Tectosilicates
albite - 9 3 , - 9 7 , - 105 anorthite - 8 3 , - 8 5 , - 89 beryl - 102 cancrinite - 86 coesite - 1 0 6 , - 110 cristobalite - 109 [3-eucryptite - 90.6 heulandite - 95 to - 108 leucite - 9 7 , - 9 1 , - 85 milarite - 102.3 nepheline - 8 5 , - 89 petalite - 1 1 0 , - 111 scolecite - 86,- 89,-96 stilbite - 9 8 , - 1 0 1 , - 108 tourmaline - 88 quartz - 107 Miscellaneous
k k f c,d n n v
f,1 t u
c,q d
f,d 1 c
analcime armenite carnegieite chabazite cordierite emerald gmelilite kalsilite microcline natrolite oligoclase scapolite sodalite thomsonite tridymite
-
92,-
96,-
-
95,-
102,-
101
1
82
u
82
m
99 -79,-10 -
-
103
f
97
d
-
85,96,-
-
89,98,-
88,-
93,-
0
93,-
-
86,-
-
109to
94
m,q
100
k
105
k
95
97,-
d,f o
d
106
p
85 89,-
c 92
d,L1
113
n
- 82.0 - 94.2 - 78.5 - 9 5 . 6 , - 100.0
x x x x
c,n
silicates
benitoite lorenzenite penkvilksite vinogradovite
-
93.4 90.0 95.6 900.6
K e y to references a M a g i et al. (1984) c Sherriff et al. (1991) e M e r w i n et al. ( 1991) g W e i s s et al. (1987) i Sanz and Serratosa (1984) 1 L i p p m a a et al. ( 1981) n Smith and Blackwell (1983) p Sherriff et al. (1987) r Smith and Steuernagel (1992) t K o h n et al. (1997) v Phillips et al. (2000)
fresnoite narsarsukite titanite zorite
b d f h j m o q s u x
Smith et al. (1983) Janes and Oldfield (1985) Sherriff et al. (1991 a) K i n s e y et al. (1985) K o m a r n e n i et al. (1986) Stebbins et al. (1986) Putnis et al. (1985) Hovis et al. (1992) F y f e and K e n n e d y (1986) A r m b r u s t e r (1999) Labouriau et al. (1998)
214
Multinuclear Solid-State NMR of Inorganic Materials
Table 4.3. A selection of 298i chemical shifts for tetrahedral Si in metal silicates, in
ppm with respect to TMS. Compound
298i shift
Li4SiO4 -64, -65, -62, -64.7, - 6 6 BazSiO4 -70 Mg2SiO4 -62 o~-CazSiO4 -70 [3-Ca2SiO4 - 71 ~/-Ca2SiO4 - 74 H-Pb2SiO4 - 94.3 M-Pb2SiO4 -96.6 et-Li2Si205 -91 [3-Li2Si205 -92.5 ot-NazSi205 -93.6 [3-Na2Si205 -87.5, -85.6 ~/-Na2Si205 - 86 ~-Na2Si205 -90 Y2SiO5 -79.8 Li6Si207 -67 Ca3Si207 -75, - 7 6 La2Si207 -83 In2Si207 -88 oL-Y2Si207 - 80.96, - 82.43, - 83.4 l, - 84.95 [~-Y2Si207 -93 [3-Y2Si207 -93.65 "y-YzSi207 -92.8 ~/-Y2Si207 -92.68 8-Y2Si207 - 81.27, - 83.00 ~/-Y2Si207 -83.0 8c28i207 -95 Pb3SieO7 -77.5, -94.3,096 LizSiO3 -75 Na2SiO3 -76.8 MgSiO3 -82 PbSiO3 - 84.2, - 86.5, -94.4
Reference Xu and Stebbins (1995) Magi et al. (1984) Magi et al. (1984) Magi et al. (1984) Magi et al. (1984) Magi et al. (1984) Bessada et al. (1994) Bessada et al. (1994) Mortuza (1989) Mortuza (1989) Mortuza (1989) Mortuza (1989) Mortuza (1989) Mortuza (1989) Dupree et al. (1988) Mortuza (1989) Janes and Oldfield (1985) Magi et al. (1984) Magi et al. (1984) Parmentier et al. (2000) Magi et al. (1984) Parmentier et al. (2000) Dupree et al. (1988) Parmentier et al. (2000) Parmentier et al. (2000) Parmentier et al. (2000) Magi et al. (1984) Bessada et al. (1994) Magi et al. (1984) George et al. (1998) Magi et al. (1984) Bessada et al. (1994)
For a fuller discussion of the effect of the various silicate structural groups on the 298i chemical, the reader is referred to Engelhardt and Michel (1987). A list of the reported chemical shift for other inorganic silicates is given in Table 4.3.
4.5. THERMAL DECOMPOSITION OF SILICATE MINERALS N M R spectroscopy, especially 298i NMR, has proved to be a valuable technique for studying thermal decomposition of minerals, especially in cases where the intermediate phases are poorly crystalline or X-ray amorphous.
29Si N M R
215
A mineral reaction of both academic and practical interest is the thermal decomposition of the related 1:1 clay minerals kaolinite and halloysite. These lose structural (hydroxyl) water at about 550~ forming an essentially X-ray amorphous phase which has been the subject of considerable discussion (MacKenzie et al. 1985a, Watanabe et al. 1987, Lambert et al. 1989, Rocha and Klinowski 1990, Massiot et al. 1995). Much of the discussion centres on the changes in the coordination state of the A1 during dehydroxylation and in the subsequent transformation to mullite, about which 27A1 NMR has provided considerable insight (see Chapter 5). The thermally-induced changes in the 29Si spectra are simpler and less contentious. On dehydroxylation, the sharp kaolinite Si peak at about - 9 2 ppm broadens considerably and shifts to about - 102 ppm (Figure 4.8), consistent with a range of Si environments of which the mean Si-O-Si angle can be estimated from the NMR spectrum (MacKenzie et al. 1985a). Rocha and Klinowski (1990) fitted the broad 298i profile of dehydroxylated kaolinite (called metakaolinite) to five Gaussian peaks, while Lambert et al. (1989) claimed a satisfactory fit with four Gaussians, and monitored the change in their relative intensities as a function of dehydroxylation temperature. Heating to higher temperatures causes metakaolinite to form either a cubic spinel or a low-temperature form of mullite (or both), depending on various factors including the crystallinity of the parent sample, the thermal regime and the impurities present in the system. Decomposition of metakaolinite is accompanied by the separation of amorphous SiO2, resulting in a 298i resonance at about - 1 1 0 ppm (Figure 4.8). The subsequent formation of well-crystallised mullite is also apparent from the evolution of its characteristic 298i NMR spectrum (Figure 4.8). 298i and 27A1 NMR have been used in studies of the effect of LiNO3 mineraliser on the thermal decomposition of kaolinite (Rocha et al. 1991) and the unh
5
650
.
970~
ll00~
.92A-108
-110
540~ ~
~~-91.5
800~j
~2 105~// 9
-60 -100 -140
-60 -100
29Si shift (ppm) w.r.t. TMS
0
I
-109 i
I
__
I
-60 -100 -140
I_
t
1
k
t
~.
-6O -100 -140
29Sishift (ppm) w.r.t. TMS
Figure 4.8. Changesin the 29Sispectra of kaolinite during its thermal decomposition, showing the progressive formation of the broad metakaolinite resonance envelope(- 99 to - 102 ppm) at 650-800~ the sudden appearance of free SiO2 (- 110 ppm) at 970~ and the formation of mullite (- 88 to - 92 ppm) above 1100~ Adapted from Mackenzie et al. (1985a) by permission of copyright owner.
216
Multinuclear Solid-State N M R of lnorganic Materials
effect of flash calcination, in which the 29Si lineshape was fitted to four Gaussian peaks which changed in relative intensity according to the residence time of the sample in the calciner (Slade and Davies 1991). 29Si and 27A1NMR have also been used to study the thermal decomposition of kaolinite under water vapour atmosphere, which was found to facilitate dehydroxylation and the subsequent formation of crystalline products, and improve the mechanical properties of the fired material (Temuujin et al. 1998e). The thermal decomposition of halloysite, a mineral closely related to kaolinite, in which the plates are rolled up into tubes, has also been studied by 29Si and 27A1 NMR spectroscopy (Smith et al. 1993). Pyrophyllite is another layer-lattice aluminosilicate mineral which decomposes on heating to mullite and cristobalite. Its thermal reactions have been studied by 29Si and 27A1NMR (MacKenzie et al. 1985, Sanchez-Soto et al. 1993). The position of the single sharp 29Si resonance at - 9 5 . 6 ppm in unheated pyrophyllite changes on dehydroxylation to - 1 0 1 ppm, corresponding to a mean Si-O-Si angle of 137.6 ~ in the dehydroxylated phase and leading to a refinement of its structure (MacKenzie et al. 1985). Heating at 1100-1150~ results in the abrupt appearance of the 29Si peaks corresponding to mullite ( - 8 7 ppm) and cristobalite ( - 1 0 9 ppm) (MacKenzie et al. 1985). The effect of grinding on the thermal decomposition of pyrophyllite has also been studied by 29Si and 27A1NMR (Sanchez-Soto et al. 1993). The reactions of several other minerals which thermally decompose to form mullite have been studied by 29Si and 27A1 NMR. These include the mica mineral muscovite, which also contained sufficient iron to permit a complementary 57Fe M6ssbauer study (MacKenzie et al. 1987), the hydroxyfluoride mineral topaz (Day et al. 1995) and the semi-amorphous aluminosilicate minerals allophane (MacKenzie et al. 1991) and imogolite (MacKenzie et al. 1989). The same combination of NMR nuclei has been used to study the thermal decomposition of other aluminosilicates including an illiterich clay (Roch et al. 1998), montmorillonite (Brown et al. 1987), and a related mineral, Fuller' s Earth (Drachman et al. 1997). NMR has also been used to study the effect of water vapour on the thermal decomposition of montmorillonite clay compacts (Temuujin et al. 2000a). 29Si in conjunction with 25Mg NMR has been used to follow the thermal decomposition of several magnesium silicates, revealing unexpectedly complex behaviour of the asbestos mineral chrysotile, in which the 29Si spectral peak at - 9 2 ppm broadens and shifts to about - 73 ppm with the formation of an X-ray amorphous dehydroxylate (MacKenzie and Meinhold, 1994a). This phase transforms to forsterite, MgzSiO4, at about 600-700~ at which temperature a second, Si-rich dehydroxylate forms, characterised by a 29Si resonance at - 9 7 ppm (Figure 4.9). This transforms to enstatite (MgSiO3) and free silica at 770-800~ with further enstatite being formed at 1150~ by solid state reaction between the silica and forsterite (MacKenzie and Meinhold 1994a). Other magnesium silicate minerals whose thermal decompositions have been
29Si NMR
217 5Mg3$1:OjlOH| 4
600~
t -g~~
730~
- 6 !.T
2MI
Mg~Si2OI(OH] i
4MgiSllOs{Ofl) 4 tSl
-61.6
650~
750~
]~
| ~..
9 ,
-60
i
..
i _ = ..... I _ : ,
-100
~
e+.s p ~ l
( ~--61.6plmm|
I
-100
29Si shift (ppm) w.r.t. TMS
,
,, t "~,o
( ~ ..gTpeml
t"
2SiO l
MgSiO 3
I~ ; , ,,,,.
[ ,ocslerlte' ]
: ...... 1:--~--I--+:
-60
[.+d+...
700 "C
2MgiSiO 4
0 0 . . _+, . . ., / C
_
Q
7M@2$10 4 1(~.
700oc ~-~,5 -,28
MgSi:O r
73 p p m ) ~670
~
7"70 800 "~:
l'MglSlO 4
,l
( ~ .,83plma)
[ (~-.1101~m|
I_ . 5 Mg. :S|O 4
5M9S|O:1
~,""-"+' 1 1 6 - - s+.s ~,m!
l ~ - . 83 mmm)
Figure 4.9. Changes in the 298i MAS NMR spectra during the thermal decomposition of chrysotile (white asbestos). Note the evidence for the two dehydroxylated phases, that at - 72 ppm forming forsterite directly, that at - 97 ppm forming enstatite by the thermal decomposition sequence shown schematically at the fight. From MacKenzie and Meinhold, (1994a), by permission of copyright owner.
studied by 29Si and 25Mg NMR include talc (MacKenzie and Meinhold 1994b) and synthetic hectorite (Mandair et al. 1990, MacKenzie and Meinhold 1994c). 4.6. RELATIONSHIPS BETWEEN Z9Si CHEMICAL SHIFT (~) AND STRUCTURE
The 29Si chemical shift values are directly related to the shielding of the 29Si nucleus by the electronic structure of its immediate environment. The chemical shift will thus be influenced by the disposition and chemical nature of the adjacent atoms. Considerable efforts have been made to relate the chemical shifts which have been reported for a large number of silicates to their structural parameters. These relationships, which are often empirical, have been sought with the Si-O bond length (Smith et al. 1983, Smith and Blackwell 1983, Smith et al. 1984, Grimmer 1985, Higgins and Woesner 1982, Grimmer and Radeglia 1984, Weiden and Rager 1985), the Si-O-Si (or T-O-T) bond angle (Smith and Blackwell 1983, Smith et al. 1984, Engelhardt and Radeglia 1984, Newsam 1987, Thomas et al. 1983, Mortuza et al. 1998), or some trigonometric function of the bond angle (Smith and Blackwell 1983, Smith et al. 1984, Newsam 1987, Mortuza et al. 1998, MacKenzie et al. 1985) and to the electronegativity of the surrounding groups (Janes and Oldfield 1985). Most of these simple relationships have been demonstrated
218
Multinuclear Solid-State NMR of Inorganic Materials
to hold for only limited groups of silicates, but by making other refinements, the chemical shifts of a much wider group of silicates can be predicted (Sherriff et al. 1991). In general it is found that the chemical shifts become less negative with increasing mean S i-O bond length, decreasing mean T-O-T bond angle (where T denotes the tetrahedral atom) or with decreasing electronegativity of the surrounding groups.
4.6.1 Relationships between ~ and the Si-O bond length Since the shielding of the Si nucleus in silicates is influenced by the degree of s-hybridisation of the oxygen bond orbitals, which in turn is directly related to the Si-O bond length, several authors have sought a simple relationship between the chemical shift ~ and the Si-O bond length in silicates. Linear relationships of this type have been reported, usually for small groups of silicates. The most extensive test of this relationship was made on a group of 20 silicates, which included representatives of all the silicate classes (Smith et al. 1983). There is considerable scatter, but the best-fit line, shown in Figure 4.10 is defined by (4.2)
= 875 (dsi-o) - 1509
where ~ is in ppm and dsi-o is the mean bond length in/k. Other small groups of samples give similar relationships with the mean value of dsi-o, but still with considerable scatter. Typical relationships, also shown in Figure 4.10, are 6 - 43.2(dsi_o) - 179 (for four silica polymorphs and a silicalite precursor) (Smith and Blackwell 1983),
(4.3)
Weiden & Rager 1985 -40 r
Grimmer & Radeglia 1984 et al. 1983
Smith et ~ m19i84t h a / "
-80
epm
~
"" -120 r~
-160 ~ m m e r |
,
Smith & Blackwel11983 gins & Woesner 1982 1985
!
1.58
l
t
,
|
i
I
i
1.62
|
!
1.66
Mean Si-O distance (,~) Figure 4.10. Relationships between the 29Si chemical shift ~ and the mean Si-O bond length
reported by various authors for groups of related silicate structures.
29Si N M R
219
6 = 1447(dsi_o) - 2432 (for Na,K feldspars, Smith et al. 1984),
(4.4)
6 = 1218(dsi_o) - 2058 (for five silicates and quartz, Grimmer 1985),
(4.5)
6 = 1372(dsi_o) - 2312 (for albite and natrolite and two silica polymorphs) (Higgins and Woesner 1982),
(4.6)
6 = 1187(dsi_o) - 2014 (for various silicates, Grimmer and Radeglia 1984),
(4.7)
6 = 1126(dsi_o) - 1909 (for Si-O bonds in single-crystal Mg2SiO4) (Weiden and Rager 1985).
(4.8)
If all these results are combined in a single plot, the resulting line is 6 = 999(dsi_o) - 1709
(4.9)
However, the scatter in this simple generalised relationship, especially for the chain silicates, reduces its usefulness, and indicates that other factors such as next-nearestneighbour interactions are playing an important role in determining the chemical shift.
4.6.2 Relationships between 8 and the Si-O-Si bond angle The electronegativity of the Si-O bond is related to the Si-O-Si bond angle (or more generally, the T-O-T angle, where T is a tetrahedral atom). This suggests the possibility of a simple relationship between g and the mean tetrahedra! bond angle oL (in degrees), or some trigonometric function of the bond angle. The linear relationships which have been derived between g and the mean T-O-T angle for small groups of samples are shown in Figure 4.11. These relationships are 6 = -0.127c~ - 91.5 (for four silica polymorphs and a silicalite precursor) (Smith and Blackwell 1983),
(4.10)
6 = - 1.18c~ + 69.2 (for Na,K feldspars, Smith et al. 1984),
(4.~1)
6 = - 0.619c~ - 18.7 (for 21 silica polymorphs and zeolites) (Engelhardt and Radeglia 1984),
(4.12)
- - 0.553c~ - 7.58 (for nine zeolites, Newsam 1987), 6 = -0.579c~ - 25.3 (for 10 zeolites, Thomas et al. 1983),
(4.13) (4.14)
220
Multinuclear Solid-State NMR of Inorganic Materials
a et al.
-80
1998
-..~sa _...~ 1987 ~'
-90 Engelhardt & " ~
a~
,I~ -lO0 Radeglia 1984 r~ -110
~
Engelhardt 1999 Kohn et al. 1997 Smith eta/. 1984
Slvadinarayana
et M "- - yana ~
:1,, 130
. Thomas .19. 9 e. t a l . 140
8
~
150
-- "h pSmit e, Blackwel11983 ,_
160
Mean Si-O-Si angle ot (o) Figure 4.11. Relationships between the 29Si chemical shift ~ and the mean tetrahedral S i - O - S i bond angle oLreported by various authors for groups of related silicate structures.
- - 0.563ce - 9.62 (for three sodium disilicate polymorphs) (Mortuza et al. 1998)
(4.15)
= - 0.79c~ + 18.18 (for 13 leucites and related compounds) (Kohn et al. 1997)
(4.16)
Although these various groups of samples are fitted by their respective lines with reasonable scatter, they are not well fitted by a single line. If such a line is plotted, its equation is given by 6 - - 1.44ce + 107
(4.17)
but the scatter in these results suggests that it would be best not to treat this as a general relationship. This does not however preclude the use of specific angular relationships which are known to hold for restricted groups of compounds. Thus, Engelhardt (1999) has demonstrated that 33 sodalites with different cage contents conform closely to the angular relationship 6 = - 0.62a - 1.09
(4.18)
and Sivadinarayana et al. (1998) have found that for 17 of the 24 crystallographically distinct sites in the monoclinic zeolite ZSM-5 a satisfactory correlation of the chemical shifts with the mean T-O-T bond angle ot is given by 6 = - 0.607c~ - 20.9
(4.19)
29Si N M R
221
This simple relationship with the bond angles was found to be more satisfactory than relationships with various electronic properties calculated by semi-empirical quantum chemical calculations based on cluster models (Sivadinarayana et al. 1998). Since the electronegativities of the s-hybridised oxygen bond orbitals are related to the Si-O-Si bond angles oL by a cosine function of the type COSOL/(COSOL-- 1), relationships of this form, or with sec c~ ( = 1/coset) have been sought. In several cases, the secant relationships shown in Figure 4.12 have been claimed to provide a better linear fit to the data sets than the simple mean angle relationship. Note that the parameter plotted is the mean of the secants of the bond angles and not the secant of the mean bond angle. The straight lines representing each of these data sets are: 6 = - 55.7 sec a - 176 (for four silica polymorphs and a silicalite precursor) (Smith and Blackwell 1983), (4.20) = - 62.7 sec a - 180 (for N a - K feldspars, Smith et al. 1984),
(4.21)
6 = - 23.3 sec a - 116 (for nine zeolites, Newsam 1987),
(4.22)
6 = - 32.9 sec a - 134 (for three sodium disilicate polymorphs) (Mortuza et al. 1998)
(4.23)
Additionally, the secant relationship (4.20) established for silica polymorphs (Smith and Blackwell 1983) has been found to hold to within two ppm for a series of 12 layerlattice aluminosilicates (MacKenzie et al. 1985) provided all T-O-T angles (including
-80 1987 -90 et al. 1998
-100 MacK
mith et al. 1984
-110
~Smith --
-1.8
.
-
'
11]6 ....
,
-1.4
,,,1
,
I
-1.2
-
~
& Blackwel11983 .
!
-1.0
M e a n sec a Figure 4.12. Relationships between the 29Si chemical shift ~ and the mean secant of the tetrahedral Si-O-Si bond angle oLreported by various authors for groups of related silicate structures.
222
Multinuclear Solid-State NMR of lnorganic Materials
the relevant Si-O-A1 angles) are used in the calculation. When all these data sets are plotted together, there is less scatter in the best-fit line (4.24)
= - 67.2 sec ce - 182
than for the simple angle relationship, but the result is still less than satisfactory. It should not be concluded, however, that this and the other empirical relationships have no use. Indeed, individual secant relationships derived for restricted groups of compounds fit the experimental data with little scatter, suggesting that useful structural information can be gained, so long as the relationship appropriate to a particular type of compound is used. Thus, the secant relationship (4.20) with the angular definition modified for aluminosilicates (MacKenzie et al. 1985) has been used to shed light on the possible existence of a Si-A1 spinel suggested to form when the clay mineral kaolinite is heated to 980~ On the basis of the known structure of the closely-related ~/-alumina spinel, the T-O-T angle of the postulated tetrahedral Si site in such a structure predicts a chemical shift of - 79 ppm for this site. Such a resonance would normally be masked by the broad resonance of the amorphous SiO2 also present, but when this material was selectively removed by leaching with KOH solution, the predicted 295i peak was detected (MacKenzie et al. 1996) (Figure 4.13). Measurements of the relative intensity of this
~
-110
befor
-110 after 1
l
20
|
t
-60
t
I
9
-140
29Si shift (ppm) w.r.t. TMS
Figure 4.13. 29Si NMR evidence for the presence of Si in the w-alumina spinel formed from kaolinite at 980~ After leaching out most of the uncombined SiO2 from the sample with KOH solution, the resonance at - 77.5 ppm indicates the presence of Si in the tetrahedral sites of the Al-rich spinel. Adapted from MacKenzie et al. (1996) by permission of copyright owner.
29Si NMR
223
peak, taken in conjunction with the known amount of SiO2 removed by leaching, suggested that the amount of SiO2 in this particular phase is quite small (3.9 wt % maximum) (MacKenzie et al. 1996).
4.6.3 More complex relationships between ~ and the structure The failure of relationships between simple geometric factors and ~ for a wide range of silicate types has led to a recognition that the electronegativity of the groups of atoms surrounding the Si must be taken more rigorously into account, for example, by utilising a linear correlation between ~ and the nett charge on the silicon atom. The latter is reflected by the electronegativities of the four ligands attached to the Si atom, which can be empirically calculated by assigning characteristic group electronegativity values to all the groups or fragments attached to the silicon (Janes and Oldfield 1985). Allowance is then made for the variation in the Si-O electronegativity due to different Si-O-Si angles by use of a linear scaling procedure and the group electronegativites are summed to give an estimate of the chemical shift ~ from the empirical relationship
(4.25)
6 = - 24.336 X E N + 279.27
where ]~EN is the sum of the group electronegativities for systems such as silicate minerals, containing ~r and ~r-bonding (so-called type P silicon). This approach was able to predict to within about 2 ppm the chemical shifts of 99 sites in 51 compounds (Janes and Oldfield 1985) (Figure 4.14A), but is difficult to use in the reverse direction, i.e. to derive structural information from the chemical shifts. A
B
-150
,~ -60 ~
r~ -100
7
-80
~ -100 ~ Q
-50 -50
. . .
-100
-150
C a l c u l a t e d 29Si shift ( p p m )
Q
-120
....
i ....
-100
Calculated
~'..,,
I,....
-80
i ....
I.
-60
298i shift
(ppm)
Figure 4.14. Observed and calculated 298i shifts for a number of different silicate mineral structural types. A. Shifts calculated by the group electronegativity approach, from Janes and Oldfield (1985), by permission of the American Chemical Society. B. Shifts calculated by the method of Sherriff et al. (1991), used by permission of copyright owner.
224
Multinuclear Solid-State NMR of lnorganic Materials
Shortcomings in the simpler relationships have led to the development of another empirical approach which takes into account the positions of the next-nearest neighbours and makes a correction for the geometry of the S i-O-X coordination triangle (Sherriff et al. 1991). The equation relates ~ to 1)', a parameter which includes a standard geometric term which is multiplied by a measure of the strength of the cationoxygen bond, summed over all the first-neighbour cations. To make the equation more general for structures containing distortions arising from highly strained tings, a correction term log(D) was introduced, where D is the distance between the Si and the nearest-neighbour cation; thus, ~O' - SI[(1 - 3cos20i)/3Ri3)(exp[(ro - ri)/0.37])(log(Di)]
(4.26)
where the term in COS20describes the interaction between the magnetic dipoles of the bond and the nucleus, and the term in ro - ri is a weighting factor to correct the dipole moment according to its bond valence (Sherriff et al. 1991). A good linear relationship has been found between ~ and 1)' for 124 Si sites in a wide variety of silicates: 6 = 701.6~' - 45.7
(4.27)
This relationship can accurately predict the chemical shifts of a wide range of silicates from a knowledge of their structures (Figure 4.14B), and has also been applied with reasonable success to a series of titanosilicates (Labouriau et al. 1998) but the introduction of higher levels of correction factors makes it difficult to use in the reverse sense to derive structural information from the chemical shift. Although simple correlations between the 29Si chemical shift and geometrical parameters can provide useful indications of structure for restricted groups of related compounds, the most precise structural information is certain to be provided in the future by comparing the NMR spectra with ab initio calculations of the NMR parameters made on the basis of assumed models. An indication of the potential of such an approach is provided by the ab initio calculations made by Bull et al. (2000) for a zeolite, siliceous ferrierite. The chemical shifts and intensities of the five Si sites in this compound were calculated for two differing published structures, and compared with the experimental NMR spectrum (Figure 4.15). This approach, which was also carried out for the 170 spectrum of siliceous ferrierite, clearly demonstrates the superiority of one of the postulated structures over the other (Bull et al. 2000). Calculations using density functional theory have also been successfully used to predict 298i chemical shifts in zeolites (Valerio et al. 1999) and in the SiO2 polymorphs coesite, low cristobalite and e~-quartz (Xue and Kanzaki 2000). Approaches to structure elucidation by NMR involving theoretical calculations will become more generally used as the calculation techniques improve.
225
29Si N M R
A
3
2
5
1
4
observed
B
3
2
5
1
4
calculated
Lewis model
C
M~176 i i. i
calculated
.
-108
....
'.
-114
4
.
-120
zgsi shift (ppm) w.r.t. TMS Figure 4.15. Schematic representation of A. observed and B.,C. calculated 29Si NMR spectra of the five Si sites in siliceous ferrierite zeolite. The ab initio calculations are based on two reported structural models, and indicate a clearly better fit to model B. From Bull et al. (2000), by permission of the American Chemical Society.
4.7. FIVE AND SIX-COORDINATED Si-O COMPOUNDS
The best-known compound containing Si(VI) is the high-pressure silica polymorph stishovite, which has a chemical shift of 191.3 ppm (Stebbins and Kanzaki 1991). Other high-pressure silicate phases also known to contain Si(VI) are shown in Table 4.4, together with their 298i chemical shifts. The natural abundance 29Si MAS NMR spectra of the two high-pressure hydrous magnesium silicates Phase B and superhydrous Phase B, obtained by cross-polarisation with 1H, show, in addition to tetrahedral resonances at - 64.0, - 75.8 and - 75.0 (Phase B) and at - 74.6 (superhydrous Phase B), the most deshielded Siw resonance positions reported for octahedral Si (Phillips et al. 1997). These unusually positive Si w shifts were attributed to the absence of oxygen sharing with other Si vt and the large degree of oxygen sharing with Mg octahedra (in both Phase B and superhydrous Phase B all 12 polyhedral edges are shared with MgO6). Attempts have been made to relate the Si(VI) NMR data to structural parameters using similar relationships to those for Si(IV) units. The more negative chemical shifts of Si(VI) can qualitatively be explained in terms of the number and field strength of the nearest-neighbour cations (Stebbins and Kanzaki 1991). The approach of Sherriff et al. (1991) (equation 4.26 without the term in log(D)) is found to give reasonably good predictions of chemical shifts, as long as the protons in the structure are not included in the calculation (Figure 4.16A). This results in a line of best fit to the Si(VI) data given by
226
Multinuclear Solid-State N M R of lnorganic Materials
Table 4.4. A selection of 2 9 8 i chemical shifts reported for compounds containing Si(VI), in ppm with respect to TMS.
29Si shift
Compound thaumasite Mg-Si ilmenite stishovite Na-Mg-Si pyroxene Mg-Si garnet wadeite Mg-Si perovskite Ca-Si perovskite CaSi205 Mg12Si4019(OH)2 (Phase B) Mg2oSi6H8036 (superhydrous Phase B)
-
.mq
B ~
-80 .~ -120
-120
[~
Si
r~ oH
o~Si
~ -16o
r~ -160
o,1)Si
-200
~
-200
,
-0.3
Grimmer (1980) Stebbins and Kanzaki (1991 ) Stebbins and Kanzaki (1991 ) Stebbins and Kanzaki (1991 ) Stebbins and Kanzaki (1991) Stebbins and Kanzaki (1991) Stebbins and Kanzaki (1991 ) Stebbins and Kanzaki (1991) Stebbins and Kanzaki (1991) Phillips et al. (1997) Phillips et al. (1997)
179.6 181.0 191.3 194.7 197.6 203.1 191.7 194.5 193.4 170.4 166.6
A
-80
Reference
-0.2
I
-0.1
,
,,
I
0
n
,
i
I
i
15
17.5
20
22.5
25
~EN
Figure 4.16. 298i chemical shifts ~ for 6-coordinated Si-O compounds plotted against: A. the structural parameter 1"~of Sherriff et al. ( 1991) (equation 4.26 without the term in log(D)), B. the group electronegativity parameter EN of Janes and Oldfield (1985). The lines for 4-coordinated Si-O compounds (without data points) are also included for both equations.
6 = 145.4gT -
173.2 (Stebbins and Kanzaki 1991)
(4.28)
Note, however, that this line does not coincide with that defined for Si(IV) c o m p o u n d s , also plotted in Figure 4.16A. T h e g r o u p e l e c t r o n e g a t i v i t y a p p r o a c h (Janes and O l d f i e l d 1985) also gives a reasonably good fit to the data (apart from that for Mg-Si perovskite), leading to the linear relationship for Si(VI) c o m p o u n d s 6 - - 13.48 , ~ E N + 108.7 (Stebbins and Kanzaki 1991)
(4.29)
29Si NMR
227
These data, plotted in Figure 4.16B, show significant scatter, although the line about which they lie represents a reasonable continuation of the line for Si(IV) compounds, and also passes through the region now known to be characteristic of Si(V). The literature contains considerable discussion of the existence of Si(V) in glassy systems, inferred from the presence of small 298i NMR peaks at about - 150 ppm in the spectra of potassium silicate glass (Stebbins and McMillan 1989, Stebbins 1991), alkali silicate glasses quenched from the liquid state at high pressures (Xue et al. 1991) and at ambient pressure (Stebbins and McMillan 1993), aluminosilicate glasses (Risbud et al. 1987, Sato et al. 1991) and "superquenched" calcium aluminosilicates (Sato et al. 1991 a). Since the structures of these amorphous materials could not be determined independently by X-ray crystallography, the existence of Si(V) and its assignment to the - 150 ppm resonance was made by inference from its position between Si(IV) and Si(VI). It was also supported by the observation that in some organic molecules the chemical shifts for Si(OR)5 groups are 43-51 ppm lower than the corresponding Si(OR)4 groups (Holmes 1990), and that quantum mechanical calculations for SiF molecules (Tossell and Lazaretti 1986) predict a similar effect accompanying a coordination change from four to five. Direct confirmation for this assignment has now come from a study of crystalline CaSi2Os, which contains a 298i NMR resonance at - 150 ppm (Stebbins and Poe 1999) (Figure 4.3) and is known from X-ray crystallography to contain Si(V) in addition to Si(IV) and Si(VI). 4.8. CROSS-POLARISATION (CPMAS) EXPERIMENTS
In cross-polarisation experiments two nuclides are excited at the correct frequencies to satisfy the Hartmann-Hahn condition (Chapter 2) so that the magnetisation of the more abundant spin system (typically 1H) is transferred to the system with the smaller signal (298i). The FID of the nuclide of interest (29Si) is acquired with a stronger signal and at the usually shorter T1 value of the protons, allowing the spectrum to be obtained more quickly and with an enhanced signal/noise level. 4.8.1 Cross-polarisation between
1H a n d 29Si
The Hartmann-Hahn condition in these experiments is usually established by using Q8M8(Si[(CH3)318Si8020) as the calibration compound which also has the advantage of providing a secondary chemical shift reference. Other reference compounds including kaolinite and sepiolite have been suggested, but the toxicity of the latter could prove to be a disadvantage. As well as providing improved sensitivity, cross-polarisation experiments between 1H and 29Si can also be used to provide additional structural information, since the Si atoms in closest proximity to protons are preferentially enhanced. The data from CPMAS experiments should, however, be regarded as qualitative rather than quantitative. If improved quantification is required, it is essential to ensure that the signal is not
228
Multinuclear Solid-State NMR of Inorganic Materials
saturated (i.e. all the 298i nuclei are allowed to recover their magnetisation completely between pulses). By taking strict precautions and using single-pulse experiments, Farnan et al. (1987) were able to identify the hydrated structural units in hydrous silica glass. The hydroxyl concentrations estimated from these experiments proved to be lower than previously indicated by other techniques. Similar work on sodium silicate glasses (Ktimmerlen et al. 1992) has indicated that H20 depolymerises the silicate network, and that both Si-OH and molecular water are present in these hydrous glasses. The improved sensitivity of 1H-29Si CP MAS has been exploited by Alma et al. (1984) to record the 298i spectra of synthetic mica and montmorillonite and by Kodama et al. (1989) in a study of ground kaolinite. In these studies, the non-CP and CP spectra were identical in shape and position, indicating that all the Si sites are in similar proximity to protons. Of greater interest from a structural point of view are examples where CP and non-CP spectra differ. In the case of the hydrated phases of pure silica (silica gel and fumed silica), the intensities of the Q2 and Q3 peaks which are bonded to hydroxyl groups should be significantly increased by comparison with the Q4 peak (unbonded silica). This was shown to be the case by Chuang and Maciel (1997) who used their 298i results to refine a model of the silanol bonding at the surface of silica gel (Figure 4.17). The technique was also used in a study of gel synthesis of albite glass (Schmelz and Stebbins 1993). Non-CP
CP
Silica ge
F u m e d s" "
!
-70
|_
!
.I
-100
i
_|_
|
-130
!
I
-70
a
I
I
-100
I
I
!
m
-130
29Si shift (ppm) w.r.t. T M S Figure 4.17. Use of cross-polarisation between ~H and 298i to investigate the surface hydration of silica gel and fumed silica. Cross-polarisation increases the intensity of the Si sites in proximity to hydroxyls, shown by the curve-fitted spectra to be the Q2 and Q3 sites. Adapted from Chuang and Maciel (1997).
29Si NMR
229
The discrimination of protonated Si sites by CP MAS was also used by Yang and Kirkpatrick (1989) in a study of the hydrothermal decomposition of albite and sodium aluminosilicate glass, and rhyolitic glass (Yang and Kirkpatrick 1990), and as a means of differentiating between the various sites in acid-treated montmorillonites (Tk~ic et al. 1994). CP MAS NMR has also been used to identify a Si site at - 100 ppm in a microporous material derived from acid-leached metakaolinite as the Q3 unit Si(OSi)3OH (Okada et al. 2000).
4.8.2 Cross-polarisation between 19F and 29Si Like 1H, 19F is another nuclide with abundant magnetisation which can be transferred to 29Si to improve its sensitivity in fluorinated Si compounds. The Hartmann-Hahn condition was established by using sodium hexafluorosilicate, for which the relaxation time (3.2 s) is rather long for convenience, but no more suitable compound has been identified (Hoffner et al. 1993). The technique has been used to study octadecasil, a siliceous clathrate compound containing fluoride ions, and a fluorinated siliceous MF1type zeolite prepared in a fluoride medium (Hoffner et al. 1993). The 19F to e9si experiment was found to be not as efficient as a 1H to e9si cross-polarisation because the dipolar interactions between F and Si are weaker due to the lower magnetogyric ratio of 19F by comparison with 1H. Nevertheless, this cross-polarisation experiment can be used to enhance the sensitivity of S i, especially for atoms directly bonded to fluorine, and was used to confirm the assignment of the two Si resonances in the octadecasil spectrum (Hoffner et al. 1993). The technique has also been used to study fluorinedoped aluminosilicate glasses (Sebald et al. 1992).
4.8.3 O t h e r cross-polarisation e x p e r i m e n t s with
29Si
Cross-polarisation to protons is only applicable to a restricted range of protonated inorganic materials (gels, some glasses and minerals), but the benefits of increased sensitivity have recently been demonstrated in non-protonated systems by cross-polarising to other nuclei such as quadrupolar eTA1 and e3Na (Shore et al. 1999). The technical difficulties arising from the use of a quadrupolar nucleus and the relatively small differences between the Larmor frequencies of 295i and eTA1 or e3Na can be overcome by optimising the efficiency of the spin-lock, which depends strongly on the stability of the spinning speed, the rf power levels and the quadrupolar coupling constants (Shore et al. 1999). The larger linewidth of the e3Na resonance makes it more difficult than eTA1 to spin-lock uniformly. Applying 27Al-e9si CP to a sample of crystalline albite (Figure 4.18A) increased the signal/noise by a factor of five, corresponding to a 25-fold reduction in the ~acquisition time. However, the two Si sites with one next-nearest neighbour A1 (at -96.1 and 103.9 ppm) are more intense than the site at - 91.8 which has two nearest-neighbour -
230
Multinuclear Solid-State NMR of Inorganic Materials
A Non-CP
23Na --~ z9Si CP
C 27A1 ~ 29Si CP ,,~
....
i ....
-90
| ....
i ....
-100
' w , ~'~ , i " "
-110
29Si shift (ppm) w.r.t. TMS Figure 4.18. Effect of cross-polarisation to 23Na and 27A1 on the 298i spectrum intensity of albite. Note that all three spectra have been scaled to be noise-equivalent. From Shore et al. (1999), by
permission of the American Chemical Society. A1 atoms; this is related to differences in the relaxation rate and the cross-polarisation rate of the - 9 8 . 1 ppm site (Shore et al. 1999), and illustrates the necessity for caution when drawing conclusions from CP intensities. A smaller but still useful signal/noise enhancement was found for this material with 23Na-29Si CP (Figure 4.18B), but the intensity ratios of the three Si sites are different again, reflecting differences in the relaxation time of the sites and the larger quadrupolar frequency of 23Na (Shore et al. 1999). Such cross-polarisation procedures to enhance the signal intensity of 298i are expected to be of use in making two-dimensional isotropic-anisotropic correlation spectroscopy more accessible to systems without protons.
4.9. GLASSES, GELS AND OTHER AMORPHOUS MATERIALS
Unlike X-ray diffraction techniques, solid-state NMR does not depend on the presence of long-range atomic periodicity in a structure, but is sensitive to short-to-medium range geometries and orderings within 10A of the observed nucleus. It is therefore particularly useful for investigating X-ray amorphous phases such as glasses, gels, and other amorphous materials. In most amorphous materials there is, generally, a much wider range in the parameters determining the local Si environments than in the corresponding crystalline form, leading to broad, overlapping peaks from which the information is extracted by peak fitting or deconvolution techniques. Since these procedures
29Si N M R
231
are critical to the extraction of data from broad overlapping 29Si NMR spectra they are considered in more depth in Section 4.9.2.
glasses 298i NMR allows the identification and quantification of the various types of structural
4.9.1 Silicate
units in silicate glasses, defined as Qn, where n is the number of Si-O-Si bridges. The presence of a range of Si-O-Si angles in each Qn group leads to broadening, which can be modelled by a variety of analytical functions, as reviewed by Dupree (1994). The peak maximum of the broad Si spectrum of pure fused silica occurs at about - 111.5 to - 1 1 2 ppm, and its width is consistent with a range of tetrahedral bond angles of about 130-179 ~ (Oestrike et al. 1987). 298i NMR measurements on amorphous silica densified at 50 kbar and 600~ (Devine et al. 1987) have been used to detect a pressure-induced shift in the mean Si-O-Si bond angle from 143 ~ to 138 ~ Analysis of the bond-angle distribution suggested that the compressive force facilitates formation of low-member ring structures by reducing the distance of closest approach of the second-nearest-neighbour oxygens. The structure of amorphous SiO powders has been invesigated by 298i NMR which indicates that by contrast to the random bonding present in evaporated SiO films, the powders consist of microscopic regions of Si greater than 20A in size and SiO2, together with a significant amount of interphase material (Dupree et al. 1984a). The addition of alkali or alkaline earth oxides to fused silica promotes the formation of non-bridging oxygens, and changes the distribution of the Qn units which have been ascertained from the 298i NMR spectra and compare well with theoretical predictions (Dupree et al. 1984). Resolution of the various Qn units in alkali silicate glasses by 298i NMR can be enhanced by exploiting differences in the chemical shift anisotropy (CSA) of the different units. Thus, Stebbins (1987) was able to distinguish a small amount of Q4 in the major Q3 unit of sodium silicate glasses using static (non-spun) spectra in which the Q4 sites are accentuated because of the small CSA of this more symmetrical unit (Figure 4.19). Differences in the CSA of various Q3 units of sodium silicate glasses have also been exploited by Duer et al. (1995) in a 2D experiment in which the normal MAS spectrum is obtained in one dimension and the chemical shift tensor associated with each Qn species is obtained in the second dimension. This technique has revealed the existence of two Q3 units of different CSA in sodium disilicate glass (Duer et al. 1995), and has potential application to many other glass systems. The situation in aluminosilicate glasses is complicated by the additional effects of next-nearest-neighbour A1 on the Si shifts. Although some Qn sites can be unambiguously assigned, others such as @(0A1) and Q4(3A1) occur in the same chemical shift range and cannot be differentiated on this basis. Systematic variations in the 298ipeak positions and widths of sodium aluminosilicate and calcium aluminosilicate glasses can
232
Multinuclear Solid-State NMR of Inorganic Materials
A
MAS
34% Na20
B
Unspun
34% N
37%Na~O~/[Q~Q ~ / ~ 37~176 "~'~"--'----41%Na20_~_,,, ~ 41%Na~/__~ , ~ -60
-80 -100 29Si shift
0
-100
-200
(ppm) w.r.t. TMS
Figure 4.19. 298iNMR spectra of sodium silicate glasses containing the indicated amounts of Na20 (mol%). A. MAS spectra, showing resolved Q2 and Q3 units. B. Unspun spectra allowing the Q4units (shaded) to become visible because of their smaller chemical shift anisotropy. From Stebbins (1987), by permission of MacMillan Magazines Ltd.
be related to the extent of Si-A1 ordering (Lee and Stebbins 1999). A downfield shift with increasing A1203 content in the 29Si spectra of rapidly-quenched sodium aluminosilicate glasses has been interpreted in terms of a lower degree of network polymerisation as the concentration of network modifiers increases (Schmticker et al. 1997). Provided appropriate fitting methods are used, NMR spectra provide quantitative information about the relative numbers of the various Q" units in binary silicate glasses which can be tested against predictions from various structural models (Eckert 1992). The distribution of Qn units in simple glasses depends on the composition, and could theoretically follow either a binary distribution of the two species present in the crystalline composition (Figure 4.20A), or a random statistical distribution (Figure 4.20B). 29Si NMR results for lithium silicate glasses are consistent with a binary distribution over the limited composition range 25-29 mol% Li20 (Figure 4.20C), with deviations above 33 mol% Li20 caused by disproportionation of Q3 to Q4 + Q2 (Dupree et al. 1990). A similar binary distribution of Q4/Q3 units has been found in lead silicate glasses below about 30 mol% PbO, but at higher PbO concentrations up to 65 mol%, the distribution of Q" units is more consistent with the random statistical model (Dupree et al. 1987). At 70 mol% PbO, the glass contains mainly isolated SiO44- groups in a lead-oxygen matrix. A 298i MAS NMR study of a series of sodium lead silicate glasses shows the presence of Q~ and Q2 units which interconnect the Pb-O-Pb network in compositions of > 50 mol% PbO. As the alkaline oxide concentration of the glass is increased, the microhardness decreases, probably due to the conversion of more rigid covalent Q3 and Q4 structural units to less-rigid Q1 and Q2 units (Shrikhande et al. 2001).
298i
233
29Si NMR A
--
B
~
C
Q3 QZQ,QO
'~176 It74AA
l,,,,,I 0
li.,,I 0
Qi 20 o
20 20
60
100
o
20
R+zO or 1/2+O (tool %)
60
/.~-'/"
""
~"...
60 " ~ .....
: 60[ ~Q3 zj
o
.
~ 100 o .
20
"..~i~...94 24
~"
QiS"
32
40
LizO (mol %)
Figure 4.20. Distributionof Qnunits in a binary silicate glass, predicted by: A. binary distribution model, and B. random statistical distribution model, from Dupree et al. (1987). C. Experimental data for lithium silicate glasses plotted against the line calculated from the binary distribution model (dashed). From Dupree et al. (1990), by permission of Elsevier Science.
298i MAS NMR together with 31p NMR has been used to study the changes occurring during thermal conversion of phosphosilicate gels to glasses (Clayden et al. 2001). Dried gels containing 10 and 30 mol% P205 have very similar siloxane frameworks containing silanol groups and trapped orthophosphoric and pyrophosphoric acid. However, the course of the subsequent structural evolution depends markedly on the P205 content, with the presence of 6-coordinated Si being observed in the glassy matrix of the higher P-content glass in which co-polymerisation of the silicate and phosphate tetrahedra occurs at lower temperatures than in the lower-P composition (Clayden et al. 2001). Octahedral SiO6 also occurs in alkaline earth phosphosilicate glasses as well as in the related crystalline phosphosilicates Si50(PO4)6, 8i3(PO4)4 and SiP207. Other glassy systems in which the distributions of the silicate units have been studied by 298i NMR include SiO2-PzOs-CaO-MgO phase-separated glasses (Oliveira et al. 2000), SiO2-LizO-P205 glasses (Holland et al. 1998), binary lead silicate glasses (Fayon et al. 1998) and SiO2-NazO-P2Os-B203 glasses (Yamashita et al. 1999). Some compositions within the latter system were found to contain 6-coordinated Si. Devitrification (formation of crystalline products) in lithium metasilicate glass has also been studied by 29Si NMR (Clayden et al. 1998), as has the devitrification of rapidly quenched glasses of cordierite composition (Okada et al. 1998). The 29Si NMR spectra of a series of sodium borosilicate glasses (Bunker et al. 1990) show that in compositions containing 30-40 mol% B203 the predominant structural unit is Q4 (~ of about - 1 l0 ppm), but at lower B203 contents the 29Si shift becomes less negative, reflecting an increase in the number of Q4(1B) ( - 105 ppm) and Q3(0B) units ( - 90 ppm). The latter component could also arise from the presence of Q4(3B)
234
Multinuclear Solid-State NMR of Inorganic Materials
units, but this possibility is ruled out by the glass stoichiometry (Bunker et al. 1990). Borosilicate glasses of various compositions have also been studied by Martin et al. (1995) and by Martens and Miiller-Warmuth (2000), who concluded on the basis of 29Si, liB and 23Na NMR spectroscopy that there is much better mixing of the silicate and borate units than previously assumed, and that the Na + is more uniformly distributed. Borate, borosilicate and boroaluminate melts have also been studied by 298i, liB and 27A1 NMR by Sen et al. (1998). Yamashita et al. (2000) have used a number of NMR nuclides including 298i in a study of alkaline earth phosphosilicate and aluminoborosilicate glasses. In some of these compositions, 29Si signals corresponding to 5 and 6-coordinated Si were detected, the intensities of which are related to the glass composition (Figure 4.21). Studies of the differences between the distribution of the silicate species in molten and quenched glasses have been facilitated by the development of high-temperature NMR probes capable of operating up to 2000~ (Taulelle et al. 1989). However, many silicate systems melt at much lower temperatures and do not require this type of laser-heated probe. Furthermore, suitably narrow spectra can be obtained from molten systems without the need for magic angle spinning, considerably simplifying the probe design (although MAS probes are now available for operation at temperatures of 600-700~ The narrow 298i NMR lines obtained from a variety of silicate liquids indicate the operation of a mechanism for rapid exchange between the various structural species known to be present in the glasses. The NMR results for K28i409 have allowed the process to be modelled and the exchange frequencies determined (Faman and Stebbins 1990).
o~)Si oV)si k ~Si i
Ca Sr
L
0
I
I
-100
.
I
-200
.
I
I
._
-300
29Si shift (ppm) w.r.t. TMS Figure 4.21. 29Sispectra of alkaline-earth phosphosilicate glasses of composition 0.3MO-0.05SiO2-0.65P2Os, showing evidence for Si in 6-fold and possibly 5-fold coordination. From Yamashita et al. (2000), by permission of Elsevier Science.
29Si NMR 0% N
i
. . . .
!
. . . .
!
235
2.3% N
. . . . .
!.
,.
i
-50 -100 -150
.
.~..
I
. . . . . . .
8. . . . . .
4.3% N
d ~
~
,
....
-50 -100 -150
!
. . . . . .
!
. . . . . . .
J_-~,
-50 -100 -150
29Si shift (ppm) w.r.t. T M S Figure 4.22. Curve-fitted 29Si spectra of Na-Si-O-N glasses showing the effect of increasing nitrogen (in atom %) on the Q1 and Q2 structural units. From Unuma et al. (1992), by permission of copyright owner.
An increasing interest in oxynitride compounds has led to NMR studies of the effect of incorporating nitrogen into glassy phases. In work on the Na-Si-O-N system, Unuma et al. (1992) have shown that increasing the N content from zero to 4.3 at % results in the apparent progressive increase in the number of Q1 and Q2 units and a decrease in the number of Q4 units (Figure 4.22). Taking into account the change in chemical shift resulting from the substitution of one N for one O in SiO4 (estimated as + 15 ppm) and the charge compensation by Na + accompanying each such substitution, it was concluded from the NMR results that about half the total nitrogen atoms are bonded to two silicons. Other studies of nitrogen-containing glasses and their recrystallisation products have been reported by Aujla et al. 1986 (Y-Si-A1-O-N glasses), Sato et al. 1990 (SiA1ON glass) and Nordmann et al. 1996 (Li-A1-Si-O-N glasses). 29Si NMR has also been used to investigate the incorporation of nitrogen into glasses in the silicon oxynitride system (Kohn et al. 1998), in which the presence of SiO4 _ xNx tetrahedra were detected (where x = 0,1,2,3,4). Glasses in the silica-rich portion of the system SiO2Si3N4-AlzO3-A1N have been shown by 29Si NMR to contain Si(O3N) tetrahedral units (McMillan et al. 1998).
4.9.2 D e c o n v o l u t i o n
of 29Si NMR
spectra
The most commonly used method for deconvoluting 29Si NMR spectra is to fit Gaussian peaks, the area of which is taken as a measure of the concentration of that particular species. Gaussian peaks describe a random distribution of parameters, a condition which may not necessarily be fulfilled in some glasses, in which regions of heterogeneity may exist. In an examination of this problem, Mahler and Sebald (1995) deconvoluted the 29Si spectra of sodium silicate glasses using Gaussian and several non-symmetric lineshapes, and found that the judgement of which gave the most acceptable fit depended on the goodness-of-fit criterion used. More importantly, the
236
Multinuclear Solid-State NMR of Inorganic Materials ~iso (ppm)
+
A
B
Q2
B
Q Q0
~, -90
,
~
Q3 . ~,
|
omq
~9 -70
,<
-60 -50 -7'5-11)0 ' '
......\ : ~ 2 ~ -50
7"3,~. .. -130 -50 -130
-80
- 100
29Si shift (ppm) w.r.t. TMS
90 ~ dim. (ppm w.r.t TMS) Figure 4.23. A. Two-dimensional 298i spectrum of CaSiO3 glass obtained by double-angle spinning at the magic angle and 90~. The cross-sectional profiles to the fight are representative of the 45 cross-sections analysed and simulated as shown by two overlapping Qn components. B. Resulting deconvoluted spectrum, constructed from the Gaussian components derived from the 2D plot. The experimental MAS spectrum is shown by the black dots, the solid line is the fitted envelope. From Zhang et al. (1997), by permission of the American Chemical Society. fitted areas under the curves differed from one model to another, leading to the conclusion that it is not possible unambiguously to quantify the numbers of each species by deconvolution procedures (Mahler and Sebald 1995). One possible solution to resolving broad 298i lineshapes has been suggested by Zhang et al. (1997), who made use of the fact that the various Qn sites show different chemical shift anisotropy (CSA). By carrying out a DAS experiment (see Chapter 2) on CaSiO3 glass, a 2-dimensional plot was constructed of the sample spun at two angles (the magic angle and 90 ~ (Figure 4.23A). Sections taken through the 2-D plot were then simulated and used to determine the contribution of the Qn units to that particular section, making the simplifying assumption that no more than two Qn units overlap at each isotropic frequency. Analysis of the final fitted lineshape (Figure 4.23B) suggested that the assumption of Gaussian distributions is probably reasonable for this glass, and the resulting relative populations of the five Qn species are reliable (Zhang et al. 1997).
4.9.3 Connectivities in glass
Most of the applications of 298i NMR to glass studies to date have been used to provide information about the nature and relative numbers of the Qn structural units present. Another important structural detail is the way in which these units are connected together in the glass. Developments in two-dimensional 298i NMR have allowed such
29Si N M R
237
connectivity relationships to be established (Glock et al. 1998). A simple one-dimensional experiment allows only the environment of closely-associated atoms or structural units to be observed, shown schematically in Figure 4.24 as a window of small size. If the size of the observation window is now increased, connectivities between more distant units can be explored, and, where homogeneous regions exist, information may also be gained about their size and disposition (Figure 4.24). By modulating the spectrum in a second dimension, 2D techniques enable the size of the observation window to be increased. By using the 2D correlation spectroscopy (COSY) sequence in this way, Glock et al. (1998) have revealed the connections between the Q3 and Q4 units in NazSi409 glass which has been isotopically enriched in 29Si (Figure 4.25). Based on the fact that differences can be detected in the isotropic shift of a Qn group depending on the type of the bonded adjacent groups, the structural units surrounding the three resolvable Q3 peaks at about - 89, - 92 and - 95 ppm have been identified (Glock et al. 1998). In the 2D spectrum, the conventional NMR signal appears on the diagonal, and if there is coupling between the different peaks, off-diagonal "cross peaks" will appear. By projecting the shifts of the cross peaks onto the two axes, the connected silicon groups can be identified. An earlier COSY study of the connectivity in glasses, including glassy sodium silicate (Knight et al. 1990) also indicated
Figure 4.24. Schematic representation of the difference in glass connectivity information obtainable from 1D NMR experiments (small observation window) and 2D experiments (larger window) for: A, B. homogeneous glasses, C, D. glasses containing regions of inhomogeneity.
238
Multinuclear Solid-State NMR of Inorganic Materials Cf~
-80
-
.L_~_~,
y'!C>'
~,~ -60 'i f!
~~ -20 .me ro~
~
.el
t
i
.,
I
i
I
,~
-80 -90 -I00 -ll0 29Si shift (ppm) w.r.t. TMS
Figure 4.25. 298idouble quantum NMR spectrum of 29Si-enriched 25Na20-75SiO2 glass. The diagonal peaks are marked by filled circles, and the off-diagonal peaks indicate the connectivities between the structural units (in this case between Q3 and Q4). From Glock et al. (1998), by permission of Elsevier Science. connections between the Q3 and Q4 units, but implications for the bond angles between the connected units arising from this earlier work have been questioned (Dupree 1991).
4.9.4 Chalcogenide glasses Silicon-containing chalcogenides are non-oxide compounds such as SiS2 and SiSe2 containing both comer and edge-sharing 8i84/2 or SiSe4/2 tetrahedral units, by contrast with oxide glasses, in which the connectivity mode is exclusively via corner-sharing tetrahedra. The resulting structures have given rise to their own nomenclature; E (~ denotes corner-shared SIS4/2 tetrahedra, E ~) denotes Si84/2 units sharing one common edge with an adjacent tetrahedron and linked to two other 8i84/2 units by comer sharing, and E <2) denotes 8i84/2 units sharing edges with two adjacent tetrahedra (Figure 4.26). Crystalline SiS2 contains only E ~2) units in one-dimensional chains, with a single 298i NMR resonance at - 21.1 ppm (Tenhover et al. 1988). In the glassy form, SiS2 contains three 298i NMR peaks, at + 7.5 ppm (assigned to E (~ units), - 7.6 ppm (assigned to E ~l) units) and - 19 ppm (assigned to E (2) units). The corresponding 298i shift in crystalline SiSe2 is - 93.0 ppm, and in glassy SiSe2 is - 29.7 ppm (E(~ - 67.6 ppm (E
239
29Si N M R
decrease in the relative numbers of edge-sharing tetrahedra (Moran et al. 1990). A similar effect can be obtained by the addition of LizSe to SiSe2 glasses, but the overall fraction of edge-sharing species is lower than in the corresponding LizS-SiS2 glasses (Pradel et al. 1992). 29Si NMR has been used to show that the addition into glassy SiSe2 of other network formers such as PzSe5 or even additional Se diminishes the degree of edge-sharing due to the formation of Si-Se-Se or Si-Se-P bonds (Moran et al. 1990). A particular interest in silicon chalcogenide glasses containing LizS, NazS or AgzS lies in their ionic conductivity. 29Si NMR has been used to monitor changes in the glass structures containing these additions, and indicates that Ag shows the greatest tendency to form edge-sharing units, followed by Li, then Na (Pradel et al. 1995). Eckert et al. (1989) have combined 29Si NMR with 6Li and 7Li NMR to study some Li-containing chalcogenide systems. The 29Si chemical shifts of glassy and crystalline chalcogenide compounds are collected in Table 4.5.
Er
E o) I j ~Si
I/
S"
]
Si - S
-->
s
E (2)
I -- '-\
~si<Ss>| ~S"sil ~S./
"-
s--s~
7.5 p p m
-7.6 ppm
-19 p p m
S i - S e --~ -29.7 p p m
-67.6 p p m
-81.8 p p m
Figure 4.26. The three possible tetrahedral Si configurations in chalcogenide glasses and the corresponding 29Si chemical shifts in sulphide and selenide glasses. Adapted from Moran et al. (1990).
Table 4.5. A selection of 29Si chemical shifts for chalcogenides and related compounds, in ppm with respect to TMS. Compound
29Si shift
Reference
SiS2 (crystalline) SiS2 (glass) LizSiS3 (HT) LizSiS3 (LT) Li4SiS4 Na4Si4Slo NazSiS3 (HT)
- 19.5 - 16.8,- 7.6,+ 7.5 5.0 -7.9 4.7,8.7 2.9,0.7 -9.5
Tenhover et al. (1988) Tenhover et al. (1988) Pradel et al. (1995) Pradel et al. (1995) Pradel et al. (1995) Pradel et al. (1995) Pradel et al. (1995)
240
M u l t i n u c l e a r Solid-State N M R o f l n o r g a n i c M a t e r i a l s
Table 4.5. (Continued)
Compound
29Si shift
Reference
Na2SiS3 (LT) NanSiS4 AgsSiS6 SiSe2 (crystalline) SiSe2 (glass) Li2SiSe3 LiloSi3Sell LiaSiSe4 Na2Si2Se5 Na2SiSe3 (HT) Na2SiSe3 (LT) Na6Si2Se7 NanSiSe4
3.2 8.6 15.1 - 93.0 - 81.8,- 67.6,- 29.7 - 86.3 - 53.3,- 55.6,- 64.4 - 65.8,- 68.4,- 71.2 - 31.0,- 33.7 - 91.4 - 51.7 - 57.1, - 59.1 - 80.7
Pradel et al. (1995) Pradel et al. (1995) Pradel et al. (1995) Tenhover et al. (1988) Tenhover et al. (1988) Pradel et al. (1995) Pradel et al. (1995) Pradel et al. (1995) Pradel et al. (1995) Pradel et al. (1995) Pradel et al. (1995) Pradel et al. (1995) Pradel et al. (1995)
HT = high-temperatureform,LT = low-temperatureform
4.9.5 Gels
Conventional syntheses of inorganic materials such as glasses and ceramics typically involve energy-intensive high-temperature solid-state chemical reactions between oxides or silicates. In the interests of conservation of energy resources, the last two decades have seen increased interest in lower-temperature processes involving more homogeneous precursors chemically derived from a variety of gels. Since these gels are typically X-ray amorphous, their constitution and thermal crystallisation reactions can conveniently be studied by NMR spectroscopy. The gels may be derived wholly from organo-metallic reagents or inorganic salts, or from a mixture of inorganic and organic reagents ("hybrid" gels), and are classified as monophasic or diphasic, depending on the degree of mixing of their components. The simplest gels to be studied by 29Si NMR are those consisting only of SiO2, in which the number and distribution of the structural Qn units can be determined in a similar manner to silicate glasses. 29Si NMR has also been used to follow the condensation and gelation reactions of silica and to determine the kinetics of such processes (Fyfe and Aroca 1995), but investigations of the early reaction stages in gels typically involve solution NMR measurements, and their detailed discussion is outside the scope of this book. Silica xerogels obtained by heating the gels have been studied by 29Si MAS NMR (Abidi et al. 1998), and the effect of added paramagnetic Mn 2+ ions on the 29Si relaxation time noted. 129Xe spectroscopy of adsorbed Xe gas suggested the localisation of the paramagnetic ions to surface sites (Abidi et al. 1998). The commercially important aluminosilicate ceramic mullite (A16Si2013) can readily be produced by heating either monophasic (homogeneous) or diphasic gel precursors. The reactivity and reaction paths of both types of mullite precursor have been
29Si N M R
241
investigated by Jaymes et al. (1996) who found that, depending on the preparation method, the more reactive monophasic gel shows a 29Si band centred at - 80 to - 9 5 ppm, in the region of aluminosilicate units (Figure 4.27A). On heating, these gels may show evidence of the separation of uncombined SiO2 (an additional broad peak at about - 1 1 0 ppm) before converting to the characteristic mullite spectrum at 1300~ Diphasic gels (Figure 4.27B) show a broad 29Si resonance centred in the region of uncombined SiO2 ( - 108 to - 110 ppm) which co-exists for a time with the typical aluminosilicate spectrum appearing at about 1200~ (Jaymes et al. 1996). The 29Si NMR resonance in a more homogeneous mullite gel prepared from all organometallic reagents was found to have its maximum in the characteristic region of mullite (about - 86 ppm), even though the sample contained no crystalline mullite (Jaymes et al. 1994). Mullite precursor gels prepared from organic reagents sometimes darken on heating, maintaining the colour to relatively high temperatures. 29Si NMR spectra of such samples reveal a shoulder at about - 50 ppm, in addition to the aluminosilicate and free SiO2 peaks at about - 86 and - 114 ppm respectively (Figure 4.28) (MacKenzie et al. 1996a). The position and thermal stability of the low-field peak have led to its identification as a stable amorphous silicon oxycarbide ("black glass") (see below). 29Si NMR has proved useful in studies of the structures of coprecipitated SiO2-TiO2 gels and their thermal decomposition products. The formation of these materials, which have applications as catalysts, has been shown by CPMAS to involve the formation of silica zones, especially when extensive pre-hydrolysis of the organometallic precursor is permitted. Molecularly mixed precursors were found to predominate only where the partial hydrolysis of the tetraethoxysilicate has been carried out in acid prior to adding the tetraisopropoxytitanate (Walther et al. 1991). The formation of TiO2-SiO2 gels has also been studied by 298i NMR by Dirken et al. (1995), Pickup et al. (1999)
A
Monophasicgel
mulliteI ~ 1 3 0 0 ~
B
Diphasicgel
mulliteI ................ ~ ~ ~ .' / ~~___1300~ 1400~
1275oc ---9. ~ ~ / / ~ _ \~---1200oC spinelI ~ / / ~ ~ ~ \ ~ 1000~ 975~ ~ j / ~ 840~ 750oC amorphousI ~500o C -40 -70 -100-130 -40 -70 -100 -130 29Si shift (ppm) w.r.t. TMS 29Si shift (ppm) w.r.t. TMS
980oc ---}~ ~ ~ \ ~'--lO00~ " - ~ - - - - ~ \ ~-~ 900~ / - ~ ~ ' ~ \ ~'~ 600~ amorphousI ~ _ _ ~ j \ ~_~ 300oc
Figure4.27. Changes in the 29Si spectra of A. monophasic, and B. diphasic gels of mullite composition on heating. Note the predominantly aluminosilicate resonance ( - 85 ppm) in the monophasic gel at lower temperatures, compared with the predominantly siliceous resonance ( - 108 ppm) in the diphasic gel. From Jaymes et al. (1996), by permission of copyright owner.
242
Multinuclear Solid-State NMR of lnorganic Materials
|
0
Figure 4.28. air at 350~
t
|
i
,
9
t
|
I
i,,
|
i_
-40
-60 -120 -160 -200 29Si shift ( p p m ) w.r.t. T M S
29Si spectrum of a dark-coloured mullite-composition aluminosilicate gel heated in fitted to three Gaussian peaks. Note the peak at - 49 ppm, attributed to a stable amorphous silicon oxycarbide. From MacKenzie et al. (1996a).
and Andrianainarivelo et al. (1996), who also used the technique to study the formation of ZrO2-SiO2 gels. 29Si NMR of the latter indicated a homogeneous distribution of the components on an atomic scale, reflected by the presence in the NMR spectra of a substantial number of Si-O-Zr bonds. A similar 29Si NMR result was reported by Pickup et al. (1999a) for SiO2-ZrO2 gels, in which the nearest neighbour environment of the Zr was found to be similar to that of cubic zirconia. 295i NMR studies of a series of silica-based gel precursors of mixed oxide catalysts including SiO2-TiO2 and SiO2-ZrO2 (Miller and Lakshimi 1998) indicate that the Ti and Zr cations are substitutionally inserted into the silica framework. 29Si MAS NMR has also been used to study the gel synthesis of other silicates, including various magnesium silicates (Hartman and Millard 1990), glass-like cordierite (Selvaraj et al. 1990) and barium aluminosilicate (celsian) (MacKenzie and Kemmitt 1999). The structural organisation in Si3B3N7 ceramics prepared by ammonolysis and pyrolysis of a 29Si-enriched gel precursor CI3SiNHBC12 has been investigated by 29Si{ I~B } REAPDOR and liB {298i } REDOR NMR spectroscopy (van Wiillen et al. 2000). These combined experimental approaches have provided evidence of two types of connectivity in these materials, characterised by regions containing mainly Si-N-Si linkages and other regions containing predominantly B-N-B linkages (van Wiillen et al. 2000).
4.9.6 Other a m o r p h o u s materials
Mechanochemical processing (high-energy grinding) is increasingly attracting attention as a method for producing inorganic materials from oxides and minerals at lower
29Si N M R
243
temperatures than required by conventional synthesis methods. Grinding together the reactants produces precursors in which the components are mixed to a high degree of homogeneity and thus require less thermal energy for their conversion to the desired product. Since grinding also disrupts the long-range atomic order of crystalline reactants, NMR is an ideal technique for studying the amorphous precursors and monitoring their subsequent thermal reactions. The formation of amorphous mullite precursors by grinding various forms of hydrated and unhydrated silica or kaolinite with the hydrated aluminas gibbsite, boehmite and bayerite has been extensively studied by 29Si MAS NMR (Temuujin et al. 1998, 1998a, 1998b,1998c, 2000). The 29Si NMR spectra have proved to be a sensitive indicator of the degree of homogeneity (and thus the reactivity) of the ground precursors, since the most homogeneous materials show, in addition to the uncombined SiO2 peak at about - 1 1 2 ppm, a peak at about - 8 4 ppm indicating the formation of A1-O-Si bonds (Figure 4.29A). On the basis of the 29Si spectra, it was shown that the most efficient mechanochemical reactions in the silica-alumina system are those in which the alumina component is highly hydrated. The degree of hydration of the silica component is less important than the type of hydration; silica gel containing a high proportion of silanol groups forms a more homogeneous precursor than a more highly hydrated silicic acid.
A
gibbsite+ silica gel
_
-84
-110
-112
g~d
-2o
'loo
q8o
29Si shift (ppm) w.r.t. TMS diphasic mullite gel
/~ //.111 ~^,~/~
V~ g r ~ 20 hr
-20
-100
-
-180
29Si shift (ppm) w.r.t. TMS
'loo
-~
29Si shift (ppm) w.r.t. TMS ~ -114 -83 [ l / ~ ~ /
f~/~. -20
^ . ~/ -100
-180
29Si shift (ppm) w.r.t. TMS
Figure 4.29. Effect of grinding on the 29Si spectra of the alumina-silica system. A. Mixture of gibbsite, AI(OH)3 and silica gel, and B. Diphasic gel of mullite composition. Note in both systems the evidence of Si-O-A1 bond formation (peak at about - 84 ppm). Adapted from Temuujin et al. (1998a) and Temuujin et al. (1999).
244
Multinuclear Solid-State N M R of lnorganic Materials A
-111
silicic acid +
Mg(OH)z
-91.5
ground
-84.3 20 hr
B
-111 silicic acid + Mg(OH) 2
~ ~ / -100
hydrothermalhr _.~ ~
-85.3 -93.8
80~ 24 1
|
-20
J
.
.
,
,
i
-100
i
-180
~9Si shift (ppm) w.r.t. TMS
t
i
-20
t
|
-100
,
- - ,
-180
z9Si shift (ppm) w.r.t. TMS
Figure 4.30. Effect of grinding on the 298i spectra of the magnesia-silica system. A. Ground mixture of Mg(OH)2 and silicic acid. B. The same mixture not ground but hydrothermally reacted. The similarity in the two results suggests that grinding induces an unusual hydrothermal reaction. Adapted from Temuujin et al. (1998d). Mechanochemical processing has also been shown by 298iMAS NMR to be capable of converting a diphasic mullite gel into a more homogeneous precursor by forming A1-O-Si bonds (Figure 4.29B) (Temuujin et al. 1999). Mechanochemical reactions within the system MgO-SiO2-H20 result in the formation of an unusual layer-lattice hydrous magnesium silicate with 29Si resonances at - 9 1 . 5 and - 84.2 ppm (Figure 4.30A) (Temuujin et al. 1998d). The peak at - 91.5 ppm could arise from the Q3 unit of a hydrated magnesium silicate (although it does not correspond precisely to talc or serpentine), while the peak at - 84.2 ppm indicates the presence of an amorphous anhydrous magnesium silicate similar to enstatite, MgSiO3. A similar 298iNMR spectrum was found in a hydrothermally reacted mixture of Mg(OH)2 and silicic acid, suggesting that the mechanochemical sample results from an unusual mechanochemically-induced hydrothermal reaction (Figure 4.30B) (Temuujin et al. 1998d).
4.10. Si-NANDSi-N-OCOMPOUNDS Silicon nitride, oxynitride and silicon aluminium oxynitride (SiA1ON) compounds are of considerable technical interest as advanced ceramics because of their stability and chemical inertness at elevated temperatures, and their excellent mechanical properties. 29Si MAS NMR has proved to be useful in studying various aspects of these compounds, including their formation and structure, the processes by which they are
29Si N M R
245
formed into useful components (e.g. sintering) and also those which degrade the material in service (e.g. oxidation). NMR can be used for phase identification ("fingerprinting") of these compounds by comparing their spectra with those of well-characterised single-phase material, for differentiating between the various polymorphs, with the possibility of making semiquantitative estimates, and for providing structural information by identifying the local atomic environments or structural sub-units. The characteristic chemical shift of tetrahedral Si-N is about - 4 8 ppm. Silicon nitride, Si3N4, occurs in two polymorphic forms, the lower-temperature o~-form differing in its stacking sequence from the [3-form to which it converts at about 1400~ ot-Si3N4 contains two inequivalent Si sites, with an average bond length difference of 0.0021 nm, which give rise to two sharp 29Si resonances at - 47.1 and - 49 ppm (Figure 4.31A). By contrast, ~-Si3N4 contains a single 29Si resonance at about - 48 ppm (Figure 4.31B). Although these differences are small, the resonances co-existing in materials of good crystallinity can be sufficiently sharp to provide an estimate of the relative amounts of each polymorph in samples sintered in the presence of various additives (MacKenzie and Meinhold 1994). Information about the relative amounts of each polymorph is also useful for predicting the sinterability of Si3N4 powders, since the c~-form is more reactive than the ~-form. Amorphous Si3N4 has a resonance at - 4 7 ppm but a much greater linewidth (Carduner et al. 1987). Recently a cubic form of Si3N4 containing both tetrahedral and octahedral Si-N in a spinel structure has been produced by high-pressure shock synthesis (Sekine et al. 2001). The 29Si MAS NMR
A
-47 r
-49
4
C cubic Si3N4
octahedral Si-N
tetrahedral
B -48
~-Si3N4
,~1
,
0
I
t
I
-40
,
I
I
si-N
!
!
~
I
f
-80
29Si shift (ppm) w.r.t. TMS
L'
t
'
I
'
O
I
I
I
I ~
I
I -225
'
'
I
'
'
-1O0
'
I
I
. . . .
II
i
-2OO
'"
!
! ' '
'
I
I-I
-300
29Si shift (ppm) w.r.t. TMS
Figure 4.31. A, B. The 29Si spectra of the two polymorphs of silicon nitride containing tetrahedral Si-N units. C. The 29Si MAS NMR spectrum of cubic Si3N4 showing both tetrahedral and octahedral Si-N resonances in the expected tetrahedral:octahedral ratio of 1:2. From Sekine et al. (2001) by permission of the copyright owner.
246
Multinuclear Solid-State NMR of Inorganic Materials
spectrum of this compound (Figure 4.31C) shows the tetrahedral Si-N resonance at - 50 ppm and a peak at - 2 2 5 ppm corresponding to octahedral Si-N sites. The relative occupancy of the tetrahedral:octahedral sites is about 1:2, consistent with the spinel structure (Sekine et al. 2001). Silicon nitride can be produced by several processes, which have been studied by 298i MAS NMR. Direct nitridation of elemental silicon powder (chemical shift - 7 9 to - 8 0 ppm) has been found to be accompanied at 1375-1425~ by a progressive change of about 10 ppm towards the less negative value of Si3N4, the resonance of which simultaneously grows at its expected position (Figure 4.32) (MacKenzie et al. 1999). This unexpected change in the chemical shift may be due either to an increase in the bandgap of the Si as the core size of the reactant grains decreases in the last stages of the reaction, or to an increase in the Knight shift resulting from a change in the donor concentration of the Si due to its interaction with the nitrogen (MacKenzie et al. 1999). Poorly crystalline nanoscale Si3N4 prepared by laser heating has been shown by 298i NMR to contain both Si-OH and Si-O bonds (Yue et al. 1996). A surface layer of Si3N4 deposited on SiO2 particles by gas phase reaction between trichlorosilane and ammonia has been characterised by 298i MAS NMR (van der Voort and Vansant 1996) and 298i MAS NMR has been used to identify the reaction intermediates accompanying Si3N4 formation by pyrolysis of polyethylsilazane within the NMR probehead (Sigmund et al. 1996).
A -80.3 .J~ .
1300"1350~
1375-1400~~ 8 ~ m.,-72 1425~ ~ 1 8
~
~ -76 o~
o
48~ ~-71.5 m -80
1475~ J
0 600 1200 Temperature (~
~ I
I
I
-40 -90 -140
29Si shift (ppm) w.r.t. TMS Figure 4.32. A. Nitridation of elemental Si monitored by 298i spectroscopy. As the Si3N4 peak (-48 ppm) grows during the reaction, the elemental Si peak at - 80 ppm progressively shifts to less negative values. At the highest temperatures some oxidation to SiO2 ( - 112 ppm) is also apparent. B. Shift of the 298i peak for elemental Si as a function of nitriding temperature. From MacKenzie et al. (1999) by permission of copyright owner.
29Si N M R
247
Sintering of Si3N4 powder is difficult to achieve because it does not form liquid phases through which diffusion and mass transport can occur. Small amounts of additives such as MgO and Y203 which form intergranular liquid phases may therefore be used to assist sintering. The interaction of these sintering aids with Si3N4 at sintering temperatures has been studied by 29Si MAS NMR, supplemented by 27A1, 25Mg and 89y MAS NMR (MacKenzie and Meinhold 1994). Since only very small amounts of intergranular phases are formed by reaction with the sintering aids, their detection by NMR spectroscopy in sintered Si3N4 can be very difficult. An interesting solution to this problem was adopted by Carduner et al. (1989) who increased the packing density in their MAS probe by using highly sintered samples machined into cylinders fitting tightly into the rotor. Only one stoichiometric silicon oxynitride phase, SizN20 is known, containing a single Si site associated with the structural unit SiN30 having its characteristic 298i resonance at - 63 ppm (Dupree et al. 1985). 298i MAS NMR has been used to investigate the constitution of amorphous silicon nitride and oxynitride fibres prepared by a gasphase process (Chollon et al. 1998). The presence of small amounts of oxygen in ot-Si3N4 fibres was distinguished by a 298i peak at - 5 6 . 4 ppm arising from SiN30 units. Other types of silicon oxynitride fibres show a broad 298i resonance centred at - 48 ppm (Si3N4) but with a tail to more negative chemical shifts ascribed to a distribution of resonances from SiN30 ( - 62 ppm), SiN202 ( - 78 ppm) and SiNO3 units ( - 9 4 ppm) (Chollon et al. 1998).
4.11.
Si-AL-O-NCOMPOUNDS
The sialons are a group of ceramic compounds of increasing technological importance for their thermal, chemical and mechanical properties. These are similar to Si3N4 but can be more readily tailored to specific requirements because of the wide range of possible sialon compositions. The sialons fall into several structural types:
4.11.1 [3-sialon, S i 6 - z A l z O z N s - z
In these compounds z can have values from zero (representing pure Si3N4) to about four. These compounds are isostructural with ~-Si3N4 and show a similar 29Si chemical shift of about - 4 9 ppm which remains constant over the composition range apart from some line broadening at higher z values (Dupree et al. 1988, Sjoberg et al. 1992, Yue et al. 1996a). The 29Si results indicate that the Si bonding is preferentially to N in the [3-sialon structure. Of the various methods which can been used to prepare [3-sialon, several have been investigated by 29Si NMR. Thus, Yue et al. (1996a) reported that the products of
248
Multinuclear Solid-State NMR of Inorganic Materials
self-propagating high-temperature synthesis from a mixture of A1, Si and A1203 under nitrogen have 29Si NMR spectra similar to compounds prepared by conventional solidstate reaction. Synthesis from the organometallic precursor polyaluminocarbosilane (Soraru et al. 1991) and the extension of the process to the production of [3-sialon fibres (Soraru et al. 1993) have both been studied by 29Si MAS NMR. However, the synthesis process most extensively studied by solid-state NMR is that of carbothermal reduction of aluminosilicate minerals such as kaolinite, which are mixed with finely divided carbon and heated in nitrogen at > 1400~ (Neal et al. 1994, MacKenzie et al. 1994a). Under carbothermal conditions the clay decomposes to a mixture of mullite and amorphous silica (MacKenzie et al. 1996b), the latter forming SiC which reacts with the mullite to form [3-sialon, in some cases via other sialon phases such as X-sialon (see below). The precise reaction sequence and the nature of the intermediates has been shown by the NMR studies to depend on various factors including the nature of the aluminosilicate starting mineral (MacKenzie et al. 1994a). In a variation on this process which has been used to prepare [3-sialons with low z-values the additional Si can be supplied by the addition of either SiO2 or elemental Si. This reaction, which is assisted by the addition of a small amount of Y203, has been studied by NMR, which shows that the sequence is unaffected by the Y203, which facilitates the formation of both mullite and [3-sialon and promotes the formation of A1-N units (MacKenzie et al. 1997). Since the carbothermal reactions take place in a flowing gas atmosphere, problems arising from vapour phase transport can occur, leading to the removal of material from the reacting mixture and its re-deposition as an amorphous coating in the cooler parts of the furnace. NMR studies (Figure 4.33) have enabled the deposit to be identified as a mixture of SiC, SiO2 and a K-feldspar, with elemental Si sometimes appearing as a result of the disproportionation of vapour-phase SiO (Ekstrom et al. 1996). The NMR identification of this product assemblage was found by thermodynamic calculations to be feasible. The sintering of sialon powders can be subject to the same difficulties as experienced with Si3N4, and similar sintering additives (MgO or Y203) have been found to assist. The nature of the intergranular phases formed by these oxides during sintering of [3-sialon has been studied by 29Si, 2VA1,25Mg and 89y MAS NMR (MacKenzie and Meinhold 1996). Oxidation is the major cause of failure of sialon ceramics used at higher temperatures in air. The kinetics and mechanism of [3-sialon oxidation have been determined in studies in which 29Si NMR were used to identify the formation sequence of the early-stage products (MacKenzie et al. 1997a, Shimada et al. 1998). The 29Si NMR spectra of partially oxidised [3-sialon (Figure 4.34) clearly indicate the initial formation of amorphous SiO2 ( - 1 1 3 ppm), followed at higher temperatures by mullite
249
29Si N M R
-19
-108 -62
-19
40 -40 -120 29Si shift (ppm) w.r.t. TMS Figure 4.33. 298i spectra of three samples of X-ray amorphous "wool" vapour deposited in the cool parts of the system during carbothermal synthesis of [3-sialon. All samples contain SiC ( - 19 ppm) and SiO2 ( - 110 ppm) and one contains elemental Si (-79 ppm). The peak at - 62 ppm is from a small amount of SizN20. From Ekstrom et al. (1996) by permission of copyright owner. unoxidised t
0
~-47 -115
IO00-1100oC /~ ~ -44 -88 -113
-47
9~
-80
-160 0 -80 -160 0 29Si shift (ppm) w.r.t. TMS
-80
-160
fully oxidised 1100-1200 oC
1300_1400oC 86 [A'112 !
t
0
-80
-160
t .... 4
0
//-110
9
|
#
-80
i
!
t
1__
-160
29Si shift (ppm) w.r.t. TMS Figure 4.34. 298i spectra of the stages in the oxidation of [3-sialon. As the sialon peak at - 47 ppm decreases, the first oxidation product is amorphous SiO2 ( - 113 ppm) followed by mullite ( - 86 ppm). From MacKenzie et al. (1997a), by permission of copyright owner.
250
Multinuclear Solid-State NMR of Inorganic Materials
( - 86 ppm). A semi-quantitative estimate of the partitioning of the Si between these two phases, made by curve-fitting the 29Si NMR spectra, indicates the ratio of Si in SiO2 to S i in mullite to be about four, in reasonable agreement with the oxidation equation
6Si7A15OsNll ~ 4A16Si2013 + 34SIO2 + 2A1203 + 33N2
(4.30)
for which the ratio is 4.25. The presence of the A1203 predicted by this equation was confirmed by 27A1MAS NMR (MacKenzie et al. 1997a).
4 . 1 1 . 2 0 - s i a l o n , Si2_xAlxO l +xN2_x
In these compounds x can have values from zero (representing pure Si2N20) to about 0.4. These compounds are isostructural with SizN20 and retain the same 298i shift of about - 63 ppm over the entire composition range (Sjoberg et al. 1992). Since O-sialon is a Si-rich phase, it can conveniently be synthesised by a silicothermal reaction between kaolinite clay, silica and elemental Si in a nitrogen atmosphere (Barris et al. 1997). During this reaction, a new compound was discovered with an XRD pattern unlike that of O-sialon. 298i NMR showed this new compound to contain the SizN20 units typical of O-sialon (Figure 4.35). This result led to the subsequent identification of the new phase as a new low-temperature form of O-sialon (Barris et al. 1997). O-sialon is technically important because of its superior resistance to attack by molten metals and to oxidation at higher temperatures. Its oxidation reactions have been studied by 298i and 27A1 MAS NMR which indicates that it is increasingly protected from oxidation at higher temperatures by the formation of a protective layer of SiO2 containing some octahedral A1 (MacKenzie et al. 1998).
O-sialon
New sialon phase -61 -113
-50 -100 -150
-50 -100 -150
~9Si shift (ppm) w.r.t. TMS
Figure 4.35. Comparison of the 298i spectrum of a previously unknown sialon phase with that of O-sialon. The diagnostic presence of oxynitride units in the new phase ( - 61 ppm) has led to its identification as a low-temperature form of O-sialon. From Barris et al. (1997), by permission of copyright owner.
29Si NMR
251
4.11.3 X-sialon, nominally Si12Al18039N 8 This sialon is isostructural with mullite, being a solid solution with Si3N4. An earlier NMR study (Klinowski et al. 1984) reported tetrahedral and octahedral 27A1 resonances but was unable to obtain a 29Si spectrum, possibly due to a long relaxation time. More recently, a 29Si resonance has been reported at - 56.5 ppm (Smith 1994), suggesting the occurrence of SiO2N2 units in the structure, in which the nitrogen preferentially occupies the tricoordinate anion sites of the mullite with Si as the nearest neighbours. The 29Si spectrum of a phase-pure X-sialon prepared by silicothermal synthesis (Sheppard et al. 1997) acquired with a long recycle delay time (3000 s) shows a broad envelope with maximum intensity at - 6 6 . 5 to - 6 8 ppm and at least three shoulders at - 57, - 76 and - 90 ppm. This spectrum is consistent with the presence of a range of Si-O-N environments with the major contribution from SiO3N units containing at least two different Si environments. The major tetrahedral Si-O resonance of mullite ( - 88 to - 90 ppm) is also present. The 29Si chemical shifts of the various oxynitride and SiA1ON units are shown schematically in Figure 4.36. Polytypoid s i a l o n and r
O-sialon
-
Si-AI-O
X-sialon
I
,
9- Si3N 4 (tet) Si3N 4 (oct) ~ --
-225
SiO2N 2 = Si2N20 9- - -
SiOaN
SiC3N II
Ill
SiC2N 2 SiCN 3 -
S i 3 N 4 (tet) Si3N 4 (oct) ----> -225 -
-
Si
SiC30 SiC
i
-
SiC20 2
-
SiCO3 SiO2
I
20
I ,
I
0
I
f
,1.
-20
-40
29Si Figure
,.
I
I
I
-60
!
-80
I
I ....
-I
-100
shift (ppm) w.r.t. T M S
4.36. Chemical shift ranges of the various Si-N, Si-C units and their related compounds.
252
Multinuclear Solid-State NMR of lnorganic Materials
S ilicothermal synthesis from a mixture of kaolinite, S i powder and ~/-A1203 has proved to be a useful method for preparing X-sialon, and has been shown by solid-state NMR to proceed in several parallel steps, including the nitridation of the Si, decomposition of the clay to mullite and amorphous SiO2, reaction between the SiO2 and the ~/-A1203, and reaction between the Si3N4 and the aluminosilicate (Sheppard et al. 1997). The effect of a number of metal oxide additives on the silicothermal synthesis and densification of X-sialon has also been studied by solid-state NMR (Sheppard and MacKenzie 1999). 298i MAS NMR (Figure 4.37) showed that the conversion of Si3N4 to the SiOzN2 and SiO3N units of X-sialon is facilitated by the presence of one wt% MgO, CaO, CeO2 (which produce samples with 298i NMR spectra typical of wellreacted X-sialon) and ZrO2. BaO and Y203 facilitate the preferential formation of SiO3N units, whereas Fe203 facilitates SiOzN2 formation (Sheppard and MacKenzie 1999). The effect of higher concentrations of MgO and Y203 on X-sialon formation has also been studied by 298i MAS NMR, augmented by 27A1, 25Mg and 89y NMR spectroscopy (MacKenzie et al. 2000). qr X-sialon
w /q Oil D
'~ l 9
'3OO"4OO~
~#.
Zr, Ce O0*C Ce~1500'
OJ~l:3 /
B a ~
~Fe
O
,
,
" -lO0
9
, ooo I r
0
I
I
"1"
-100
Figure 4.37. Effect of 1 wt% of various metal oxide additives on the silicothermal formation of
X-sialon at 1500~ monitored by 29Si NMR. As the reaction proceeds, the shape of the spectra change according to changes in the various regions corresponding to particular structural units as marked: * -- silicon oxynitrides, O = S i 3 N 4 , [::] : S i O 2 , 9 = mullite. Note that not all oxide additives produce the characteristic X-sialon spectrum at 1500~ Adapted from Sheppard and MacKenzie (1999).
29SiNMR
253
29Si NMR has shown that X-sialon powder is less stable to high-temperature oxidation than O-sialon, beginning to lose nitrogen from the SiO2N2 units below 940~ and forming mullite at 1200-1300~ (MacKenzie et al. 1998).
4.11.4 Polytypoid sialons,
(SiraOm(O,S)m+l
These are a range of sialons with structures closely related to A1N in which the nonmetal atoms in excess of that required by the 1:1 wurtzite A1N layer structure are accommodated by the formation of an octahedral layer and the half-filling of the tetrahedral layers. Several 29Si NMR peaks have been reported for various polytypoid sialons, with isotropic chemical shifts ranging from about - 3 5 to - 4 9 ppm (Sjoberg et al. 1992, Klinowski et al. 1984, Butler et al. 1984, Smith 1992). Some of the SiN4 tetrahedra in the polytypoid sialons are edge-sharing, accounting for the more highly positive chemical shifts than those in Si3N4.
4.11.5 oL-sialons,
MxSi12_(m+n)Alm+nOnN16_n
The structure of these sialons is based on that of oL-Si3N4, but requires the presence of other metal ions (M) for stabilisation. The M ions are typically yttrium and rare-earths, but can also be Mg or Ca. The values of m and n in the formula depend on the metal ion present. The oL-sialons, which have extremely good mechanical properties, also form technically useful solid solutions with [~-sialons. The 298i chemical shift of oL-sialon (about - 48 ppm) is similar to that of Si3N4 and appears to be insensitive both to the sialon composition and the nature of the M ion (Leach et al. 1990). Carbothermal synthesis has been used to prepare yttrium a-sialon from mixtures of halloysite clay and Y203 with either SiO2 or elemental Si. The reaction mechanism, particularly the role played by intermediate SiC phases, was monitored by 298i NMR (Ekstrom et al. 1998). The preparation of fine-grained Ca oL-sialon has also been studied by 298i NMR (Hewitt et al. 1994). The reported 298i chemical shifts for these and related compounds are collated in Table 4.6.
4.12. OTHER METAL SILICON NITRIDES AND OXYNITRIDES
Three compounds have been identified in the system Y-Si-N; these are YSi3Ns, Y2Si3N6 and Y3Si6N11 (Ekstrom et al. 1997). These phases have been produced by a carbothermal process and their 29Si and 89y NMR spectra reported (Table 4.6). A range of silicon oxynitrides is also formed with heavy elements such as yttrium and lanthanum. The yttrium compounds include Y2Si303N4 (N-melilite), Y4Si207N2 (N-YAM or J-phase), YsSi3012N and YSiO2N. The 29Si NMR shifts of these
254
Multinuclear Solid-State NMR of lnorganic Materials
Table 4.6. A selection of 298i chemical shifts for silicon nitride, silicon oxynitride and SiA1ON compounds and related metal silicon nitrides and oxynitrides, with respect to TMS. Compound oL-Si3N4 13-Si3N4 cubic Si3N4 Si2N20 [3-SiA1ON O-SiAION X-SiAION Polytypoid SiAION oL-SiA1ON YsSi3012N LasSi3012N Y4Si207N2 Y4SiA1OgN LaaSizOvN2 Y2Si303N4 Y2Si2.9Alo.lO3.1N3.9 SmzSi2.sAlo.zO3.2N3.8 YSiO2N LaSiO2N La3Si3A13OI2N2 La3Si804N11 Y2SiA1OsN MgsSi3A1Ol iN LiSiON YSi3N5 LaSi3N5 Y2Si3N6 Y6Si3Nlo Mg2SiN2 MgSiA1N3 LiSi2N3
29Si shift (ppm)
Reference
-49.0. -47.1 Harris et al. (1990) - 48.5 Harris et al. (1990) - 50, - 225 Sekine et al. (2001) -62 Dupree et al. (1985) -48.5 Dupree et al. (1988a) -60.9 to -61.3 Sjoberg et al. (1992) -66.5 to - 6 8 , - 5 7 , - 7 6 , - 9 0 Sheppard et al. (1997) - 35 to -49 Smith (1992) -48 Leach et al. (1990) -67.5, -74.8 Dupree et al. (1988) -77.7 Dupree et al. (1989) -74.4 Dupree et al. (1988) - 74.8 Smith (1999) - 84.2 Dupree et al. (1989) -56.7 Dupree et al (1988) -57 Chee et al. (1995) - 35 Chee et al. (1995a) -65.3 Dupree et al. (1988) - 72.4 Dupree et al. (1989) -68, -77 Thompson et al. (1991) - 56.5, -68.2 Thompson et al. (1991) -72.2 Kruppa et al. (1991) -62.0, - 75.1 Smith (1999) -51.0 Thompson et al. (1991) -42.3, -45.5 Dupree et al. (1988) -57.3, -65.4 Dupree et al. (1989) - 58.3, -60.8, -65.3 Ekstrom et al. (1997) -35.3, -37.7 Ekstrom et al. (1997) -44.4 MacKenzie and Meinhold (1994) -42.5 MacKenzie and Meinhold (1994) -49.3 Leach et al. (1990)
compounds, shown in Table 4.6, typically indicate a change of 10-15 ppm for each nitrogen atom substituting for oxygen. 298i NMR has been used to investigate the details of YzSi303N4 formation from Y203 and Si3N4 under nitrogen (Dupree et al. 1988), indicating that the reaction occurs via the intermediate formation of YSi3Ns. The single 298i signals obtained from YzSi303N4 and Y4SizOvN2 suggested that their structures contained a single Si site (Dupree et al. 1988), but more recent 15N NMR work on Y4SizO7N2 (Hauck et al. 1993) showed two N resonances, implying that the single 298i peak must be composed of two overlapping resonances. Similarly, the 15N spectrum of YzSi303N4 (Koroglu et al. 1996) can be fitted to three overlapping peaks, consistent with a structure containing two distinct Si sites which cannot however be distinguished by 298i NMR spectroscopy.
255
29Si NMR
4.13. Si-C,Si-C-O AND Si-C-NCOMPOUNDS The 298i resonances of Si-C bonds occur at significantly lower fields than Si-O and Si-N, allowing ready differentiation of these species. Silicon carbide is a diamond-like network solid occurring in a large number of polytypes with different layer stacking sequences. These polytypes occur in two structural forms, hexagonal oL-SiC with the wurtzite-type structure, and cubic [3-SIC with diamond or zincblende structure. The first neighbours of the Si in both the hexagonal and cubic forms are four carbon atoms at the comers of a tetrahedron, but differences in the next-nearest neighbour configurations produce different 298i NMR spectra (Figure 4.38). The hexagonal structure contains three different next-nearest neighbour configurations, leading to three 298i resonances at -13.9, - 2 0 . 2 and - 2 4 . 5 ppm, whereas the cubic form contains 12 next-nearest neighbours located at the comers of a cuboctahedron, giving rise to only one 298i peak at - 18.3 ppm (Finlay et al. 1985). These peaks may be shifted upfield in whiskers and samples of small particle size (Wagner et al. 1989). Differences in the areas of the 298i peaks found for the various polytypes are related to the structure (Guth and Petuskey 1987, Hartman et al. 1987), and were used to predict the spectra of other SiC polytypes. However, the relaxation times of pure SiC samples can be extremely long due to the lack of an effective relaxation mechanism in dilute spin-l/2 rigid solids. The use of insufficiently long recycle delay times can lead to the 298i (and 13C) signals of high-purity components of SiC mixtures being anomalously small or missed entirely. The presence of impurities such
-18 [}-SIC -20 ~-SiC
"11~ ~ - 2 5
amorphous SiC - i ~ ~ . ~ ~ . ~
-i
,
.
i
i
i
-40
29Si shift (ppm) w.r.t. TMS
Figure 4.38. 29Sispectra of the oLand [3-polymorphsof SiC, and amorphous SiC. From Guth and Petuskey (1987), by permission of the AmericanChemical Society.
256
Multinuclear Solid-State NMR of lnorganic Materials
as nitrogen can have widely different effects on the relaxation times of various sites in hexagonal SiC polytypes (Hartman et al. 1994). Commercial SiC powders can contain stacking faults which cause broadening of the 295i spectrum (Figure 4.38), suggesting that NMR is a useful method for characterising the microcrystalline disorder of commercial SiC powders (Carduner et al. 1990). Cross-polarisation between 29Si and 1H has been used to demonstrate the presence of protonated species in plasma-produced SiC powder (Dando and Tadayyoni 1990).
4.13.1 Silicon oxycarbide species
Silicon oxycarbide glasses, or"black glasses", produced by a sol-gel process from organo trimethoxysilanes, have been characterised by 29Si MAS NMR (Zhang and Pantano 1990). In these glasses, which have the general formula SiCxO2(1- x), one tetravalent carbon substitutes for two divalent oxygen atoms, leading in the amorphous network to the formation of C(Si)4 units; part of the carbon in gel-derived glasses can also be present as a dispersed free carbon phase (Soraru et al. 1995). The 29Si NMR spectra of black glasses formed from gel precursors can show a range of Si-O-C units, from SiC30 ( - 5 ppm), SiC4 ( - 16 ppm), SIC202 ( - 35 ppm), SiCO3 ( - 70 ppm) and SiO4 (about - 108 ppm) (Soraru et al. 1995). The 29Si chemical shifts for these species are shown schematically in Figure 4.36. A similar range of silicon oxycarbide species can be seen in the 29Si NMR spectrum of commercial "Nicalon SGN" silicon oxycarbide fibres (Hatfield and Carduner 1989) (Figure 4.39) and were shown by 29Si NMR to be present as impurities in commercial SiC fibres (Inkrott et al. 1986). 29Si NMR has been used to confirm the presence of SiCO3 units in amorphous silicon oxycarbide fibres produced by a sol-gel method from an organically modified silicate (ORMOSIL) precursor (Hu 2000). SIC,,
ISiOzC2 SiOC3
T
-
.
.
.
.
-50
-150
29Sishift (ppm)w.r.t. TMS Figure4.39. 29Sispectrum of commercial Nicalon SGN silicon oxycarbide fibre, showingpeaks corresponding to a range of oxycarbide units. From Hatfield and Carduner (1989), by permission of copyright owner.
29Si NMR 4.13.2 Silicon carbonitride
257
species
Interest in composite materials formed between Si3N4 and SiC has led to the use of 295i NMR to identify the Si-C-N structural units present in these compounds. The 29Si NMR spectrum is believed to change progressively as the nitrogen of Si3N4 is substituted by carbon, but many of these studies have been made on amorphous materials derived by pyrolysis of organic gel precursors resulting in broad overlapping 29Si carbonitride peaks, making assignments difficult. Suzuki et al. (1995) suggest that the 29Si resonance of SiC3N units is at + 30 to - 5 ppm, that of SiC2N2 is at - 5 to - 27.5 ppm (overlapping with SIC), and that of SiCN3 is at - 27.5 to - 42.5 ppm (note that there appears to be a misprint in this paper). These ranges are in general agreement with Mocaer et al. (1993), who in pyrolysis studies of polycarbosilazane precursors assigned a peak at - 10 ppm to SiCzN2 units, and with the assignment to SiCN3 units by Lewis and Maciel (1995) of a peak at - 25 ppm in pyrolysed hydridopolysilazane. 295i NMR has also been used to identify the pyrolysis intermediates and products from polyhydridomethylsilazane (Seitz et al. 1996) and functionalised chlorosilanes (TraB1 et al. 2000).
4.14. OTHER MATERIALS
4.14.1 Biologically compatible
glasses
Bioglass| materials are complex bioactive Na-Ca-P-silicate glasses which are currently used to replace soft tissue and bone (Hench 1991). Their structure has been studied by 29Si, 23Na and 31p NMR (Lockyer et al. 1995) and was shown to consist of Q2 [Si(OSi)2(O-)2] and Q3 [Si(OSi)30-] glass networks, the latter associated preferentially with Na + and the former with Ca 2+. This partitioning is suggested to be responsible for the bioactivity of the glass, which undergoes extensive surface reorganisation in vivo to form the bone mineral hydroxyapatite. The presence of the Ca-Q 2 regions in the glass moderates the dissolution of the Na-Q 3 regions, allowing gel formation which is followed by migration of Ca 2+ and PO4 3- to form the biocompatible layer (Lockyer et al. 1995).
4.14.2
Cements
Portland cement is typically composed of about 25% [3-dicalcium silicate (lamite), and 50% tricalcium silicate with the balance made up of various calcium aluminates and calcium iron aluminate (brownmillerite). Setting occurs when the cement is hydrated; all the components show varying degrees of reactivity with water, but the most significant hydraulic activity is associated with the tricalcium silicate, which forms a cohesive mixture of calcium hydroxide and calcium silicate hydrate (C-S-H)
258
Multinuclear Solid-State NMR of lnorganic Materials
with water. The structure and formation mechanism of C-S-H from tricalcium silicate is a subject of considerable interest in cement chemistry, and has been investigated by 29Si MAS NMR, which shows that as hydration proceeds beyond the induction stage, a dimer is formed, evidenced by the appearance of a Q1 resonance but no Q2 resonance (Lippmaa et al. 1982, Clayden et al. 1984). The presence of fine, reactive silica has been shown by 29Si MAS NMR to increase the degree of silicate polymerisation during tricalcium silicate hydration (Dobson et al. 1988, Young 1988). The formation of C-S-H hydrates from colloidal silica and lime water has also been studied by 29Si MAS NMR (Grutzek et al. 1989). Since the setting of tricalcium silicate involves the formation of hydrates, crosspolarisation studies between 29Si and ~H have been used to provide information about the association of protons with the silicate units in the early stages of the reaction of 29Si-enriched tricalcium silicate (Rodger et al. 1988). The results indicate the formation of a small amount of hydrated material during the induction period, the end of which is therefore not marked by the sudden onset of hydration, but by a change in the product silicate species, from monomers to dimers. 29Si NMR has also been applied to a hydration study of actual Portland cements, the broad spectra of which are composed of the spectra of the component tricalcium silicate and [3-dicalcium silicate (Figure 4.40) (Barnes et al. 1985). Integration of the 29Si spectra of the discrete cement silicates and Portland cement hydrated under identical conditions allowed a comparison of the hydration characteristics (Figure 4.41) and
Unhydrated
Hydrated
-->
opt
C3S
1
3 __
i
-40
i
C ,
9
-70
~ ,
_J
,
-100
29Si shift (ppm) w.r.t. TMS
-40
0
-100
29Si shift (ppm) w.r.t. TMS
Figure 4.40. Changesin the 298i spectra of the cement minerals tricalcium silicate (C3S) and [3--dicalciumsilicate (C2S), and ordinaryPortland cement (OPC) containingthese minerals brought about by hydration at 20~ for one week. From Barnes et al. (1985), by permission of copyright owner.
29Si NMR
259
OPC
100 m
~ ~
C3S
15
25
6O
o
20
-0.5
0.5
Log10 reaction time (days) Figure 4.41. Comparison of the hydration reactivity of the cement minerals C3S, ~-C2S and ordinary Portland cement (OPC) determined as a function of hydration time by integrating their 29Si spectra. From Barnes et al. (1985), by permission of copyright owner.
showed that [3-dicalcium silicate is much less reactive to water than tricalcium silicate, which is responsible for the bulk of the hydraulic activity. 295i NMR has also been used to study the hydration of class G Portland cement (largely tricalcium silicate) (Masse et al. 1993), and Reactive Powder Concrete (RPC), a blend of Portland cement with silica fume and crushed quartz, in which a Q3 peak appearing in samples heated at 250~ was attributed to a crystalline hydrate phase xonoffite (Zanni et al. 1996).
4.14.3 I n o r g a n i c p o l y m e r s
Inorganic polymeric materials (called 'geopolymers') can be formed by condensing aluminosilicates derived from dehydroxylated clays with sodium silicate under extremely alkaline conditions with careful control of the water content (Barbosa et al. 2000). The polymers are three-dimensional Si-O-A1 structures built up from units such as [-Si-O-A1-O-], [-Si-O-A1-O-Si-O-] or [-Si-O-A1-O-Si-O-Si-O-], dictated by the overall composition, with charge balancing hydrated Na + or K + located in the framework cavities. These polymers cure at ambient temperatures, forming hard, strong dimensionally stable materials with good high-temperature properties up to at least 1100~ They can also be used to bond a variety of ceramic and mineral powders into useful inorganic composites. Since these polymers are X-ray amorphous, their formation mechanism and structure has been studied by 29Si, 27A1 and 23Na NMR (Barbosa et al. 2000). The broad 29Si NMR spectra (Figure 4.42) suggest a range of Si environments but with a predominance of Q4(3A1) units, forming an open framework which accommodates the hydrated Na + ions. The thermal stability of these polymers is also indicated by the comparative lack of change in the 29Si spectra even after heating at 1100~ (Figure 4.42).
260
Multinuclear Solid-State NMR of Inorganic Materials
metakaolinite
geopolymer
geopolymer
-105 /~ polymerisation "~1 & curing .~ -40
~t65C~, -120
/~
i ~
1300~
93
' ~
, ~
-40 -120 -40 ~9Si shift (ppm) w.r.t. TMS
-120
Figure 4.42. Formationand thermalstabilityof sodiumpolysialatepolymerpreparedfrom metakaolinite.Afterpolymerisationand curing,the broadspectrumis centredin the Q4(3A1)region and remainsessentiallyunchangedon heatingto 1100~ reflectingthe stabilityof this open frameworkstructure.Adaptedfrom Barbosaet al. (2000), by permissionof copyrightowner.
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Multinuclear Solid-State NMR of Inorganic Materials
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Chapter 5 27A1 NMR General Considerations Chemical Shifts in 27A1Spectra 5.2.1 27A1Chemical Shifts in A1-O Environments 5.2.2 27A1Chemical Shifts in Aluminosilicates 5.2.3 Relationships between 27A1Chemical Shift (~i~o) and Structure 5.3. Five-Coordinated A1-O 5.3.1 A1~v) in Well-Defined (Crystalline) Environments 5.3.2 A1~v) in Non-Crystalline Environments 5.3.3 A1~v) in Zeolites 5.4. Aluminium Oxides 5.5. Amorphous Aluminium Compounds 5.5.1 Aluminate Gels 5.5.2 Glasses 5.5.3 Other Amorphous Systems 5.6. Aluminophosphates 5.7. Aluminium Borate and Molybdate 5.7.1 Aluminium Borate 5.7.2 Aluminium Molybdate 5.8. Aluminium Fluorides 5.9. Thermal Decomposition Reactions 5.10. Cements 5.11. Nitride and Oxynitride Compounds 5.12. Sialon Compounds 5.12.1 Polytypoid Sialons 5.12.2 [3-Sialons 5.12.3 O-Sialons 5.12.4 X-Sialons 5.12.5 oL-Sialons 5.12.6 Sialon Glasses References 5.1. 5.2.
271 272 273 274 279 281 281 283 287 291 294 294 299 303 304 307 307 307 3O8 310 313 316 317 317 318 320 321 322 323 324
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Chapter 5 27A1 NMR 5.1. GENERAL CONSIDERATIONS
Aluminium is an abundant element in nature, in which it is combined with silicon in a wide range of naturally occurring aluminosilicates (silicon, aluminium and oxygen together constitute almost 90 atomic % of the earth's crust). Aluminium is therefore an important element in geochemistry, mineralogy and a wide range of inorganic materials. 27A1 has a spin I = 5/2, and hence a nuclear quadrupole moment arising from a nonspherical distribution of nuclear electrical charge which can interact with electric field gradients at the nucleus (see Chapter 2). These quadrupolar interactions cause broadening and distortion of the spectral peaks and displacement from the isotropic (true) chemical shift. Since these effects decrease with the square of the magnetic field strength, it is often advisable to acquire 27A1 spectra at the highest possible field (although some situations may arise where overlap is better resolved at lower field strengths). Magic angle spinning can also narrow the 27A1 resonances by a factor of about four; although the second-order quadrupolar broadening is not completely removed by this means, other methods such as double rotation (DOR), dynamic angle spinning (DAS) and multiple quantum (MQ) experiments can be applied for this purpose (see Chapter 2). Despite these limitations which are common to all quadrupolar nuclei, 27A1 is a very favourable nucleus for NMR spectroscopy since it has a natural abundance of 100 percent and often quite fast relaxation times which enable short delay times to be used, allowing good quality spectra to be acquired in relatively short measurement times (although it should be noted that in some pure aluminates with rigid structures and absence of protons, surprisingly long relaxation times may be encountered). These factors have made it one of the most thoroughly studied nuclides, together with 29Si, with which it frequently occurs in minerals and other inorganic materials. Spinning side bands which occur at multiples of the spinning frequency are observed in 27A1 spectra, arising from the quadrupole interaction. Spinning sidebands of increased intensity are often found in the 27A1 spectra of natural aluminosilicates containing impurities. The origin of these intense spinning sidebands has been investigated by Oldfield et al. (1983) who concluded that in some cases they may not be due to chemical shift anisotropy alone, but to the presence of large magnetic susceptibility broadening which could be reproduced by adding small amounts of ferromagnetic iron oxide. Whatever their origin, spinning sidebands can interfere with the spectra containing both octahedral and tetrahedral A1 at spinning speeds up to about 3-4 kHz; these should be removed by spinning as fast as possible (at least 10-12 kHz), especially if 271
272
Multinuclear Solid-State N M R o f Inorganic Materials
estimates of the relative amounts of octahedral and tetrahedral A1 are being made. The advent of modem MAS probes with rotors of less than 4 mm diameter has made even faster spinning speeds (ca. 20 kHz) routinely possible, and such speeds are necessary, especially with glassy materials, to obtain spectra which accurately reflect the distribution of aluminium between the different A1 sites. An alternative method for removing interfering spinning sidebands has been suggested by Carduner (1989), who successfully applied to 27A1 a pulse sequence for the total supression of spinning sidebands (TOSS) developed for use with spin I = 1/2 nuclides.
5.2. CHEMICAL SHIFTS IN 27AI SPECTRA As is the case with all quadrupolar nuclei where strong quadrupolar interactions exist, the position of the central transition resonance is shifted away from the position of its isotropic shift 8iso (the "true" position it would occupy if unperturbed). Thus, the reported 27A1 peak positions (8) may not necessarily coincide with the isotropic shifts (Siso), which are typically displaced to less shielded (more positive) values with respect to the observed peak maxima. At a field strength of 11.7 T this displacement in crystalline phases is between 1 and 10 ppm, typically about 5 ppm. The magnitude of the difference between the observed and isotropic shifts in 27A1 increases with increasing electric field gradient at the nucleus. To determine 8iso requires a knowledge of the quadrupolar coupling constant • and the asymmetry parameter xl (see Chapter 2). Although at the magnetic fields currently used for 27A1 NMR spectroscopy the central NMR transition is generally broadened only by the second-order quadrupolar interaction (see Chapter 2), it is possible that in some environments the 27A1 chemical shifts may be orientation-dependent. However, evidence for such chemical shift anisotropy (CSA), which would appear as an additional interaction to the quadrupole coupling, is very sparse for 27A1. Based on double rotation (DOR) spectra, 27A1 CSA has been reported for one of the pentacoordinated sites in the aluminium phosphate A1PO4-21 and its magnitude has been estimated as ca. - 40 ppm from simulations of spectra obtained at spinning angles other than the magic angle (Samoson et al. 1993). A more clearcut demonstration of 27A1 CSA was derived from a single-crystal 27A1 NMR study of sapphire (ot-A1203) for which a value of - 17.3 ___0.6 ppm was obtained (Vosegaard and Jakobsen 1997). A third example of 27A1 CSA is provided by a study of a crystalline 1:1 complex of aluminium chloride with phosphoryl chloride. The stationary (non-spun) 27A1 spectrum revealed a lineshape containing a combination of CSA and second-order quadrupolar interactions. Simulation of this spectrum gave a value of 60 ppm for the CSA component (Schurko et al. 1998). Notwithstanding the complications associated with quadrupolar nuclei, the observed 27A1 shifts provide useful information, since they strongly depend on the A1
27A1NMR ---
273
AIN 4
A~O,,N4.x .....
AIO4 ------
AIO4 (AIO6)
AIO4 Q 3 ( 3 S i ) '
AIO4 Q 4 ( 4 S i ) -- A I ( O B ) 4 "-'-
AI(OP4) - ~
AIO5
9- - -
AI(OP), AIO6 ~ AIF 6
--
AI(OB)6 ~
_
t
I
100
I
80
I
I
60
I
I
40
[
!
20
i
I
0
I
I
AI(OP)6 I
-20
27A1 s h i f t ( p p m ) w.r.t. AI(H20)63+
Figure 5.1. Range of 27A1chemical shifts in various A1 compounds. coordination number and the nature of the atom coordinated to the A1 (Figure 5.1). 27A1 chemical shifts are quoted with respect to the hydrated A1 ion Al(H20)63+ but the octahedral A106 resonance of yttrium aluminium garnet, Y3A15O12 provides a useful secondary standard since it has a sharp peak at 0.7 ppm with respect to Al(H20)63+, and also provides a simultaneous check of the magic angle setting and excitation conditions (see Chapter 3).
5.2.1 27AI chemical shifts in Al-O environments In A1-O environments, the 27A1 chemical shifts for 4-coordinated and 6-coordinated A1 are well separated, the former occurring at about 50-80 ppm and the latter at about - 10 to 15 ppm (Figure 5.1). The isotropic chemical shifts of 5-coordinated A1 in welldefined crystalline compounds fall between these two ranges at about 30-40 ppm (see Table 5.1). The ability to distinguish between octahedral and tetrahedral A1 in inorganic compounds, and to be able to make even a semi-quantitative estimate of their relative amounts can provide valuable information, as in the case of spinels such as MgAI204. The spinel structure contains two types of site, called A and B which can contain tetrahedral or octahedral cations (or both). Normal spinels have the configuration
274
Multinuclear Solid-State NMR of Inorganic Materials
A4B6204, where the superscripts denote the coordination number. Inverse spinels have the configuration Ba(AB)604, but most spinels show a degree of disorder between these two end-members, with formulae expressed as (A~_ xBx)4(B2 _ xAx)604, where x is the degree of disorder. Since x is related to the octahedral:tetrahedral ratio R by x-
2/[1 + R]
(5.1)
an estimate of this important parameter can be made from the 27A1 spectrum, provided satisfactory peak-fitting or integration methods are used, and the spinning speed is sufficient to ensure non-interference by the spinning side bands (Figure 5.2A). This method was first used by Gobbi et al. (1985), Dupree et al. (1986) and Wood et al. (1986) to determine the extent of Mg/A1 ordering and the vacancy distribution in a series of MgA104 spinels. Subsequently, the same principle has been used by Millard et al. (1992) to determine the dependence of the degree of disorder in MgA1204 on the temperature from which the samples were quenched. The results compare well with a theoretical curve generated by a thermodynamic model (Figure 5.2B) (Millard et al. 1992). This method has been extended by Maekawa et al. (1997) to measurements of x in MgAI204 at temperatures up to 1600~ by in situ high-temperature 27A1 NMR spectroscopy. 27A1 MAS NMR has also been used to study the effect of gallium on the inversion parameter of spinel solid solutions of the type MgAI2_ xGaxO4 (x ~< 2) (Ito et al. 2000) and the exchange kinetics of A13 + between the tetrahedral and octahedral sites of MgAlzO4-ZnAI204 mixtures (Kashii et al. 1999). The aluminium coordination in the yttrium aluminates Y4A1209, YA103 and Y3A15012 has been determined by 27A1MAS NMR (Florian et al. 2001). Two equallypopulated tetrahedral A1 sites were resolved in Y4A1209, one site with ~iso = 78.2 ppm, • = 1.622 MHz, ~1 - 0.48, and the other with ~iso = 76.2 ppm, XQ = 1.554 MHz and xI = 0.77. YA103 contains only octahedral A1 (~i~o = 10.7 ppm), but Y3A150~2 contains both tetrahedral and octahedral A1 in the ratio 60:40. The parameters of the tetrahedral site are ~i~o = 77.5 ppm, XQ = 6.153 MHz and Xl = 0.05, and the ~iso value of the octahedral site is 10.7 ppm.
5.2.2 27A1chemical shifts in aluminosilicates The two most common classes of aluminosilicates are tectoaluminosilicates and layerlattice aluminosilicates. Tectoaluminosilicates, which include the feldspar minerals, have a three-dimensional framework structure of SiO4 and A104 tetrahedra linked by corner-sharing, with the negative charges arising from tetrahedral substitution of A1 for Si compensated by the presence of extra-framework cations. The A104 tetrahedra in the tectoaluminosilicates are normally connected exclusively to SiO4 tetrahedra, since
275
27 A 1 N M R
A
AlCW)
0.5
M
0.4 0.3
w
150
~
~
l
~)
~
'
-150
27A1shift (ppm), w.r.t. AI(H20)63+
.~
0.2
~
0.1 0
I
400
I......
800
I
I
.
1200
I
. .I
1600
Temperature (K)
Figure 5.2. A. 27A1MAS NMR spectrum of MgA1204spinel showing the octahedral and tetrahedral resonances from which the inversion parameter x can be calculated. B. Inversion parameters for MgAI204spinels calculated from the 27A1MAS NMR spectra, as a function of the sample quenching temperature. The solid line represents the value of x calculated from thermodynamic theory. From Millard et al. (1992), by permission of the Mineralogical Society of America.
direct links between A104 tetrahedra are forbidden by Loewenstein's Rule. The range of tetrahedral chemical shifts is about 55-68 ppm (Figure 5.1). The structures of layer-lattice aluminosilicates consist of two-dimensional octahedral A106 sheets alternating with tetrahedral SiO4 sheets in which some replacement of Si by A1 can also occur, the charge imbalance being compensated by the presence of interlayer cations (Figure 5.3). In this case the A1 is surrounded by three SiO4 units, and has a tetrahedral chemical shift range of about 70-80 ppm (about 10 ppm less shielded than in the framework aluminosilicates). An approximately linear relationship (but with considerable scatter) has been reported for a number of layer lattice aluminosilicates between the chemical shifts 6 of the tetrahedral A1 and the tetrahedral composition, expressed as the ratio Si/(Si(IV) + AI(IV)) (Kinsey et al. 1985) 6 = - 11.5[Si/(Si(IV) + AI(IV))] + 79.3
(5.2)
A similar linear relationship (again with considerable scatter) has been plotted by Kirkpatrick et al. ( 1 9 8 5 ) for the tetrahedral A1 in 39 framework aluminosilicates (faujasites, other zeolites and aluminates)"
276
Multinuclear Solid-State N M R o f lnorganic Materials
B
A
C~
t Tetrahedral sheet L
1:1 layer lattice 9 -- Tetrahedralcation @ -- Octahedralcation
I Octahedral sheet Tetrahedral sheet
2:1 layer lattice O" Oxygen O " Oxygen+ hydroxyl O - Hydroxyl
Figure 5.3. Side view (along the a-axis) of ideal structures of two common configurations of layer-lattice aluminosilicates. A. 1:1 layer lattice, consisting of alternative octahedral and tetrahedral sheets. B. 2:1 layer lattice, consisting of two tetrahedral sheets sandwiching the octahedral sheet. Where the octahedral cations are trivalent, only two out of three octahedral sites are occupied and the mineral is dioctahedral. Where the octahedral cations are divalent, all octahedral sites are occupied and the mineral is trioctahedral. 6 = - 18.5[Si/(Si(IV) + AI(IV))] + 72.3
(5.3)
The chemical shifts of the octahedral A1 are typically in the range 0-10 ppm, but no systematic change of the octahedral chemical shift with aluminosilicate composition was detected (Kinsey et al. 1985). The various polymorphs of aluminium silicate are framework silicates containing A1 in several different sites. One aluminosilicate of considerable technological importance is mullite, A16Si2013, in which columns of octahedral A106 units are cross-linked by SiO4 and A104 tetrahedra. The charge imbalance in this structure is compensated for by the formation of characteristic oxygen vacancies in conjunction with three distorted tetrahedral A1-O groups (a so-called "tricluster" or AI* unit). The 27A1NMR spectrum of mullite contains an octahedral resonance at ca. - 0.9 to 3.5 ppm and a resonance in the tetrahedral region which often shows signs of splitting into two components, at ca. 57-63 ppm (the normal A104 groups) and at ca. 42-48 ppm (ascribed to the tetrahedral AI* units) (Merwin et al. 1991). A more detailed structural analysis of mullites of different composition (AlaSiO7) predicts on the basis of Si-A1 ordering principles the occurrence of two distinguishable tetrahedral units associated with the triclusters, designated T* and T' (Kunath-Fandrei et al. 1994). Although these could not be distinguished by conventional MAS NMR, a greater degree of detail could be obtained by satellite transition spectroscopy (Kunath-Fandrei et al. 1994), in which the spinning sideband structures extending over more than 1 MHz were analysed (Figure 5.4A). Simulation of the 27A1 lineshapes for several of the satellite transition sidebands
277
27 A 1 N M R
(Figure 5.4B) yielded a reliable quantitative estimate of the A1 occupancy of the various tetrahedral sites in good agreement with the average structural model for this mullite (Rehak et al. 1998). Satellite transition spectroscopy has also been used to study the A1 configuration of sillimanite, a polymorph of AlzSiO5 which is structurally related to mullite (Rehak et al. 1998). Andalusite, another polymorph of A12SiOs, contains A1 in both 5 and 6-fold coordination. The 6-coordinated site has a much larger nuclear quadrupolar coupling constant XQ (15.6 MHz) than the 5-coordinated site (5.9 MHz), necessitating the use of high spinning speeds and high applied magnetic fields to avoid overlap with the spinning sidebands of the 5-coordinated A1 site and allow the quadrupolar lineshape of the 6-coordinated site to be observed (Dec et al. 1991). The observed and simulated 27A1 spectrum of andalusite are shown in Figure 5.5A. A recent 27A1 MAS NMR study of andalusite at a high magnetic field (18.8 T) and fast spinning speeds (34 kHz) has significantly narrowed the extremely broad octahedral signal and provided an improved estimate of the relative amounts of A1(v~ and A1(v~ present (Alemany et al. 1999a). Another polymorph of AlzSiOs, kyanite, contains four crystallographically inequivalent A106 sites with widely differing quadrupolar interactions but spread over a relatively small chemical shift range. A battery of 27A1 NMR techniques, including MAS at different fields, satellite transition spectroscopy (Smith et al. 1994), MQ MAS NMR (Alemany et al. 1999, 1999a) and double rotation (Xu and Sherriff 1993, Smith et al. 1994) has been used to resolve the four sites and allow the simulation of the complete spectrum (Figure 5.5B). An unusual pentameric aluminosilicate unit A1Si4(O,OH)]6 occurs in the mineral harkerite, Ca24Mgs[A1Si4(O,OH)1612(CO3)8(BO3)8(H20,C1). This pentamer consists
B
A
AlOe observ
AIO4
A
Octahedral AlOe simulat
~8000
"
6000
4000
2 0 0 0 ....
27A1 s h i f t ( p p m ) w . r . t . A I ( H 2 0 ) 3 +
'
' 4000 . . . .
27A1s h i f t
(ppm)
3~100'
'
w.r.t. AI(H20)63+
Figure 5.4. A. 27A1satellite transition spectrum of crystalline mullite. B. Portion of the satellite transition spectrum of mullite, showing observed sideband spectrum (top), simulated spectrum (middle) and the one octahedral and three tetrahedral line profiles used in the simulation. After Rehak et al. (1998), by permission of the Mineralogical Society of America.
278
Multinuclear Solid-State N M R o f Inorganic Materials
B
A Ale~ obs
observe simulate~/-/A
AI~
~~.___ ~
" +site 1
simulated
site 2 site 3 -- site 4
9
I
-
i
-
i
!
-
200
27A1 shift (ppm)
I
0
v . . . .
,~
~
--
-200
w.r.t. AI(H20)63+
I
20
|
I
0
I
,
|
- -
-20
27A[ shift ( p p m ) w.r.t. AI(H20)63+
Figure 5.5. Observed and simulated 27A1MAS NMR spectra of A12SiO5 polymorphs. A. Andalusite (top) and simulation (bottom) according to the following parameters: Al(V): • = 5.9 MHz, xl = 0.7, 8iso = 36 ppm. Al(VI): XQ = 15.5 MHz, TI = 0.12, 8iso = 12 ppm. From Dec et al. (1991). B. Kyanite (top) with simulated spectrum and the four A1(w) simulation components (below) with the following parameters: Site 1, XQ = 3.6 MHz, rl = 0.85, 8i~o - 4.0 ppm, Site 2, 8Q = 6.6 MHz, TI = 0.59, 8i~o = 7.7 ppm, Site 3, XQ = 9.2 MHz, ~ = 0.38, 8i~o = 11.0 ppm, Site 4, • = 10.1 MHz, r I = 0.27, 8iso = 14.9 ppm. From Smith et al. (1994). Both figures used by permission of the copyright owners.
of four Qt(1A1) silicon atoms arranged around one Q4(4Si) aluminium atom, resulting in an A1-O-Si angle of 176 ~ The 27A1 N M R spectrum contains a single peak with 8iso = 44 ppm, falling b e t w e e n the generally accepted ranges for A104 and A105 (Dirken et al. 1995). From the point of view of the A1 atom, harkerite resembles a f r a m e w o r k structure because each AIO4 group is surrounded by four Si(O,OH)4 tetrahedra. The low value of 8iso must therefore arise from the A1-O-Si angle of almost 180 ~ and fits well with the angular correlation with 8iso found for other f r a m e w o r k silicates (equation 5.4 below). Zunyite, All2SisOzo(OH,F)~8C1 is an aluminosilicate mineral containing a Si5016 pentameric unit. Its 27A1 N M R spectrum contains, in addition to a 71.4 p p m resonance from the central A104 tetrahedron and a major 7.7 ppm resonance from the twelve surrounding A106 octahedra, another signal at 46.1 p p m (Dirken et al. 1995). The similarity of this peak to the pentameric A1 resonance in harkerite has led to the suggestion that it arises from excess A1 incorporated into the Si5016 pentamer, giving an A1Si4016 unit in which the A1-O-Si angle is predicted from equation 5.4 to be 171 ~ -+ 2.5 ~ (Dirken et al. 1995).
279
27 A1NMR 5.2.3 Relationships between 27A1c h e m i c a l shift
(~iso) a n d
structure
For the limited case of framework aluminosilicates, relationships have been determined between the 27A1 isotropic chemical shift ~i~o and structural parameters such as the mean tetrahedral A1-O-Si bond angle oL(in degrees) and the mean distance between the tetrahedral atoms. Based on 17 framework aluminosilicates, Lippmaa et al. (1986) established the angular correlation to be 6i~o = - 0 . 5 0 a + 132 (ppm)
(5.4)
Relationship (5.4) was found to hold for nepheline-kalsilite solutions (Hovis et al. 1992) and very similar angular relationships have also been demonstrated for other limited groups of framework aluminosilicates 6iso = - 0 . 5 3 a + 138.2 (3 sites in leucite, Phillips et al. 1989)
(5.5)
6iso = - 0 . 5 3 8 a + 138.8 (3 leucites and analcime, Phillips and Kirkpatrick 1994)
(5.6)
6iso = - 0 . 5 0 9 a + 132.24 (10 leucite analogues, Kohn et al. 1997)
(5.7)
The similarity of all these relationships suggests they may validly be combined into a single line (Figure 5.6A) described by 6i~o = - 0 . 5 3 2 a + 137
(5.8)
A slightly different linear relationship was found by Weller et al. (1994) between 8i~o and the mean tetrahedral A1-O-A1 bond angles in a series of 7 aluminate sodalites 6i~o = - 0 . 2 2 2 a + 108
(5.9)
It should be noted that in this case the bond angles are defined as A1-O-A1. A linear relationship has also been established between giso and the mean distances between the tetrahedral atoms (T-T) in angstroms for 11 sites in a series of leucites and related framework aluminosilicates (Kohn et al. 1997). It should be noted that this study was made at a high field strength (14.1 T) resulting in narrow lines, the positions of which were taken as being reasonable approximations to giso without further correction. The resulting linear relationship (Figure 5.6B) has the equation 6iso = - 59.965(T-T) + 246.39
(5.10)
280
M u l t i n u c l e a r Solid-State N M R o f l n o r g a n i c M a t e r i a l s
A
oo
1 66
o
&
@
oo
t~
P~
_ 130
140
150
3.00
Mean T-O-T angle (~
i 3.05
. . . .
! 3.10
,
-
i 3.15
3.20
Mean T-T distance (.~)
Figure 5.6. A. 27A1isotropic chemical shift as a function of the mean tetrahedral angle for a number of framework aluminosilicates, plotted from the data from Lippmaa et al. (1986), Hovis et al. (1992), Phillips et al. (1989), Phillips and Kirkpatrick (1994) and Kohn et al. (1997). B. 27A1isotropic shifts for a series of framework aluminosilicates as a function of the mean distance between the tetrahedral cations, from the data of Kohn et al. (1997). In view of the fact that 27A1 is a quadrupolar nucleus, more general relationships have been sought between the 27A1 nuclear quadrupole coupling constant • (see Chapter 2) and structural parameters related to the A1-O bond length and the O-A1-O bond angle (Ghose and Tsang 1973). The parameter based on the A1-O bond lengths 1 is called the longitudinal strain Io~1,defined as led - Elln(lJlo)l
(5.11)
where li are the actual A1-O bond lengths and lo is the ideal bond length of a polyhedron having the same volume as the coordination polyhedra. The parameter related to the bond angle is called the shear strain I~1, defined as I~ = ~ltan (0i-0o)l
(5.12)
where 0i are the actual O-A1-O bond angles and 0o is the ideal bond angle (90 ~ for octahedra and 109.5 ~ for tetrahedra). These strain parameters provide a measure of the deviation from ideality of the bond lengths and angles in the A1-O polyhedra. On testing these parameters for both tetrahedral and octahedral sites in a range of mineral structures for which XQ data were available, Ghose and Tsang (1973) found for tetrahedral sites a very poor correlation between XQ and loll, but a better linear relationship with the bond angle function I~1 (Figure 5.7A), defined by XQ = 7.781qA + 1.63
(5.13)
281
e7 A 1 N M R
The data of Weller et al. (1994) for 7 aluminate sodalites have also been found to show a linear relationship between XQ and I~1 (5.14)
XQ = 7.831~ + 0.476
By contrast, for octahedral A1-O units the correlation between XQ and Iq~lwas found to be very poor, but a reasonable correlation was found with the bond length function Ic~l (Figure 5.7B), given by XQ = 48.71cel + 1.3
(5.15)
This relationship did not appear to hold however for corundum, spodumene and the A12(m) site of chrysoberyl, but possible reasons for this were not discussed by Ghose and Tsang (1973).
5.3. FIVE-COORDINATED AI-O 5.3.1 A1 (v) in well-defined (crystalline) environments
A1 occurs in well-defined pentacoordinated sites in a number of crystalline materials, including andalusite (Alemany and Kirker 1986, Lippmaa et al. 1986, Dec et al. 1991),
A
B
12
. ~
N
=
o j . ,
t'N
15
o
< ~.. e.I
4 84
o 0
0
I
0.4
I
0.8
Tetrahedral AI
'
1.2
0
9 0
9 0.1
, 0.2
. . . . . 0.3
i 0.4
Octahedral AI
Figure 5.7. A. 27A1nuclear quadrupolar coupling constant for tetrahedral A1 sites in aluminosilicates as a function of the mean shear strain, reflecting the distortions in the tetrahedral bond angles. B. 27A1nuclear quadrupole coupling constants for octahedral A1 sites in aluminosilicates as a function of the mean longitudinal strain, reflecting differences in the A1-O bond lengths. Plotted from the data of Ghose and Tsang (1973).
282
M u l t i n u c l e a r Solid-State N M R o f l n o r g a n i c M a t e r i a l s
A12Ge207 and LaA1Ge207 (Massiot et al. 1990), grandidierite (Smith and Steuernagel 1992), dehydroxylated pyrophyllite (Fitzgerald et al. 1989), SrAll2019 (Jansen et al. 1998), CaA112019 (Gervais et al. 2001), the phosphate minerals augelite and senegalite (Bleam et al. 1989), A1PO4-25, a molecular sieve precursor (Jelinek et al. 1991) and AllsB4033 (Kunath et al. 1992). The five-coordinated A1 in these compounds occurs in configurations approximating either to trigonal bipyramidal or square pyramidal, resulting in significant electric field gradients at the nucleus, and quadrupolar lineshapes which are sufficiently well developed to allow the quadrupolar parameters to be extracted by spectral simulation techniques (Chapter 2). Some of these compounds contain A1 in sites other than pentacoordinate, resulting in complex lineshapes (Figure 5.8). The structure of pyrophyllite dehydroxylate contains only pentacoordinated A1, and can be simulated by a single quadrupolar lineshape (Figure 5.8A), whereas andalusite contains both a pentacoordinate and octahedral site, the latter showing a much larger XQ value than the former (Figure 5.8B). Andalusite is one of the few compounds containing A1~v) for which a b initio calculations of the electric field gradients have been made (Bryant et al. 1999). The calculated NMR parameters for both the A1~v) and A1~w) sites show excellent agreement with the experimental values (Table 5.1). The 27A1 spectrum of augelite also shows two quadrupolar sites, corresponding to A1~v) and A1~v~), the latter indicating a slightly smaller XQ value than the former (Figure 5.8C). A more complex situation is found in grandidierite, which contains two A1(vI) sites in addition to the A1~v) site (Figure 5.8D), and in SrAll2019 and CaAll2019, both of which contain three A1~vI) and one A1r site in addition to the A1Cv) site. The NMR parameters of the compounds containing well-characterised A1Cv) are collected in Table 5.1.
Table 5.1. 27A1NMR parameters of compounds containing well-characterised A1~v) sites. Compound
XQ(MHz)
xl
8iso (ppm)
Reference
andalusite andalusite andalusite andalusite pyrophyllite dehydroxylate A12Ge207 LaA1Ge2Ov SrAll2019 CaAll2Ol9 augelite grandidierite AllsB4033 (2 sites) A1PO4-21(2 sites)
5.9 5.6 5.9 5.9 10.5
0.7 0.76 0.7 0.69 0.6
35 35.2 36 35.6 29
Alemany& Kirker(1986) Alemanyet al. (1999) Dec et al. (1991) Lippmaaet al. (1986) Fitzgeraldet al. (1991)
8.8 7.2 2.1 4.2 5.7 8.7 1.08, 1.42 5.9, 7.4
0.4 36 Massiot et al. (1990) 0.3 35 Massiot et al. (1990) 0.7 18 Jansen et al. (1998) 0.0 20.0 Gervaiset al. (2001) 0.6 29 Bleam et al. (1989) 0.95 41 Smith& Steuernagel (1992) 0, 0.49 52, 43 Kunathet al. (1992) 0.68, 0.52 14, 16 Jelineket al. (1991)
283
27 A 1 N M R
A
B
D served
d
observed
observed
......S A ~ _ _ ~
...............~
d
ulated
J ~
simulated
Al~
1
_______j~/~---..._AI ~v~site 2 :_
~
Al c~ site 3 !
'
'
1 '~
100
9
,"~
i
~
' ,
0
,
!
--~--,
-100
T
I
i---
200
9
- '
9
0
-200
100
0
-100 100
0
-100
27A| shift (ppm) w.r.t. AI(H20)63+ Figure 5.8. 27A1MAS NMR spectra of a series of crystalline compounds containing A1 in well-defined 5-coordinated sites. A. pyrophyllite dehydroxylate, after Fitzgerald et al. (1989), by permission of the Mineralogical Society of America, B. andalusite, after Dec et al. (1991), C. augelite, after Bleam et al. (1989), D. grandidierite, from Smith and Steuernagel (1992). By permission of the copyright owners. Jansen et al. ( 1 9 9 8 ) have demonstrated an approximately linear relationship between the isotropic chemical shifts of a number of these crystalline A1(v~ compounds and a parameter R describing the angular distortion of the A1 polyhedron from an ideal trigonal bipyramid configuration, defined as R = ZI 0i-0ol/~ 0~
(5.16)
Where 0 i are the actual polyhedral O-A1-O bond angles and 0o are the bond angles in the ideal A1 polyhedron (in this case assumed to be a trigonal bipyramid). Figure 5.9 shows the resulting relationship, which can be described by 6is o -- 372R + 6.93
(5.17)
These results suggest that distorted trigonal bipyramidal configurations are most prevalent in these compounds, with the exception of a complex hydroxy-aluminosilicate, vesuvianite (Phillips et al. 1987), in which the A1(v~ site is closer to a square pyramidal configuration.
5.3.2 A I fv)
in non-crystalline environments
In addition to the crystallographically well-defined compounds, there are a number of other compounds which contain a broad 27A1 resonance at about the position of the 8iso values for A1(v~ (25-40 ppm). The compounds containing this resonance are often
284
Multinuclear Solid-State N M R o f lnorganic Materials
O~
50
J
40 0
0 0
30
20 0
o
o 10
-
I 0.050
.
.
.
J 0.075
,
= 0.100
Angular distortion function R Figure 5.9. 27A1isotropic chemical shift for the well-defined A1(v) sites in a number of crystalline compounds, as a function of the angular distortion from ideal trigonal bipyramidal conformation, R, defined in equation 5.16. From Jansen et al. (1998) by permission of the American Chemical Society. poorly crystalline or X-ray amorphous, and include quenched aluminosilicate glasses (Risbud et al. 1987), aluminosilicate gels (Yasumori et al. 1990, Selvaraj et al. 1993, Taylor and Holland 1993), aluminate gels (MacKenzie and Kemmitt 1999, MacKenzie et al. 1999, MacKenzie et al. 2000), dehydroxylated minerals such as metakaolinite (Lambert et al. 1989, Sanz et al. 1988, Rocha and Klinowski 1990), amorphous p-alumina (Meinhold et al. 1993) and compounds made amorphous by grinding, including ground gibbsite (MacKenzie et al. 1999a), ground boehmite and ~/-alumina (MacKenzie et al. 2000a), ground mullite (Schmticker et al. 1998) and various ground mixtures of hydrated aluminas with oxides and silicates (Temuujin et al. 1998, Temuujin et al. 2000, Temuujin et al. 2000a, MacKenzie et al. 2000b). Figure 5.10 shows a selection of typical 27A1 NMR spectra of amorphous aluminosilicates from different sources, all containing the resonance at ca. 30 ppm. Because the chemical shifts of the resonance in these amorphous materials are similar to the chemical shifts of A1~v~ in the crystalline compounds (Table 5.1), it has become commonplace to assign all broad peaks in this spectral region to pentacoordinated A1. Two other aluminium compounds, barium aluminate glycolate (Cruickshank et al. 1986, Herreros et al. 1994) and senegalite (Bleam et al. 1989) which contain A1~v~ also show a similar resonance at about 36 ppm which, because of its featureless lineshape, cannot be unambiguously simulated. There are however problems with such a blanket assignment of peaks at ca. 30 ppm to A1~v~,since they generally show only a very small magnetic field dependence (Meinhold et al. 1993a), by contrast with the strong field dependence of the crystalline A1~v~ compounds (Jansen et al. 1998). Furthermore, a consequence of such an assignment is that the position of these peaks coincides with the isotropic chemical shift, implying
285
27 A1 N M R
AI(OH)3+ silicagel ground120min. I/
Monophasic alumin~ 0
Spraypyrolysed ~ A alumin~ \q/~.________
AluminosilicateA glas~ ~ _ ~
f,/ \
80
-80 27A1 shift (ppm)
1
80
-80
w.r.t.AI(H20)3+
Figure 5.10. 27A1MAS NMR spectra of a number of amorphous aluminosilicates of approximately mullite composition derived from various sources, showing the peak at ca.30 ppm attributed to A1~v) (arrowed). From McManus et al. (2001), by permission of Elsevier Science.
only a small electric field gradient at the nucleus. This appears to be inconsistent with the distortions present in normal 5-coordinated A1 sites and the broadness of the peaks. An alternative explanation has been proposed (Schmticker and Schneider 1996), that this resonance in amorphous aluminosilicates arises from A1 in distorted tetrahedral sites in the vicinity of the tricluster oxygen defect occurring, as in mullite, as a charge-balancing mechanism for the substitution of tetrahedral Si by A1. In mullite, this resonance occurs at ca. 42-48 ppm (Merwin et al. 1991), but the less constrained environment of amorphous gels could result in its displacement upfield (Schmficker and Schneider 1996). Further indirect evidence supporting this explanation for amorphous aluminosilicates comes from grinding experiments (Schmficker et al. 1998) in which the transformation of crystalline mullite to an X-ray amorphous material is accompanied by the appearance of the 30 ppm resonance which gradually returns to the tricluster position when the mullite is thermally recrystallized (Figure 5.11). During these structural rearrangements, the positions of the tetrahedral and octahedral peaks remain largely unchanged but the apparent interchangeability of the tetrahedral tricluster resonance with the 30 ppm peak suggests a degree of relationship in the sites giving rise to these resonances. A second piece of indirect evidence relating the 30 ppm peak in aluminosilicates to A1~v) rather than A1~v) species comes from large angle X-ray scattering (LAXS) experiments on aluminosilicate glasses (Schmficker et al. 1999). The aluminium site occupancies of a number of glass compositions were determined from their 27A1 NMR spectra and used to calculate the pair distribution function (PDF) profile which was
286
Multinuclear Solid-State N M R o f lnorganic Materials A 5S 47
[ ~ 1.1
~
L
~
j
l
Crystalline mullite
I
,
120 40 -40 27A1shift (ppm)w.r.t. AI(H~O)s3+ ground 1200 min. ~
2 S~
..12
A
5S
6oooc
L
I
80
I
I
0
!
!
l
-80
2
soooc
ti
I,
80
I
fX
___/
I
0
I
-80
27A1 shift ( p p m )
I
...
80
I
I
0
looo~
!
55 47
i
I
-80
i,
80
!
0
!
J
-80
w.r.t. AI(H20)63+
Figure 5.11. Effect of grinding and subsequent thermal recrystallisation on the 27A1 MAS NMR spectrum of synthetic crystalline mullite. Note the progressive change of the 47/32 ppm resonance. The small resonance at 13 ppm is from a residual corundum (a-alumina) impurity. After Schmticker et al. (1998), by permission of the copyright owner. A
B
.
1.5
1.9 r (A)
.
.
.
!
2.3
!
1.5
1.9
2.3
r (A)
Figure 5.12. Pair distribution function of the first maximum derived from large-angle scattering measurement of a 50 mol% A1203 aluminosilicate glass (solid line) compared with the calculated profile (dotted line) derived from 27A1MAS NMR estimates of the AI site distributions. A. Based on a model in which both the 60 and 30 ppm resonances are assumed to be tetrahedral, B. Based on a model in which the 30 ppm resonance is assumed to be from 5-coordinated A1-O. After Schmticker et al. (1999), by permission of the copyright owner.
then c o m p a r e d with the e x p e r i m e n t a l profile d e t e r m i n e d by L A X S e x p e r i m e n t s (Figure 5.12). Over the whole range of glass compositions studied, the fit to the L A X S profile was distinctly better for the m o d e l which a s s u m e d contributions from only
287
27 A 1 N M R
tetrahedral and octahedral A1 (Figure 5.12A) than for the model in which the 30 ppm NMR resonance is assigned to A1~v~ (Figure 5.12B). Although distorted tetrahedral sites associated with tricluster oxygen defects may provide one possible explanation for the broad 27A1NMR peak in amorphous aluminosilicates, the resonance occurs in other systems where there is no obvious mechanism for tricluster formation. Because of its broad featureless characteristics, the peak cannot be simulated to obtain unique values of • and xl. In such cases, it is possible to derive a combined parameter, the quadrupolar product PQ (sometimes called the composite quadrupolar coupling constant h) which contains both • and xI '!72 1/2
P Q - XQ(1 + -3-)
(5 18)
Values of PQ for lineshapes too featureless to be simulated may conveniently be extracted from MQMAS data (Chapter 2). 27A1 MQMAS NMR spectra have been obtained for the various A1 sites in the range of amorphous aluminosilicates for which the 1-dimensional MAS NMR are shown in Figure 5.10 (McManus et al. 2001). The PQ and ~iso values for the ca.30 ppm sites are essentially independent of the type of sample, occurring in the range PQ = 3.0-4.3 MHz and ~iso = 37-43 ppm. The PQ values are very similar to that determined by 27A1 MQMAS NMR for the distorted tetrahedral tricluster site in crystalline mullite (3.2 MHz, Bodart et al. 1999), but the chemical shift of the latter (49 ppm) is too positive for these sites to be identical. 2VA1MQMAS NMR studies of the dehydration and rehydration of other aluminosilicate gel materials have, however, led to the conclusion that the A1 resonance which develops at ca. 30 ppm when the gels are heated at 100-200~ is not caused by five-coordinated A1 species, but by distorted tetrahedral A1 experiencing large quadrupole induced shifts and small chemical shifts due to conformational changes in the polymeric network (Peeters and Kentgens 1997). Since the quadrupole interactions in these materials are strong, offresonance nutation NMR spectroscopy was used in this case to confirm the MQMAS NMR results.
5.3.3 A1 cv) in zeolites
The catalytic activity and stability of aluminosilicate zeolites is strongly influenced by removal of A1 from the framework sites into extraframework positions. This can be accomplished by dehydroxylation or calcination followed by steam treatment, the latter process producing "ultrastable" zeolites. Early 2VA1NMR studies indicated the appearance of the 30 ppm resonance attributed to A1~v) when faujasite and other zeolites are steamed (Gilson et al. 1987). Double rotation 27A1NMR studies at two magnetic fields (Ray and Samoson 1993) indicated that the nature of the A1 species depends on the method of dealumination; the 30 ppm resonance in zeolite-Y samples treated
288
Multinuclear Solid-State N M R o f lnorganic Materials
A
B
R
90
30
-30
27A1 shift (ppm) w.r.t. Ai(H20)63+
80
40
0
-40
27A1shift ~pm) w.r.t. Al(H20)63+
Figure 5.13. 27A1NMR spectra of thermally-dealuminated ammonium-exchanged Y-zeolite. A. 14.1T MAS NMR spectrum, B. 11.7T DOR spectrum, from Ray and Samoson (1993), by permission of Elsevier Science.
in a single thermal step arises from non-framework A1 in distorted tetrahedral sites with significant second-order shifts arising from large quadrupolar couplings. The MAS spectrum (Figure 5.13A) shows an octahedral non-framework A1 resonance at ca. zero ppm, a tetrahedral framework AI signal at ca. 60 ppm and the third resonance (probably non-framework A1) at ca. 30 ppm. The double rotation spectrum (Figure 5.13B) averages the anisotropy of the second-order quadrupolar interactions as well as first-order broadening, revealing intrinsic differences in the total isotropic chemical shifts (i.e. the isotropic chemical shift plus the second-order quadrupolar isotropic shift). This procedure reveals the presence of a second tetrahedra! peakat 47.5 ppm, corresponding to a • of 6.2 MHz. Spin-lattice relaxation measurements on steam-dealuminated zeolites suggest that the 30 ppm peak is a mixture of at least three non-framework A1 species with different relaxation characteristics from the octahedral and tetrahedral species (Yang 1995). An MQMAS NMR study of the coordination state of the A1 species giving rise to the 30 ppm peak in thermally activated mordenite (Chen et al. 2000) suggests that in samples calcined up to about 600~ the principal contribution to the 30 ppm peak is from distorted 4-coordinated A1 sites with a XQof about 5.8 MHz, this identification being made on the basis of the position of the signal in the isotropic dimension of the triple-quantum spectrum (Figure 5.14). Increasing the calcination temperature, or using two-step calcination significantly increases the contribution of pentacoordinated A1 to the 30 ppm resonance (Figure 5.14) (Chen et al. 2000). A major problem with 27A1 NMR spectroscopy of steamed zeolites (and with dehydroxylated minerals such as metakaolinite (MacKenzie et al. 1985)) is the inability to detect all the A1 present at lower applied fields and slower MAS speeds. The presence
289
27 A 1 N M R
Unheated
600~ O
. .' .
Al-,r(,,
~~
,,
.
~
.
/ ~
"~
I~1
" i -
-1---
60
-
io
**e, Al~V) 9 ~
AlOe') ~ ~
"~" AI(~)Al(,V)
i(rr
650~176
700~
.....
'"" Ai0r ~1
O
'!"
"~
"i }
~ ~ ] ~ ~ A 'v)ill ')
"~~
Aiov)
I~1
1(]
I~l
"~.~, I~1
i
60
-2o
20
-20
60
20
-20
60
20
-20
MAS dimension (ppm) Figure 5.14. 27A1triple-quantum MAS NMR spectra of commercial zeolite mordenite in the hydrogen form, unheated and heated to various temperatures followed by hydration. Spinning side bands are marked by asterisks. The final spectrum is of a sample heated in two steps, of 650~ and 500~ with an intermediate rehydration step. After Chen et al. (2000), by permission of copyright owner.
A
-20
~ AI0n)
<
~3A!m 0*
/i
-.-20 .:-
simulation
......................................
v~ ....
i .... i'"'1
80
....
i ....
i'"'1
.... f ....
40
! 40 i .... i .... |'"~1
0
F2 (ppm)
.... i .... i .... I ....
-40
deconvolution i
i
120 27A1
.
1
8'0
|
,
40
shift (ppm)
|
0
9
1
I
-40
w . r . t . AI(][-I20)63+
Figure 5.15. A. 27A1MQMAS spectrum of steam-treated faujasite zeolite H-Y obtained at 18.8T. B. Single-pulse 18.8T MAS NMR spectrum (top) simulated with two tetrahedral, one A1(v) and one octahedral resonance (lower profiles) using intensities derived from the MQMAS spectrum (A). Asterisks denote spinning side bands. After Fyfe et al. (2000), by permission of the Royal Society of Chemistry.
of "invisible aluminium" leads to problems in quantification even when small pulse angles are used, and has hindered the identification of the A1 species associated with the 30 ppm peak. A recent MQMAS NMR study of steamed faujasite at a very high magnetic field (18.8 T) has enabled all the A1 to be detected (Figure 5.15A), and allowed the unambiguous resolution of four A1 sites in this material (Fyfe et al. 2000). Spectral simulations based on the MQMAS NMR spectrum (Figure 5.15B) distinguish
290
Multinuclear Solid-State NMR of lnorganic Materials
two tetrahedral sites with isotropic chemical shifts of 60.7 and 60.4 ppm, a 5-coordinate species (~iso = 32.2 ppm) and an octahedral site (~iso = 3.2 ppm). Carrying out these MQMAS experiments at three magnetic fields (9.4 T, 14.4 T and 18.8 T) has made it possible to unambiguously simulate the MAS spectra and confirm the values of the NMR parameters corresponding to the characteristic A1 sites in these materials (Fyfe et al. 2001). 27A1 MAS spectroscopy at an even higher field (19.6 T) has been used to determine the number of A1 sites in zeolite MCM-22, and to monitor their changes during delumination by hydrothermal treatment or calcination (Ma et al. 2001). These high-field experiments enabled four different tetrahedral framework A1 sites, at 61.5, 57.0, 53.5 and 50.0 ppm, to be identified in the parent zeolite, and showed that the A1 in the 57 ppm site is preferentially expelled during hydrothermal treatment with the concomitant appearance of a five-coordinated extraframework A1 resonance at 30 ppm (Ma et al. 2001). 27A1 MQMAS NMR has also been used to study zeolite-Y stabilised by lanthanum and the results compared with those for ultrastabilised zeolite-Y (van Bokhoven et al. 2000). The results show that the framework A1 is polarised in both zeolites, by the presence of La 3§ in the former and by extraframework octahedral A1 in the latter, which also shows evidence of the presence of A1Cv) (van Bokhoven et al. 2000). Another approach to the problem of detecting "invisible aluminium" in sites with very large quadrupole coupling constants which broaden the 27A1 signal beyond detection is to use a double resonance method known as TRAPDOR (TRAnsfer of Population in DOuble Resonance). This technique has been used to study the catalytic sites in dehydrated zeolite HY by exploiting the interaction between the ~H and 27A1 nuclei (Grey and Vega 1995). In the TRAPDOR experiment, the intensity of the 1H MAS echo signal is decreased by applying 27A1 radiation during the evolution period of the echo sequence. Analysis of the signal reduction allows the determination of the spininteraction parameters of 27A1 nuclei such as the A1 located at the surface of catalysts and in alumina catalyst support materials which otherwise would be invisible to MAS NMR (Grey and Vega 1995). The catalytic activity of aluminosilicate zeolites and aluminas appears to be directly related to the concentration of 30 ppm sites present, leading to the development of "super-five" materials displaying large NMR signals at this position (Wood et al. 1990). Since catalysis depends on the chemical nature of the A1 at the surface, which may not be the same as in the bulk, cross-polarisation experiments between ]H and 27A1 have been used to distinguish between the surface and bulk species (Coster et al. 1994). Since chemisorption of water provokes extensive surface reconstruction, a more suitable proton source for the CP experiments was found to be ammonia adsorbed on the surface. The results showed the presence of two kinds of surface Lewis sites associated with the non-framework A1 (a tetrahedral site at ca.58 ppm with a XQ of about 6 MHz, and an A1Cv) site at ca. 40 ppm with a slightly smaller XQ). Lewis sites either
27 A 1 N M R
291
accept or donate a pair of electrons, the former constituting a Lewis acid and the latter a Lewis base. The framework A1 is associated with BrCnsted sites (sites which either accept or donate a proton, defined as BrCnsted acids or bases respectively) (Coster et al. 1994).
5.4. ALUMINIUMOXIDES The thermodynamically stable form of alumina (ot-A1203 or corundum) consists of a hexagonal close-packed sublattice of oxygen anions with 2/3 of the octahedral interstices occupied by A1 cations in an ordered array. The octahedral NMR resonance has the parameters ~iso = ca.16 ppm, XQ = 2.38 MHz (Jakobsen et al. 1989). In addition to corundum there is, however, a number of metastable alumina polymorphs. One series, with structures based on face centred-cubic oxygen packing, contains the cubic polymorphs ~/-A1203 and qq-Al203, monoclinic 0-A1203, and ~-A1203, which may be either tetragonal or orthorhombic. A second series, based on hexagonal close-packed oxygens, contains the orthorhombic polymorph K-A1203 and the hexagonal x-A1203, in addition to the stable form oL-A1203. Several other variants on these transition aluminas have also been reported, including an X-ray amorphous form, p-A1203, which results from the dehydration of AI(OH)3 (gibbsite) at 100-400~ under reduced pressure (Slade et al. 1991). All the transition aluminas contain A1 in both tetrahedral and octahedral sites, but the chemical shifts and the distribution of the cations over the octahedral and tetrahedral sites differ from one polymorph to another. In progressing from ~/--> ~ ~ 0-A1203 a gradual decrease in the occupancy of the tetrahedral A1 sites in favour of the octahedral sites is expected from transmission electron microscopy (Wilson 1979) but this trend is not confirmed by 27A1 NMR spectroscopy (Table 5.2). It should be noted that both the observed chemical shifts and the site occupancies are magnetic field-dependent (Hughes et al. 1993), and, in addition, quantification of the latter is beset with difficulty, making it inadvisable to use these parameters directly as a means of differentiating between the polymorphs. Improved quantification of the A1 site occupancies in an amorphous alumina from partially dehydrated gibbsite (similar to p-alumina) has been obtained by using a combination of central and satellite transition MAS NMR (see Chapter 3), suggesting that this method may be generally applicable to the other transition aluminas (Kunath-Fandrei et al. 1995). Even though the differences between the 27A1NMR spectra of the transition aluminas are subtle, the technique has been used to study the thermal transformation sequences of the hydrated aluminas gibbsite, AI(OH)3 (Slade et al. 1991, Meinhold et al. 1993), boehmite, ~/-A1OOH (Slade et al. 1991a, Meinhold et al. 1993, Pecharrom~in et al. 1999), pseudoboehmite (Meinhold et al. 1993) and bayerite, AI(OH)3 (Meinhold et al. 1993, Pecharromfin et al. 1999).
292
Multinuclear Solid-State NMR of lnorganic Materials
Table 5.2. Chemical shifts of hydrous, transition and anhydrous alumina, in ppm, with respect to Al(H20)63+. Phase
Field (T)
A1Iv
A1v
A1vI
Oh/Td*
Reference
Gibbsite Gibbsite Gibbsite Boehmite Boehmite Pseudoboehmite Bayerite ~/-A1203 ~-A1203 3~-A1203 p-A1203 p-A1203 x-A1203 x-A1203
7.0 11.7 11.7 7.0 14.1 7.0 11.7 7.0 11.7 14.1 7.0 11.7 7.0 11.7
x-A1203 w
11.7
K-A1203
7.0 11.7 9.4 11.7 9.4
63.0 64.6 67 54.4 66 63.8 66.8 71.5 64.6 65.3 65 65.1 -
27.2 23 38 -
4.4-6.0 6.0 8.3 3.4 9 11.2 8.8 5.4 6.7 10 1.9 5 5.7 7.9 11.5 4.0 6.6 5 5.5 16 82
2.7 2.2 2.3 2.7 2.6 2.6 1.6 -
Slade et al. (1991) Meinhold et al. (1993) MacKenzie et al. (1999a) Slade et al. ( 1991) MacKenzie et al. (2000a) Slade et al. ( 1991) Meinhold et al. (1993) Slade et al. (1991 a) Meinhold et al. (1993) MacKenzie et al. (2000a) Slade et al. (1991) Meinhold et al. (1993) Slade et al. (1991) Meinhold et al. (1993) Kunath-Fandrei et al. (1995) Slade et al. (1991) Meinhold et al. (1993) Pecharrom~in et al. (1999) Meinhold et al. (1993) Jakobsen et al. (1989)
xI-A1203
"q-A1203 0-A1203 Corundum
* Ratio of octahedral:tetrahedral AI site occupancy-estimate only (see t e x t ) ~ This amorphous sample was derived from gibbsite under reduced pressure and may be more accurately described as p--A1203
82~iso value
O n the basis o f the p r e s e n c e o f a N M R r e s o n a n c e at ca. 30 p p m , a l u m i n i u m in 5 - f o l d c o o r d i n a t i o n has b e e n r e p o r t e d in s o m e t r a n s i t i o n a l u m i n a s , p a r t i c u l a r l y the a m o r p h o u s p-A1203 ( S l a d e et al. 1991, M e i n h o l d et al. 1993). A m o r p h o u s p r e c u r s o r s o f TI-A120 3 d e r i v e d f r o m h y d r o l y s i s o f a l u m i n i u m in s o l u t i o n s h o w strong r e s o n a n c e s at ca. 35 p p m , attributed to 5 - c o o r d i n a t e a l u m i n i u m arising f r o m the m o v e m e n t o f A1 f r o m o c t a h e d r a l to t e t r a h e d r a l sites with the c r e a t i o n o f o x y g e n a n i o n v a c a n c i e s a d j a c e n t to o c c u p i e d o c t a h e d r a l sites ( W o o d et al. 1990). A s i m i l a r but v e r y w e a k r e s o n a n c e r e p o r t e d at 30 p p m in ~/-A1203 h e a t e d at 6 0 0 ~ has b e e n a t t r i b u t e d to 5 - c o o r d i n a t e d a l u m i n i u m a t o m s o n the s u r f a c e o f the o x i d e p a r t i c l e s ( P e c h a r r o m ~ i n e t al. 1999). C r o s s - p o l a r i s a t i o n e x p e r i m e n t s b e t w e e n 27A1 N M R a n d the p r o t o n s f r o m s u r f a c e - a d s o r b e d p y r i d i n e ( M o r r i s a n d Ellis 1989) or a m m o n i a ( C o s t e r et al. 1994) h a v e b e e n u s e d to i n v e s t i g a t e the surface state o f cata l y t i c a l l y a c t i v e ~/-A1203. N o N M R e v i d e n c e h a s b e e n f o u n d o f the h y p o t h e t i c a l 3 - c o o r d i n a t e d A1 t h e o r e t i c a l l y p r e d i c t e d to o c c u r at the a l u m i n a surface. E v e n a s s u m ing for this species a 8iso of 9 5 - 1 0 0 p p m , a •
o f 10 M H z and an xl value o f one, C o s t e r
et al. (1994) f o u n d no e x p e r i m e n t a l e v i d e n c e for its existence, p r o m p t i n g a recent expla-
nation based on density functional calculations that the 3 - c o o r d i n a t e d surface A1 a t o m s
293
27 A 1 N M R
become 6-coordinated by dropping into vacant octahedral interstices in the second layer (Sohlberg et al. 1999). 27A1 NMR has proved to be a powerful technique for investigating the nature of the aluminium oxide films formed on the surface of aluminium metal by anodising. Interestingly, the acid used in the anodising electrolyte has a major effect on the A1 species formed in the film; chromic acid produces an oxide layer containing only A106 units (Figure 5.16), whereas the films formed in sulphuric, phosphoric and oxalic acids contain varying proportions of 4, 5 and 6-coordinated A1 (Faman et al. 1989). The peak position of the A106 species formed in chromic acid (6.5 ppm at a field of 8.45 T) suggests a similarity with boehmite (A1OOH) or pseudoboehmite. The appearance of additional A1(Iv) and A1(v~ species when using other acids as the electrolyte is thought to be the result of disorder accompanying the random introduction of other OH groups into the structure (Faman et al. 1989). The mobility of the OH groups in the anodic film may vary with the acid used, with phosphoric acid tending to favour the formation of A1~v~ and A1(v~ and sulphuric and oxalic acid facilitating the formation of all three coodination states (Figure 5.16). When the anodic film formed in oxalic acid is immersed in electrolyte solution without further electrolysis, the A1(Iv~ and A1(v~ species are lost and the structure reorganises into the fully six-fold coordination of pseudoboehmite (Farnan et al. 1989a). Two double-resonance techniques, REDOR and TRAPDOR (see Chapter 2), have been used to investigate the interaction between the surface of ~-A1203 and an impregnated phosphate layer (van Eck et al. 1995). Both experiments exploit the dipolar coupling between the 31p and 27A1 nuclei. In the REDOR experiment the value of the dipolar coupling is changed by applying w-pulses of rf irradiation during the echo, but in the TRAPDOR experiment the spins are irradiated continuously (see Chapter 2). REDOR and TRAPDOR experiments on P-impregnated ~-A1203 allowed the A1 atoms in close association with P to be distinguished from the 27A1 spectrum of the AI(vI)
A~ AI~') AIm )
J
~alic
.... ~
A,2i! /IlL
Sulphuric
%...._C hr~ !
.....
1
' 4;0' O -'4;0
-4aO "
o, '
27A1shift (ppm) w.r.t. AI(H20)63+
Figure5.16.
27A1 MAS N M R spectra of anodic films formed on aluminium surfaces in different acid electrolytes. Adapted from Faman et al. (1989).
294
Multinuclear Solid-State N M R of lnorganic Materials corundum u o
o o w~ "r ~q
+
40 30 20
10 0
o
b0ehmite gibbsite
0
2
4
6
8
10
36 p p m 27A1 peak area (%) Figure 5.17. Relationship between the area of the 27A1 resonance at ca.36 ppm and the total water
content of ground aluminas, after MacKenzie (2000). bulk oxide and showed that the surface layer of AIPO4 has a slightly higher degree of ordering than amorphous A1PO4 (van Eck et al. 1995). When mechanically activated by grinding, the hydrous aluminium oxides gibbsite, AI(OH)3 (MacKenzie et al. 1999a), boehmite, ~-A1OOH and to a lesser extent, ~/-A1203 (MacKenzie et al. 2000a) all develop 27A1NMR resonances at about 35 ppm. The development of this NMR resonance is related to the bound water content of the alumina, since it occurs at maximum intensity in the most highly hydroxylated form (gibbsite) but does not develop at all in anhydrous ot-A1203 (corundum) (Figure 5.17). 27A1 NMR has been used to study the mechanism by which grinding can lower by up to 400~ the temperature of corundum formation from ground gibbsite (MacKenzie et al. 1999a), boehmite and ~/-A1203 (MacKenzie et al. 2000a).
5.5. AMORPHOUS ALUMINIUM COMPOUNDS Because of its ability to probe the atomic environment in compounds which do not possess long-range order, MAS NMR has become a technique of choice for studying the constitution of amorphous materials, including gels and glasses which contain relatively sensitive NMR-active nuclei, and their thermal transformation into crystalline compounds.
5.5.1 Aluminate gels The aluminosilicate compound mullite, A16Si2013, is one of the most commercially important materials to be conveniently prepared by thermally decomposing gel precursors. The reactivity of the precursor gel depends on the preparation method, which
295
27 A 1 N M R
determines how homogeneously its constitutent atoms are combined. The most homogeneous precursors show three 27A1 NMR resonances, at ca. 56 ppm (tetrahedral), ca. 0.5 ppm (octahedral) and ca. 30 ppm (attributed either to 5-coordinated A1 or tetrahedral A1 associated with the tricluster structure surrounding an oxygen vacancy (Schmticker and Schneider 1996)). On heating, the 30 ppm resonance increases in intensity at the expense of the octahedral and, to a lesser extent, the tetrahedral peaks (MacKenzie et al. 1996), up to about 950~ when it abruptly disappears leaving only the tetrahedral and octahedral resonances (Figure 5.18). Heating to higher temperatures results in the splitting of the tetrahedral peak into two (Figure 5.18), the component at ca. 66 ppm due to the A1 in normal tetrahedral coordination, and that at ca. 43 ppm ascribed to A1 involved in the tricluster unit (Merwin et al. 1991). Less homogeneous precursors, which may also correspond to diphasic gels, have the 27A1 characteristics of the unreacted starting material and behave on heating as discrete mixtures of oxides or hydroxides. The constitution of the ~/-A1203 phase produced by heating such gels has been investigated by 27A1 NMR (Schneider e t al. 1994). Other precursor gels may show behaviour intermediate between fully homogeneous monophasic gels and diphasic gels, but the presence or absence of the 30 ppm 27A1 NMR resonance can provide a qualitative judgement of their degree of homogeneity. Mechanochemical activation (grinding) has been shown by Temuujin e t al. (1999) to be capable of converting a gel with diphasic 27A1NMR characteristics into a more homogeneous monophasic-like gel (Figure 5.19) which thermally converts to mullite ca. 200~ lower than the unground diphasic gel.
100 0 -100
100 0 -100
200
0 -200
200
0 -200
ol-O---o\ 60
/
o---_o
40 oJm 20
400
800
1200
1600
Temperature (~ Figure 5.18. Changes in the 27A1site occupancies of homogeneous aluminosilicate gel during the thermal evolution of mullite, as a function of heating temperature. From data of MacKenzie et al. (1996a) and Schneider et al. (1992).
296
Multinuclear Solid-State NMR of Inorganic Materials
8.4 unground
3.9 ground I
I
_
!
I
0
80
.-
-80
27A1shift (ppm) w.r.t. AI(H20) 3+ Figure 5,19, 27A1MAS spectrum of a diphasic aluminosilicate gel showing its conversion to the characteristic spectrum of a homogeneous monophasic gel after grinding for 20 hours. Asterisks denote spinning side bands. From Temuujin et al. (1999).
6.3
~ ~
0.8 71.4
_
60
,
,
."7"-
60
0
-60
,---Y
0 -60
L_
9
60
4,
,-~__
,-"-
0 -60
r
so eml
um
40 "Tetrahed~ ~~pp'~m
______
~ ~.Octahedral . . . . .
t-e4
0-i
0
,
.....
300
,,
600
900 1200
Temperature (~ Figure 5,20, Changes in the 27A1 MAS spectra and Al site occupancy of yttrium aluminium garnet gel during thermal transformation to crystalline Y3A15O12. After MacKenzie and Kemmitt (1999).
The appearance and growth of a 27A1 NMR resonance intermediate between the octahedral and tetrahedral peaks is a common feature of other gel precursors of important aluminate compounds. In an amorphous precursor of yttrium aluminium garnet, Y3A15O12, this resonance, at 38 ppm, appears at ca. 600~ and remains in the spectra until the crystallisation of the garnet at 900~ (Figure 5.20). 89y NMR of these gels suggests that the large Y atoms move into their final positions in the garnet structure well after the aluminate structure has been established (MacKenzie and Kemmitt
27 A 1 N M R
297
Figure 5.21. Stacked and contour representations of the 27A1MQMAS NMR spectra of lanthanum aluminate gels of (A) low-lanthanum content, in which three resonances assigned to A1(w), A1(v~ and A1(v~)can be distinguished, and (B) high-lanthanum content gel, in which only the octahedral and tetrahedral A1 resonances are observed. The solid line indicates the isotropic chemical shift, the dashed line shows the direction of the anisotropic quadrupolar line broadening and the dotted-dashed line indicates the direction of the quadrupolar-induced shift. From Iuga et al. (1999), by permission of the American Chemical Society. 1999). A similar result has been observed in the gel synthesis of lanthanum hexaluminate, LaAl11018, in which the 36 ppm resonance is even more intense, but disappears abruptly as the spinel structural units form at 1000~ The movement of the La ions into the mirror planes of the magnetoplumbite structure at 1200~ was monitored by 139La NMR (MacKenzie et al. 1999). 27A1 MQMAS NMR has been used to resolve ambiguities in the MAS spectra of a series of lanthanum aluminate gels of composition (1-x)A1203.xLa203 where x = 0 to 0.7 (Iuga et al. 1999). Except for the sample with the lowest La content, hardly any AI(V) was detected (Figure 5.21), but for all samples the concentration of AI(VI) decreased on heating until crystallisation took place (Iuga et al. 1999). A peak at 36-38 ppm in the 27A1 spectrum of the gel precursor to the analogous magnetoplumbite-structure compound CaAll2019 shows similar changes on heating, as does the gel precursor to CaAlaO7, a ceramic material with usefully low thermal expansion properties (MacKenzie et al. 2000). The aluminium configurations in the final crystallised calcium aluminate products from these two precursors have been determined in detail by 27A1 MQ MAS NMR at three magnetic fields (Gervais et al. 2001). Spectral simulation of CaA1407 at different fields (Figure 5.22A) using data
298
Multinuclear Solid-State N M R of Inorganic Materials A
~
T ~ ~ 8 ' 4 5
~j_~
__~ I
....
',
....
I
....
I.-...~
....
I
....
I
....
t
~--..
_ ~I1.7T
~ I...
T
I...I
14.1T ...t,..
i...,
200 100 0 -100 80 40 0 -40 27A1shift (ppm) w.r.t. Al(H20)63+ 27A1shift (ppm) w.r.t. AI(H20)63+ Figure 5.22. A. 27A1MAS spectrum of CaA1407 with simulated spectrum and the individual components at three different applied magnetic fields. * = spinning side bands, ,l, indicates satellite transition contribution. B. 27A1MAS spectra of CaAlleO19 with simulated spectrum and the individual components at three different applied magnetic fields. From Gervais et al. (2001). obtained from MQ MAS measurements allowed the two tetrahedral sites in this structure to be distinguished and characterised. A small octahedral A1 signal probably arises from an X-ray undetected impurity aluminate (Gervais et al. 2001). Similar experiments on crystalline CaAl12019 (Figure 5.22B) were able to resolve and characterise the one tetrahedral, three octahedral and one 5-coordinated A1 sites expected for this structure (Gervais et al. 2001). The isotropic chemical shift determined for the 5-coordinated A1 site (20 ppm) is similar to that reported by Jansen et al. (1998) for the isomorphous compound SrAll209 (18 ppm); these are amongst the lowest shifts yet reported for AI(V) (excluding those for the aluminium phosphates) and extend down into the range normally associated with A106 units, necessitating caution in interpreting aluminate peaks in this spectral region. Although the NMR results suggest that the growth and abrupt disappearance of the "pentacoordinated" resonance is a feature common to all hybrid aluminate gels, the peak attributed to pentacoordinated A1, although present, is not as clearly defined in other aluminate gel precursors, including those of the barium aluminosilicate celsian, BaA12Si208 (MacKenzie and Kemmitt 1999a) and the magnesium aluminosilicate cordierite, Mg2A14SisO~8 (Selvaraj et al. 1990). The thermal transformation of cordierite gels prepared by two different methods has also been studied by NMR by Livage et al. (1997), who reported differences (but no pentacoordinated A1) in the 27A1 spectra of precursors prepared by an inorganic route and in hybrid (organic/inorganic) precursor gels.
27 A 1 N M R
299
Knowledge of the aluminium species present under alkaline conditions is of critical importance to industrial chemical operations such as the Bayer process in which the aluminium present in bauxite ores is dissolved in concentrated sodium hydroxide solution. 27A1NMR has been used to investigate the A1 coordination states in the resulting sodium aluminates after freeze drying (Bradley and Hanna 1994). At an OH:A1 ratio > 4.4, the predominant species is Na[AI(OH)4] (QO) with a 27A1resonance at 86.6 ppm. A broader resonance at 71.3 ppm is probably composed of a variety of other polyoxoanionic species such as [AI(OHz)(OA1)2] x- (Q2), [AlzO(OH)612- (Q1) and [A1Oa]- (Q4). A water-soluble phase formed at an OH:A1 ratio between 4.4 and 4.1 was shown by 27A1 NMR to contain both tetrahedral and octahedral A1 species, while at lower ratios, the product becomes water insoluble and contains 5-coordinate A1 in addition to 4 and 6-coordinated (Bradley and Hanna 1994). 27A1NMR has also proved useful in studying the thermal transformation mechanisms of these aluminates (Bradley and Hanna 1994). The thermal development of amorphous sialon fibres from polyaluminocarbosilane gels at 1000~ and their crystallisation to ~-sialon fibres at 1400-1500~ has been studied by 27A1and 29Si NMR (Sorar~ et al. 1993). The gel precursor shows the typical 3-peak 27A1 spectrum, with resonances at 3.2, 32 and 57.4 ppm but the amorphous fibres formed at 1000~ contain only tetrahedral A1. On crystallisation of [3-sialon, the tetrahedral 27A1 resonance broadens due to the occurrence of mixed A104- xNx sites (SorarO et al. 1993).
5.5.2 G l a s s e s
27A1 NMR has proved useful for providing information about the A1 species present and, in some instances, a qualitative or semi-quantitative estimate of the relative site occupancies. Binary aluminosilicate glasses are difficult to prepare without phase separation, especially with A1203 contents > 15 wt% but using roller-quenching methods, Risbud et al. (1987) prepared a series of glasses which contained 27A1 NMR signals attributed to A1 in 4, 5 and 6-fold coordination. Although the relative amounts of A1 in these sites were not estimated, the results suggested that charge compensation of AI(IV) by AI(V) occurs in the Si-rich phases and by AI(VI) in the Si-poor phases (Risbud et al. 1987). A series of roller-quenched binary aluminosilicate glasses containing 10-60 mol% A1203 showed similar 3-peak 27A1 NMR spectra in which the relative A1 site occupancies appeared to be independent of composition (Schmticker et al. 1997). The rate at which the binary aluminosilicate glass is quenched was shown by Sato et al. (1991) to influence the structure and thus the type of 27A1 spectrum obtained. The characteristic 3-peak spectrum is obtained only with extremely rapid quenching, whereas at slower quenching rates the glass contains only tetrahedral and octahedral resonances. Quantification of the spectral intensities
300
Multinuclear Solid-State N M R of Inorganic Materials
suggested that 15-30 % of the A1 in the 3-peak spectra of the rapidly quenched samples was in sites too distorted to be detected by the NMR experiment (Sato et al. 1991). The 27A1 spectra of a series of framework aluminosilicate glasses (Oestrike et al. 1987) indicate the presence of only tetrahedral A1, corresponding to the structures of the corresponding crystalline aluminosilicates, but with broadened A1 resonances. As with the crystalline framework aluminosilicates, the A1 becomes progressively deshielded as the compositional parameter Si/(Si(IV) + AI(IV)) decreases, leading to a linear relationship between this parameter and the chemical shift (5.19)
6 = - 21.6[Si/(Si(IV) + AI(IV))] + 70.5
The progressive incorporation of Na into a series of roller-quenched aluminosilicate glasses has been shown by Schmiicker et al. (1997) to lead to a progressive decrease in the 30 ppm 27A1 component coupled with an increase in the 50 ppm tetrahedral component of the typical binary aluminosilicate glass spectrum (Figure 5.23A). A semiquantitative estimate of the changes in the relative site occupancies made by curve-fitting the 27A1 spectra indicates the final disappearance of the 30 ppm resonance at a Na20 content of about 7.5 mol% (Figure 5.23B), corresponding to the Na + content required
A
B e% Na20 80
"--
40
~0 .5 mole% Na20 '~ 80 ~ p m
.~ 4o 4 mole% Na20 9 "~
~
~ 80
ppm,14 ppm
1"-
80
2O
-40
27AI shift (ppm) w.r.t. AI(H20) 3+
I
2
t
I
6
'
|
10
sodium oxide (mole%)
Figure 5.23. A. Observed and curve-fitted 27A1MAS NMR spectra of aluminosilicate glasses containing 10 mol% A1203 and differing Na20 contents. B. 27A1site occupancies of sodium aluminosilicate glasses of 10 mol% A1203 content, as a function of Na20 content. Shaded regions are intended only as a guide to the eye. From Schmticker et al. (1997) by permission of copyright owner.
27 A 1 N M R
301
for charge balance in a glass of this composition. These results were cited as evidence that the 30 ppm resonance in these glasses arises from charge balancing tricluster defects which become progressively unnecessary as this function is taken over by Na + (Schmticker et al. 1997). 27A1NMR studies made at high temperatures (up to 1320~ of alkali aluminosilicate melts (Stebbins and Farnan 1992) show that the 27A1isotropic chemical shifts decrease with increasing temperature, reflecting significant short-range structural changes such as increasing mean coordination number. At high temperatures, a few percent of AI(VI) is present in sodium aluminosilicate glass (Stebbins and Farnan 1992). An earlier attempt to observe AI(VI) in a glass formed by melting jadeite, NaA1Si206 (Hamilton et al. 1986) was unsuccessful, indicating that the totally octahedrally coordinated A1 in the jadeite crystals becomes tetrahedral in the glass of the same composition. This conclusion has significant consequences for igneous petrology in which the structure of precipitated crystals is assumed to reflect the structure of the melt. 27A1NMR has been used to investigate the local structures of a series of glasses in the CaO-AI203 system, in which fast-quenched glasses of composition CaO:AI203 < 1 were found to contain AI(VI) and AI(V) in addition to the predominant AI(IV) (McMillan et al. 1996). The average value of ~iso increases with increasing CaO content, possibly reflecting a decrease in the polymerisation state of the aluminate units. Structural differences between CaO-AI203 glasses prepared by conventional glass melting and sol-gel methods have been studied by 27A1MAS NMR (Kerns et al. 1998) which indicate that the conventional glasses contain only AI(IV) whereas the gelderived glasses always contain a proportion of AI(VI), associated with the hydroxyl content of the gel glasses. A series of lanthanum aluminosilicate glasses has been shown by 27A|MAS NMR to contain principally AI(IV) which therefore acts as a network-former, with small amounts of AI(V) and AI(VI) appearing in glasses of higher A1203 content (Clayden et al. 1999). A 27A1NMR study of calcium aluminosilicate glasses has revealed the presence of A1 in 4, 5 and 6-fold coordination in samples with CaO:A1203 ratios < 1 (Sato et al. 1991a). This study also underlined the importance of fast spinning speeds for resolving the 5-coordinated resonance which appeared as a shoulder at 33.4 ppm on the side of the 48.6 ppm tetrahedral A1 peak; at lower spinning speeds (5 kHz) this feature was barely visible, but became progressively better resolved as the spinning speed was increased. At a spinning speed of 14 kHz all the aluminium in this site could be quantitatively observed (Sato et al. 1991a). A 27A1NMR study of a series of magnesium aluminosilicate glasses provides a good example of the use of the newer techniques to quantify the relative amounts of A1 species in poorly-resolved spectra (Toplis et al. 2000). The 27A1MAS NMR spectra of these glasses show a poorly-defined A105 shoulder on the main broad tetrahedral A1
302
Multinuclear Solid-State NMR of Inorganic Materials
B
A
0
AlOO
AIor) f
50
A!ov) , , , , ' 1 , , , , i , ~ , , i , , , ,
25
-25
F2 (ppm) _
AlOO f v-,4 ,
I
loo
,
'
50
I
o
'
'
-5o
Z7Alshift ~pm) w.r.t. AI(H20)63§
50
AIov) ,,,,i,,,,i,,,,i,,,,
75
25
-25
F2 (ppm) Figure 5.24. A. 27A1 MAS and B. the corresponding triple-quantum MQ MAS NMR spectra of two magnesium aluminosilicate glasses containing 50 mol% SiO2 and different AI:Mg ratios. After Toplis et al. (2000), by permission of the copyright owner.
peak (Figure 5.24A). The 27A1 MQ NMR spectra of these glasses (Figure 5.24B) allowed these species to be separated and quantified by projecting the 2D data on to the isotropic axis and fitting the resulting one-dimensional spectrum to two Gaussian peaks (Toplis et al. 2000). These results show that for glasses with Mg/2AI = 1, up to 6% of the A1 is 5-coordinated, throwing into doubt previous calculations of the polymerisation state of these liquids made on the assumption that all the A1 is in tetrahedral coordination charge balanced by Mg 2+. Aluminosilicate glasses containing rare-earth elements such as Sm are of interest as potential optical storage materials. 27A1MAS NMR studies of a series of sol-gel-derived Sm-doped aluminosilicate glasses indicate that at A1203 contents of N1 mol% all the A1-O units are octahedral, whereas at higher alumina contents, tetrahedral A1-O units are also formed, enhancing the homogeneity of Sm dispersion in these glasses (Jin et al. 2000). Fluorine-containing aluminosilicate glasses have been examined by 27A1 MAS NMR and 19F-27A1 cross-polarisation experiments (Kohn et al. 1991) which demonstrate the presence of AI in (IV), (V) and (VI) coordination states. The presence of F in the glasses does not exert a large effect on the chemical shift of the AI(VI) suggesting that this coordination is relatively remote from the F. By contrast, the AI(V) signal is
27 A 1 N M R
303
enhanced in the CP spectra, leading to the suggestion that this entity is present as a previously unknown A1F52- complex (Kohn et al. 1991). 27A1 MAS NMR has been used to study a series of rare-earth doped chlorofluoroaluminate glasses in the system A1-Ba-Ca-Y-Eu-F-C1 which are of interest as potential laser oscillators, optical fibre amplifiers, photochemical hole burning devices and UVvisible luminescence conversion devices (Sakida et al. 2000). The 27A1 MAS NMR spectra indicate that the presence of the C1 does not affect the anion coordination environment of the A1, with only A1-F bonds being present in all the glasses. The A1F6 octahedral units form chains and/or rings by corner-sharing, the resulting A1-F coordination being highly symmetrical as evidenced by the narrow 27A1line profiles of these glasses (Sakida et al. 2000). A 27A1 MAS NMR study of a series of lanthanum aluminium phosphate glasses has shown the presence of A1 in 4-fold, 5-fold and 6-fold coordination. The average A1 coordination number increases with increasing A1/La ratio and decreases with increasing O/P ratio, as can be understood in terms of the tendency of the structure to avoid forming A1-O-A1 bonds (Karabulut et al. 2001). A series of barium aluminofluorophosphate glasses has also been studied by 27A1 NMR (Fletcher et al. 1990). The spectra show the presence of AI(IV), AI(V) and AI(VI) in these glasses, the relative abundance of AI(VI) coordinated to P increasing with increased fluorine content, consistent with the preferred coordination of A1 in the fluorides and the apparent coordination of F by both A1 and Ba. The AI(IV) chemical shift becomes about 5 ppm more shielded with the addition of fluorine, suggesting the formation of AI(IV)-F linkages. A series of tellurite glasses in the system TazO3-A1203-TeO2 has been studied by 27A1 MAS NMR (Youngman and Aitken 2001). All these glasses were found to contain 4, 5 and 6-fold coordinated A1, with speciation independent of the Ta/A1 ratio but sensitive to the TeO2 content. Octahedral A1 predominates in TeO2-rich compositions, with an increasing proportion of tetrahedral A1 formed as the TeO2 content decreases. The proportion of 5-fold coordinated A1 is constant over the whole composition range. The average A1 coordination number was also shown by 27A1 MAS NMR to be influenced by the thermal history of the glasses (Youngman and Aitken 2001).
5.5.3 O t h e r a m o r p h o u s systems
The detection and estimation of non-crystalline components in mineral and ceramic systems has long posed an important but difficult problem. A number of possible techniques for estimating the non-crystalline component of ground kaolinite, including 27A1 and 29Si MAS NMR have been evaluated by Kodama et al. (1989). It was observed that the amount of non-crystalline material determined by chemical methods was directly related to the intensity of the AI(IV) resonance, suggesting a possible
304
Multinuclear Solid-State NMR of Inorganic Materials
Metakaolinite
Geopolymer
RT
00 ~
/\
f 9
I
80
,,
,
i
20
L ,
"
i " - ~ - - i
-40
80
. i
20
-
*
-40
80
20 -40
80
20
-40
27A1shift (ppm) w.r.t. AI(H~O)63+ Figure 5.25. Changes in the 27A1NMR spectrum of metakaolinite on the room temperature formation of sodium polysialate geopolymer and its subsequent service at high temperatures. Note the immediate conversion to tetrahedral A1 upon room-temperature polymerisation, and the thermal stability of the tetrahedral A1 in the geopolymer framework structure. The 13 ppm resonance at 1300~ arises from a small amount of corundum (e~-A1203). Based in part on Barbosa et al. (2000). means of estimating the non-crystalline component of this system from NMR measurement of the ratio AI(IV)/[AI(IV) + AI(VI)]. Inorganic geopolymers based on aluminosilicate polysialate units constitute a class of ceramic-like materials which cure and harden at room temperature but have excellent high-temperature properties. When formed from metakaolinite and sodium silicate in highly alkaline conditions, the characteristic 27A1 NMR spectrum of metakaolinite immediately changes to predominantly 4-coordinated A1 (Figure 5.25) which persists even up to high temperatures, indicating that although the structure contains no longrange atomic order, the cross-linked tetrahedral AIO4 and SiO4 framework from which it is composed is extremely stable (Barbosa et al. 2000).
5.6. ALUMINOPHOSPHATES
Like the aluminosilicates, the aluminophosphates comprise a group of compounds in which the A1 may occur in 4, 5 and 6-fold coordination. Because of the difference in electronegativity between phosphorus and silicon, the range of A1 shifts corresponding to these coordination states in the aluminophosphates is generally more negative by up to 25 ppm than in the aluminosilicates (Figure 5.1). The aluminophosphates (A1PO4s) encompass a large group of compounds of considerable practical importance as molecular sieves and catalyst materials. Like the zeolites, these framework structures are based on a three-dimensional network of tetrahedrally coordinated atoms linked via bridging oxygen atoms to give porous structures containing cavities of molecular dimensions. The cavity openings and thus the adsorption properties are controlled by the ring size, of which a large variety has been
27 A 1 N M R
305
reported. Silicon can also be incorporated into these structures, giving a series of framework silicoaluminophosphates (SAPOs), while other metals such as Mg, Mn, Fe, Co and Zn can also enter into the A1PO4 structure, to give metalloaluminophosphates (MeA1POs). Solid state NMR has proved particularly useful in providing a detailed understanding of the structure and framework ordering in these important materials and has been reviewed by Barrie (1993). The first of the aluminophosphates to be extensively characterised is designated A1PO4-5, the nomenclature carrying over from the original Union Carbide patents on these materials. Early 27A1 NMR work showed a fairly broad resonance reflecting second-order quadrupolar interactions; multiple-field simulations indicated XQ = 2.3 MHz, xl = 0.95 and 8iso = 44 ppm, characteristic of tetrahedral AI(OP)4 units (Miiller et al. 1985). This early 27A1 work and a complementary 31p study provided unequivocal evidence for the strict alteration of aluminium and phosphorus over the framework sites. 27A1NMR has also been used to study the effect of prolonged exposure to water, which converts some of the tetrahedral A1 occurring at 21-33 ppm (in a 4.7 T field) to the octahedral species Al(PO)4(OH2)2 with a peak at ca. - 17 ppm (Meinhold and Tapp 1990). Cross-polarisation experiments between 27A1 and 31p have been used by Fyfe et al. (1996) to demonstrate that the framework structure remains intact during the hydration process, by studying the connectivities in the system. These experiments make use of the fact that magnetisation can be transferred reversibly between a quadrupolar nucleus such as 27A1 and a spin-~/2 nucleus such as 3~p (Fyfe et al. 1992), allowing transferred-echo double-resonance (TEDOR), rotational-echo doubleresonance (REDOR) and dipolar-dephasing (DD) experiments (see Chapter 2) which enable connectivity relationships between the various framework atoms to be established (Fyfe et al. 1996). In the case of hydrated A1PO4-5, both two-dimensional 27A1 ~ 3~p cross-polarisation and two-dimensional TEDOR experiments (Figure 5.26) clearly show the connection of phosphorus to the tetrahedral and octahedral aluminium atoms, interpreted as a random distribution of both A1 coordinations around each phosphorus (Fyfe et al. 1996). A further bonus from the two-dimensional CP experiment is its demonstration that a small 5-coordinated A1 resonance at 8 ppm (Figure 5.26) is also associated with the phosphorus, supporting a suggestion that it arises from the coordination of one water molecule to a framework A1 atom (Fyfe et al. 1996). Another A1PO4 which has been extensively studied is designated VPI-5, named after the Virginia Polytechnic Institute rather than the Union Carbide equivalent A1PO4-54. This material is of considerable interest since its channels are surrounded by 18-membered rings, giving it a large aperture of about 12A. 27A1 NMR spectroscopy indicates that this is a hydrated compound containing both tetrahedral and octahedral A1 sites (Wu et al. 1990). DOR experiments have revealed additional detail, resolving two tetrahedral sites (Wu et al. 1990). The connectivities between the A1 and P atoms in this structure have been studied by REDOR and two-dimensional TEDOR
306
Multinuclear Solid-State NMR of Inorganic Materials
A
31p
~
9 r r
o
m
:I O
'Et0
g~
,,I
....
I,,,,1
0
....
I ....
-50
f ....
~1
o',
-100
Frequency (ppm from
H3PO4)
_1,,,,11,,,I,1,11
....
ILIJ
25 -25 -75 Frequency (ppm from H3PO4)
Figure 5.26. Two-dimensional 27A1--->31p spectra of A1PO4-5. A. Cross polarisation (CP) experiment, B. TEDOR experiment. The dashed lines indicate the interconnected resonances. The spinning side bands in (A) are indicated by asterisks. After Fyfe et al. (1996) by permission of Elsevier Science.
experiments employing cross-polarisation between the 27A1 and 31p (Fyfe et al. 1992a), and similar studies have also been made on A1PO4-8, a phase which results from the heat-treatment of VPI-5 at 400~ (Fyfe et al. 1993). These experiments show that in both VPI-5 and its dehydrated form A1PO4-8 both the tetrahedral and octahedral A1 sites are connected via dipolar couplings to each of the three resolved 3~p sites in the second dimension. A1PO4-21 is a crystalline aluminophosphate which contains a single tetrahedral A1 site and two distorted 5-coordinated AI(OP)4(OH) sites, the latter being subject to large quadrupolar interactions which cause severe line broadening. 27A1 DOR experiments have enabled all three sites to be resolved (Jelinek et al. 1991). The resulting spectral simulations indicate ~iso and • values of the two 5-coordinated sites of 0.4 and - 5.4 ppm and 5.9 and 7.4 MHz respectively, while the tetrahedral site has ~iso = 42.2 ppm and • = 3.7 MHz (Jelinek et al. 1991). When heated, A1PO4-21 transforms to A1PO4-25. This contains A1 in two tetrahedral environments which cannot be resolved by DOR at a field of 11.7 T, since they have similar isotropic chemical shifts and their isotropic quadrupolar shifts are comparatively small. At a lower magnetic field (4.2 T) larger isotropic quadrupolar shifts occur which allow resolution by DOR (Jelinek et al. 1991). The silicon-substituted aluminium phosphate SAPO-37 has a faujasite framework structure and shows a sharp tetrahedral and octahedral 27A1 resonance, with an underlying broad tetrahedral resonance (Fyfe et al. 1995). By using 27A1 ~ 3 ] P and 27A1~ 298i TEDOR experiments, Fyfe et al. (1995) have shown that the main Si environment in SAPO-37 is Si[4A1], and have been able to identify the environments of the
307
27 A 1 N M R
three A1 sites, including the asymmetric environment giving rise to the broad resonance with large anisotropy. Such experiments which allow the atomic connectivities to be established are thus capable of providing extremely powerful structural information.
5.7. ALUMINIUMBORATE AND MOLYBDATE 5.7.1 A l u m i n i u m
borate
A118B4033 is a complex aluminium borate with an interesting crystal structure containing chains of octahedral A106 units cross-linked by A104 and two different types of A105 units. The resulting 27A1 spectrum is complex, with fine structure resolvable at higher field strengths (Figure 5.27A). An initial simulation was made by Kunath et al. (1992) from measurements at three different magnetic fields, also using satellite transition spectroscopy (SATRAS) to increase the spectral resolution by analysing the fine structure of the sidebands. A subsequent DOR and MAS study going to higher fields and spinning speeds (Massiot et al. 1995) has led to a revised simulation (Figure 5.27B) and slight changes in some of the resulting quadrupolar parameters.
5.7.2
Aluminium molybdate
Aluminium molybdate, A12(MoO4)3, is an important compound in catalyst technology. Although the crystal structure contains four equally populated inequivalent A1 sites, an
A observed
simulated
/~~
7.OT
A/~ observed
\
~~
9 ,
-
14.1I T ..... Q'~M
-
-;
ted
A! AI A I ~ ~ 75 I__
100
0
-100
100
0
27Al shift (ppm) w.r.t. AI(H20)63+
-100
i
25
~ A I fvI) -25
27A1 shift (ppm) w.r.t. AI(H20)6 a§
Figure 5.27. A. Experimental and simulated 27A1NMR spectrum of Al18B4033 at a range of magnetic fields and spinning speed of 15 kHz. B. The experimental and simulated spectrum at 17.6 T showing the individual components of the four aluminium sites. After Massiot et al. (1995) by permission of copyright owner.
308
Multinuclear Solid-State N M R o f Inorganic Materials
A
B
observed
M
simulated site 1 site 2 site 3 !
. . . .
!
. . . .
!
. . . .
-11
!
. . . .
!
. . . .
-12
!
. . . .
i
. . . .
-13
i
. . . .
!
. . . .
-14
!
. . . .
site 4
i
-15
10
0
2VAl shift (ppm) w.r.t. AI(I-I20)63+ Figure 5.28. A. Expanded portion of the 2VA1MAS and DOR spectra of A12(MoO4)3acquired at 9.4 T, after Haddix et al. (1993), by permission of the American Chemical Society. B. Detail of the second sideband at 11.7 T, with the simulated spectrum and the fitted components corresponding to the four octahedral sites. The frequency scale in (B) is arbitrary. After KunathFandrei et al. (1995a), by permission of Elsevier Science.
earlier 27A1 NMR study (Han et al. 1992) showed only two inequivalent A1 sites. Subsequently, 27A1 DOR proved capable of resolving all four sites (Figure 5.28A) (Haddix et al. 1993). Further detail of the structure of these sites extracted from the satellite transitions by SATRAS experiments has allowed all the quadrupolar parameters and isotropic chemical shifts of these four sites to be deduced (Figure 5.28B). Simulation of the sideband spectra shows that the XQ values of all four sites fall within a narrow range (0.78 to 1.21 MHz), as do the ~iso values ( - 1 0 . 3 to - 1 3 . 4 ppm) (Kunath-Fandrei et al. 1995a).
5.8. ALUMINIUM FLUORIDES Aluminium in the aluminium fluorides and alkali fluoroaluminates is 6-coordinated, but with various possible arrangements of the A1F6 octahedra resulting in isotropic chemical shifts ranging from + 1.4 to - 13.2 ppm (Figure 5.1). Dirken et al. (1992) have proposed a nomenclature for these octahedra; by analogy with the Qn (quaternary) symbol used for tetrahedral units, the symbol S" (senary) is suggested, where n represents the number of bridging F atoms per octahedron. A1F3 occurs in two polymorphs, designated oLand [3; in both forms the A1F6 octahedra are condensed into three-dimensional (S 6) networks, with ~iso and XQ values of -- 13.2 ppm and 2.8 MHz (a-form) and - 12.5 ppm and 3.4 MHz ([3-form) (Dirken et al. 1992).
309
27 A 1 N M R
One of the most commercially important fluoroaluminates is cryolite, Na3A1F6, used as a molten solvent in electrolytic aluminium smelting. This compound has an S o structure in which all the A1F6 octahedra are isolated, resulting in Siso and XQ values of 1.4 ppm and 2.0 MHz respectively (Dirken et al. 1992). Since the main interest in cryolite is in its molten state, high-temperature 27A1NMR has proved useful for investigating the melting behaviour and quantifying the abundance of the structural A1 species in cryolite-containing melts (Stebbins et al. 1992a). High-temperature lineshape and relaxation time measurements of cryolite using 27A1 in conjunction with 23Na and 19F NMR have been used to study the atomic motions associated with the monoclinic-to-orthorhombic displacive phase transition occurring in cryolite at ca. 550~ (Spearing et al. 1994). 27A1, 23Na and 19F NMR has also been used to investigate the ionic mobility of cryolite up to 300~ (Lacassagne et al. 1998). This work showed a marked decrease in the 27A1 linewidth at 200-300~ related to the increased mobility of the associated fluorine which averages the dipolar interactions causing 27A1 linebroadening at lower temperatures. Chiolite, NasA13F14 is a fluoroaluminate with a structure containing sheets composed of distinguishable S 2 and S 4 A1F6 octahedra. Although the isotropic 27A1 shifts of these two octahedral environments are very similar they can be resolved. For the S 2 unit, 8 i s o = - 1 ppm and X Q -'- 8.2 MHz while for the S 4 unit 8 i s o = - - 3 ppm and XQ = 6.5 MHz (Dirken et al. 1992). High-temperature 27A1 NMR has been used to study melts of chiolite and its eutectic with A1F3 (Stebbins e t al. 1992a). As the temperature increases, the 27A1 spectrum of chiolite sharpens dramatically and the • values for both octahedral sites increase significantly. Since there is no evidence of a phase transition at these temperatures, the increased site distortion is probably due to thermal expansion of the lattice (Spearing et al. 1994). The 27A1 interaction parameters of a number of crystalline fluoroaluminate compounds are shown in Table 5.3. Table 5.3. 27A1interaction parameters of A1-F compounds. Compound
~iso(ppm)*
ot-A1F3 [3-A1F3 Na3A1F6
- 13.2 - 12.5 -0.8, 1.4 - 1.2 - 19.5 -0.8 - 13.4 - 11.7 -1 -3
K3A1F6 KA1F4 T12A1F5 ot-BaA1F5 Ba3A12F12 NasA13F14(site 1) (site 2)
XQ( M H z ) 2.8 3.4 ND 2.0 ND ND ND ND ND 8.2 6.5
* Chemicalshiftsquotedwithrespectto AI(H20)63+
Reference Dirken et al. (1992) Dirken et al. (1992) Sakida et al. (2000), Dirken et al. (1992) Sakida et al. (2000) Sakida et al. (2000) Sakida et al. (2000) Sakida et al. (2000) Sakida et al. (2000) Dirken et al. (1992) Dirken et al. (1992)
310
Multinuclear Solid-State NMR of lnorganic Materials
5.9. THERMAL DECOMPOSITION REACTIONS Heating plays an important role in the processing of many inorganic compounds, especially the production of ceramics and related materials from minerals. During thermal reactions of Al-containing materials, the long-range atomic ordering may be destroyed, making the solid-state processes difficult to monitor by X-ray diffraction. In these instances, 27A1 NMR has proved extremely valuable in monitoring changes in the A1 environment. Because of its technical importance for clay-based ceramics, the thermal decomposition reaction of kaolinite clay to form mullite has been studied extensively by both 29Si NMR (see Chapter 4) and 27A1 NMR. The earliest A1 study, at low magnetic field and slow spinning speeds, recognised the fact that when kaolinite loses its structural (hydroxyl) water to form amorphous metakaolinite at about 550~ two new 27A1NMR peaks appeared at about 66 ppm (tetrahedral AI) and 34--39 ppm, but almost 90% of the total 27A1 NMR signal was lost, indicating that most of the A1 atoms in metakaolinite were in sites which were too distorted for NMR detection under the experimental conditions then available (MacKenzie et al. 1985). Subsequent work at higher fields and faster spinning speeds has indicated that at least some of this "missing" A1 is in sites characterised by the resonance at ca.30 ppm variously attributed to AI(V) (Gilson et al. 1987, Lambert et al. 1989) or to tetrahedral A1 in incipient distorted tricluster units (Schmticker and Schneider 1996) (see discussion in section 5.3.2). A triple-quantum NMR study of kaolinite at various stages of its thermal decomposition sequence (Rocha 1999) has presented evidence of the presence of distorted AI(IV), AI(V) and AI(VI) sites in metakaolinite, which transforms to a mixture of amorphous material, poorly crystalline mullite, ~-alumina and a small amount of unreacted metakaolinite on heating to 950~ (Figure 5.29). Although the metakaolinite spectrum was not simulated in detail, and no quantitative estimates of the A1 population of the various sites were made, values of ~iso and the quadrupolar product PQ estimated from the MQ MAS spectra were respectively 63 ppm and 5.4 MHz for the 4-coordinated site and 37 ppm and 5.0 MHz for the 5-coordinated site (Rocha 1999). These gi~ovalues are comparable with those of McManus et al. (2001) for a different metakaolinite (64 ppm and 37 ppm respectively) but the PQ values of McManus et al. (2001) are smaller for both sites (2.9 and 3.8 MHz respectively), indicating considerable variations in the quadrupolar measurements for different metakaolinite samples. On heating to > 980~ the 30 ppm 27A1 metakaolinite resonance which formed at the expense of the octahedral kaolinite resonance disappears abruptly, leaving the octahedral and tetrahedral peaks of a typical aluminosilicate spectrum (Rocha and Klinowski 1990, Temmujin et al. 1998a) (Figure 5.30). 27A1NMR offers little resolution of an ongoing debate over the nature of a cubic spinel phase which often forms at about 980~ since, although the spectra of samples heated at this temperature have
27 A 1 N M R
A
B
kaolinite i./
0
C
metakaolinite
~
. ..... ',e'.~
js
20
311
.......
-20-
"~
+i. +-
+
60
F2 (ppm)
+'~
spinel/mullite
0
,0
_ .......
-60
60
F2 (ppm)
0
1
-60
F2 (ppm)
Figure 5.29. Triple-quantum 27A1MAS NMR spectra of the stages in the thermal decomposition of kaolinite. Line A depicts the anisotropic axis, QIS denotes the quadrupole-induced shift axis and marks the isotropic shift axis. A. Kaolinite, recorded at 30 kHz spinning speed with high-power proton decoupling. B. Metakaolinite, recorded at 15 kHz spinning speed. C. Metakaolinite heated at 950~ 15 kHz spinning speed. The small residual five-coordinated A1 signal is suggested to arise from some residual unreacted metakaolinite. From Rocha (1999), by permission of the American Chemical Society. 100
8o
v octahedral
,. 9 60
r~
~.~ 40
~v
o
r
20
o 400
600
800
1000
Temperature (~ Figure 5.30. Changes in the 27A1site occupancy of kaolinite during its thermal decomposition, after Temuujin et al. (1998a) by permission of the copyright owner.
chemical shifts and octahedral:tetrahedral ratios typical of "y-A1203, they may not differentiate between this and other possible amorphous or poorly crystalline aluminosilicate phases. As heating progresses to higher temperatures, the typical mullite 27A1 chemical shifts and octahedral:tetrahedral ratios develop. 27A1 NMR has been used to study the flash calcination of kaolinite (Slade and Davies 1991, Meinhold e t al. 1992) and the closely-related tubular mineral halloysite (Bastow e t al. 1995) as well as the effect of lithium nitrate mineraliser on the thermal reactions of kaolinite (Rocha e t al. 1991). An 27A1 and 298i NMR study of the influence of various gas reaction atmospheres on the formation of mullite from kaolinite has found that vacuum and reducing
312
Multinuclear Solid-State N M R o f Inorganic Materials
100 ~
.pl c~
9e,i
80 60 40
v
.~
20
,
o
tetrahedral
o
-
o
-
,
I
0.2
,
.
|
0.4
,
,
i
0.6
0.8
Water vapour pressure (arm)
Figure 5.31. Effect of water vapour pressure on the thermal decomposition of kaolinite held at 500~ for two hours, based on changes in the 27A1site occupancy. After Temuujin et al. (1999a) by permission of the copyright owner. atmospheres produce greater amounts of mullite which also contains a higher proportion of the tricluster T* A1 sites (MacKenzie et al. 1996). When the reaction is carried out under positive pressures of water vapour, dehydroxylation, as monitored by the appearance and growth of the 27A1 metakaolinite resonance at ca. 27 ppm, can be accomplished at temperatures as low as 500~ (Figure 5.31), and the subsequent thermal crystallisation and mechanical properties of the fired product are enhanced (Temuujin et al. 1998a). Another thermal decomposition reaction which has been comprehensively studied by 27A1 and 298i NMR is that of the dioctahedral layer-lattice aluminosilicate mineral pyrophyllite, which has a crystal structure similar to the micas, but without tetrahedral Al-for-Si substitution and therefore no charge-balancing interlayer cations. An earlier NMR study (Mackenzie et al. 1985) found that, as with kaolinite, dehydroxylation at ca. 700~ was accompanied by the loss of almost 90% of the 27A1 spectral intensity, with the five-coordinated A1 expected from structural considerations to be formed on dehydroxylation escaping detection (MacKenzie et al. 1985, Sfinchez-Soto et al. 1993). The presence of AI(V) in dehydroxylated pyrophyllite has been confirmed by 27A1NMR at the higher field of 14.1 T (Fitzgerald et al. 1989) (see section 5.3.1 and Table 5.1). 27A1 and 29Si NMR has also been used to monitor the structural changes brought about in pyrophyllite by grinding, and the effect on the subsequent thermal decomposition reactions (S~nchez-Soto et al. 1997). The thermal decomposition reactions of several other layer-lattice clay minerals have also been studied by 27A1 NMR in conjunction with other nuclides. These include a montmorillonite which contained sufficient iron for a complementary study to also be made by Mossbauer spectroscopy (Brown et al. 1987), Fuller's Earth (another
27A1NMR
313
montmorillonite mineral) (Drachman et al. 1997), muscovite mica (MacKenzie et al. 1987) and an illite clay related to the mica family (Roch et al. 1998). The thermal decomposition has been studied by 27A1 and 29Si NMR of two poorly crystalline aluminosilicates, allophane (MacKenzie et al. 1991) and a related tubular form imogolite (MacKenzie et al. 1989). Other minerals and materials whose thermal decomposition reactions have been studied by 27A1 NMR include topaz, AlzSiO4(F,OH)2, which forms mullite on heating at > 1100~ (Day et al. 1995) and synthetic hydrotalcite, Mg6Alz(OH)16CO3.4H20, a layer-lattice compound with useful catalytic properties in which A1 is substituted for Mg in the octahedral layers, with charge balance being achieved by the presence of the interlayer carbonate ions. A combination of 27A1 and 25Mg NMR indicates that on dehydroxylation this compound forms an assemblage of poorly crystalline MgO containing substituent A13+ and vacancies, and a spinel-type transition alumina which subsequently forms MgAI204 (MacKenzie et al. 1993). Aluminium titanate (tielite, AlzTiOs) has excellent thermal shock resistance but poor mechanical strength which can, however, be improved by reinforcing with whiskers of a related phase such as potassium hollandite (KzAlzTi6016). Such composites can be formed by thermal decomposition of sol-gel precursors, reaction sintering of the two phases or by thermal treatment of an appropriate glass-ceramic material. 27A1 MS NMR has been used to study the co-formation of these two phases during thermal treatment, and indicates that hollandite crystallises as whiskers within the tielite matrix (Kohn and Jansen 1998).
5.10. CEMENTS Portland cement and high-alumina cements contain, in addition to calcium silicate phases, calcium monoaluminate, CaAI204 (or CA in cement chemist's shorthand, where C = CaO and A = A1203). The 27A1 NMR spectra of this compound, in which the A1 is exclusively in tetrahedral coordination, and a number of other calcium aluminates have been determined (Mtiller et al. 1986), and more recently, using satellite transition spectroscopy (SATRAS) which has allowed the multiple tetrahedral sites in the various calcium aluminates to be distinguished (Skibsted et al. 1993). The NMR parameters for the synthetic aluminates and a number of their hydration products are shown in Table 5.4. The calcium aluminates, especially CaAl204 (CA) and Ca3A1206 (C3A) react readily with water, contributing to the hydraulic activity of the cement. The A1 in the hydrated phases is exclusively in six-fold coordination, making 27A1 NMR a convenient method for monitoring the progress of hydration of both the pure aluminate phases and alumina cements (Figure 5.32A). This technique has been used to study the
314
Multinuclear Solid-State NMR o f lnorganic Materials Table 5.4. 27A1 NMR parameters for calcium aluminates and their hydration products in Portland and high alumina cements, from Skibsted et al. (1993). Chemical shifts relative to Al(H20)63+. Compound
XQ (MHz)
"q
~(ppm)
CaA1407 (CA2)
6.25 9.55 2.50 2.60 2.60 3.32 3.37 4.30 9.7 3.8 8.69 9.30 2.4 0.705 1.8 0.36 1.7
0.88 0.82 0.2 0.75 0.95 0.53 0.39 0.47 0.4 0.7 0.32 0.54
75.5 69.5 81.9 83.8 86.2 82.7 81.6 81.2 85.9 80.2 79.5 78.3 10.2 12.36 10.2 13.1 11.8
CaA1204 (CA)
CaI2Al14033 (Cl2AT) Ca3AI206 (C3A) CaA12H2oO14 (CAHlo) Ca3A12HI2OI2 (C3AH6) Ca4A12H2602o (C4AHI3) Ca6AI2S3H64Oso (C6AS3H32) Ca4AI2SH24022 (C4ASHI2)
0.09 0.19
A
I
200
I
l
100
0
..,I
,
~
1.0
-100
1:1 water:cementN~ complete hydration
o
0.8
f
(b)
~ (a) % O
0.6 %% ~ ~ ..
I
I
I
200
100
0
.
I
-100
o~
~
0.4
o
0
4
8 12 T i m e (hr)
O
16
0
2o
27A1shift (ppm) w.r.t. AI(H2O)63+ Figure 5.32. A. 27A1NMR spectra of (top) unhydrated alumina cement (principally monocalcium aluminate), and (bottom) product of full hydration with demineralised water at a cement: water mass ratio of 1:1. Asterisks indicate spinning side bands. B. Change in the percentage of four-coordinated AI in alumina cement during hydration, as a function of time estimated by 27A1 MAS NMR. Open symbols (a): hydration with demineralised water, Filled symbols (b): hydration with 0.5 mass percent Li2CO3 solution. After Luong et al. (1989), by permission of the American Ceramic Society.
27 A 1 N M R
315
hydration of C3A (Lippmaa et al. 1982) and the temperature dependence of CA hydration (Mtiller et al. 1984, Rettel et al. 1985). 27A1 NMR has also been used to study the hydration of alumina cement and the effect on the reaction of lithium-containing setting accelerators (Luong et al. 1989). The lithium was shown by this means to eliminate the induction period of the hydration without changing the rate of hydration (Figure 5.32B). Cements often contain small amounts of heavy metals carried over from the raw materials from which they are produced. The location of these impurities in the hydrated cement phases may affect their subsequent leachability, and is therefore of environmental interest. A detailed analysis of the 27A1NMR sideband structure of C3A hydrated in the presence of chromium and zinc ions has proved useful in determining the way in which these species enter into the structure of the hydration products of C3A (Moulin et al. 2000). 27A1 NMR has also been used to study the environment of A1 as a guest ion in the two calcium silicates which constitute the other major phases of Portland cement, Ca3SiO5 (alite) and CazSiO4 (belite). Low levels of A1 (well below 1%) substituted for Si in both minerals have been detected (Skibsted et al. 1994). The 27A1NMR spectrum of Al-substituted belite (Figure 5.33A) indicates a single quadrupolar tetrahedral site with a value of g (96.1 ppm) representing the most deshielded chemical shift reported for A104. The octahedral resonance in this spectrum was attributed to an amorphous aluminate impurity phase (Skibsted et al. 1994). The 27A1 NMR spectrum of A1 substituted for Si in alite (Figure 5.33B) shows a principally tetrahedral A1 resonance, broadened due to the superposition of several lineshapes arising from A1 in different tetrahedral sites in alite (the monoclinic alite structure contains 18 different tetrahedral Si sites). A low-intensity A106 resonance was ascribed to substitution for octahedral Ca or vacancies in the alite structure (Skibsted et al. 1994).
A
B
simu ~ o b s.............................................................. e ~ ,.J., ~ :~.~= 120
60
0
-60
~ ~..,__,~] ~_...., 80... 40 300 100 -100 -300
27A1shift (ppm) w.r.t. AI(H20)63+ Figure 5.33. A. 9.4T 27A1NMR spectrum of A1 guest ions in Ca2SiO4 (lower) and (upper) simulation of the partly overlapping lineshapes using an AI(W~:A1(vI~intensity ratio of 58:42 with a single A1~ site and a Gaussian distribution of lineshapes for the A1(vI~site. B. 9.4T 27A1 NMR spectrum of A1 guest ions in Ca3SiO5 with (inset) the expanded central transition for the A1(~v)resonance. The asterisk marks the A1(vI) resonance corresponding to a total A1 intensity of 3%. From Skibsted et al. (1994), by permission of the Royal Society of Chemistry.
316
Multinuclear Solid-State N M R o f Inorganic Materials
The substitution of A1 into poorly crystalline calcium silicate hydrate C-S-H has been studied using 27A1 MQ MAS NMR (Faucon et al. 1998). This reaction is facilitated by the presence of Na + which lodges in the interlayer spaces of the C-S-H structure and compensates the charge imbalance when the tetrahedral S i is substituted by A1. The 27A1 NMR spectra distinguished two substitution sites corresponding to the bridging and non-bridging positions in the tetrahedral chains of C-S-H, with the bridging sites being more favoured at higher aluminium concentrations (Faucon et al. 1998).
5.11. NITRIDE AND OXYNITRIDE COMPOUNDS
Aluminium nitride (A1N) has a wurtzite-type structure composed of A1N4 units in a hexagonal lattice. Its 27A1 spectrum consists of a single resonance at 114 ppm (Dupree et al. 1988). Analysis of the 27A1 satellite peaks in the spectra indicates a value for XQ and r I of 1.913 MHz and ca. 0 respectively, with an unusual positive temperature coefficient of • which may be related to the wurtzite structure (Bastow et al. 1998). A1N is commonly prepared by nitridation of A1 metal, and may contain oxide impurities which can be detected by 27A1 NMR with extremely high sensitivity (down to about 0.05 wt%) (Haase et al. 1989). Surface reactions of A1N with moisture form aluminium hydroxides of Which the AI(VI) can readily be detected by 27A1 NMR, and in ultrafine A1N powders can constitute the major feature of the A1 spectrum (Hayashi et al. 1987). Oxygen can dissolve in the A1N structure to form a defect spinel related to ~-A1203 but more thermally stable on account of the presence of the nitrogen. The region of phase stability occurs around 35.7 mol% A1N, corresponding to A123027N5, with the 27A1 NMR spectra of these compounds containing resonances corresponding to A1N4 (114 ppm), A104 (65 ppm) and A106 (12 ppm) structural units (Dupree et al. 1988). The 14.1 T 27A1NMR spectrum of an A1ON sample prepared with 50 mol percent A1N obtained at spinning speeds of 18-20 kHz (Figure 5.34) shows evidence of several overlapping resonances between 50 and 110 ppm which appear to arise from mixed A1-N-O units (Fitzgerald et al. 1994). Simulation of this spectrum has led to the suggested peak assignment: 114-117 ppm = A1N4, 106 ppm = A1N30, 96 ppm = A1N202 or A1NO3, 66 ppm - A104 (Fitzgerald et al. 1994). The 27A1 peak positions for A1-N, A1-O-N and Si-A1-O-N compounds are collected in Table 5.5. A new low-temperature method for producing ultrafine powders of A1N and A1ON involves the reaction of aluminium sec-butoxide and anhydrous hydrazine in acetonitrile at 80~ (Kim et al. 2000). The amorphous precursor is then heated in nitrogen, argon or ammonia and forms crystalline products < 1000~ 27A1 NMR has provided useful insights into the structural changes occurring during heating, revealing the presence of A104, A105 and A106 groups which give way to the single A1N resonance at ca. 113 ppm as nitridation progresses. The 27A1 NMR results suggest that the formation of
317
27 A 1 N M R
AINzO2 AINO3 IAIO~ AIN30 / I .
.
.
A'AI/A,O41{/: : ~ / components
.
..'./'Yl
.;..~5,,~..a
.
;
;-~
/i\~-i~-
150 50 -50 -150 27A1 shift (ppm) w.r.t. AI(H20)63+ 150
50
-50
-150
27A1shift (ppm) w.r.t. AI(H20) 3+ Figure 5.34. 27A1MAS NMR 14.1T spectrum of A1ON powder containing 50 tool % A1N. Observed spectrum (upper left), simulated spectrum (lower left) and peak components from deconvolution analysis (right). From Fitzgerald et al. (1994) by permission of the American Chemical Society. Table 5.5. 27A1 peak positions for A1-N, AI-O-N and SiA1ON compounds, relative to Al(H20)63+. Compound
8peak (ppm)
A1N 114 A1ON 14,67,106 15R polytypoid SiA1ON 10,93,112 21R polytypoid SiA1ON 10,112 [3-SiA1ON, z = 1 75,89,108 [3-SiA1ON, z = 4 8,75,89,108 O-SiA1ON, x -- 0.24 64.7 O-SiA1ON, x = 0.2 60 X-SiA1ON 2.8,59 X-SiA1ON 0.05-0.8,61.9-63
Reference Dupree et al. (1988) Fitzgerald et al. (1994) Smith (1992) Smith (1992) Dupree et al. (1988) Dupree et al. (1988) Sjoberg et al. (1992) Barris et al. (1997) Smith (1994) Sheppardet al. (1997)
A105 groups in the precursor results from the replacement of alkoxy groups by hydrazide species, and once formed, A1Os appears to facilitate the incorporation of nitrogen during heat treatment (Kim et al. 2000).
5.12. SIALON COMPOUNDS 5.12.1
Polytypoid sialons
Of the various known structures for the extensive series of compounds of Si, A1, O and N (the sialons), those most closely related to A1N are the polytypoid sialons (Si,A1)m(O,N)m+l. The non-metal atoms in excess of the 1:1 ratio required by the
318
M u l t i n u c l e a r Solid-State N M R o f l n o r g a n i c Materials
wurtzite structure are accommodated in the polytypoids by the incorporation of octahedrally coordinated layers in the structure and by half-occupancy of some of the tetrahedral layers. Earlier 27A1 NMR studies of the polytypoid sialons showed an A1N4 resonance at ca. 110 ppm and an A106 peak at ca. 10 ppm (Klinowski et al. 1984), sometimes with an underlying broad signal (Butler et al. 1984). Faster spinning speeds have revealed a second tetrahedral resonance at ca. 90 ppm (Smith 1992), assigned to tetrahedral A1N30 units on the basis of a model in which only A1 occupies the octrahedral layer and the A1 in the tetrahedral layers bond preferentially to O, by comparison with the Si (Figure 5.35A). Analysis of these 11.7 T polytypoid sialon spectra has led to the identification of the chemical shifts of the A1N30, A1N202 and A1NO3 resonances as occurring at ca. 93, 89 and 75 ppm respectively. The differences in these shift ranges from those found for A1ON (Fitzgerald et al. 1994) may reflect the fact that the sialon spectra were less well resolved than those of A1ON, possibly due to chemical shift dispersion effects arising from the presence of Si as well as A1 next-nearest neighbours.
5.12.2
fS-sialons
These compounds are the most commercially exploited sialons to date. They are isostructural with [3-Si3N4 and have a range of composition Si6-zAlzOzN8-z, where z can take values from zero (pure Si3N4) to ca. 4. From structural considerations, the A1 is expected to occur in a range of tetrahedral coordinations, but the 27A1 spectra are very field-dependent. At lower fields (4.7 T) reasonably narrow lines are seen at approximately 103 and 66 ppm, corresponding to A1N4 and A104 units respectively, but these are superimposed on a broader envelope and correspond to only about 10% of the
A
21Rpolytypoid sialon
B
13-sialon z --- 2.7
10A t
200
-I
O-sialon x = 0.24
D
x-sialon
6
If
I
C
3/0
.8
112
I
0
I
I
-200
1
!_
200
!
I
0
I
_1
-200
27A1 shift (ppm) w.r.t. AI(I-I20)63+
I
I
200
|-
I
0
l___J
-200
1__
200
i__
I
0
_l
_
I,L
-200
27A! s h i f t ( p p m ) w . r . t . A I ( H 2 0 ) 3+
Figure 5.35. Typical 27A1MAS NMR spectra of sialons. A. 21R polytypoid sialon. B. [3-sialon, z - 2.7, C. O-sialon, x = 0.24, D. X-sialon. Asterisks indicate spinning side bands. Spectra A-C from Sj6berg et al. (1992), by permission of the Royal Society of Chemistry. Spectrum D from Sheppard et al. (1997).
27 A l N M R
319
total A1 signal (Dupree et al. 1988). At higher fields (11.7 T) some detail emerges, but the spectra are still not well resolved (Figure 5.35B). The resonance at ca. 110 ppm is thought to be unlikely to arise from pure A1N, as local charge compensation requires at least one of the A1 nearest neighbours to be oxygen; evidence of a second peak at ca. 106 ppm reported in higher-A1 ~-sialon samples (Sj6berg et al. 1992) is consistent with more than one type of A1-N environment. The tetrahedra127A1 resonance at ca. 65 ppm in all [3-sialons is broad and probably corresponds to mixtures of overlapping bands from A1NxO4-x units. Small octahedral A1 resonances often found in the 27A1 spectra of [3-sialons are not expected from structural considerations and are normally ascribed to impurity phases. The 27A1 NMR spectra of the various [3-sialons appear to be independent of the synthesis method, being similar for samples prepared by sintering (Sj6berg et al. 1992), self-propagating high-temperature synthesis (Yue et al. 1996) or carbothermal synthesis from kaolinite (MacKenzie et al. 1994) or halloysite (Neal et al. 1994). Carbothermal synthesis and its variant, silicothermal synthesis, have proved attractive routes for preparing sialons from readily available clay mineral raw materials. The clay is mixed with fine carbon and/or silicon powder and reacted in a stream of purified nitrogen at > 1400~ The [3-sialon product carbothermally synthesised from kaolinite has the composition Si3A1303N5 (z = 3), controlled by the SIO2:A1203 ratio of the clay 3A12Si2Os(OH)4 + 15C + 5N2---> 2Si3A1303N5 + 6H20 1" + 15CO 1"
(5.20)
The complex sequence of reactions preceding the formation of the sialon has been studied by 27A1 and 29Si NMR (MacKenzie et al. 1994a, MacKenzie et al.~ 1996) as has sialon formation itself (Neal et al. 1994, MacKenzie et al. 1994). [3-sialons with lower z-values, which are of interest because of their Si3N4-1ike physical properties combined with sintering benefits due to the presence of the A1, can be carbothermally prepared from kaolinite with the necessary extra Si added either as SiO2 or elemental Si. 27A1NMR has been used to study the changes in the A1 environment during these reactions, and the effect of adding three mass% Y203 as a mineralising agent (MacKenzie et al. 1997). The formation of amorphous volatile products during carbothermal reduction processes can cause problems by condensing in the cooler parts of the furnace and altering the composition of the original mixture. 27A1 NMR shows that these amorphous phases all contain A1 solely in tetrahedral coordination, with chemical shifts consistent with the potassium feldspars microcline and sanidine (54-58 ppm). Thermodynamic analysis confirmed the feasibility of vapour-phase transport of both A1 and alkali metal impurities from the clay which can then condense to form short-range feldspar-like units (Ekstr6m et al. 1996).
320
Multinuclear Solid-State N M R o f lnorganic Materials
Since [3-sialons can be difficult to sinter, metal oxides such as A1203, MgO and Y203 may be added to assist the process. 27A1 NMR has been used in conjunction with 29Si, 25Mg and 89y NMR to study the effect of these additives, both singly and in combination (MacKenzie and Meinhold 1996). The results indicate that Y203 by itself or in combination forms Y3A150~2 (yttrium aluminium garnet) which reacts at higher temperatures to form polytypoid sialon and Y-containing glass. MgO initially forms MgAI204, thereby removing some A1 from the sialon and lowering the z-value, but at higher temperatures a glass is formed, containing Mg in both octahedral and tetrahedral coordination. A1203 reacts with the [3-sialon, increasing its z-value (MacKenzie and Meinhold 1996). When placed in service at higher temperatures under oxidising conditions, sialons progressively degrade. The oxidation of carbothermal [3-sialon powder has been studied by 27A1 and 298i NMR (MacKenzie et al. 1997a). A progressive change in the A1 environment during oxidation (an increase in the octahedral A1 peak and a shift of the tetrahedral A1 resonance from ca. 67 ppm in the sialon to ca. 55 ppm) is related to the formation of mullite, while the appearance of a small amount of oL-A1203 at 14 ppm is consistent with the known stoichiometry of the oxidation reaction (MacKenzie et al. 1997a).
5.12.30-sialons
These sialons are structurally related to silicon oxynitride, Si2N20, and have the composition Si2-xAl~O~+~N2-x, where x varies from zero to ca. 0.4. Over the whole composition range the 27A1 spectra show a tetrahedral A104 resonance at 65 ppm, and in many cases a minor octahedral peak at ca. 2 ppm which is attributed to an impurity phase (Figure 5.35C) (Sj6berg et al. 1992). A new compound formed during a novel silicothermal synthesis of O-sialon from clay, silica and elemental Si was found to have similar 27A1 and 29Si NMR spectra to O-sialon. This NMR observation led to its subsequent identification as a low-temperature form which was previously unknown because other synthesis methods require higher temperatures, at which the low-temperature form is unstable, and conversion to the more usual high-temperature form has occurred (Barris et al. 1997). Because of their high SiO2 content, O-sialons show superior resistance to oxidation. The oxidation of O-sialon powder has been studied by 27A1 and 295i NMR, which indicates that the octahedral A1 thought to be present as an impurity phase in the unoxidised sialon is retained at all oxidation temperatures (MacKenzie et al. 1998). Some of this octahedral A1 is present as mullite which forms at 120(O1500~ but at higher temperatures, the oxidation is progressively hindered by the formation of a protective fused layer which is found by 27A1 NMR to contain an unexpectedly high proportion of octahedral A1 in the amorphous silica-rich material formed at 1600~ (MacKenzie et al. 1998).
27 A 1 N M R
321
5.12.4 X-sialons
This compound, which may be considered as a solid solution between Si3N4 and mullite, has a nominal composition of Si12Al18039Ns, and contains both tetrahedral and octahedral 27A1 NMR resonances. An earlier study reported that the A104 and A106 peaks occur at 66.9 and 0.8 ppm respectively (Klinowski et al. 1984), but more recently, the use of faster spinning speeds has revealed distinct 27A1 signals at 59 and 2.8 ppm (Smith 1994). Similar sharp 27A1 NMR peaks (at 62-63 ppm and 0.5-0.8 ppm) have been found in the spectra of X-sialons produced by carbothermal and silicothermal syntheses (Figure 5.35D) (Sheppard et al. 1997). 27A1 NMR spectroscopy has also been used to study the various stages in the silicothermal reaction sequence leading to the formation of X-sialon (Sheppard et al. 1997) and the effect of a number of metal oxide additives on the silicothermal formation and sintering of X-sialon (Sheppard and MacKenzie 1999). The silicothermal formation of X-sialon from clay, ~/-A1203 and Si proceeds via the formation of mullite 3AlzSi2Os(OH)4 + 6~/-A1203 ---->3A16Si2013 + 6H20 ]"
(5.21)
which then reacts with Si3N4 formed in situ by nitridation of the Si 3A16Si2013 + 6Si + 4N2 --~ Si12AllsO39N8
(5.22)
During this reaction sequence the tetrahedral A1 chemical shift should vary from about 65 ppm in ~-A1203 to about 59 ppm in mullite and back to about 63 ppm in X-sialons (Figure 5.36A), with a corresponding change in the tetrahedral:octahedral A1 ratio from about 0.5 in ~/-A1203 through about 1.1 in mullite to about 1.3 in X-sialons (Figure 5.36B). Measurements of the tetrahedral A1 shift and the tetrahedral:octahedral ratio as a function of temperature for reaction systems containing a number of different metal oxide additives have been used as an indicator of the way in which the various additives influence this complex reaction sequence (Sheppard and MacKenzie 1999), and indicate that the modification of the Si(A1)-O portions of the mullite structure to form X-sialons is assisted by all the common metal oxides except BaO, ZrO2 and Fe203 (Figure 5.36). 27A1 NMR has been used to monitor the oxidation of X-sialon powder (MacKenzie et al. 1998), and shows at about 1200~ an abrupt change in the relative amount of tetrahedral A1 from the typical value for X-sialon (ca. 61%) to the typical value for mullite, measured at the same field strength (ca. 52%) (Figure 5.37A). The accompanying change in the tetrahedral A1 chemical shift is more gradual, from the typical X-sialon value of ca. 62 ppm at 950~ to ca. 50 ppm at 1450~ (MacKenzie et al. 1998), the latter representing the mean of the two tetrahedral shifts of pure crystalline mullite (Figure 5.37B).
322
Multinuclear Solid-State N M R o f Inorganic Materials
B
A l~ 66.5 ,~
64.5
.m
62.5 F , ~ / ~ / ~ ~ ~ . . 60.5
~9 3 ~
1.5
.* y-AlzO3
58.5 1200
,
y
/-"
~
X-sialon mullite
Ms
1.1
X-sialon
o
0.7
i~ 1300
. , , i_ ~mulfite 1400 1500 Temperature (*C)
y-Al203
1200
1300
1400
Temperature
1500 (~
Figure 5.36. Modification of the silicothermal synthesis of X-sialon by metal oxide additives monitored by A. changes in the tetrahedral 27A1shift as a function of synthesis temperature, and B. changes in the tetrahedral:octahedral 27A1ratio as a function of synthesis temperature. The heavy line in both graphs refers to the additive-free control sample. From Sheppard and MacKenzie (1999).
B
A
62
/ *.' 58 o o
60 .~ ,g:l
r
54 900
\o
r~
~ 52 1100
1300
Oxidation temperature
1500 (~
\
15'00 1100 1300 91P0 Oxidation temperature (~
Figure 5.37. Oxidation of X-sialon powder monitored by A. changes in the amount of tetrahedral 27A1 in the sample as a function of oxidation temperature, and B. changes in the tetrahedral 27A1 shift as a function of oxidation temperature. From MacKenzie et al. (1998).
5.12.5 a - s i a l o n s
These c o m p o u n d s have the structure of o~-Si3N4 but need to be stabilised by the presence of other cations (typically Y, Ca, Mg or the rare earths). 27A1 and 29Si N M R have been used to study the carbothermal and silicothermal synthesis of a series of Y-containing oL-sialons YxSile-4.sxAln.5xOi.5xNl6-1.Sx, where x = 0.3, 0.5 and 0.7 (Ekstr6m et al. 1998). W h e n hot-pressed at 1800~ the 27A1 N M R spectra of the powders produced in the original synthesis (essentially a mixture of e~ and [3-sialon) were considerably broadened (Figure 5.38). The strong octahedral peak at ca. 0.8 ppm arises from the development of polytypoid sialon, while the broadening and shift of the original A1N resonances at ca. 113 ppm to ca. 9 0 - 1 0 6 ppm suggests the development of a
323
27 A 1 N M R
fromSiO2 j 113
1800~ .
k....._A~___...... from Si /113
32 Mpa 2 hr 1~
92 ~ 0 " 7
J~ ~ 13 32 Mpa 2 hr ...... 120 40 -40 120 40 -40 27A1shift (ppm) w.r.t. AI(H20)63+
,----","-'r---f~',"r--:r-__
Figure 5.38. Effect of hot-pressing on the 27A1MAS NMR spectra of yttrium-oL-sialon synthesised by carbothermal and silicothermal synthesis using SiO2 as the silica source (top) and Si as the silica source (bottom). Asterisks denote spinning side bands. From Ekstr6m et al. (1998).
continuum of A1-O-N units (predominantly A1ON3, but with smaller numbers of A1OzN2 and A103N) (Ekstr6m et al. 1998). N-melilites are yttrium or rare-earth silicon oxynitrides of general formula RzSi303N4 which often form when rare earth oxides are used to assist sintering in Si3N4-based ceramics. N-melilites can also take A1 into their structure, but NMR studies of the resulting phases are often hampered by the large magnetic moments possessed by many of the rare earth ions which cause severe broadening of the NMR signals of nearby nuclei. Since these effects are smaller for samarium, it has been possible to study the incorporation of aluminium into samarium N-melilite by 27A1 and 298i NMR (Chee et al. 1995). The results indicate that Al-substituted samarium N-melilite contains predominantly A104 and SiOzN2 units, irrespective of the extent of A1 solubility (Chee et al. 1995).
5.12.6 Sialon glasses
The incorporation of nitrogen into aluminosilicate glass results in an improvement in the physical properties (increased hardness, refractive index, softening temperature, mechanical strength and resistance to chemical attack). Pressureless melting and quenching at a controlled rate results in the formation of dense sialon glasses with compositions ranging from almost pure SiO2 to Si3.8All.307.4N (McMillan et al. 1998). The structural units in a sialon glass of typical composition Si25A16.6055N have been studied by 27A1 and 29Si NMR (Sato et al. 1990), which indicate the presence of AI(IV), AI(V) and AI(VI) species coordinated to oxygen, by contrast with the corresponding nitrogen-flee aluminosilicate glass which contains only AI(IV) and AI(VI) (Sato et al. 1990, McMillan et al. 1998). The presence of trivalent nitrogen in the glass is thought
324
Multinuclear Solid-State NMR of Inorganic Materials
to lead to cross-linking in the anion sub-network, increasing the viscosity of the melt at the glass transition temperature and allowing the 5-coordinated A1 sites to be more readily quenched in (Sato et al. 1990). Yttrium-sialon glasses have also been investigated by 27A1 NMR (Jin et al. 1994). Although the interpretation of these results was limited by the slow spinning speeds used, the absence of A1-N bonding was noted, with the A1 occupying tetrahedral A104 sites within the glass structure.
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S~inchez-Soto, P.J., P6rez-Rodriguez, J.L., Sobrados, I. & Sanz, J. (1997) Chem. Mater., 9, 677. Sanz, J., Serratosa, J.M., Moya, J.S. & Aza, S. (1988) J. Amer. Ceram. Soc., 71, C418. Sato, R.K., Bolvin, J. & McMillan, P.F. (1990) J. Amer. Ceram. Soc., 73, 2494. Sato, R.K., McMillan, P.F., Dennison, P. & Dupree, R. (1991) J. Phys. Chem., 95, 4483. Sato, R.K., McMillan, P.F., Dennison, P. & Dupree, R. (1991a) Phys. Chem. Glasses, 32, 149. Schmiicker, M. & Schneider, H. (1996) Berichte Bunsenges Phys. Chem., 100, 1550. Schmticker, M., MacKenzie, K.J.D., Schneider, H. & Meinhold, R.H. (1997) J. Non-Cryst. Solids, 217, 99. Schmticker, M., Schneider, H. & MacKenzie, K.J.D. (1998) J. Non-Cryst. Solids, 226, 99. Schmticker, M., Schneider, H., MacKenzie, K.J.D. & Okuno, M. (1999) J. European Ceram. Soc., 19, 99. Schneider, H., Merwin, L. & Sebald, A. (1992) J. Mater. Sci., 27, 805. Schneider, H., Voll, D., Saruhan, B., Schmticker, M., Schaller, T. & Sebald, A. (1994) J. European Ceram. Soc., 13, 441. Schurko, R.W., Wasylishen, R.E. & Phillips, A.D. (1998) J. Mag. Reson., 133, 388. Selvaraj, U., Komarneni, S. & Roy, R. (1990) J. Amer. Ceram Soc., 73, 3663. Selvaraj, U., Komarneni, S. & Roy, R. (1993) J. Solid State Chem., 106, 73. Sheppard, C.M., MacKenzie, K.J.D., Barris, G.C. & Meinhold, R.H. (1997) J. European Ceram. Soc., 17, 667. Sheppard, C.M. & MacKenzie, K.J.D. (1999) J. European Ceram. Soc., 19, 535. Sj6berg, J., Harris, R.K. & Apperley, D.C. (1992) J. Mater. Chem., 2, 433. Skibsted, J., Henderson, E. & Jakobsen, H.J. (1993) Inorg. Chem., 32, 1013. Skibsted, J., Jakobsen, H.J. & Hall, C. (1994) J. Chem. Soc. Faraday Trans., 90, 2095. Slade, R.C.T. & Davies, T.W. (1991) J. Mater. Chem., 1, 361. Slade, R.C.T., Southern, J.C. & Thompson, I.M. (1991) J. Mater. Chem., 1, 563. Slade, R.C.T., Southern, J.C. & Thompson, I.M. (1991a) J. Mater. Chem., 1, 875. Smith, M.E. (1992) J. Phys. Chem., 96, 1444. Smith, M.E. & Steuernagel, S. (1992) Solid State Nucl. Mag. Reson., 1, 175. Smith, M.E., Jaeger, C., Schoenhofer, R. & Steuernagel, S. (1994) Chem. Phys. Lett., 219, 75. Sohlberg, K., Pennycook, S.J. & Pantelides, S.T. (1999) J. Amer. Chem. Soc., 121, 10999. Sorar~, G.D., Mercadini, M. & Dal Maschio, R. (1993) J. Amer. Ceram. Soc., 76, 2595. Spearing, D.R., Stebbins, J.F. & Farnan, I. (1994) Phys. Chem. Minerals, 21,373. Stebbins, J.F. & Farnan, I. (1992) Science, 255, 586. Stebbins, J.F., Farnan, I., Dando, N. & Tzeng, S-Y. (1992a) J. Amer. Ceram. Soc., 75, 3001. Taylor, A. & Holland, D. (1993) J. Non-Cryst. Solids, 152, 1. Temuujin, J., Okada, K. & MacKenzie, K.J.D. (1998) J. European Ceram. Soc., 18, 831. Temuujin, J., Okada, K., MacKenzie, K.J.D. & Jadambaa, T.S. (1998a) J. European Ceram. Soc., 19, 105. Temuujin, J., Okada, K. & MacKenzie, K.J.D. (1999) Ceram. Int., 25, 85. Temuujin, J., MacKenzie, K.J.D., Jadambaa, T.S., Namjildorj, B., Olziburen, B., Smith, M.E. & Angerer, P. (2000) J. Mater. Chem., 10, 1019.
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Temuujin, J., MacKenzie, K.J.D., Schmficker, M., Schneider, H., McManus, J. & Wimperis, S. (2000a) J. European Ceram. Soc., 20, 413. Toplis, M.J., Kohn, S.C., Smith, M.E. & Poplett, I.J.F. (2000) Amer. Mineralogist, 85, 1556. van Bokhoven, J.A., Roest, A.L., Koningsberger, D.C., Miller, J.T., Nachtegaal, G.H. & Kentgens, A.P.M. (2000) J. Phys. Chem. B, 104, 6743. van Eck, E.R.H., Kentgens, A.P.M., Kraus, H. & Prins, R. (1995) J. Phys. Chem., 99, 16080. Vosegaard, T. & Jakobsen, H.J. (1997) J. Mag. Reson., 128, 135. Weller, M.T., Brenchley, M.E., Apperley, D.C. & Davies, N.A. (1994) Solid State Nucl. Mag. Reson., 3, 103. Wilson, S.J. (1979) Proc. British Ceram. Soc., 28, 281. Wood, B.J., Kirkpatrick, R.J. & Montez, B. (1986) Amer. Mineralogist, 71,999. Wood, T.E., Siedle, A.R., Hill, J.R., Skarjune, R.P. & Goodbrake, C.J. (1990) Mater. Res. Soc. Symp. Proc., 180, 97. Wu, Y., Chmelka, B.F., Pines, A., Davis, M.E., Grobet, P.J. & Jacobs, P.A. (1990) Nature, 346, 550. Xu, Z. & Sherriff, B.L. (1993) Appl. Mag. Reson., 4, 203. Yang, X. (1995) J. Phys. Chem., 99, 1276. Yasumori, A., Iwasaki, M., Kawazoe, H., Yamane, M. & Nakamura, Y. (1990) Phys. Chem. Glasses, 31, 1. Youngman, R.E. & Aitken, B.G. (2001) J. Non-Cryst. Solids, 284, 9. Yue, Y., He, H., Klinowski, J., Wu, Y. & Zhuang, H. (1996) J. Mater. Chem., 6, 1391.
Chapter 6
170 NMR 6.1. Introduction 6.2. Background 6.2.1 Enrichment Schemes 6.2.2 Experimental NMR Methodology 6.2.3 Relationships between NMR Parameters and Structure 6.3. Binary Oxides 6.3.1 Crystalline Materials 6.3.2 Sol-Gel Produced Samples 6.4. Crystalline Ternary Ionic Systems 6.5. Silicates and Germanates 6.5.1 Crystalline Materials 6.5.1.1 Silica and Germania 6.5.1.2 Ternary Silicates 6.5.1.3 Silicates and Germanates of Zirconium and Titanium 6.5.2 Amorphous Materials 6.5.2.1 Silica and Germania 6.5.2.2 Metal Silicate and Germanate Glasses 6.5.2.3 Gel-Based Silicates 6.6. Aluminium- and Gallium-Containing Systems 6.6.1 Alumina and Aluminates 6.6.2 Crystalline Alumino- and Gallosilicates 6.6.3 Amorphous Aluminosilicates 6.7. Boron-Containing Systems 6.7.1 Borates 6.7.2 Ternary and Quaternary Systems 6.8. Other Systems 6.9. Hydrogen-Containing Samples 6.9.1 Crystalline Hydroxides and Other Hydrogen-Containing Materials 6.9.2 Hydrous Gels and Glasses 6.10. High Temperature Ceramic Superconductors References
333 334 334 337 346 349 349 352 355 359 359 359 361 365 366 366 367 369 372 372 375 379 381 381 382 384 386 386 387 388 390
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Chapter 6
170 NMR 6.1. INTRODUCTION Oxygen is a ubiquitous component of the inorganic compounds of fundamental importance to materials technology (advanced, structural and functional ceramics, catalysts, etc.). This is hardly surprising since oxygen is the most abundant element in the earth's crust (~62.5 at%), 2.95 times more abundant than silicon and 9.67 times more plentiful than aluminium, the two next most abundant elements. This also clearly illustrates the important place of oxygen in mineralogy, and minerals are the source of many raw materials for advanced processing. Metals are usually refined from natural ores, many of which are oxides (e.g. bauxite, rutile). The production of aluminium by the Bayer process is an oxide-based industry with an annual production of almost 20 billion tonnes. Thus, oxygen NMR should have many important applications; since it is a technique sensitive to short-range interactions of the oxygen atoms located throughout the structure, it can provide information about the structure in its entirety (other nuclei are often further away from the centres of structural change). However, until the last few years, reported 170 NMR studies of solids have been relatively few. There are a number of reasons for this. Of the three stable oxygen isotopes (~60 with 99.76 % natural abundance, 170, with 0.037 % abundance and ~SO, with 0.2 % abundance), only ~70 has a nuclear spin (I - 5/2) and is accessible to NMR. Hence for routine observation isotopic enrichment is necessary, involving both cost and effort which hindered the initial development of ~70 NMR of solids. However, the importance of oxygen and in particular the oxygen sites to materials' properties led to a few early high-resolution solid-state 170 NMR studies, the first one published in 1983 (Schramm, Kirkpatrick and Oldfield 1983). A second problem with 170 is the significant broadening which can affect the resonances of this quadrupole nucleus, the quadrupole interaction being a strong function of the covalency of the M-O bond. Historically, the first group of compounds studied by ~70 NMR (silicates and zeolites) was unfortunately one of the most difficult. The ability to characterise the many types of oxygen sites in silicates and aluminosilicates would be extremely useful, but these sites have relatively large quadrupole interactions that can only partially be narrowed by MAS. Furthermore, these sites show a very small shift range (--~40 ppm) for ~70. These factors meant that the early 11.7 T 170 NMR MAS spectra of silicates and zeolites did not show much promise because of the lack of resolution, often detecting only Si-O-Si and Si-O-A1 rather than all the inequivalent sites expected from crystallography. Hindsight shows that the use of 170 333
334
Multinuclear Solid-State NMR of lnorganic Materials
NMR to solve such problems was correct, but required improvements in the technique which were not then available. Unfortunately these early experiments cast an unduly pessimistic perspective on the use of 170 NMR as a probe of silicate materials. The much smaller quadrupole interactions in more ionic oxides, and even in glassy materials, and the very large shift range for 170, allow the different sites readily to be distinguished. 170 is probably best characterised as a nucleus with a small quadrupole moment and a large shift range, making it attractive for solid state NMR. The 1990s brought great advances in the methodology for 170 NMR, including improved procedures for isotopic enrichment, much wider availability of fast MAS and high applied magnetic fields, and the development of specialised techniques for removing second-order quadrupole interactions, including DAS, DOR and MQ MAS. There now exist in the literature elegant examples of the application of all these techniques to 170, bringing impressive gains in resolution with as many as ten different oxygen sites being resolved. In the last few years the literature has seen an explosion in the number of papers using 170 solid state NMR and provided the cost of the isotope does not become prohibitive, this trend is likely to continue, assuring 170 NMR of an important role in materials characterisation.
6.2. BACKGROUND
6.2.1 Enrichment schemes A wide range of schemes for l Vo enrichment of samples is now available. As these techniques have become almost routine this is the first step in making 170 NMR a more widely used tool for analysis of oxides. The two readily available 170-labelled sources are O2 and H20, with typical 170-enrichment levels of 10-75 at %. The enrichment level of choice for any given experiment is a compromise between cost and the required sensitivity. Our experience shows that 20 at % enrichment of the precursor provides sufficient sensitivity for 1D spectroscopy and at current prices l g of the labelled oxide product typically costs --~s to manufacture. For MQ and DOR experiments, higher enrichments (35-45 at %) are desirable. A convenient route for producing certain oxides is by hydrolysis of the chloride. SiO2 can be prepared this way, as outlined by Bray et al. (1982). SIC14 is dissolved in ether and held at ice temperature under a dry nitrogen atmosphere. 170-labelled water is then added dropwise to this mixture while stirring. After addition of all the water the mixture is allowed to gradually warm to room temperature, again stirring. The resulting precipitate must be washed thoroughly several times to remove residual chloride and is then vacuum dried. The subsequent heat treatment depends on the target product but is typically 250~ for 1 day followed by calcination at 600~ for a few hours
170 NMR
335
to produce very clean, dry amorphous silica gel, or at 1500~ for 15 minutes to produce well-crystallised low cristobalite. Other oxides can also be enriched in this way but care must always be taken to control the temperature of the reaction as some chlorides hydrolyse vigorously. An alternative route becoming increasingly popular with the wider commercial availability of a variety of alkoxides is to start with a metal alkoxide (M(OR)x, where R is an alkyl chain). The alkoxide is dissolved in alcohol (our laboratory normally uses propanol) to act as a mutual solvent for the alkoxide and the water which is then added dropwise at room temperature while stirring to initiate a hydrolysis reaction (Brinker and Scherer 1990).
M(OR)x + xH20 ---->M(OH)~ + xROH
(6.1)
This reaction provides an efficient method for attaching the oxygen label to the metal. The hydroxyl groups on the metal then react with each other to cross-link the structure:
M(OH)x + M(OH)x --> (OH)x_ ~MOM(OH)~_~ + H 20
(6.2)
This results in the formation of a metal-oxygen-metal bond by a process which continues, resulting in the growth of the metal clusters and the removal of the hydroxyls. The nature of the liquid changes, first becoming a colloidal suspension, then a sol, and eventually turning into a gel. The alcohol and water produced must be removed by stirring at room temperature until the sample has dried to a crumbly solid. The sample is then powdered and vacuum dried. 13C NMR has revealed that even at this stage residual organic fragments are attached to the oxide, but these can be removed by gradually heating to 250~ and then 500~ The key is to heat gently to remove the carbon, producing an amorphous finely divided oxide, as the porous structure does not sinter which would seal in the carbon. Another route for the formation of the oxides of metals that have insoluble hydroxides is to start with a soluble salt such as CaC12 or Mg(NO3)2 and add a soluble hydroxide such as KOH in the presence of 170-labelled water. The insoluble hydroxide precipitate is washed prior to thermal dehydroxylation to form the oxide. Carbonates can also be enriched by sealing in a thick walled glass tube with ~70-labelled water and heating at 80-110~ for typically --~1-2 weeks. The exchange reaction can be facilitated by the addition of a small amount of NH4C1 flux (a typical mixture would be 1.23 g of CaCO3, 0.33g of water and 0.077g of NH4C1). The flux and any excess water can be removed by subsequent drying under nitrogen at 450~ for 1 hour. K2CO3 can be enriched this way and then used to prepare other sparingly soluble metal carbonates by an exchange reaction with the chloride of the required metal, the solubility of the KC1 facilitating its removal from the metal carbonate. Similar exchange reactions can be carried out with
336
Multinuclear Solid-State NMR of lnorganic Materials
hydroxides by sealing them with enriched water. This is a common route for preparing enriched alumina from AI(OH)3, or more commonly A1OOH, by sealing them in either a quartz or gold tube with enriched water and heating at 200-350~ for a few days. These syntheses readily produce enriched binary oxides from which more complex phases can be formed by mixing the appropriate oxides, hydroxides or carbonates and heating to a temperature where solid state reaction takes place. However 170 can be easily lost during this heat treatment, necessitating the use of different methods at temperatures higher than 1000~ Below 1000~ the powder can be heated in an open alumina boat or platinum crucible under a nitrogen atmosphere, but exchange and loss of oxygen becomes an increasing problem as 1000~ is approached. Above 500~ the use of a dry nitrogen atmosphere produces significant advantages over a normal atmosphere. Above 1000~ the loss of labelled oxygen becomes very rapid, and the best strategy is to seal the mixture in a platinum or gold foil capsule. An approach often used in the preparation of silicates is to mix and homogenise the oxides by forming a glass which is then crystallised to form the product. Even for sealed samples it is important to keep the time spent at temperatures in excess of 1000~ as short as possible. Another key to securing successful solid state reactions is to achieve intimate mixing between the different metal oxide components. In this respect, the alkoxide route is attractive, since the components are homogeneously mixed in the initial solution. Care must be taken to match the hydrolysis rates of the different metal alkoxides, but techniques have now been developed by sol-gel chemists for overcoming such problems (Brinker and Scherer 1990). Gas exchange can be used to enrich many oxides. The oxygen in high-temperature ceramic superconductor phases such as YBa2Cu3Ov_x is mobile and some can be removed by heating under vacuum (Oldfield et al. 1989). The system can then be back filled with 170-labelled 02 before cooling back down to room temperature. Heating under a 1702 atmosphere has been found to be a very widely applicable approach for enriching oxides (Yang et al. 1989). A scheme typically used with zeolites is to heat to 500-750~ and evacuate to 10-4-10 -5 Torr for 12-24 hours. 1702 is then introduced and the sample left for about another day before cooling back down to room temperature. Water can also be exchanged with zeolites. 170 NMR has been used to study in detail the kinetics of this process in both stilbite (Xu and Stebbins 1998) and analcime (Cheng et al. 2000). The powders were ground and the 44-75 txm fraction used for the reaction. The samples were initially enriched by sealing with 170-labelled water in a gold tube and heated to the appropriate temperature for 10 days to 6 weeks to ensure uniform labelling of the sites. In both stilbite and analcime it was found that the rate of reaction was determined by the exchange step and not by diffusion in these finegrained porous materials. The samples were then evacuated and backreacted with normal water for varying times at various temperatures, using the 170 NMR signal to monitor the kinetics of the equilibrium
170 NMR M-170- M' + 92160 ~ M - 1 6 0 - M' + 02170
337 (6.3)
where M,M' = Si,A1. For stilbite in the temperature range 150-200~ the Si-O-A1 fragments were shown to exchange faster than the Si-O-Si. For analcime at 400~ SiO-A1 exchanges faster than Si-O-Si but by 500~ the rates of exchange become comparable, suggesting that Si-O-Si exchange has a higher activation energy.
6.2.2 Experimental N M R methodology 170 NMR provides an ideal opportunity for the exploitation of the full range of solid-
state NMR methodology now available. The relative contributions of the interactions, particularly the chemical shift and quadrupole interaction, vary so widely that no single approach is correct for ~70 NMR of all samples. The most straightforward experiments are one-pulse acquisition and a spin-echo sequence, used with both static and MAS conditions. If only relatively narrow lines are present, as is the case for ionic systems, one pulse acquisition tends to be sufficient, although the moderate Larmor frequency of oxygen makes the use of a spin-echo sequence advisable at lower fields (-< 8.45 T) even with MAS. When echoes are combined with MAS, the echo spacing should be set to an integral number of rotor periods. In samples where there is significant quadrupole interaction, such as silicates and borates which contain more covalent links, broader lines are encountered and echo techniques are commonly used. 170 shows a wide variation in relaxation times, making it necessary to consider carefully the relaxation delay to be used. A delay of 0.25 s is often sufficient for gel-formed precursor materials, but much longer T1 times are encountered in more rigid structures where there are no efficient relaxation mechanisms. The relaxation time of BaHfO3 was found to be 32 s, a value typical of this type of material. Figure 6.1A shows the 170 NMR spectra of albite glass acquired at 7.05 T, in which the Si-O-Si and Si-O-A1 signals completely overlap under both static and MAS conditions. However, the static spectrum shows better-defined features (the singularities in the quadrupole lineshape), allowing the two contributions to be unambiguously separated. Combination of the 170 MAS NMR data for a crystalline titanodiphenylsiloxane acquired at various magnetic fields (Figure 6.1B) with the static NMR data (Figure 6.1C) has allowed the interactions in this material to be deduced (Gervais et al. 2000). In materials containing X-O-Y linkages with very different interaction parameters, the choice of an optimum magnetic field is not straightforward, particularly where there is a large variation in the quadrupole interaction at different sites and the material is amorphous. The magnetic field variation of an amorphous TiO2-SiO2 based material is shown in Figure 6.2. This material has a complex cross-linked structure giving five distinct 170 NMR signals, OTi3, OTi4, two types of Si-O-Ti and Si-O-Si (Gervais et al. 2001). The resonances arising from oxygen bonded only to titanium are
338
Multinuclear Solid-State NMR of Inorganic Materials A
B
Staticecho
5.6T
~~/u ~_J
500
0
-500 MAS
C
~ated
, 17.6T ~ /~
0
-5O0
170 shift (ppm) w.r.t. H20
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observed
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~
obser::
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i0o
170 shift (ppm) w.r.t. H20
....
400 ........
~ted 300 .....
170 shift (ppm) w.r.t. H20
Figure 6.1. A. 170 static echo and MAS spectra of albite glass. Note the singularities in the static spectrum which allow the contributions from the Si-O-Si and Si-O-A1 groups to be separated. Asterisks denote spinning side bands. From Dirken et al. (1997) by permission of the copyright owner. B. Observed and simulated 170 MAS NMR spectra of a crystalline titanodiphenylsiloxane at three magnetic fields. Asterisks denote spinning side bands. C. Observed and simulated 170 14.1 T static NMR spectrum of the same titanodiphenylsiloxane. From Gervais et al. (2000) by permission of the copyright owner.
very ionic and their MAS linewidths are dominated by chemical shift dispersion (CSD), making these peaks very much broader and more difficult to discern at the highest field. On the other hand, the Si-O-Si MAS lineshape is completely dominated by second-order quadrupole effects, becoming very much narrower and more prominent at higher fields. The Ti-O-Si signals show significant contributions from both secondorder quadrupolar and CSD effects. The advantage of the highest field is its resolution of two different Ti-O-Si environments. It is clear from Figure 6.2 that the appearance of the spectrum changes considerably from 5.6 to 14.1 T and could well influence the interpretation of the data. One of the greatest advantages of NMR lies in the quantitative integrity of the spectra; if great care is taken, as in the case of this study, quantitative information can be extracted from the quadrupolar 170 NMR spectra. At 5.6 T the intensity of the Si-O-Si linkages will be underestimated by NMR because the factor pQ2/poVr (see Section 3.5.4) will be much higher for this fragment, causing some of the (1/2,- l/z) transition intensity for this site to appear in the sidebands. By using this factor to make a correction, satisfactory agreement can be achieved between the intensity distribution determined by the two techniques. This example illustrates the need for careful correction of intensities and the benefits of making measurements at more than one magnetic field; data from one field interpreted superficially can be very misleading.
170 N M R Si-O-Ti
339
Si-O-Si
OTi3
8.45 T
*
14.1
600
200
-200
170 shift (ppm) w.r.t. H20 Figure 6.2. Observed and simulated 170 MAS NMR spectra of an amorphous TiO2-SiO2 material at three magnetic fields, showing resolution of the various structural sites. Asterisks denote spinning side bands. From Gervais et al. (2001) by permission of the copyright owner. Table 6.1. 170 NMR linewidths of X-O-Y fragments in the amorphous sol-gel sample measured at 5.6 and 14.1 T, as shown in Figure 6.2. X-O-Y fragment Av (kHz) (+ 0.1) at 5.6 T Av (kHz) (+_ 0.1) at 14.1 T Si-O-Si Ti-O-Si (1) Wi-O-Si (2) OTi4 OTi3
7.3 3.6 4.2 2.8 1.9
3.2 3.9 4.6 6.9 5.2
Multiple field data can be used to determine the dominant interaction and to provide an estimate of the quadrupole interaction even when a distinct quadrupole lineshape cannot be discerned. As an example, the linewidths of the Ti-O-Si fragments in the above amorphous gel determined at 5.6 and 14.1 T are shown in Table 6.1. Table 6.1 shows the lines corresponding to OTi3 and OTi4 are much broader at the higher field, indicating that CSD dominates in these sites. The opposite is true for the S i-O-Si line, showing that second-order quadrupolar effects dominate in this case. However, there is only a slight change in the linewidths of the Ti-O-Si groups between
340
Multinuclear Solid-State NMR of Inorganic Materials
the two magnetic fields, indicating that both chemical shift dispersion and the secondorder quadrupolar interaction make significant contributions. If at one field (Box) the two contributions are AvcsD and AVQ then the total linewidth at that field (in Hz) will be given by
(AVTot,Bol)2 _ (AVcsD)2 + (AVQ)2
(6.4)
If measurements are taken at a second magnetic field (Bo2) so that ~ = (Bo2/Bol) then the total linewidth at the second field may be written as
(AVTot.Bo2)2_
~2
(AVcsD)2 +
(AVQ) 2
~2
(6.5)
Hence, measurements at the two fields allow these two contributions to be estimated, from which the quadrupole interaction can also be estimated by simulating the line derived by superposition of the two contributions. 170 spectra are usually referenced directly against the primary shift substance (H20 at 0 ppm). MAS is the method of choice in ionic materials where there is generally a small XQ value and the centreband is narrow and structureless. In crystalline materials of this type shift differences as small as 4 ppm can be detected (Bastow et al. 1996). The quadrupole interactions can be determined from the envelope of spinning sidebands of the satellite transitions, a method which has been used in ~70 NMR studies of boehmite (Walter and Oldfield 1989, Skibsted et al. 1991) and silica gel (Jfiger et al. 1993). The singularities of the (_+ 3/2, ~___1/2) transitions in BaZrO3 are clearly visible (Figure 6.3) and the intensity distribution of the sidebands that can be assigned to the central transition is determined by the CSA (Bastow et al. 1996). Thus, in ionic materials such as this the change in the relative size of the quadrupole and chemical shift interaction for the central transition (CSA dominant) and the satellite transitions (first-order quadrupole dominant) allows the two interactions to be independently deduced (Bastow et al. 1996). Where XQ becomes significant, as in SiO2, the 170 NMR spectra of silica gel show significantly better resolution of the (+ 3/2, + ~/2) transition than of the central transition (Jager et al. 1993). However, although the satellite transition approach is more easily implemented than DAS and DOR it has not been used with 170 to resolve different sites, and has now been largely superseded for 170 by the development of MQ MAS. Among the first applications of DAS and DOR were ~70 studies of crystalline silicates, in which spectacular gains in resolution of up to two orders of magnitude were obtained (Chmelka et al. 1989, Mueller et al. 1991, Mueller et al. 1992). In the case of woUastonite, nine sites were resolved, in agreement with the site multiplicity expected from the crystal structure (Figure 6.4). The use of field-dependent data to deduce isotropic information from changes in the isotropic position has now been supplemented by MQ MAS NMR. In the second dimension of the 2D DAS and MQ data sets
170 NMR
~ j
!
9
~
I
a
,
9
9
1000
,f
, ........
341
|
I
500
i
i
0
,
9
!
_ t
l
1
-500
170 shift (ppm) w.r.t. 1120 Figure 6.3. MAS NMR spectrum of BaZrQ showing a sideband intensity distribution resulting from the CSA. From Bastow et al. (1996a) by permission of the copyright owner. 170
DAS
DOR j gSiz06 .I .t..
~
diopside
100
0
100
0
forsterite l g ~ S i ~ 60
20
60
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20 J,II
25
S
i
~
75
25
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0
100 larnite
140
100
0 J/
140
100
170 shift (ppm) w.r.t. H~O Figure 6.4.
170
DAS and DOR NMR spectra of several silicates, from Mueller et permission of the American Chemical Society.
al.
(1991) by
342
Multinuclear Solid-State NMR of lnorganic Materials
the quadrupole interaction parameters can be deduced by simulating the anisotropic slices. The direct use of high-resolution data to obtain quantitative information presents problems; the normal approach is to use the interactions thus derived to fit the static and MAS 1D data, sometimes combining this with multiple applied magnetic fields to ensure that the fit is as unambiguous as possible. The 1D data provide the most reliable quantitative information. Several studies have compared the different approaches for extracting information from 170 NMR. 1D static, MAS, DAS and 3Q MAS data for the zeolite stilbite have been compared (Xu and Stebbins 1998). Although there are ten crystallographically distinct oxygen sites in the structure, Si,A1 disorder allows only the Si-O-Si and Si-O-A1 signals to be distinguished by any of these methods. The disorder leads to increased width in the isotropic dimension of the 2D data sets; in silicate glasses this isotropic width can be typically an order of magnitude larger than in the corresponding crystalline silicate. The full 2D DAS dataset contains anisotropic information in the second dimension, providing a correlation between the isotropic and anisotropic information. Since DAS data are often collected at other than the magic angle (usually 79.2 ~ the anisotropic information is composed of a mixture of interactions, but is dominated by the second-order quadrupolar. DAS data in the isotropic dimension clearly distinguish bridging and non-bridging oxygens in glassy silicates. The width of the isotropic dimension provides a measure of the distribution of the environments (Farnan et al. 1992). Values of XQ and xl can be deduced from the information in the anisotropic dimension, allowing important structural information to be derived if the relationship between the bond angle and these parameters is known. A correlation between the isotropic shift and the quadrupole information and hence the bond angle distribution has been deduced (Figure 6.5) for glasses (Faman et al. 1992) and zeolites (Xu and Stebbins 1998). The isotropic shift in silicate glasses was found to be related to the asymmetry. D~tB(2) ~'iso,Q) ~ D2(PQ) ~ D3(a)
(6.6)
where a is the Si-O-Si bond angle. In the case of stilbite the 3Q isotropic dimension was found to give better resolution than DAS. Simulation of the data from the different methods showed a decrease in the • values calculated from the static, DAS and MQ experiments, reflecting the decrease in the additional anisotropic broadening detected by these different approaches. The value deduced from MQ data would be expected to agree with the 1D MAS data. The ratio of Si-O-Si:Si-O-A1 was greatest when derived from static and DAS data, but was lower in the 3Q experiment, resulting from the loss of intensity from the sites of larger XQ due to dependence of the excitation on XQ. For zeolite Na-A it was found that 5Q yielded better resolution than 3Q, even at a higher field (Mildner et al. 1999). A comparison of DOR, 3Q and 5Q MAS made for several low-silica zeolites (Freude et al. 2001) demonstrated that the isotropic chemical
170 NMR
343
-40
-20
0
20
40 I
[
100
I
I
0
I
I
-100
oh(ppm) Figure 6.5. Use of 170 DAS NMR to determine details of the bridging oxygen resonance in glass. Slices taken parallel to the to2 anisotropic axis are shown at the right, together with multi-parameter least-squares fits (broken lines) giving the variation of the magnitude and asymmetry of the electric field gradient across the isotropic lineshape (oJ1).From Faman et al. (1992) by permission of MacMillan Magazines Ltd. K28i409
shifts effectively determine the isotropic linewidth. DOR and 5Q measurements give similar resolution, superior to that from 3Q data. The disadvantage of 5Q compared to 3Q data is its much lower sensitivity (reduced by a factor of ---40 in some zeolites) (Freude et al. 2001). Improved excitation in MQ sequences can be obtained by the use of sequences such as RIACT and FAM2. A comparison of FAM with normal pulse excitation for a series of zeolites showed FAM to give better sensitivity (Zhao et al. 2001). This study noted that the XQ-dependence of the excitation and detection was exaggerated for 5Q compared to 3Q experiments. Resolution of the different oxygen sites in siliceous zeolites has provided a challenge for NMR. Detailed studies of the four sites in faujasite (Bull et al. 1998) and the ten sites in ferrierite (Bull et al. 1999) have presented a significant test of the technique, especially since in ferrierite the sites occur in a shift range of < 15 ppm. 3Q MAS data (Figure 6.6) provided anisotropic information which was combined with DOR at five magnetic fields to give six data points from which PQ and 8i~o,c~were derived, via the Vo-2 dependence (Figure 6.7). The interactions deduced from these data were then correlated with the structure (Section 6.2.3).
344
Multinuclear Solid-State NMR of Inorganic Materials
40
6O .,.1,.
50 ....... 0 ....... is0
ppm
70
0
-70
170 shift (ppm) Figure 6.6. 2D ~70 MQMAS NMR spectrum of ferrierite showing in the vertical direction (the projection along the y-axis) the MQMAS scaled isotropic spectrum and in the horizontal direction the anisotropic lineshapes for the five resolved peaks with their simulations (light broken lines) from which the 170 NMR interaction parameters were derived. From Bull et al. (2000) by permission of the American Chemical Society.
11.7 T D O R 2x10 -16
R 5 T DOR
lxl~
}Y/7/A~
j'-
17.s5 T o o a
0
20
20 -~_
170 shift (ppm)
-20
14.04 T M Q M A S
_ l x l 0 -16
Figure 6.7. Combination of 170 DOR data for ferrierite at five magnetic fields with MQMAS data. The intercept on the x-axis corresponds to the isotropic chemical shift for each site and the gradient is proportional to the quadrupolar product PQ. From Bull et al. (2000) by permission of the American Chemical Society.
345
170 N M R
CP experiments with 170 provide useful information about materials in which the oxygen site is protonated. Static and MAS 1H --+ 170 CP has been used to investigate a range of compounds including Mg(OH)2, Ca(OH)z, A10(OH), talc and amorphous silica (Walter et al. 1988). Detailed studies of the CP dynamics showed that T10 can be short, giving typically short optimum contact times (< lms). Because of the modulation of the magnetisation transfer due to rotational effects, the CP signal can be very strongly attenuated under MAS conditions. Mg(OH)2 is a useful set-up compound for determining the Hartmann-Hahn match condition. 170 CP NMR has been used to identify the different sites in boehmite, in which the A1-OH signal is significantly enhanced by CP compared with the A1-O-A1 signal (Figure 6.8A). Dipolar oscillations can often be observed in these systems since the relative isolation of the protons causes the magnetisation to oscillate between the proton and the oxygen (Figure 6.8B). These oscillations contain information about the oxygen-proton distance. If homonuclear dipolar decoupling is applied in a static experiment while the 170 magnetisation is being accumulated, the effect of 1H-170 dipolar coupling can be seen in the spectrum (Figure 6.8C). Singularities additional to the normal second-order quadrupolar interaction are seen, and the inclusion of the dipolar interaction in the simulation gives a measure of the 1 H - 1 7 0 distance (van Eck and Smith 1998). 27A1--->170 CP has also been demonstrated in oL-AI203, giving a large increase in the signal-to-noise ratio (Haase and Oldfield 1994).
A AI-O-AI Non-CP
obs
r~
AI-OH
=
.
~
/
~
~
k~ oJ 4
sim.late./ 0.1
r~
, ,
r
200
0
-200
270 shift (ppm) w.r.t. H20
0
.
.
.
.
.
.
.
.
0.3 1.5
Contact time (ms)
3.0
. . . .
1000
i
. . . .
,
0
. . . .
~
. . . .
,
. . . .
J
-1000
170 shift (ppm) w.r.t. H20
Figure 6.8. A. 170 MAS NMR spectra of boehmite, with and without CP. Asterisks denote spinning sidebands. Note the significant enhancement of the A1-OH signal under CP conditions. From Walter et al. (1988). B. 1H-170 cross-polarisation curve for the hydroxyl group singularity at 323 ppm in the spectrum of Mg(OH)• as a function of the contact time. The inset shows the experimental data of the first 300 txs and the best fit to the data (solid line). From van Eck and Smith (1998). C. Observed and simulated 1H-170 CP echo NMR spectrum of Mg(OH)x(OCH3)2-x without using proton decoupling during acquisition. From van Eck and Smith (1998). All figures used by permission of the copyright owners.
346
Multinuclear Solid-State NMR of Inorganic Materials
6.2.3 Relationships between N M R parameters and structure After acquisition, the NMR data must be interpreted in terms of the structure of the sample material with the aim of providing information about the local atomic environment. Much of the interpretation has been based on the semi-empirical Townes-Dailey approach which relates the NMR parameters to the covalency of the bonds by consideration of the p- and d-orbital occupancy (Townes and Dailey 1949). Early applications to 170 NMR of this approach were made for borates (Jellison et al. 1977), silicates (Geissberger and Bray 1983) and zeolites (Janes and Oldfield 1986). Other treatments have applied quantum mechanical calculations (Tossell and Lazzeretti 1988, Tossell 1990). Steinberg examined the 170 Ti-values of A-O-A bonds using a geometrical approach which made no assumptions about the electron distribution or the p-orbital occupancy since these effects cancel, although the electron distribution must have certain symmetry (Sternberg 1993). The work is applicable to Si-O-Si linkages, allowing the A-O-A bond angle (ct) to be deduced from its relationship with ~q, determined to be:
3(cos a + 1) 1/=- 3cosa-1
(6.7)
Comparison of this relationship with ab initio calculations showed the largest difference occurs as the Si-O-Si bond angle approaches 90 ~ Although the geometric formula gives slightly lower values, the same trends were found in both approaches. There is a need to increase the number of experimental data points to test these relationships and allow their refinement. One of the most elegant studies has been of the SiO2 polymorph coesite, containing five distinct oxygen sites with bond angles ranging from 137.22 ~ to 180~ (Grandinetti et al. 1995). A high quality 170 DAS spectrum of coesite was accumulated over a period of more than 15 days (Figure 6.9), for which the resonances were assigned on the basis of the general increase of XQ with bond angle and a consideration of the intensity of the different peaks. These data suggested the relationship Zo - ZQ (180 ~ 2 cos a cos a - 1
(6.8)
Ab initio calculations show that the x and z axes of the efg tensor lie in the plane of the Si-O-Si bond with the x-axis bisecting the Si-O-Si bond angle. Ab initio calculations have also been used to examine the effect of cations coordinated to Si-O-Si bonds by considering the (OH)3Si-O-Si(OH)3 unit with lithium and sodium coordinated to the Si-O-Si site. Both XQ and xl were found to have the same functional dependence on oLirrespective of the presence of the cations, although there was an offset depending on the number of coordinating cations and their field strength (Vermillion et al. 1998). The value of XQ was found to shift systematically to lower
170 N M R
347 observed
;~
/
-20
'" 9
.~ ~ ~
~
~
%~o
.
.
,mq
simulated
o.
~ m l ~ i ~ ~ : : ~ '
o
"0"
20
~
60
-60
0
~...,...,..~,.
......
60
Frequency (ppm)
0
,.~,...0...
T...,..=~...,
-60
....
60
0
.,.,-., .... ,...,...
-60
170 shift (ppm) w.r.t. H20
Figure 6.9. 2D 170 DAS NMR spectrum of coesite, with cross-sections taken parallel to the anisotropic dimension for all five sites in coesite and the best-fit simulations (extreme right). From Grandinetti et al. (1995) by permission of the American Chemical Society.
0 Li §
N -6
1.0
1 Li §
2 Li§
0.6 1="
CY-4 100
2 Li §
-4
0 Li § 140
100
180
Si-O-Si (~
140
0 Na + 1 Na +
2 Na +
cy-4 180
Si-O-Si (o)
0.2
0.6
~
ONa +
J 1
0.6
100
,Na 4t
2 Na § . . . . 140
Si-O-Si (~
1.0
11
1.0
0.2 140
180
Si-O-Si (o)
-6
100
1 Li +
0.2
0 Na +
180
0.2
0.6
1.0
q
Figure 6.10. Results of ab initio calculations of the 170 NMR interaction parameters XQ and ~q of the (OH)3Si-O-Si-(OH)3 unit in sodium and lithium silicates. From Vermillion et al. (1998) by permission of the copyright owner.
values as the number of cations and the field strength of the coordinating cation increased (Figure 6.10), as described by the relationship"
xQ a(l+cosCOS
348
Multinuclear Solid-State NMR of Inorganic Materials
On the other hand, TI shifts to much higher values when one cation is added but shows a much smaller effect when there are two coordinating cations. The value of ~q was described by the function: r / = b ( 12 cosC~ - 1 ] q+2gIN
(6.10)
A comparison made of the dependence of XQ and xI on the orientation of the oxygencation internuclear vector showed that for a single cation, the value of XQ is insensitive to this orientation whereas -q shows a degree of orientation dependence, making it a less reliable parameter for the estimation of e~ in these systems. These calculations predicted that for cations of smaller radius and high field strength (e.g. Mg 2+) the dependence of XQ on oLcan be significantly different. A difficulty of all this work is the relative lack of experimental measurements of systems containing bridging oxygens against which these relationships can be tested. Ab initio calculations were extended to a series of related clusters containing central M-O-M bonds. A linear relationship was found between XQ and both the bond distance and the cation group number. These parameters were suggested to be better for predicting XQ in systems containing bridging oxygens than arguments based on electronegativity (Clark and Grandinetti 2000). Sophisticated methods developed for calculating these parameters (Palmer and Blair-Fish 1994) have been extended by ab initio density functional theory using the WIEN code and applied to ~70 NMR calculations for fosterite (Winkler et al. 1996). In recent studies of J70 NMR data for zeolites (Bull et al. 1998, 2000) the efg tensor has been calculated using the Moloch module of the TURBOMOLE programme (Ahlrichs et al. 1989) and by using density functional theory within the GAUSSIAN programme (see references within Bull et al. 1998, 2000 for more details). The question of correlating the ~70 ~o,c~ values with the structure has also been addressed. Shift calculations use various approaches including the CPHF-GIAO method (H~iser et al. 1992). Examination of the sites within siliceous faujasite and ferrierite has led to the conclusion that there is no obvious correlation of the shift with the structure. The isotropic chemical shifts in coesite which span a range of 29 ppm are not consistent with the expected monotonic relationship between ~i~o,c~and oL, nor with the predicted shift range of 10 ppm (Grandinetti et al. 1995). However, in another series of zeolites the ~i~o,c~values were found to decrease as o~ increased (Freude et al. 2001), suggesting, for a restricted ranges of materials such as the sites in sodium A and LSX, the relationship cos a
5(ppm) = - 2 1 4 ~
cos a - 1
+ 136
(6.11)
Currently it appears that shift correlations may have only limited application to 170 NMR in groups of materials with closely related structures.
170 N M R
349
One of the most powerful applications of these correlations between NMR parameters and structure is to provide a better understanding of the structure of amorphous materials which are very difficult to study by other techniques. Silicate glasses have been studied by relating the giso,cs value of 298i to the bond angle distribution (Dupree and Pettifer 1984, Pettifer et al. 1988). In the case of 170, the parameter -q has been used to determine the Si-O-Si bond angle distribution (Farnan et al. 1992) using the relationship: D3(a ) - D4 (r]) ~
.
(6.12)
This has provided important structural information about glasses (Section 6.5.2).
6.3.
BINARY
OXIDES
6.3.1 Crystalline materials The large 170 NMR shift range in diamagnetic oxides (Table 6.2) indicates the sensitivity of 170 NMR to structural effects. Oxygen can take a large range of coordination numbers (typically 2-6), the 170 shift becoming more shielded with increasing coordination
Table 6.2. 170 NMR peak positions in diamagnetic binary oxides. Compound
Coordination
Chemical shift* (ppm)
Reference
BeO MgO
OBe4 OMg6
26 47 47 294 390 629 141 - 18 - 18.7 121 289 293 247 246 105 334 557 552 584 591 591
Turner et al. (1985) Turner et al. (1985) Bastow & Stuart (1990) Turner et al. (1985) Turner et al. (1985) Turner et al. (1985) Turner et al. (1985) Turner et al. (1985) Bastow & Stuart (1990) Turner et al. (1985) Oldfield et al. (1989) Bastow & Stuart (1990) Oldfield et al. (1989) Bastow & Stuart (1990) Oldfield et al. (1989) Oldfield et al. (1989) Oldfield et al. (1989) Bastow et al. (2000) Bastow et al. (2000) Oldfield et al. (1989) Bastow et al. (1993)
CaO SrO BaO CdO (200~
OCa6 OSr6
OBa6
ZnO
OZn4
Yellow-HgO PbO
OHg4 OPb6
SnO
OSn3
SnO2
OSn 3
BaO2 TiO2, anatase brookite, site 1 site 2 rutile
OTi3 OTi3 OTi3 OTi3
350
Multinuclear Solid-State NMR of Inorganic Materials
Table 6.2. (Continued) Compound
Coordination
Chemical shift* (ppm)
Reference
ZrO2, cubic Tetragonal Monoclinic, site 1 site 2 HfO2 Monoclinic, site 1 site 2 CeO2
OZr4 OZr4 OZr4 OZr3 OHf4 OHf3 OCe6
VO2,
OV3 OV3 OCu4
355 378 324 401 266 334 877 878 755 815 - 181 193 - 277 496 467 467 584 584 305 196 364 355 356 359 383 377 346 313 242 97 455 305
Bastow & Stuart (1990) Bastow & Stuart (1990) Bastow & Stuart (1990) Bastow & Stuart (1990) Bastow & Stuart (1990) Bastow & Stuart (1990) Oldfield et al. (1989) Bastow & Stuart (1990) Bastow & Stuart (1990) Bastow & Stuart (1990) Oldfield et al. (1989) Bastow & Stuart (1990) Bastow & Stuart (1990) Bastow & Stuart (1990) Bastow & Stuart (1990) Yang et al. (1992) Bastow & Stuart (1990) Yang et al. (1992) Yang et al. (1992) Oldfield et al. (1989) Oldfield et al. (1989) Oldfield et al. (1989) Oldfield et al. (1989) Florian et al. (1995) Florian et al. (1995) Florian et al. (1995) Florian et al. (1995) Florian et al. (1995) Florian et al. (1995) Oldfield et al. (1989) Pickup et al. (2000)
site 1 site 2
Cu20
-
Ag20 Ti203 La203,
OAg4 site 1
OLa6
site 2
OLa4
Lu203 Bi203 T1203 Sc203 C-Y203
OLu4 OBi4 OT14 OSc4 OY4
B-Y203, site 1 site 2 site 3 site 4 site 5 In203 Ta205, site 1 site 2
OY4 OY4 OY4 OY5 OY6 OIn4 OTa3 OTa2
9- all shiftsin Chapter6 are referencedto H20.
number. The sensitivity of 170 N M R to structure is well illustrated by ZrO2, which principally occurs in three polymorphic forms, cubic (OZr4), tetragonal, (OZr4) and monoclinic (OZr3, OZr4). The 170 N M R spectra of the tetragonal and monoclinic forms are readily observable even at natural abundance (Bastow and Stuart 1990) (Figure 6.11A). The monoclinic form shows two very sharp resonances separated by 77 ppm, with linewidths of --~ 2 ppm. The tetragonal phase shows a single peak --~10 ppm wide at an intermediate position. The cubic phase is not a stable form of pure ZrO2 at room temperature, but can be stabilised by the addition of a second oxide such as MgO or Y203. The
351
170 NMR A tetragonal
T(~
1A
6OO
mznol~nic [ [ t A
anatase 800
rutile
Mg-stabilised
1000
400
tA -200
170 shift (ppm) w.r.t. H20
A
rutile
650
550
450
170 shift (ppm) w.r.t. H20
800
400
0
170 shift (ppm) w.r.t. H20
Figure 6.11. A. 170 natural abundance MAS NMR spectra of zirconia polymorphs. The peak marked A is from oxygen in the alumina rotor. The asterisks denote spinning side bands. From Bastow and Stuart (1990) by permission of Elsevier Science. B. ~70 MAS NMR spectra of titania gel heated to various temperatures, showing the evolution of rutile at the expense of anatase. From Bastow et al. (1993) by permission of the Royal Society of Chemistry. C. Static and MAS 170 NMR spectra of cubic Y203. Adapted from Florian et al. (1995).
resulting solid solution contains a range of next-nearest neighbours resulting in atomic disorder which is reflected in the NMR spectrum as a shift from the resonance position of the tetragonal phase and a much increased linewidth (84 ppm) arising from chemical shift dispersion. Whereas XRD has difficulty distinguishing the tetragonal and cubic phases, this distinction can readily be made by 170 NMR. NMR also shows that additions of MgO to ZrO2 produce no MgO signal, indicating that the magnesium enters the network and does not form a separate phase. The effect of the nearest element on the 170 NMR spectra can be gauged by comparing the shifts of the isostructural compounds ZrO2 and HfO2, in which the corresponding sites show a shift difference of --~ 60 ppm. The various polymorphs of TiO2 differ in only the relative disposition of the TiO6 units but their 170 shift differences span a range of ---35 ppm. The solid state conversion of anatase to rutile can be readily followed by 170 NMR (Figure 6.11B). The more distorted structural units in rutile give rise to a noticeable field gradient (Figure 6.11B). The value of XQ has been determined from the MAS spectrum to be 1.50 +_ 0.03 MHz, with xl = 0.87 +_ 0.03 (Bastow e t al. 1996), agreeing accurately with an earlier single crystal study (Gabathuler e t al. 1973). Some of the Group II oxides with NaC1 structures show very narrow resonances arising from the cubic structure containing the
352
Multinuclear Solid-State NMR of Inorganic Materials
spherically symmetric 0 2 - ion. The value of XQ in CaO, SrO and BaO is close to 0 (Turner et al. 1985). The 170 shifts of the group IIA and liB oxides show correlations with the cation radius r (,~)"
~ ( p p m ) - 2571.3 + 11 and ~(ppm)- 150r 3 - 7 8
(6.13)
HgO, which is much more electronegative than the other metals, has a much larger XQ value (---7 MHz) (Turner et al. 1985). On the basis of a larger data set, a more general correlation of ~ with the cationic radius has been suggested:
~(ppm) = -439r + 3205
(6.14)
Although it is not obvious how to rationalise these correlations, they provide guidance to the variations expected in such compounds. In a similar way, XQ values for 170 have been correlated with the bond ionicity; as the bond becomes more ionic the oxide ion becomes more O2--like and the field gradient decreases according to
ZQ(MHz) = -0.2031(%) + 14.8
(6.15)
where I is the bond ionicity (Schramm and Oldfield 1984, Oldfield et al. 1989). The single oxygen site in cubic C-form of Y203 has a CSA (Figure 6.11 C) with a span of 115 ppm (Florian et al. 1995). In the monoclinic B-form there are five oxygen sites (three OY4 sites, one OY5 and one OY6). All five sites can be clearly resolved in the 170 MAS NMR spectrum (Florian et al. 1995) and have values of XQ which vary between 0.32 and 1.80 MHz. The OY5 site shows the greatest distortion with a hint of second-order quadrupolar structure in the MAS centrebands. A good correlation has been found between the 170 isotropic chemical shift and the Y-O bondlength for all the OY4 sites in these two compounds.
6.3.2 Sol-gel produced samples The sol-gel process has become one of the most important routes for 170-enriching binary oxides. One of the spin-offs is that the oxides formed by this process evolve into the final crystalline state via a series of amorphous structures that can be directly probed by 170 NMR, providing a great deal of structural detail which would be difficult to derive by other methods. An early example illustrating the utility of 170 NMR in such studies was an investigation of the sol-gel formation of ZrO2 (Bastow et al. 1992). The three peaks in the spectra indicated the presence in the amorphous material of predominantly monoclinic-like ZrO2 with some tetragonal-like regions. On crystallisation the spectrum significantly narrowed and indicated the presence of predominantly
170 N M R
353
tetragonal phase. The nature of the oxygen sites is unequivocally indicated by the 170 shifts, providing in this case a precise identification of tetragonal zirconia. This is an interesting result, since the stable phase in pure undoped ZrO2 at room temperature is monoclinic, except in the case of very fine, nanocrystalline ZrO2 in which the tetragonal phase is stabilised by a surface energy effect. A subsequent, much more detailed study of 170 NMR of ZrO2 combined with Zr K--edge EXAFS and XANES (Chadwick et al. 2001) compared nanocrystalline ZrO2 formed by the sol-gel and hydroxide precipitation methods. The 170 MAS NMR spectra (Figure 6.12A) show two peaks at 405 and 303 ppm with an intensity ratio ---1:1 at positions very close to crystalline monoclinic. These resonances persist to the point of crystallisation at 360~ of the tetragonal phase, which predominates until the particle growth above 500~ brings about a reversion to the monoclinic form. The data reveal that the amorphous precursor has a monoclinic-like local structure prior to crystallisation, thereafter becoming a complex mixture of tetragonal- and monoclinic-like sites (Chadwick et al. 2001). A
B
OTi3
T(~
T(~ ,[
OTi4 $
300
250
J^
'\
360 (15h)
200
o
____) ';oo'
in
\...e.to. ,,"~'
~lll,ll,lT
3;o
IY'| I
2;0
170 shift (ppm) w.r.t. H20
800
400
0
170 shift (ppm) w.r.t. H 2 0
Figure 6.12. A. 170 MAS NMR spectra of undoped ZrO2 gel heated to various temperatures. Note the change from the twin resonances of the monoclinic form, which are replaced by the single tetragonal resonance upon recrystallisation at 360-380~ but revert to the monoclinic form above 500~ From Chadwick et al. (2001) by permission of the American Chemical Society. B. 170 MAS NMR spectra of titania gel heated to various temperatures. Note the gradual loss of the OTi4 resonance on heating. From Bastow et al. (1993) by permission of the copyright owners.
354
Multinuclear Solid-State N M R o f lnorganic Materials
Other examples of the use of 170 NMR to monitor the evolution of structure in sol-gel samples include a study of TiO2 (Bastow et al. 1993), V205 (Pozarnsky and McCormick 1994), La203 (Ali et al. 1996), HfO2 (Bastow et al. 1996), MgO (Chadwick et al. 1998) and Ta205 (Pickup et al. 2000). The results clearly indicate that there is no well-defined evolution route for the structure and each oxide system has its own peculiarities. HfO2 is chemically very similar to ZrO2, and shows a 170 NMR spectrum of the gel with two resonances close to the monoclinic position. The only difference between the spectra of these two compounds is that on crystallisation HfO2 shows very much narrower monoclinic peaks and no indication of the formation of the tetragonal phase (Bastow et al. 1996). The formation of La203 by the sol-gel process proceeds via LaO(OH), which is readily identified by the ~70 peak corresponding to the OLa4 site at 546 ppm (Ali et al. 1996). TiO2 is a very important system with many technological applications which have encouraged much research, especially into the formation of nanocrystalline forms with increased surface area and higher activity. Two 170 NMR resonances are observed in sol-gel produced TiO2 at 514 and 368 ppm which have been assigned by careful work carried out on 170-enriched molecular titanium-oxo-organo clusters (Day et al. 1991, 1992, 1993, Scolan et al. 1999). These results indicate that the OTix coordinations have shifts in the range 650-850 ppm for x = 2,450-650 ppm for x = 3,250--450 ppm for x = 4 and < 250 ppm for x - 5. Variable field NMR clearly indicates that for all sites bonded only to titanium, the spectra are dominated by chemical shift effects, with almost no contribution from quadrupole effects (Bastow et al. 1993, Scolan et al. 1999). The OTi2 sites are characterised by an extremely large CSA with a typical span o f - 650 ppm (Scolan et al. 1999). On heating there is a gradual decrease in OTi4 until only OTi3 remains at crystallisation (Figure 6.12B, Bastow et al. 1993). Nanoparticles of TiO2 showed a range of OTix environments, one of the OTi3 environments perhaps showing evidence of a contribution from second-order quadrupole effects (Scolan et al. 1999). The nanoparticles are characterised by a bulk OTi3 signal, surface signals from OTi2, OTi3 and a signal from species coordinated to acetylacetone. ~70 NMR has also been used to study structure development in titania gels where the reactions are controlled by regulating the hydrolysis ratio and by the use of complexing agents (Blanchard et al. 2000). The nature of the ligand was found to change the local structural units in the titania core. 170 MAS NMR provided a direct quantitative measure of the different OTix units present, providing an insight into the structure. 170 NMR gives a direct indication of the changes in the proportion of the different oxygen environments in amorphous gels during heating as well as other information about subtle changes in the structure which are provided by observing other common trends in the 170 NMR data. For example, the linewidths of the resonances in the initial gel tend to increase up to the point of crystallisation. Since the linewidth is largely determined by chemical shift dispersion, this indicates a wider range of environments
170 NMR
355
and hence a more disordered structure. Furthermore, the 170 resonances of the initial gel tend to show a monotonic increase in shift up to the point of crystallisation, as illustrated by the OZr, and Hf4 resonances in zirconia and hafnia gels respectively; the former resonance shifts from 303 to 321 ppm and the latter from 245 to 258 ppm. The chemical shift reflects the electron density in the bonds, and is determined by factors such as coordination number, hybridisation and bond length. The changes in the shifts of these gels can be interpreted in terms of a decrease in the mean bond length, illustrating the use of NMR as a detailed structural probe of the amorphous state.
6.4. CRYSTALLINETERNARY IONIC SYSTEMS Inspection of the 170 MAS NMR spectra of ternary oxygen-containing materials reveals a large variation in linewidth, with some spectra showing extremely high resolution and revealing minor differences in the site crystallography. A selection of typical spectra of titanates and zirconates are shown in Figure 6.13 and those of the corresponding hafnates are shown in Figure 6.14. Comparison of the shift ranges of titanates (372-564 ppm), zirconates (298-376 ppm) and hafnates (237-332 ppm) indicates that in compounds of the form AxBOy, the B ions principally determine the 170 shift range. This is illustrated by a comparison of isostructural zirconates and
SrTiO3
~ __._t__
CaTiO3 ,
,
5
SrZr03
~
Li2TiO3 .
0
~_~
CaZr03
Li2Zr03
340 320
600
400
200
600
400
200
600
400
200
170 shift (ppm) w.r.t. H20 Figure 6.13. A selection of typical 170 MAS NMR spectra of metal titanates and zirconates. From Bastow et al. (1996) by permission of the copyright owner.
356
Multinuclear Solid-State NMR of Inorganic Materials
BaHfO 3 t
,,
i
400
!
|
200
!
!
1
0
170 shift (ppm) w.r.t. H20 Figure 6.14. A selection of 170 MAS NMR spectra of metal hafnates. From Bastow et al. (1996b) by permission of the copyright owner.
hafnates for which the ratio of the isotropic shifts (Hf/Zr) falls in the very narrow range 0.84--0.88 (Bastow et al. 1996b). The average value for zirconates and titanates (Zr/Ti) is 0.74 + 0.03, very close to the ratio of the polarising powers of these two elements (0.7) (Bastow et al. 1996). Alternatively, the ratio of the bond lengths in these compounds may be considered. The symmetry of a structure is reflected by the number of oxygen sites it contains. For example, the cubic perovskites (SrTiO3, BaZrO3) show only a single sharp oxygen resonance, reflecting the equivalence of all the oxygen sites. In BaTiO3 there is a slight distortion of the structure, in which the titanium and barium are slightly displaced from their ideal positions, giving rise to two inequivalent oxygen sites with a ratio of 2:1. 170 MAS NMR clearly resolves these two sites at room temperature but as the temperature is raised the resonances move together. The cubic phase formed in the tetragonal-to-cubic transition at 130~ shows only a single peak at 538 ppm (Spearing and Stebbins 1994). The splitting of these two resonances in materials such as CaTiO3 with the GdFeO3-structure is typically 5 ppm. In monoclinic materials of lower symmetry (e.g. Li2TiO3) there are still only two signals but these show greater differences in shift (--~20 ppm) (Bastow et al. 1996).
170 N M R
357
Table 6.3. 170 NMR data for crystalline ternary oxide compounds. Compound
Peak position (ppm)
Reference
Li2TiO3 K2Ti409 KzTi6013 MgTiO3 CaTiO3 SrTiO3 SrzTiO4 BaTiO3
405.8,372.3 250, 350, 660 320, 440, 590 398 448.0, 443,4 465 426, 407 564, 523 553,530 303 298 301 301,273 420 465 298.4, 280 308.9, 286.0 336, 329 343.7,340.2 376.0 375 237.8, 249.2 239.8, 258.4 292.7,284.1 296, 298 331.9 85 423 213, 198 504 350 160", 215 375 1197 422, 429, 437
Bastow et al. (1996) Bunker et al. (1997) Bunker et al. (1997) Millard et al. (1995) Bastow et al. (1996) Bastow et al. (1996) Bastow et al. (1996) Anuradhi (1990) Spearing & Stebbins (1994) Millard et al. (1995) Millard et al. (1995) Millard et al. (1995) Millard et al. (1995) Bunker et al. (1997) Bunker et al. (1997) Bastow et al. (1996) Bastow et al. (1994) Bastow et al. (1996) Bastow et al. (1996) Bastow et al. (1996) Bunker et al. (1997) Bastow et al. (1996b) Bastow et al. (1996b) Bastow et al. (1996b) Bastow et al. (1996b) Bastow et al. (1996b) Bastow et al. (1996) Bastow et al. (1996) Bastow et al. (1996) Bastow et al. (1996) Bunker et al. (1997) Adler et al. (1994) Oldfield et al. (1989) Schramm & Oldfield (1984) Schramm & Oldfield (1984)
Cubic MgzTiO4 Tetragonal MgzTiO4 Cubic ZnzTiO4 Tetragonal ZnzTiO4 BazTiO4 PbTiO3 Li2ZrO3 NazZrO3 CaZrO3 SrZrO3 BaZrO3 PbZrO3 LizHfO3 Na2HfO3 CaHfO3 SrHfO3 BaHfO3 LiSnO3 SrSnO3 Sr2SnO4 LiNbO3 PbNb206 Ba2In205 BaBiO3 KMnO4 K2WO4
* - isotropic chemical shift In ionic materials, where broadening of the central transition is dominated by CSA, the chemical shift tensor can be characterised. The 170 CSA values for such compounds are summarised in Table 6.4. The chemical shift tensors in the titanates tend to have larger spans than the zirconates, reflecting the influence of the B cation on the CS tensor. Many oxides have properties such as ionic conductivity, making them suitable for use as solid electrolytes and oxygen sensors. These properties depend on ionic motion in the material and can be studied by lVO NMR. Such a study has been made between room temperature and 1200~ of BazIn205 which contains three inequivalent oxygen sites
358
M u l t i n u c l e a r Solid-State N M R o f I n o r g a n i c M a t e r i a l s
Table 6.4. 170 chemical shift tensor components of ternary oxide compounds. Compound BaZrO3 SrTiO3 SrZrO3 CaTiO3 CaZrO3 LiNbO3 KzWO4, site 1 site 2 site 3
8iso,cs(ppm) Span(ppm) 367.2 468.1 340.1 443.9 329.3 444.7 437 429 422
337 473 332 388 356 610 347 365 353
Skew
Reference
-0.49 - 0.73 -0.67 - 0.60 - 0.42 - 0.34 -0.80 - 0.73 - 0.64
Bastow et al. (1996) Bastow et al. (1996) Bastow et al. (1996) Bastow et al. (1996) Bastow et al. (1996) Bastow et al. (1996) Schramm & Oldfield (1984)
within the orthorhombic structure (Adler et al. 1994). The spectra show two oxygens with similar shifts of --~160 ppm and large XQ value, and a third site characterised by a Gaussian line at 215 ppm. The spectra show an order-disorder transition at 925~ above which temperature the population of mobile anions increases. At --~1075~ the structure becomes cubic and at 1200~ a single Lorentzian resonance is seen at 210 ppm (Adler et al. 1994). Similar high temperature studies have been carried out on the highly atomically disordered perovskites Ba(Ino.67Zro.33)Oy and Ba(Ino.67Ceo.33)Oy. DAS measurements showed that the oxygen is displaced from the position of cubic symmetry and that there are relatively few mobile oxygen atoms below 800~ (Adler et al. 1994a). Defect sites can have profound effects on the properties of materials. BaFBr and BaFC1 are modem storage X-ray phosphor materials. Since both compounds scavenge oxygen from their environment they usually contain oxygen impurities. Detailed knowledge of the oxygen location in the structure is necessary to understand its substitutional preference and any tendency to form oxygen interstitials. A complex lVo MAS NMR spectrum has been observed for oxygen-doped BaFBr, containing four sharp lines at 630, 610, 597 and 587 ppm, together with a much broader resonance at 197 ppm (Bastow et al. 1994a). The corresponding spectrum for BaFC1 contained signals at 181 and 161 ppm. The lines at about 600 ppm were assigned to O 2- substitution on the anion sites with a preference for fluoride displacement, but some substitution also occurs on the bromine site. The splitting of these resonances is probably due to next-nearest neighbour and vacancy displacement effects. The other peak which shows a large shift is probably from a peroxide anion 022-. The BaFC1 spectra show no evidence of O 2-, but suggest the presence of 022- in two different sites; the one corresponding to the resonance at 181 ppm is probably a substitutional site and the other, at 161 ppm, arises from an interstitial anion. The solid state chemistry of mixed oxide systems can be directly probed by 170 NMR. The formation of nanocrystalline TiO2 has been studied by doping the system
170 N M R
359
with SnO2 which has the rutile structure. A 170 N M R study of the sol-gel reaction in this system showed two peaks at 595 and 375 ppm. Since the latter remained even after extensive heat treatment, it was not an O(Ti,Sn)4 resonance such as seen in T i Q gels (Bastow et al. 1995). Neither do the peaks match the 170 N M R spectrum of SnO2. The major peak is therefore in the position of OTi3 in rutile and the other peak probably arises from OX3, where X is a combination of titanium and tin. The most striking aspect of this result is the presence in the initial gel prior to heat treatment of oxygen in a rutile-like OTi3 configuration rather than the anatase-like configuration found in a pure TiO2 gel. The ~70 N M R spectra of a series of solid solutions in the PbZrxTi~-xO3 system showed distinct peaks arising from nearest-neighbour zirconium and titanium environments, lVo N M R was used to monitor the formation of stable phases resulting from changes in the composition of the system at room temperature (Klemperer and Richard 1998).
6.5. SILICATES AND GERMANATES
materials and germania. Because of the special place occupied by SiOa as the
6.5.1 C r y s t a l l i n e 6.5.1.1 S i l i c a
archetypal network glass former, there have been extensive efforts using many experimental techniques to understand its structural details. The crystalline polymorphs used to calibrate these experimental probes have been studied in detail by ~70 NMR. Their parameters are summarised in Table 6.5. The N M R data for N M R coesite were derived from the anisotropic dimension of a DAS data set (Figure 6.9, Grandinetti et al. 1995). The N M R parameters for all the other samples were taken from 1D static and MAS data. The static measurements of the SiO2 Table 6.5. 170 NMR parameters for crystalline SiO2 and GeO2 polymorphs. Sample
~i.... s(ppm)
XQ (MHz)
~l
Si-O-Si bond angle
Cristobalite
40 + 2 44 38 + 2 29 + 1 41 53 + 1 57 + 1 58 + 1 109 + 1 70 + 5 160 + 5
5.3 _+ 0.1 5.3 5.25 + 0.1 6.05 + 0.05 5.43 +_ 0.05 5.52 _+ 0.05 5.45 _+ 0.05 5.16 _+ 0.05 6.5 _+ 0.1 7.3 _+ 0.1 7.5 _+ 0.1
0.125 _ 0.005 0 0.18 _+ 0.02 0.000 + 0.005 0.166 _+ 0.005 0.169 +_ 0.005 0.168 _+ 0.005 0.292 + 0.005 0.125 _+ 0.05 0.48 + 0.05 0.10 + 0.05
146.4~
Quartz Coesite, site 1 site 2 site 3 site 4 site 5 Stishovite Quartz-GeO2 Rutile-GeO2
Reference
Spearing et al. (1994) Timken et al. (1986) Dupree (2000) 180.00~ Grandinetti et al. (1995) 142.56~ 149.53~ 144.46~ 137.22~ OSi3 Xue et al. (1994) 133.0~ Hussin et al. (1999) OGe3 Hussin et al. (1999)
360
Multinuclear Solid-State NMR of Inorganic Materials
polymorphs require the inclusion in the fitting of a CSA contribution. The characteristic span and skew of the tensors have been determined to be 80 ppm and --~ 0.4 for quartz (Dupree 2000), 70 _+ 5 ppm and 1 for cristobalite (Spearing et al. 1992) and 94 ppm and --~ 0.45 for stishovite (Xue et al. 1994). In going from the SiO4 units in quartz, through cristobalite to coesite and to the SiO6 units in stishovite, both the isotropic chemical shift and • markedly increase. The two forms of GeO2 show only a very small difference in • but the two coordinations can be distinguished by a change of--~ 90 ppm in the isotropic shift. If these experimental data are then plotted to test the functions in Section 6.2.3, ~q shows good agreement with Equation 6.7 (Figure 6.15), providing a strong basis for the use of NMR data to provide information about the structure of amorphous materials. The experimental data have also been used to test the reliability of ab initio molecular orbital calculations which were found to accurately reproduce the results, suggesting they could also be used to elucidate the structure of amorphous silicates (Xue and Kanzaki 2000). Variable temperature 170 NMR has been used to examine the c~<--->[3phase transition in cristobalite at 254~ The T1 value decreases from 150 s at room temperature to ---29 s just below the transition temperature, dropping still further to 1.5 s at the transition point. Analysis of the 170 NMR lineshape showed that ~q decreased to zero (within the accuracy of the measurements) at the transition temperature. These results imply that the transition is related to reorientational fluctuations of the SiO4 groups and that the [3-phase is characterised by dynamical averaging of twin domains on the unit cell scale (Spearing et al. 1992).
0.4 [-
0.2
0 I
I
140
I
!
c.
. . .I .
160
t
1
L
I
180
Si-O-Si angle (o) Figure 6.15. Relationship between the 170 asymmetry parameter "q and the Si-O-Si bond angle for the silica polymorphs cristobalite, coesite and quartz. The solid line corresponds to Equation 6.7. From Dupree (2000) by permission of John Wiley and Sons Ltd.
170 NMR
361
6.5.1.2 Ternary Silicates. The ternary silicates were amongst the first inorganic
systems to which 170 MAS NMR was applied. Such materials contain several oxygen sites, and although the MAS spectra showed features from each site, there was overlap between the resonances (Figure 6.16A). However, spectral simulation was able to provide values of the interactions for each site. These materials were ideal candidates for high resolution techniques such as DOR, DAS and MQ MAS, which allowed much higher resolution and were able to distinguish the individual resonances. In the 1D MAS NMR spectrum of forsterite the resonances of the three non-bridging oxygens (nbo) all overlap, but the corresponding 3Q data resolve the three separate lines and allow the interaction parameters to be determined from the anisotropic dimension (Figure 6.16B,C) (Ashbrook et al. 1999). In crystalline materials with many different inequivalent oxygen sites the 1D data can only be crudely apportioned to different sites whereas DOR and DAS show all the inequivalent sites, (six in clinoenstatite and nine in wollastonite (Mueller et al. 1991, 1992). Table 6.6 summarises the 170 NMR interaction parameters from such crystalline materials.
B
A
20 i
i
i
,
i
f
| , ,
i , ,
80
i
i , ,
!
40
z,
~,~
0
~
_
60 . . . . . . . . . 80 40
F2 (ppm) components
J
,,,
~~~,~ t
'
1
80
'
J
s
I
'
~
'
I
40
'
'
'
I
'
'
'
J
0
t----I
1 kHz
170 shift (ppm) w.r.t. H20 Figure 6.16. A. Observed and simulated 170 MAS NMR centre band spectrum of crystalline diopside (CaMgSi206) showing the individual fitted components. From Timken et al. (1987) by permission of the American Chemical Society. B. Three-quantum 170 MAS NMR spectrum of forsterite showing resolution of the three non-bridging oxygen sites. C. Cross-sections parallel to the F2 axis of the MQMAS spectrum in B from which the • and xl values were derived by computer simulation of these peak shapes. From Ashbrook et al. (1999) by permission of the Mineralogical Society of America.
362
Multinuclear Solid-State NMR of lnorganic Materials
Table 6.6.
170 NMR
parameters of crystalline silicates and germanates.
Sample
Site
~i.... s(ppm)
XQ (MHz)
~q
Reference
Li2Si205
bo(1) bo(2) nbo(1) bo(1) bo(2) nbo(1) bo(1) bo(2) nbo(1) nbo bo bo(1) bo(2) nbo(1) bo(1) bo(2)* bo bo(1) bo(2) nbo(1) bo(1) bo(1) nbo(2) nbo(1) nbo(2) nbo(3) nbo(1) nbo(2) nbo(3) nbo(1) nbo(2) nbo(3) nbo(1) nbo(2) nbo(3) nbo(4) nbo(1) nbo(2) nbo(3) nbo(4) nbo(5) bo(1) nbo(1) nbo(2) nbo(1) nbo(2) nbo(3) nbo(4)
108 _+ 10 35 + 10 38 +_ 2 55 + 5 55 + 2 34 + 1 52 + 10 74 + 10 36 + 2 45 + 5 42.6 114 _+ 10 69 _+ 10 72 _+ 2 62.5 _+ 1 97 _+ 1 51 124 _+ 10 59 _+ 10 93 _+ 2 62 _+ 2 60 + 2 42 + 2 61 _+ 2 62 _+ 2 47 _+ 2 ND ND ND 64 _+ 1 61 + 1 48 _+ 1 63 + 1 60 _+ 2 59 + 2 52 _+ 1 64 _+ 1 61 _+ 1 60 + 1 52 _+ 1 49 _+ 1 75 + 2 94 + 2 91 _+ 2 134 _+ 2 128 + 2 122 _+ 2 122 _+ 2
5.60 + 0.3 4.05 _+ 0.3 2.45 + 0.1 5.7 + 0.3 4.7 _+ 0.2 2.35 + 0.1 5.74 + 0.2 4.67 + 0.2 2.40 + 0.1 ND 5.10 5.1 _+ 0.4 4.7 _+ 0.4 2.1 _+ 0.2 4.45 _+ 0.05 4.90 _+ 0.05 4.95 4.4 + 0.4 4.7 _+ 0.4 1.9 + 0.2 5.1 _+ 0.2 3.2 _+ 0.2 3.2 _ 0.2 2.35 + 0.2 2.35 _+ 0.2 2.70 _+ 0.2 2.53 2.42 2.77 2.5 + 0.1 2.5 _+ 0.1 2.9 + 0.1 2.5 + 0.1 2.3 _+ 0.1 2.3 _+ 0.1 2.7 _+ 0.1 2.5 _+ 0.1 2.4 _+ 0.1 2.4 _+ 0.1 2.8 _+ 0.1 2.7 _+ 0.1 3.8 _+ 0.2 2.1 +_ 0.2 2.3 _+ 0.1 2.9 + 0.2* 2.7 _+ 0.2* 2.5 + 0.2* 2.8 + 0.2~f
0.55 + 0.2 0.0 +_ 0.1 0.1 _+ 0.1 0.0 + 0.2 0.25 _+ 0.2 0.1 + 0.1 0.2 + 0.1 0.3 + 0.1 0.2 + 0.1 ND 0 0.1 + 0.1 0.2 _+ 0.1 0.5 _+ 0.1 0.35 _+ 0.05 0.20 + 0.05 0.1 0.1 _+ 0.1 0.5 _+ 0.1 0.5. _+ 0.1 0.3 _+ 0.1 0.0 _+ 0.1 0.0 _+ 0.1 1.0 + 0.1 0.2 _+ 0.1 0.3 _+ 0.1 0.39 0.18 0.28 0.4 + 0.1 0.2 _+ 0.1 0.3 _+ 0.1 0.3 + 0.1 0.2 _+ 0.1 0.3 _+ 0.1 0.2 _+ 0.1 0.3 _+ 0.1 0.2 + 0.1 0.3 _+ 0.1 0.2 _+ 0.1 0.3 _+ 0.1 0.2 _+ 0.1 0.1 +_ 0.1 0.1 _+ 0.1 ND ND ND ND
Maekawa et al. (1996)
e~-Na2Si205
e-NazSi205 Sodium ilerite K2Si205
K2Si409 (wadeite) KHSi205 Rb2Si205
MgSiO3 (clinoenstatite) Mg2SiO4 (forsterite)
chondrodite
clinohumite
e~-CaSiO3 (wollastonite) CaeSiO4 (larnite)
Xue et al. (1994) Maekawa et al. (1996)
Xue et al. (1994) Brenn et al. (2000) Maekawa et al. (1996)
Xue et al. (1994) Oglesby et al. (2001) Maekawa et al. (1996)
Timken et al. (1987)
Schramm & Oldfield (1984) Fritsche et al. (1986)
Ashbrook et al. (1999)
Ashbrook et al. (2001)
Ashbrook et al. (2001)
Timken et al. (1987)
Mueller et al. (1992)
170 N M R
363
Table 6.6. (Continued) Sample
Site
oL-SrSiO3
bo(1) nbo(1) nbo(2) bo(1) nbo(1) nbo(2) bo(1) nbo(1) nbo(2) bo(1) nbo(1) nbo(2) bo(1) bo(2) bo(3) bo(4) bo(1) bo(2) bo(3) bo(4) bo(5) bo(6) bo(7) bo(8) bo(9) bo(10) bo nbo
BaSiO3
CaMgSi206 (diopside)
faujasite
ferrierite
NazGeO4
8iso,c~(ppm) 80 • 2 108 ___2 105 +__2 87 ___2 169 • 2 159 ___2 69 ___2 84 ___2 63 __+2 69 • 2 86 ___2 64 ___2 47.3 ,,, 0.3 42.3 ___0.3 37.2 • 0.3 34.8 ___0.3 43.1 ___0.3 41.6 _ 0.3 40.7 ___0.3 39.6 __+0.3 39.0 ___0.3 37.0 +__0.3 37.0 __+0.3 35.9 ___0.3 34.8 ___0.3 28.0 _+ 0.3 70 _+ 5 47 • 5
XQ (MHz)
Xl
4.1 _ 0.2 0.4 ___0.1 2.1 ___0.2 0.1 ___0.1 2.2 +__0.2 0.1 +__0.1 3.7 ___0.2 0.4 ,,, 0.1 2.1 • 0.2 0.1 ___0.1 1.6 ___0.2 0.1 ___0.1 4.4 _ 0.2 0.2 ___0.1 2.7 __+0.2 0.0 • 0.1 2.7 _ 0.2 0.1 ___0.1 4.39 +__0.05 0.36 • 0.05 2.83 _ 0.05 0.13 ___0.10 2.74 + 0.05 0.00 --- 0.10 5.14 ___0.03 0.1 ___0.05 5.10 ___0.03 0.3 ___0.05 5.39 _+ 0.03 0.2 ___0.05 5.28 ___0.03 0.2 ___0.05 5.62 ... 0,03 t 5.22 ... 0.03 t 5.35 • 0.03 t 5.29 ___0.03 t 5.38 • 0.03 t 5.27 ... 0.03 t 5.32 __+0.03 ~ 5.46 ... 0.03* 5.64 ___0.03 t 5.57 + 0.03 t 5.2 + 0.5 0.5 _+ 0.1 2.5 + 0.5 0.5 + 0.1
Reference Timken et al. (1987)
Timken et al. (1987)
Timken et al. (1987)
Mueller et al. (1992)
Bull et al. (1998)
Bull et al. (2000)
Hussin et al. (1998)
* is a bridging oxygenis betweena tetrahedraland an octahedralunit, and t refers to the quadrupoleproductnot XQ"
It is i m m e d i a t e l y a p p a r e n t f r o m T a b l e 6.6 that bo a n d n b o c a n be r e a d i l y d i s t i n g u i s h e d b y their XQ v a l u e s w h i c h fall into d i s t i n c t r a n g e s ( 3 . 7 - 5 . 8 M H z for b o a n d 1 . 6 - 3 . 2 M H z for nbo). XQ is related to the differences in the ionicity o f the c a t i o n - o x y g e n b o n d s . T h e n b o are m o r e i o n i c b o n d s c o n t a i n i n g less p - o r b i t a l c o n t r i b u t i o n , a n d h e n c e XQ is s m a l l e r . T h u s , the XQ v a l u e s o f b o t h the b o a n d n b o units c a n be c o r r e l a t e d w i t h the c a t i o n e l e c t r o n e g a t i v i t y for g r o u p s o f r e l a t e d s i l i c a t e s (e.g. t h e a l k a l i n e - e a r t h c o m p o u n d s ) ( F i g u r e 6 . 1 7 A , T i m k e n et al. 1987), a n d the giso,cs v a l u e s are also r e l a t e d to the c a t i o n r a d i u s ( F i g u r e 6 . 1 7 B , T i m k e n et al. 1987). T h e c a t i o n t h u s p l a y s a c e n t r a l r o l e in d e t e r m i n i n g the ~70 N M R i n t e r a c t i o n p a r a m e t e r s for b o t h the b o a n d n b o g r o u p s , a l t h o u g h the e f f e c t is s t r o n g e r for the n b o units. T h e i m p o r t a n c e o f t h e s e e x p e r i m e n t a l d a t a is t h a t t h e y a l l o w the c o r r e l a t i o n function for the N M R p a r a m e t e r s to b e t e s t e d b e f o r e b e i n g u s e d to p r o b e the s t r u c t u r e o f
364
Multinuclear Solid-State NMR of Inorganic Materials A
bridging N
160
:I~ 4.0 M g , C ~ t--
2.0
n o n ~
80
oo
Sr C a / . 1 4 r ~ Ba ~ non-bridging
Q
-
O
Mg,Ca
-
t~ ,..r
40
Mg I_
t
0.9
1.1
i
_
I
1.0
1.3
,
I
1.2
~
1.4
I
1.6
Cation radius (.~)
Cation electronegativity
Figure 6.17. A. Relationship between the 170 nuclear quadrupole coupling constant • and the cation electronegativity for the bridging and non-bridging oxygens in the alkaline earth metasilicates. B. Relation between the logarithm of the 170 chemical shift and the cation radius for bridging and non-bridging oxygens in the alkaline earth metasilicates. From Timken et al. (1987) by permission of the American Chemical Society.
6.0 6.0 N
N T
4.0
O
4.0
n
•
El
•
9
100
l
i .
.
140
I
,
I
I
180
-1.0
.
Si-O-Si (o)
I
,
I
,
I
-0.8
,
!
-0.6
cos(Si-O-Si)
Figure 6.18. Relationship between the nuclear quadrupole coupling constant XQ of a number of silicates and the bridging Si-O-Si angle (left) and cos(Si-O-Si) (fight). From Maekawa et al. (1996) by permission of the American Chemical Society. glasses. S o m e of the initial functions used by Farnan et al. 1992 and Grandinetti et al. 1995 for XQ did not fit the data (Figure 6.18), leading to the suggestion of an alternative function which gave a m u c h i m p r o v e d fit: ZQ ( M H z ) - - 5 . 8 9 2 c o s ( S / - O - Si) + 0.248
(6.16)
170 N M R
365
In the case of siliceous zeolites with complex structures, increasing the number of data points did not result in a strong correlation of the interaction parameters (Bull et al. 1998, 2000). The ab initio calculations used in this work provided a very useful understanding of the spectra. Two structural models for the compound ferrerite were used to calculate the N M R parameters, which were then compared with the experimental data. The sensitivity of the N M R parameters to the structure constituted a means of testing of the limitations of the crystallography, thus providing genuine crystallographic input to elucidate the correct structure.
All three oxygen atoms in Na4Zr2Si3012 show 170 XQ values in the range for nbo (Bastow et al. 1996) indicating that zirconium acts as a network modifier in silicates. Since only two of the oxygen sites in Na4Zr2Si3012 are inequivalent, one of the three observed peaks must arise from a secondary phase. The ~70 N M R spectrum of Na2ZrSiO5 also showed two signals at 120 and 30 ppm, with evidence of some structure in the spectrum. Since materials containing bonds between titanium and silicon have a number of important technological applications, the ability to characterise them directly is of great practical interest. Studies have shown that the XQ value in these systems is larger than in Zr-O-Si. Titanium is an intermediate oxide which can act as both a network
6.5.1.3 S i l i c a t e s a n d g e r m a n a t e s o f z i r c o n i u m a n d t i t a n i u m .
Table 6.7. 170 NMR interaction parameters for silicates containing zirconium and titanium. Sample
Site
~i..... (ppm)
Na4Zr2Si3012
ZrOSi(1) ZrOSi(2) ZrOSi(3) ZrOSi Si-O-Ti Si-O-Si Ti-O-Si Apical O Si-O(1)-Si Si-O(2)-Si SiO(1)-Si Si-O(2)-Si
169.5 118.0 126.0 160* 190" ND 157 741" 333 +_ 1 363 • 1 305 _+ 1 370 • 1
Si-O-Ti Si-O(1)-Si Si-O(2)-Si Ti-O-Ge Apical
295 • 1 69 +_ 1 54 • 1 148 749*
ZrSiO4 Ba2TiSi208 (fresnoite) LiTiOSiO4 [Ti(acac)20]2 [OSi(C6H5)212 [Ti(acac)O1.5]2 [OSi(C6H5)213. 2C4H802 TiOz[O4Si4(C6H5)812 LiTiOGeO4
* denotesthe peakpositionratherthanthe isotropicchemicalshift.
•
(MHz)
"q
Reference
2.68 2.75 2.80 ND ND 3 3.05 --~0 3.7 _+ 0.1 3.3 • 0.1 3.3 • 0.1 3.0 • 0.1
0.0 0.1 0.2 ND ND --~0 0.35 ND 0.1 + 0.05 0.1 _ 0.05 0.0 • 0.05 0.0 • 0.05
Bastow et al. (1996)
3.0 • 0.1 4.7 • 0.1 5.3 • 0.1 4.80 --~0
0.0 • 0.05 Gervais et al. (2000) 0.0 -+ 0.05 0.0 • 0.05 0.22 Bastow et al. (1999) ND
Dirken et al. (1995) Bastow et al. (1999) Gervais et al. (2000) Gervais et al. (2000)
366
Multinuclear Solid-State NMR of Inorganic Materials bridging O Li2TiOGeO4
rutile apical O
gO Li2TiOSiO4
"~utile ,
t
1000
,
.i
,
I
600
,
I
,
|
200
I
.
170 shift (parts in 106) w.r.t. H20 Figure 6.19. 170 MAS NMR spectra of Li2TiOSiO4 and Li2TiOGeO4 showing the resonances from the apical and bridging oxygen atoms. The asterisks denote spinning side bands from the apican oxygen resonances. From Bastow et al. (1999) by permission of the copyright owner.
former, with typically 4-fold Ti coordination, or as a network modifier, with Ti in 6-fold coordination. Titanium can also adopt 5-fold coordination. It is important to know the sensitivity of the 170 NMR interaction parameters to the coordination of the titanium, but the data set presently available is too small to be definitive. Important insight into this question has been provided by a series of crystalline cyclic diphenylsiloxanes, which, although essentially large organic molecules, have titanosilicate cores. Their XQ values lie in the range 3-3.5 MHz, not very different from those of Si-O-TiO3 and Si-O-TiOs. Although it is possible that these units might be discriminated, Si-O-TiO3 has a smaller shift (Gervais et al. 2000); more data points are required to test this possibility. LiTiOSiO4 contains TiO5 units in a square pyramid configuration in which the apical oxygen shows a very distinct 170 resonance. In this case the value of • is too small to be determined but the spectrum contains spinning sidebands due to CSA spread over --~1000 ppm (Figure 6.19) (Bastow et al. 1999).
6.5.2 Amorphous materials 6.5.2.1 Silica a n d germania. The values of XQ, T] and the 170 shift for silica glass are --~5 MHz, --~0.2 and 37 ___ 3 ppm respectively (Geissberger and Bray 1983, J~iger et al. 1993), indicating that the chemical environment in glass is very similar to
170 NMR
367
the environments in the crystalline silica polymorphs quartz and cristobalite. The relationships between the structural parameters and the NMR interactions for these crystalline modifications (Equations 6.8, 6.9 and 6.12) allow the bond angle distribution to be deduced in glass. A similar process has been followed for germania. The resulting distribution of angles was fitted by a Gaussian function with a mean Ge-O-Ge bond angle of 133 ~ and a width of 4 ~ (Hussin et al. 1999). This distribution is much narrower than the values found for both SiOa and GeO2 glasses from a combination of highenergy photon data and neutron diffraction data (Johnson et al. 1983 and Grimley, Wright and Sinclair 1990) which gave values of 148.3 ~ and 7.5 ~ for SiO2 and 133 ~ and 8.3 ~ for GeO2 (Neuefind and Liss 1996). The NMR interaction parameters XQ = 5.3 MHz, Xl = 0 and giso,cs = 42 ppm have been determined for Si-O-Si in a silica gel (van Eck et al. 1999). 6.5.2.2 M e t a l silicate a n d germanate glasses. Although the subtle crystallographic
distinction between the sites is lost in a glass because of the distribution of environments, 170 NMR can provide information about the structure of glasses by being able to distinguish gross features, including bridging and non-bridging sites and the effect of cation coordination. Both MAS and static NMR spectra can be used, the more distinct lineshapes in the latter being sometimes easier to simulate (Figure 6.20). The spectra of a series of disilicate glasses were modelled by a Gaussian distribution of parameters (although not using the physical basis described in Section 6.2.3). This then allowed the mean Si-O-Si bond angle and the angular distribution to be determined. The maximum of the distribution profile shifted from 143 ~ in lithium silicate glasses to 136~ for caesium. Potassium silicate glasses showed the narrowest distribution (Maekawa et al. 1996). In a series of glasses quenched at ambient and high pressure (> 6 GPa), both the nbo and bo in potassium tetrasilicate glass were shifted to higher frequency in the higher pressure sample by ~ 3 ppm, the nbo peak becoming ~20% broader. This shift is consistent with an increase in the mean Si-O-Si bond angle, and may indicate the formation of new species such as O3Si-O-SiO5 with the presence of tricoordinate sites (Xue et al. 1994). In metal silicate glasses the intermediate range order is determined by both the Si-O-Si bond angle distribution and the ordering of the cations, both of which factors can be elucidated by 170 DAS. Cation ordering has profound implications for many properties of a glass such as the configurational entropy of the system and transport characteristics. The bond angle distribution in K2Si409 derived from 170 DAS data showed a maximum at 143 ~ and had a halfwidth of 21 ~ (Faman et al. 1992). Studies of crystalline materials have shown that the 170 shift of both oxygen sites, but especially the nbo site, is sensitive to the local coordination of the metal. Comparison of the isotropic dimension of the spectra of K2Si409 and KMgo.sSi409 glasses showed no significant increase in width in the mixed cation glass, but the peak position was shifted
368
Multinuclear
Solid-State NMR
of lnorganic
Materials
red
_-.. . . . = . . . .
~
a
t
e
d non-bridgingoxygen component bridgingoxygen nent
~ ,
I
I
,
1
1000 0 -1000 170 shift (ppm) w.r.t. H20 Figure 6.20. Observed and simulated static 170 NMR spectrum of Li2Si205 glass showing the bridging and non-bridging oxygen components of the simulation. From Maekawa et al. (1996) by permission of the American Chemical Society.
Table 6.8.
170 NMR parameters for metal silicate and germanate glasses.
Sample
Site
Li2Si205
bo nbo bo nbo bo nbo bo nbo bo nbo bo nbo bo nbo bo nbo bo nbo
NazSi205
Na2Si307 NazSi409
K2Si205 K28i409
Cs28i205
~iso.c~(ppm) 68 + 2 42 + 2 65 _+ 5 40 +_ 2 61 _+ 2 39 _+ 2 60 + 5 39 _+ 2 50 + 4 36 _+ 2 51 _+ 2 38_+2 60 +_ 2 84 _+ 2 52 + 4 76 + 2 68 _+ 2 145 + 2
XQ (MHz)
rl
Reference
5.0 _+ 0.1 2.55 + 0.1 5.0 _+ 0.3 2.3 +_ 0.1 4.9 + 0.1 2.35 _+ 0.1 5.0 _+ 0.3 2.3 + 0.1 5.0 _+ 0.3 2.3 _+ 0.1 5.1 _+ 0.1 2.4_+0.1 4.7 _+ 0.1 2.5 _+ 0.1 4.9 + 0.2 2.3 _+ 0.1 4.55 + 0.1 3.1 _+ 0.1
0.15 _+ 0.1 0.2 _+ 0.1 0* 0* 0.1 _+ 0.1 0.2 _+ 0.1 0* 0* 0* 0* 0.0 _+ 0.1 0.25_+0.10 0.25 _+ 0.1 0.45 _+ 0.1 0* 0* 0.3 + 0.1 0.55 _+ 0.1
Maekawa et al. (1996) Xue et al. (1994) Maekawa et al. (1996)
Xue et al. (1994) Xue et al. (1994) Maekawa et al. (1998)
Maekawa et al. (1996) Maekawa et al. (1996) Maekawa et al. (1996)
170 NMR Table 6.8. (Continued)
170
369
NMR parameters for metal silicate and germanate glasses.
Sample
Site
~isoxs(ppm)
XQ(MHz)
xl
Reference
CaO.SiO2 (Ca/Si = 0.7) NazGe9019 Na4Ge9020
bo nbo bo bo
104-112 60-75 165 _+ 5 80 + 5
4.6 + 1 2.1 + 1 7.0 + 0.5 6.0 + 0.5
0.0 +_ 0.5 0.2 + 0.5 0.5 + 0.1 0.5 + 0.1
Cong & Kirkpatrick (1996) Hussin et al. (1998) Hussin et al. (1998)
* - value assumed in the fitting procedure.
from the position in the pure potassium glass. This suggested a high degree of cation ordering with the non-bridging oxygen (nbo) atoms probably coordinated by two potassiums and one magnesium ion. The isotropic dimension of the 170 DAS spectra of a series of KxNal- xSi205 glasses showed resonances from both the bo and the nbo units which appeared at 16.1 and 68.7 ppm respectively in the glass with x = 1, and at 11.8 and 27.0 ppm respectively in the glass with x = 0 (Florian et al. 1996). The shift of the nbo peak could be modelled if the nbo unit is assumed to be coordinated by four alkali cations occupying these sites randomly. The isotropic dimension of an MQ MAS spectrum can provide the same information. Comparison of the 1 7 0 3Q MAS spectra of a series of calcium and magnesium silicate glasses indicated that the nbo units are coordinated by three randomly mixed cations (Stebbins et al. 1997). 170 NMR can also provide information about the cations coordinated to the bo units. No 170 NMR evidence was found for nbo units in two sodium germanate glasses containing _< 18.2 mol % Na20. In the glass with 10 mol % Na20, 170 MAS NMR suggests that the dominant coordination is GeO6 but in the 18.2 mol % Na20 glass it is GeO4 (Hussin et al. 1998). 6.5.2.3 G e l - b a s e d silicates. A 1 7 0 MAS NMR study of hydrothermally synthesised crystalline calcium silicate hydrates has identified the sites within amorphous samples (Cong and Kirkpatrick 1996). Hydrated calcium silicate gels are important products formed in the hydration reactions of Portland cements. Complex 170 1D MAS NMR spectra were obtained, many of which showed 6 peaks, 2 from nbo units, 1 from a bo unit, and others from the H20 and hydroxyl groups associated with the calcium and silicon. Cong and Kirkpatrick (1993, 1996a) also used lVO NMR to determine the evolution time of the various sites. The chemical shifts of both the nbo and bo resonances decreased with increasing calcium/silicon ratio in the sample, reflecting the decreasing polymerisation of the network. The 170 NMR spectra of the bo units suggest that an increase in the calcium/silicon ratio results in a decrease of the average Si-O-Si bond angle. All the 170 NMR data could be interpreted in terms of the defect tobermorite model of hydrated calcium silicate gels.
370
Multinuclear Solid-State NMR of Inorganic Materials
Sol-gel reactions provide a means of producing oxides mixed at the atomic level. NMR can be used to monitor the reaction from the stage of the initial solution through to the final dense solid. In the case of silica, the presence of small amounts of a second oxide can change the properties such as the refractive index or catalytic behaviour, making an understanding of the mixing homogeneity of vital importance. Where small amounts of TiO2 are added to SiO2, it is important to know whether the titanium is atomically dispersed, what sites it occupies and whether there are discrete nanoparticles present. Another important consideration is to match the rates of hydrolysis of the components in the initial reaction to maximise the number of cross-linked M-O-Si groups. The extensive work of Babonneau has shown that solution-state 170 NMR provides a direct measure of this cross-linking and information about its variation with time in the initial sol. One of the important factors in the process is the change of reactivity when the functional groups are changed. The solution-state NMR studies of the SiO2-TiO2 and SiO2-ZrO2 systems have been summarised by Delattre and Babonneau (1997) and the work on siloxane-oxide hybrid materials by Babonneau and Maquet (2000). Once formed and dried, the amorphous gel is a porous solid which changes in structure when heated, eventually recrystallising from the amorphous state. In silicate gels such as MO-SiO2, a key question is the dispersion of the metal, which can be monitored by ~70 NMR measurements of the relative number of M-O-M, M-O-Si and SiO-Si links. The large shift range of the 170 resonances allows all these different fragments in the TiO2-SiO2 system to be readily resolved in the 1D MAS NMR spectra. The sensitivity of this method makes it applicable even when only small amounts of TiO2 are added. The NMR data can clearly show the presence of nanoscale phase separation, since some samples show only Ti-O-Si linkages, whereas additional Ti-OTi resonances occur in other samples (Smith and Whitfield 1994). A more detailed study showed that gels containing 7-41 tool % TiO2 remain amorphous up to 600~ according to X-ray diffraction. Preparation of a 7 tool % TiO2-SiO2 gel by both a oneand two-step process showed an absence of Ti-O-Ti linkages in the latter, indicating atomic-scale mixing of the metals in that case (Dirken et al. 1995). The technical importance of the TiO2-SiO2 system has led to a number of detailed studies in which NMR was combined with EXAFS, XANES and diffraction. To improve the catalytic activity, the surface area of these materials can be increased to 450 m 2 g - 1 by washing with heptane. ~70 MAS NMR has been used to demonstrate the complete mixing of titanium with the silica in all these samples (Holland et al. 2000). The reactivity of the titanium alkoxide can also be improved by complexing with acetylacetone to encourage a greater degree of Ti-O-Si crosslinking. Even samples initially containing 18 tool % TiO2 showed no sign of TiO2 phase separation, the 170 NMR spectra indicating that almost all the titanium remained as Ti-O-Si even after heating at 750~ (Figure 6.21A) (Pickup et al. 1999). The atomic dispersion can be further improved by
170 NMR
371
A T(~C) *~~J~l .
'
750
500
~
ed ,,
5O0 170
0
-500
shift (ppm) w.r.t. H20
500
0
-500
170 shift (ppm) w.r.t. H20
Figure 6.21. A. 170 MAS NMR spectra of (TiO2)o.18(SiO2)o.s2 xerogel after heating at various temperatures. The asterisks denote spinning side bands. From Pickup et al. (1999). B. 170 MAS NMR spectra of (Ta205)o.25(SiO2)o.75 xerogel after heating to various temperatures. From Pickup et al. (2000). All spectra used by permission of the Royal Society of Chemistry.
functionalising the precursor (e.g. R'xSi(OR)4-x) in addition to using acetylacetone. The ~70 NMR spectra of all such gels are complex, showing up to five resonances (Figure 6.2) (Gervais et al. 2001). 170 NMR showed that samples of the type Si(OR)4 (x = 0) give the highest (Si-O-Ti)/(Ti-O-Ti) ratio, indicating that these are the most favourable for titanium incorporation. The addition of CH3 groups increases the hydrophobic properties of the system, allowing the Si-O-Ti bonds to be more readily broken. Samples in which x = 0 show two 170 Ti-O-Si NMR resonances, one at --- 250 ppm and the other at 180-110 ppm. It is likely that 250 ppm resonance corresponds to Si-O-TiO3 framework sites, with the other resonance probably due to 4-coordinate Ti attached but in a much less thermally stable unit. Other systems studied by 170 NMR include ZrO2-SiO2. Since zirconia acts as a network modifier, it is retained in much higher concentrations in the system than TiO2. Both Zr-O-Zr and Zr-O-Si bonds are clearly observed (Dirken et al. 1995a). The 170 NMR spectrum of a sample containing 10% ZrO2 showed, in addition to the main Si-O-Si peak, a Zr-O-Si resonance at ---150 ppm. At higher zirconia concentrations phase separation occurs (Pickup et al. 1999a). The same behaviour was found in
372
Multinuclear Solid-State NMR of lnorganic Materials
Ta2Os-SiO2 gels in which even at high Ta205 concentrations (--~25 mol%) where significant phase separation has occurred, appreciable Ta-O-Si bonding is detected (Figure 6.21B, Pickup et al. 2000). 170 MAS NMR spectra have been reported in more complex nanocomposite materials from spirooxazine and spiropyran-doped matrices which contain polysiloxane and zirconium oxopolymer domains. Although the MAS speeds were quite low (5 kHz), the Zr-O-Si bonding between the distinct nanodomains could be detected as a peak at 160 ppm (Schaudel et al. 1997). As polydimethlysiloxane-zirconium oxo nanocomposite samples were dried, changes in the intensity of the different 170 signals showed that complex reorganisations of the structure were occurring (Guermeur et al. 1999). 170 MAS NMR has also shown that in the sol-gel reaction of HfO2 with SiO2 and GeO2 there is a greater tendency for HfO2 to form a solid solution with GeO2 (Bastow et al. 1996). By selectively enriching the siloxane oxygen in elastomeric polydimethylsiloxane-vanadate hybrid materials, a sharp 170 NMR signal was observed at 70 ppm above Tg, indicating that there is sufficient motion to average the interaction. Below the glass transition temperature (Tg) the 170 parameters were found to be XQ = 4.9 _+ 0.1 MHz and ~1 - 0.15 _+ 0.05, these values as expected for a Si-O-Si linkage. The intensities of the quadrupole echoes were sensitive to the motional correlation times occurring in the region of the glass transition. The estimate of Tg made from the 170 NMR spectrum at 184 _+ 5 K was consistent with the calorimetrically determined value of 177 _+ 1 K (Alonso et al. 2000).
6.6. ALUMINIUM-AND GALLIUM-CONTAININGSYSTEMS 6.6.1 A l u m i n a a n d aluminates
Walter and Oldfield (1989) reported the 170 MAS NMR spectra from o~-A1203, transitional aluminas and hydrous aluminas. Most oxygens atoms were in OA14 configurations, but a signal in 0-A1203 with a slightly higher shift and a significantly larger XQ (Table 6.9) was assigned to OA13. The values for the quadrupole parameters of o~-A1203 agree very well with a single crystal NMR study of ruby (Brunet al. 1970). In all the transitional aluminas the presence of cation vacancies result in the formation of OA13 sites, showing up as a broader line underlying the much narrower OA14 resonance. However, only in the more ordered transitional alumina 0-A1203 could sufficient structure be observed for XQ to be deduced. In the other transitional aluminas the distribution of parameters results in blurred lineshapes. The identification of OA13 with a site of larger quadrupole interaction has been confirmed by calculations of the 170 NMR interaction parameters for isostructural [3-Ga203 (Walter and Oldfield 1989). Although all the values in the gallium system were larger, reflecting the relative electronegativity of gallium and aluminium, the OGa3 sites showed larger XQ values.
170 N M R
373
Table 6.9. 170 NMR interaction parameters from aluminas and aluminates. Sample
Site
oL-A1203
OA14
~/-A1203 xI-A1203 ~-A1203 0-A1203
OA14 OA14 OAI4 OA14 OA13 OA14 OA12H OA12H
A10(OH) (boehmite) AI(OH)3 (bayerite) LaA103 NaA102 CaA1204
OA12La2 OA12 OA12,site 1 site 2 site 3 site 4 site 5 site 6 site 7 site 8 CaA1407 OA12,site 1 site 2 site 3 OA13 Y3A150~2 OA12Y2 YA103, site 1 OA12Y3 site 2 OA12Y2
8i~o,cs(ppm) Xe (MHz)
Xl
Reference
ND 75 _+ 2 72 ___ 1 73 _+ 1 73 _+ 1 72 _+ 1 72 _ 1 79 + 5 70 _ 1 40 ___5 40 _ 5
2.167 2.17 _ 0.05 2.13 _ 0.03 1.8 _ 0.2* 1.6 _+ 0.2* 1.6 _+ 0.2* 1.2 _ 0.2* 4.0 ___0.2 1.20 ___0.05 5.0 + 0.2 6.0 ___0.2
0.517 0.55 _+ 0.05 0.50 _ 0.05 ND ND ND ND 0.6 __+0.1 0.1 _ 0.1 0.5 _ 0.1 0.3 ___0.1
Brunet al. (1970) Walter & Oldfield (1989) Florian et al. (2001) Walter & Oldfield (1989) Walter & Oldfield (1989) Walter & Oldfield (1989) Walter & Oldfield (1989)
170.2t 30.9 ___0.5 86.8 _+ 1 80.7 __+ 1 76.2 ___ 1 72.4 _ 1 69.1 ___ 1 66.7 ___ 1 61.8 _ 1 57.3 ___ 1 71.6 _+ 2 61.5 _+ 2 56.8 + 2 40.6 _+ 2 14 _+ 1 143 _+ 1 165 + 1
ND 1.81 _ 0.1" 1.5 _ 0.2* 1.6 +_ 0.2* 1.6 _ 0.2* 1.6 _ 0.2* 1.3 ___0.2* 1.6 ___0.2* 1.4 _ 0.2* 1.4 +__.02* 1.9 + 0.2 1.8 + 0.2 2.1 + 0.2 2.5 _+ 0.2 1.49 _+ 0.03 1.57 + 0.03 1.65 + 0.03
ND ND ND ND ND ND ND ND ND ND 0.7 _+ 0.3 0.5 + 0.3 0.5 + 0.3 0.4 + 0.3 0.99 _+ 0.05 1.00 _+ 0.05 0.35 + 0.05
Bastow et al. (1996) Stebbins et al. (1999) Stebbins et al. (1999)
Walter & Oldfield (1989) Walter & Oldfield (1989)
Stebbins et al. (2001)
Florian et al. (2001) Florian et al. (2001)
* -determinedfromthe quadrupoleshift so representsthe quadrupoleproductPQand t is the peakpositionnot the isotropic chemical shift.
P r o t o n a t e d sites in t h e s e c o m p o u n d s h a v e m u c h h i g h e r XQ v a l u e s , w h i c h c a n be e x p l a i n e d in t e r m s o f the i n c r e a s e d c o v a l e n c y o f the bonds. A k e y m i n e r a l o g i c a l p r o b l e m w i t h a l u m i n a t e s is the q u e s t i o n o f cation d i s o r d e r i n g in spinels. N u m e r o u s 27A1 M A S N M R studies h a v e b e e n carried out on MgA1204. O n e ~70 M A S N M R study o f spinels q u e n c h e d f r o m b e t w e e n 7 0 0 and 1400~ i n c r e a s e in d i s o r d e r w i t h t e m p e r a t u r e . B o t h the
170 a n d
s h o w e d an
27A1 M A S N M R s p e c t r a
s h o w e d a s i m i l a r i n c r e a s e in d i s o r d e r but the v a l u e d e t e r m i n e d f r o m ~70 N M R was s y s t e m a t i c a l l y l o w e r ( M i l l a r d et al. 1992). ~70 M A S has b e e n u s e d to m o n i t o r the solid state N M R r e a c t i o n o f s o l - g e l f o r m e d L a A 1 0 3 ( F i g u r e 6.22). H e a t i n g to 4 5 0 ~ p r o d u c e d 170 signals f r o m L a O ( O H ) and an a l u m i n i u m o x y h y d r o x i d e , but p e a k s f r o m
374
Multinuclear Solid-State NMR of Inorganic Materials LaOOH
T(~ $ OAI4
La~O~~
450
I La~O~
LaAIO3 ]
I
800
1
I
!
I
400
,
!
t
I
I
,
!
!
0
~70 shift (ppm) w.r.t. H20 Figure 6.22 170 MAS NMR spectra of LaA103 gel heated to various temperatures. From Bastow et al. (1996a) by permission of the copyright owners. La203 and A1203 were seen at 800~ resulting from the formation of a very finely divided mixture of the unreacted individual oxides. At 950~ some unreacted oxides remained but there was evidence of significant formation of LaA103. An extensive multinuclear study has been made of the Y203-A1203 system. In the structurally simpler compounds Y3A15012 and YA103, one and two inequivalent oxygen sites respectively are observed (Table 6.9). Y2A1409 has nine inequivalent sites (OY2A12, OY3A1, OYzA12 and OY4 coordinations). Seven lines were observed in the 170 MAS NMR spectrum of this phase, two with approximately twice the intensity of the others. The shifts of these resonances span a range of 146 ppm. The data show a decrease in the 170 shift with increasing oxygen coordination number, correlated with the sum of the inverse bond lengths (Florian et al. 2001). The crystallisation of YA103 and the structure of vitreous Y3A15OI2 have also been investigated by 170 NMR. An MQ MAS spectrum showed five 170 lines, three of which were readily observable. The linewidths of the oxygen data were dominated by chemical shift dispersion. No simple model of the local oxygen coordination could be proposed to explain the relative intensities of the different peaks. Stebbins et al. (1999) have carried out important 170 MAS NMR studies of the calcium aluminates including CaA1204, which, like NaA102 has a stuffed tridymite
170 NMR
375
structure containing continuous A104 networks with well-defined A1-O-A1 links. The chemical shift and quadrupole parameters deduced for these materials (Table 6.9) indicate that the interactions of the A1-O-A1 are quite different from those of S i-O-A1 and Si-O-Si units in aluminosilicates (Stebbins et al. 1999). Analysis of the NMR data for CaA1407 took into consideration the presence of both A1-O-A1 linkages and welldefined A103 tricluster units. The position of the tricluster resonance was found to be shifted by --- 20 ppm and it has a significantly larger • value (Table 6.9), which should facilitate its identification in glassy materials (Stebbins et al. 2001).
6.6.2 Crystalline a l u m i n o - a n d gallosilicates Aluminosilicates have importance in mineralogy and in materials technology, as ceramics and microporous catalysts. One of the most important questions is the ordering of silicon and aluminium within the structure. The resolution of the various Qn(mA1) resonances in the 29Si MAS NMR spectra has provided insights into these crystalline materials. 170 NMR provides an alternative view of this ordering. Early work combining static and MAS measurements showed that the 170 resonances from Si-O-Si and Si-O-A1 can be separated, although resolution of small crystallographic differences was not possible (Timken et al. 1986, 1986a). The static measurements were necessary since the MAS often did not resolve the different fragments, although the spectra could be fitted on the basis of parameters derived from the static measurements. The 170 XQ values estimated from the static measurements were found to be larger than from the MAS spectra, and the • values from static measurements at lower fields were smaller again, indicating the presence of CSA effects which broadened the lines. The work of Timken et al. (1986) showed that the less covalent Si-O-A1 fragments have significantly smaller • values and slightly smaller shifts (Table 6.10). These trends have been correctly predicted both by the Townes-Dailey approach and by the empirical considerations of Schramm and Oldfield (1984) since both these treatments are based on the hybridisation/ionicity of the bonds. Zeolites are ideal materials on which to test structural relationships as they consist of TO4 units with only bridging T-O-T bonds and often without the complications associated with coordinating cations. For simple bridging sites (i.e. no coordinating cations) Timken et al. (1986a) developed a relationship for these materials between their XQ values and the average ionicity difference between the T components (Figure 6.23), assuming that the degree of w-back-bonding, the bonding angle and the hybridisation of the cr-orbitals remains constant. This approach correctly predicted that the • values for Si-O-Ga links should be ---20% larger than for Si-O-A1 (Table 6.10). Consideration of similar zeolites (e.g. Y) (Table 6.10) shows that the cations exert an influence at a secondary level on the NMR interaction parameters; for example, large Group II cations shift the 170 resonance to more deshielded values.
376
Multinuclear Solid-State NMR of Inorganic Materials
Table 6.10. 170 NMR interaction parameters in crystalline alumino- and gallosilicates. Sample
Site
~i ..... (ppm)
XQ (MHz)
xl
Reference
Na-A, R = 1 Na-Y, R = 2.74
Si-O-A1 Si-O-Si Si-O-A1 Si-O-Si Si-O-A1 Si-O-Si Si-O-A1 Si-O-Si Si-O-A1 Si-O-Si Si-O-A1 Si-O-Si
32 44 31 48 31 51 31 50 34 52 40 45
3.2 4.6 3.1 5.0 3.2 5.0 3.2 5.0 3.2 5.1 3.4 5.2
0.2 0.1 0.2 0.1 0.2 0.1 0.2 0.1 0.2 0.15 0.4 0.2
Timken et al. (1986) Timken et al. (1986)
Si-O-Si Si-O-Ga Si-O-AI(1) Si-O-AI(2) Si-O-AI(3) Si-O-Si Si-O-AI Si-O-Si Si-O-A1 Si-O-AI(1) Si-O-AI(2) Si-O-AI(3) Si-O-AI(4) Si-O-AI(1) Si-O-AI(2) Si-O-AI(3) Si-O-AI(4) Si-O-A1(1) Si-O-AI(2) Si-O-AI(3) Si-O-Si Si-O-Ga Si-O-Si Si-O-Ga
49 29 42.5 29.7 39.5 43 33 40 30 50.3 41.7 45.0 36.9 50.6 45.2 42.1 36.8 60.7 53.4 75.5 51 29 51 29
5.0 4.0 3.4 3.4 3.4 5.1 3.5 5.3 3.5 3.2 3.3 3.4 3.6 3.3t 3.4* 3.3* 3.6* 3.3 t 3.6t 3.2t 5.1 4.0 5.1 4.0
0.0 0.3 0 0 0.25 0.18 0.28 0.12 0.29 0.4 0.3 0.3 0.15 0.3 0.3 0.2 0.15 0.15 0.05 0.2 0.0 0.3 0.0 0.3
Timken et al. (1986)
NH4-Y, R = 2.92 NHa-Y, R = 4.98 NHa-Y, R = 7 . 5 1 Ba,Na-Y, R = 2.74 Dealuminated, Na-Y, R > 25 Ga-X, R = 1 . 6 3 Na-A, R = 1
Stilbite NHa-ZSM5, R = 19 Na-LSX, R = 1
Na,K-LSX, R = 1
T1-A, R = 1
Ga-sodalite Ba,Na Ga-sodalite
Timken et al. (1986) Timken et al. (1986) Timken et al. (1986) Timken et al. (1986) Timken et al. (1986)
Pingel et al. (1998)
Xu & Stebbins (1998) Amoureux et al. (1998) Pingel et al. (1998)
Freude et al. (2001)
Freude et al. (2001)
Timken et al. (1986a) Timken et al. (1986a)
t - value is the quadrupole product PQ based on D O R field variation measurements.
T h e h i g h e r - r e s o l u t i o n t e c h n i q u e s D A S , D O R and M Q - M A S
have provided
additional resolution of these sites, allowing the different S-O-A1 sites to be distinguished in zeolite structures (Pingel et al. 1998, Freude et al. 2001). A combination of D O R and 3 Q - M A S (Figure 6.24) has allowed the relevant N M R interactions to be deduced. For the zeolites N a - A and N a - L S X , a direct correlation b e t w e e n ~iso,cs and the Si-O-A1 b o n d angle has been p r o p o s e d based on this limited range of c o m p o u n d s
170 NMR 00
--~
/
..... / . . . .
7 MHz /
40
.NI
r
o~,,4
I
/
/
/'
I
6 MHz /
..... /
377 ,'
/
/
J
//
'
' / .....
5 MHz
/'"
/
/
/
!
I
/
-/1
/
/1
,,"/ """/ "" / '" I ////;///
20
t '
I
I
I i m
0
J
/ i I
,
/ i ~
I I
. I
.
J s I.
I
/ / I
/ I i
I I
i m
!
45
I
t
/ / I
I
! I
..
i
/
a t I
.
/
i
i
3M.z / / / I
.I
55
65
Average ionicity (%) Figure 6.23. Expected dependence of the 170 XQ values (shown in MHz on the curves) on the ionicity of the tetrahedral cations in the T-O-T' linkage. From Timken et al. (1986a) by permission of the American Chemical Society.
5iso(ppm) 35.2
~ 20
43.9
___.___~ 60
40
47.8 20
40
W 60 i
60
40
82 (ppm)
20
i
i
.......
i
60 40 20 0 170 shift ( p p m ) w.r.t. H 2 0
Figure 6.24. 170 MQMAS NMR spectrum of hydrated zeolite Na-A showing at right anisotropic slices of the 2D spectrum with their corresponding 8iso values. The MAS spectrum at lower right was fitted in accordance with the values derived from the 2D data. From Pingel et al. (1998) by permission of Elsevier Science.
378
Multinuclear Solid-State NMR of Inorganic Materials
(Pingel et al. 1998). This relationship was subsequently superseded by a correlation with the hybridisation parameter (Eq. 6.11) (Freude et al. 2001). On dehydration of the zeolite, typical shifts of --- 8ppm are observed, resulting from a combination of a change in the bond angle at the sites and the polarisation of the framework by the water molecule. As the cation is varied, the 170 NMR chemical shift becomes more positive with increasing cation radius (Freude et al. 2001). The silicon-aluminium ordering of aluminosilicate structures appears to obey Lowenstein's rule stating that A104 units avoid sharing comers with each other. In some zeolites a small excess of Si-O-Si arising from the presence of A1-O-A1 would be expected if Lownstein' s rule is not strictly obeyed. The increased resolution of 170 MQMAS can now be used to monitor such deviations. The 3Q NMR data for stilbite have provided unequivocal evidence for the presence of A1-O-A1 units, amounting to about 3% of the oxygen in this configuration (Figure 6.25) (Stebbins et al. 1999). The presence of such small amounts of A1-O-A1 is extremely important for the detailed thermodynamic modelling of these systems. Back-reacting a ~70-enriched stilbite sample showed that the A1-O-A1 bonds react preferentially with moisture (Stebbins et al. 1999). However, no A1-O-A1 could be detected in the zeolites 4A, 13X and natural analcime (Zhao et al. 2001). Variable amounts of A1-O-A1 have been detected in analcime, depending on the temperature at which the sample was synthesised (cf. Cheng et al. 2000, Zhao et al. 2001).
Si-O-AI .~AI-O-Al
Si-O-Si
/ /~ AI-O-Si
//"~ original
-, 9 20
Si-O-Si
ID
I( ili:li j .,
o j,,,~
ra~ <
l/
bac
1
40 ,
~ , ,
w,
~
i
-20
J,
~ , ,
i
r
i
r]
-30
....
,,
-4~0
Isotropic dimension (ppm)
.....
0
......
i .... -20
i ....
i ....
i ....
J
-40
170 shift (ppm) w.r.t. H20
Figure 6.25. ~70 MAS NMR spectrum of the zeolite stilbite, showing at fight the projections of the isotropic dimension of the 3QMAS spectra of the original isotopically-enriched sample, and after back-reaction with isotopically normal water vapour. From Stebbins et al. (1999) by permission of the Mineralogical Society of America.
170 N M R
379
6.6.3 Amorphous aluminosilicates Aluminosilicate glasses are the most widely manufactured glass composition, and also act as models for geological melts. 170 NMR has been used as a key technique in recent years to gain an understanding of the framework of amorphous aluminosilicates. Prior to the development of 170 NMR to the point of being able to resolve the different structural units in aluminosilicates, it was difficult to test the aluminium avoidance of a disordered structure, to investigate the Si-A1 ordering or the presence of non-bridging oxygen (nbo). 170 MQ-MAS has now provided complete resolution of the Si-O-A1 and Si-O-Si fragments and, from the slope of the MQ data (19/12) indicates that the quadrupole interaction is the dominant broadening mechanism. By fitting the anisotropic slices to second-order quadrupole interactions, the resulting values can then be used to fit the 1D MAS patterns and determine a reliable quantitative distribution of the various fragments. This procedure was first carried out for an albite glass (Dirken et al. 1997) in which the resolved Si-O-Si and Si-O-A1 resonances (Figure 6.26A) occurred with an intensity ratio of l: 1 (within the accuracy of the measurements), consistent with conventional models of this structure which do not include A1O-A1. In the so-called fully compensated glasses such as NaA1Si304 and CaA12Si2Os, the framework (A104)- units are exactly charge-balanced by the cations. In principle this corresponds to a structure consisting of a continuously-connected framework (i.e. A
Double FT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Sheared -25 ]
B
-25
Si-O-AI / ~ si-o-si NaAISi3~ ~
25 75
NaAISiO4 / \ AI-O~~/o~..,.~
%
50
-50 -150 F2 dimension
400
0
-400
75 i~,,
50 -50 -150 F2 dimension
400
170 shift (ppm) w.r.t. H20
0
-400
NaA 0
-30
-60
170 shift (ppm) w.r.t. H20
Figure 6.26. A. 170 MQMAS NMR spectra of albite glass, showing the data presented in both direct double FT and sheared format, with the observed and simulated one-dimensional slices below. From Dirken et al. (1997), by permission of the copyright owner. B. Isotropic projections of the triple-quantum MAS spectrum of sodium aluminosilicate and lithium aluminosilicate glasses. From Lee and Stebbins (2000), by permission of Elsevier Science.
380
Multinuclear Solid-State NMR of lnorganic Materials
only Q4 units) with no nbo units. 170 3Q MAS of CaA12Si208 has provided clear evidence of a small number (--~5%) of nbo units, contrary to expectations (Stebbins and Xu 1997), indicating that the commonly accepted models of these glasses are in error and that they may contain other structural units such as triclusters. 170 NMR measurements have been used to compare cation effects in LiA1SiO4, NaA1SiO4 and CaA12Si208 glasses (Lee and Stebbins 2000). In all these glasses, the isotropic projections of the 170 3Q MAS data showed three peaks, two from the expected Si-O-Si and Si-O-A1 units (the glasses have Si/A1 = 1), and a small A1-O-A1 peak (Figure 6.26B). Although the degree of aluminium avoidance in these glasses was not complete, the NMR results indicated that it exceeded 90%. The degree of disorder was smaller in the sodium glass, indicating an increase in disorder with increasing cation field strength. This study was extended by varying the Si/A1 (R) ratio (Lee and Stebbins 2000a). The quadrupole interaction did not vary significantly with R but ~iso,cs increased with decreasing R and increasing cation field strength, leading to the suggestion that the glass structure is much more strongly affected by the cation field strength. There may also be nbo units present, but these were not readily distinguished in the spectra, presenting a continuing challenge to the technique to detect all the oxygen species present in such a glass. In other sodium and calcium aluminosilicate glasses clear evidence has been found in the 170 3Q MAS data of significant formation of A1-O-A1 units. (Stebbins et al. 1999). The ~70 NMR spectra of yttrium and lanthanum aluminosilicate glasses show peaks at 54-58 ppm and 143-158 ppm, the former corresponding to bo and the latter to nbo units. Additional intensity found at 210 ppm in the spectrum of the yttrium-containing glass was taken to indicate nbo units with a higher number of coordinated yttrium ions (Schaller and Stebbins 1998). There was no evidence of intensity in the region expected for Y203 and La203, suggesting the absence of strong cation clustering in these glasses. The XQ value for the Si-O-A1 oxygen unit was estimated to be Table 6.11. 170 NMR interaction parameters in aluminosilicate glasses Sample NaA1Si308 (albite) CaA12Si208 NaA1SiO4 (nepheline) LiA1SiO4 ([3-eucryptite)
Site Si-O-Si Si-O-A1 Si-O-A1 nbo Si-O-Si Si-O-A1 A1-O-A1 Si-O-Si Si-O-A1 A1-O-AI
+ PQ ratherthanXQdetermined.
8iso,cs(ppm) XQ(MHz) 49 + 1 33 +__1 61 113 _ 2 55 42 25 55 40 20
5.1 _ 0.3 3.5 --_ 0.2 3.5 2.9 +__0.2 4.9+ 3.3+ 2.7+ 4.7 3.3 2.9+
~
Reference
0.15 _ 0.05 Dirkenet al. (1997) 0.05 _+ 0.05 ND Stebbins& Xu (1997) ND ND Lee & Stebbins (2000a) ND ND ND Lee & Stebbins (2000a) ND ND
170 NMR
381
.J O(Si3) 4. N
A
=
O(Si2AI)
A
4
**J O(SiAlW2)
~
==
* [] "/r.,, *O(SiAIWAIv) '*'J~,, Si-O(Ca)-AI
O
t
O(Al~)
:L, , , 1
9 .
.
.
.
20
1
AI-O(Ca)-AI .
.
.
.
i
a
,
,
i
i
i
J
40
,
,.l
~
60
,
,
,
I
.
.
.
.
80
170 ~ (ppm) Figure 6.27. Relationships between the 1 7 0 nuclear quadrupole coupling constant XQ and the 1 7 0 chemical shift ~ for various oxygen sites present in aluminosilicate glasses and melts. The triangles denote Si-O-Si groups, squares denote Si-O-A1, circles denote A1-O-A1 and diamonds denote tricluster units. From Xue and Kanzaki (1999) by permission of the American Chemical Society.
--~ 3.1 MHz, much smaller than in sodium and calcium glasses (Table 6.11). This is probably because the increased strength of interaction with the bo of the trivalent ion decreases the Si-O-A1 bond angle and hence decreases the Xo value. Ab initio calculations have been carried out on both (Si,A1)-O-(Si,A1) and O(Si,A1)3 tricluster groupings (Xue and Kanzaki 1999). The parameters calculated for simple bridging units agree well with experimental values. An interesting implication of these calculations is that some triclusters could strongly overlap with some of the simple bridges (Figure 6.27), but more experimental work is required to clarify these important points.
6.7. BORON-CONTAINING SYSTEMS
6.7.1 Borates
Two oxygen sites have been detected by 170 MAS NMR in crystalline NaBO2; one is a nbo site (Po = 4.1 +_ 0.2 MHz, ~iso = 78 + 2 ppm) and the other a bo site (Po = 3.5 _+ 0.2 MHz, ~iso = 93 +_ 2 ppm). Sodium and barium borate glasses show large variations in the quadrupole interaction for both bo and nbo sites, the relative magnitudes of which have also been shown to change (Stebbins et al. 2000). In sodium and barium borate glasses the isotropic projection revealed 170 resonances arising
382
Multinuclear Solid-State NMR of Inorganic Materials
from both bo and nbo units. The bo signal must contain contributions from various BO3 and BO4 linkages. As with the silicates, both the bo and nbo units vary with the cation, this effect being significantly larger for the nbo units. The widths of the 2D data sets indicate a much greater distribution of interactions at the nbo site. An early static 170 NMR study of vitreous B203 (Jellison et al. 1977) suggested the presence of two oxygen sites, consistent with a structure composed of randomly oriented boroxyl rings. A later DOR study suggested at least three oxygen sites (Youngman et al. 1995) which cannot be resolved by 1D MAS or even MQ NMR. Two of the peaks detected by DOR should appear near - 74 ppm in the isotropic dimension, while the third DOR peak would be close to - 60 ppm in the MQ NMR spectrum. It is likely that all the 170 peaks in glassy B203 would overlap in the MQ data (Wang and Stebbins 1999), which suggest, however, that there is a degree of intermediate range order in glassy B203.
6.7.2 Ternary and quaternary systems
Borosilicate glasses have an extremely wide range of applications, including nuclear waste storage, high temperature sealing, optical and chemically resistant glassware. They pose an intriguing structural problem, since the network could contain BO3, BO4 and SiO4, with bridging bonds between these units, and the possible distribution of nbo units. 11B NMR data were used in conjunction with earlier structural models to develop a comprehensive concept in which the borosilicate glasses were divided into distinct compositional regions (Dell et al. 1983). An extensive multinuclear study of sodium borosilicate glasses included 170 NMR (Bunker et al. 1990), which however suffered from problems associated with the very strong overlap in the 1D static and MAS spectra of the four components used in the simulation. MQ MAS NMR provided better 170 resolution in these systems, and although it did not completely overcome the problems arising from the overlapping oxygen sites, it suggested that some of the initial 1D assignments of the B-O-B and Si-O-B resonances were probably incorrect. The 170 3Q MAS NMR spectrum of a pure borosilicate glass shows three clearly separated resonances (Wang and Stebbins 1998). One of these is an intense Si-O-B resonance, indicating significant mixing between silicon and boron. This contradicts earlier structural models involving strong phase separation, with little mixing of BO3 and SiO4 units, and also has implications for metal borosilicates with low metal concentrations. In a sodium borosilicate glass, the amount of B-O-B (whose position was confirmed by the work on alkali borate glasses) was much less than predicted, again supporting the concept of strong mixing of BO3 in the silicate network. One possible complication in alkali borosilicate systems is the possibility of significant overlap between the borate nbo and the Si-O-Si peaks. Since, from the dependence of the NMR parameters on the
383
170 N M R
A BO
Si-O-B [""B-O-B
Ba-O-B
Si-O~,,) 100 ,
-25
9 ~Ba-O-B
-125
Si-O-B
200
. . . .
,
. . . . .
.
.-..
c....,..,
-75
-125
-75
-125
.. B-O-B
Si0
~
-25 100
-125
Si-O-B,
-~~ Ba-O-Si - - - - ~ ~
Ba-O-B
,
.-B-O-B
Ba-O-Si
S i - O ~ 200 o
..i
'is'0 .......
'-i00 ......
-is0
Isotropie dimension (ppm)
....
, ....
i ....
i ....
J ....
-75 -125 -125 170 shift (ppm) w.r.t. H20
-25
F i g u r e 6.28. A. 170 MQMAS NMR spectra of two barium borosilicate glasses. The black square indicates the expected position of the Ba-O-Si peak. B. Projections of the MQMAS NMR spectra of three barium borate glasses, showing the bridging oxygen region on the left and the non-bridging oxygen region on the right. From Zhao et al. (2000) by permission of Elsevier Science.
nature of the cation, barium is known to produce the biggest shift difference between the oxygen sites, 3Q 170 MAS spectra of barium borosilicate were investigated. These showed a bo signal composed of Si-O-Si, Si-O-B and B-O-B units, and separate nbo Ba-O-Si and Ba-O-B signals (Figure 6.28A,B), the positions of which were confirmed by comparison with binary barium silicates and borates. The nbo population in these barium borosilicate glasses is higher than predicted by conventional models, suggesting that they are very disordered (Zhao et al. 2000). 170 NMR signals from A1-O-B and B-O-B units were seen in the 3Q spectra of sodium boroaluminate glasses, together with A1-O-A1 and nbo signals (Wang and Stebbins 1999). In a more complex NazO-AlzO3-B203-SiO2 glass, 170 3Q MAS NMR revealed signals from all the different units present. Reaction of the glass with water brought about a restructuring of the framework, with the loss of boron-containing linkages leaving only the Si-O-Si and Si-O-A1 resonances (Angeli et al. 2001).
384
Multinuclear Solid-State NMR of lnorganic Materials
Table 6.12. Some representative 170 NMR interaction parameters for boron-containing materials. Linkage Si-O-Si
B-O-B
Si-O-B Si(B)-O-Na Si(B)-O-K Si-O-Ba B-O-Ba A1-O-B AI(B)-O-Na
8iso,cs(ppm) PQ ( M H z ) 37 51 48 40 92 82 72 104 113 79 62 64 20 57 35 38 76 158 197 59 16
5.4 4.9 5.0 5.1 5.5 5.1 6.0 5.0 4.3 5.0 5.6 5.6 4.3 5.6 2.2 2.5 2.1 2.3 3.6 4.1 1.7
Reference Wang & Stebbins (1999) Bunker et al. (1990) Angeli et al. (2001) Wang & Stebbins (1999) Bunker et al. (1990) Youngmanet al. (1995) Angeli et al. (2001) Wang & Stebbins (1999) Bunker et al. (1990) Angeli et al. (2001) Wang & Stebbins (1999) Bunker et al. (1990) Wang & Stebbins (1999) Zhao et al. (2000) Zhao et al. (2000) Wang & Stebbins (1999) Wang & Stebbins (1999)
6.8. OTHER SYSTEMS
There have been relatively few 170 NMR studies of other oxygen-containing systems. Some of the original high-resolution ~70 studies of microporous solids included aluminium phosphates (Table 6.13, Timken et al. 1986a). The XQ value for the A1-O-P bridging bonds in these materials was quite large (5.6 MHz). Another 170 NMR study of the nbo units in Na3PO4 produced quite a large value for • (Table 6.13, Witchas et al. 2001). This study recorded the changes in the 170 static NMR spectra of samples heated up to 585 K, at which temperature the spectra were significantly narrowed and showed a Lorentzian lineshape. Above 364 K, increasingly rapid PO4 rotation was observed by a combination of multinuclear NMR studies, neutron scattering and electrical conductivity which indicated the occurrence of anion rotation and cation transport over a wide temperature range (Witchas et al. 2001). Two 170 NMR signals with an intensity ratio of 1:1 were observed in Ca5(PO4)3OH which is known to contain three inequivalent oxygen sites in the ratio 1:1:2. Examination of the P-O distances shows however that two of the oxygen sites are very similar; the NMR observation is thus not inconsistent with the crystal structure (Wu et al. 1997). The P-O bond
170 N M R
385
Table 6.13. 170 NMR interaction parameters in other oxygen-containing systems. Sample A1PO4-5 A1PO4-11 A1PO4-17 Lt-Na3PO4 Cas(PO4)3OH, site 1 site 2 CaHPQa.2H20, site 1 site 2
KHzPO4 NH4HzPO4 CaCO3 LaSiOzN
La4Si207N2, site 1 site 2 La4SiA1OgN, site 1 site 2 site 3 LasSi3OlzN, site 1 site 2
~i..... (ppm)
• (MHz)
qq
Reference
61 63 63 ND 108 + 2 115 _+ 2 98 _+ 2 96 +_ 2 92 _+ 2 93 + 2 204 +_ 3 215 _+ 5 575 + 5 220 _+ 5 570 + 5 311 +_ 5 246 + 5 596 +_ 5 184 _+ 5
5.7 5.7 5.6 4.6 4.0 _+ 0.2 4.1 _+ 0.2 4.2 +_ 0.2 4.3 + 0.2 5.2 _+ 0.2 5.1 _+ 0.2 6.97 _+ 0.03 2.4 --~0 2.4 --~0 1.8 3.1 < 0.1 1.9
0.0 0.0 0.0 0 0.00 + 0.10 0.10 _+ 0.10 0.00 + 0.10 0.00 _+ 0.10 0.55 + 0.10 0.55 +_ 0.10 0.98 + 0 . 0 2 ND ND ND ND ND ND ND ND
Timken et al. (1986a) Timken et al. (1986a) Timken et al. (1986a) Witchas et al. (2001) Wu et al. (1997) Wu et al. (1997) Wu et al. (1997) Wu et al. (1997) S m i t het al. (1995) Harris et al. (1992) Harris et al. (1992) Harris et al. (1992) Harris et al. (1992)
distances in the PO4 unit of CaHPO4.H20 are 1.69, 1.58, 1.69 and 1.34 A, providing an explanation of the two 170 NMR peaks in the ratio 1:3 observed in the spectrum of this compound (Wu et al. 1997). In KHzPO4 and NH4HzPO4 all the oxygens in the PO4 unit are crystallographically equivalent and only a single lVO NMR resonance was observed (Wu et al. 1997). The 170 NMR signals from the bo and nbo units in binary phosphate glasses can be distinguished, and in ternary phosphate systems a third signal from P-O-A1 and P-O-Pb units has been detected (Montagne et al. 2001). The value of Xo in CaCO3 is quite large (--~ 7 MHz) with an asymmetry parameter of nearly 1 (Smith et al. 1995). These NMR interaction parameters may be related to the tricoordinate COCa2 oxygen environment in this compound. 170 NMR signals were reported from the crystalline oxynitride ceramic yttrium and lanthanum sialons (Harris e t al. 1992), typically showing nbo signals but not bo, probably reflecting the nitrogen site preference. Purely ionic environments such as in the binary oxides were also observed. Data for the lanthanum sialons determined at two applied magnetic fields have been used to estimate the NMR interaction parameters. A XQ value of --~2.5 MHz was found for the nbo units (Table 6.13). The large 170 shift range in these compounds allows the signals from the nbo and ionic sites to be completely separated. The 170 shift of the ionic sites is very similar to that of the OLa4 site in La203 (Table 6.2).
386
Multinuclear Solid-State N M R o f Inorganic Materials
6.9. HYDROGEN-CONTAINING SAMPLES 6.9.1 C r y s t a l l i n e h y d r o x i d e s a n d o t h e r h y d r o g e n - c o n t a i n i n g m a t e r i a l s
The interaction of water with inorganic materials can have profound consequences for their structure. Much effort is devoted to understanding the resulting structure changes and their implications for the properties of materials. Since the interaction with water is a local atomic scale process involving oxygen, ~VO NMR is an ideal probe for such reactions. Hydroxyl groups play an important role in these processes. The NMR interaction parameters for well-defined crystalline hydroxide compounds are shown in Table 6.14. It should be noted that some of the early studies used NQR which provides no shift information. ~70 NMR was able to distinguish the signal from the SiOH group in sodium ilerite, which was well separated in both the 1D and 3Q MAS spectra from the signal associated with the Si-O-Si sites (Brenn et al. 2000). A signal at - 2 9 ppm with 16.5% of the total oxygen intensity was associated with the mobile intercalated water. The ~70 NMR interaction parameters for the silanol group in KHSi205 were found to be very similar to those of other nbo units in silicates (Oglesby et al. 200 l) but XQ is much smaller than in many of the hydroxy compounds in Table 6.14. The decrease in XQ is probably due to the "sharing" of a particular oxygen between the proton and the metal atoms in the system. Relatively small • values have also been found in calcium silicate gels (Cong and Kirkpatrick 1996). In organic systems, XQ is known be very strongly dependent on Table 6.14. 170 NMR interaction parameters from hydroxide groups in crystalline materials. Sample
8i..... (ppm)
XQ(MHz)
~1
Reference
NaOH* KOH* LiOH Mg(OH)2
ND ND ND 20 25 25 + 1 -25 62 71 ND ND 0 25 + 3 25 + 3 61.2 60 10 37
7.59 7.14 7.01 6.8 6.8 6.8 + 0.1 7.25 6.5 6.5 6.55 7.27 7.3 6.6 _+ 0.1 7.0 _+ 0.1 5.10 3.5 4.1 4.4
0.07 0.08 0.05 0.0 0.0 0.0 + 0.1 0.0 0.0 0.3 0.12 0.08 0.0 0.1 _+ 0.2 0.2 _+ 0.2 0 0.35 0.6 0.0
Poplett (1982) Poplett (1982) Poplett (1982) Walter et al. (1988) van Eck & Smith (1998) Ashbrooket al. (2001) van Eck & Smith (1998) Wu et al. (1997) Cong & Kirkpatrick (1996) Poplett (1982) Poplett (1982) Walter et al. (1988) Ashbrooket al. (2001) Ashbrooket al. (2001) Brenn et al. (2000) Oglesby et al. (2001) Walter et al. (1988) Walter et al. (1988)
Mg(OH)x(OCH3)2_• Ca(OH)2 Ba(OH)2* Sr(OH)2* MgzSi4010(OH)2
chondrodite clinohumite Sodium ilerite KHSi205 (C6Hs)3SiOH Silica gel * - data r e c o r d e d at 77 K.
170 N M R
387
the degree of hydrogen-bonding. A detailed 170 3Q MAS NMR study using variable fields has been reported for chondrodite and clinohumite (hydrated forms of forsterite). An assignment was made of all the peaks, including that of SiOH, which showed a significantly larger XQ value and was clearly identified by means of a ~H ~ 170 CP experiment (Ashbrook et al. 2001). A correlation between ~iso,cs and the Si-O bondlength has been suggested on the basis of these assignments.
6.9.2 Hydrous gels and glasses The effect of hydration on the network structure of hydrous silicates and aluminosilicate glasses is much debated. NMR has played an important role in gathering experimental data on this problem but much of the evidence is indirect. However, since the interaction occurs via oxygen, 170 NMR can potentially provide direct information about the processes occurring. A b initio calculations on silicate structures suggest that the XQ values for silanol groups should be in the range 6.9-8.3 MHz with ~i~o,c~values in the range 14.2-27.9 ppm, and that XQ decreases as the O-H-O distance decreases (Xue and Kanzaki 1998). It should be noted that these calculations did not take into account the effect of additional cation interactions. In pure alkali silicate glasses a significant loss of the 170 nbo signal is observed upon hydration (Xu et al. 1998, Maekawa et al. 1998, Oglesby et al. 2001). The data for KHSi205 clearly show that this is due to a strong overlap of the Si-O-(H,M) peak with the bo signal (Oglesby et al. 2001). Very small changes in the 170 3Q-MAS NMR spectra of sodium aluminosilicate glasses have been analysed for evidence of disruption by water of the aluminosilicate framework. Both the 1D and MQ MAS 170 NMR spectra of dry and hydrous aluminosilicate glasses with the same composition show a striking similarity. In albite glass, the Si-O-Si:Si-O-A1 ratio remained 1:1 within experimental error (Kohn et al. 1997), agreeing with the findings of Maekawa et al. (1998) who were unable to detect any significant change in the 170 MAS spectra of dry and hydrous albite. 170 NMR of sanidine (KA1Si308) also shows that the hydration state has no significant effect on the 1D MAS data, and the projection of the isotropic slice in the Si-O-Si region of the 3Q MAS spectrum showed no additional intensity in this region, as would be expected from the silanol group in KHSi205 (Figure 6.29) (Oglesby et al. 2001). The main conclusion from these results is that although water strongly interacts with the framework of silicate glasses it does not change fully polymerised aluminosilicate glasses. Although both 1H and 298i MAS NMR of silica gel indicate the presence of significant Si-OH intensity, the presence of silanol groups was difficult to discern from the 170 spectra (van Eck et al. 1999). By using echoes with very short delays and determining the relaxation time, a liquid-like signal with essentially no quadrupole interaction was detected. An MQ MAS NMR experiment with a very short z-filter delay
388
Multinuclear Solid-State NMR of Inorganic Materials
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.
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,
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,
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,
i
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,
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MAS dimension (ppm) Figure 6.29. MAS projection of the Si-O-Si peak in the 170 MQMAS NMR spectrum of dry and hydrous sanidine (KA1Si3Os) glass. Note the lack of additional intensity in the spectrum of the hydrous glass (denoted by the broken line) indicating the absence of significant silanol concentration. From Oglesby et al. (2001) by permission of the Mineralogical Society of America. showed a weak additional signal with X Q " " 3 MHz, indicating that this silica gel contained a distribution of hydroxyl sites. Since the majority of these were very mobile, with T~--~ 0.1 ms, the quadrupole interaction is averaged away. A small subset of oxygen sites showed a larger value of • (van Eck et al. 1999).
6.10. H I G H T E M P E R A T U R E C E R A M I C S U P E R C O N D U C T O R S
The oxide-based ceramic superconductors first discovered in 1986 have been a fruitful area for study by ~70 NMR. Oxygen is at the core of these structures and plays a key role in the superconductivity. Large ~70 shifts are found between the different oxygencontaining sites, providing good resolution even from static spectra. Changes in the oxygen shift and relaxation times provide important information about the charge correlations and dynamics of the system and how these change with temperature, but a detailed discussion is beyond the scope of this book. A comprehensive review of the application of NMR (including 170) to ceramic superconductors has been given by Rigamonti et al. (1998). 170 NMR spectra have been reported from Lal.85Sro.15CuO4-x (Went and Reimer 1990, Kitaoka et al. 1989), YBa2Cu307-x (Oldfield et al. 1989, Hammel et al. 1989, Horvatic et al. 1989, Takigawa et al. 1991), T12Ba2CaCu208
389
~70 N M R
(Oldfield et al. 1989) and Bi2Sr2Can_lCUnO4+2n where n = 1,2 and 3 (Oldfield et al. 1989, Trokiner et al. 1990, 1991, Reven et al. 1991, Dupree et al. 1991, Howes et al. 1992). Typical static 170 NMR spectra show a broad asymmetric peak with a shift of 1500-2000 ppm, as well as narrower peaks much closer to the shift range of the related diamagnetic oxides. By comparison with structurally similar compounds, the resonance with the large shift is attributed to the oxygen in the CuO2 planes (Figure 6.30A) (Oldfield et al. 1989). Simulation of the different sites in such materials suggests that the CuO2 plane has a significant quadrupole interaction but is dominated by shift anisotropy which is related to the charge carriers in the system (i.e. a Knight shift). The 170 NMR spectrum of TlzBazCaCu208 shows, in addition to the resonance from the CuO2 planes, two overlapping resonances in the diamagnetic region with shifts of 315 and 350 ppm and • values of 8.8 and 5.35 MHz respectively. These were assigned to the T10 and BaO layers (Oldfield et al. 1989). A 170 NMR study of the bismuth-containing n = 3 phase made at four applied magnetic fields has allowed the different interactions to be deconvoluted. Three separate A
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Figure 6.30. A. 170 NMR spectra of various high-Tc oxide superconductors. The asterisk marks I(1) sites in a small population of aligned crystallites. From Oldfield et al. (1989) by permission of the copyright owner. B. Observed and simulated 170 NMR spectrum of Bi2Sr2Ca2Cu3Olo superconductor showing the individual fitted components of the spectrum. From Dupree et al. (1991) by permission of Elsevier Science.
390
Multinuclear Solid-State NMR of lnorganic Materials
resonances were identified; one had a XQ value of 4.6 MHz, a shift of 265 ppm and a shift anisotropy < 50 ppm The second resonance, at 1285 ppm, showed a Gaussian lineshape and the third resonance had a • value of 2.8 MHz, a shift of 1590 ppm and a shift anisotropy of 1300 ppm (Figure 6.30B) (Dupree et al. 1991). All the bismuthcontaining phases showed only one distinct resonance in the diamagnetic region, although signals would be expected from the BiO and SrO layers by analogy with the thallium analogue. It was suggested (Trokiner et al. 1990) that incommensurate modulation of the BiO layers broadened the ~70 signal beyond detection. In the n = 3 phase there are three CuO2 layers, two outer and an inner plane with a much smaller anisotropy (Dupree et al. 1991). The anisotropy in these materials is due to polarisation of the 2p,~ orbitals on the oxygen. This anisotropy is caused by either dopant holes in the structure or the interaction between these 2p~ orbitals and the copper 3d~x2 y2) orbitals. The higher symmetry of the central plane presumably reduces the mixing of the orbitals. On cooling these samples, the CuO2 planes show large decreases in their oxygen shift due to the change of the carrier density in these planes, indicating that almost all the shift at room temperature is due to charge carriers and that the chemical shift contribution is small. In the n = 3 bismuth phase, the shifts of the two CuO2 planes show different temperature dependences, indicating that a common spin susceptibility cannot be used to describe the system (Howes et al. 1992). This is in contrast to the yttrium-based superconductor which seems to show "one-spin fluid" behaviour (Barriquand et al. 1991) although recent studies up to 700 K suggest that oxygen, yttrium and copper show contrasting behaviour (Nandor et al. 1999). These results emphasise the complexity of the electronic properties of the superconducting materials and the usefulness of multinuclear NMR in their study. -
-
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170 NMR
395
Went, M.S. & Reimer, J.A. (1990) Chem. Mater., 2, 389. Winkler, B., Blaha, P. & Schwarz, K. (1996) Amer. Mineralogist, 81, 545. Witchas, M., Eckert, H., Freiheit, H., Putnis, A., Korus, G. & Jansen, M. (2001) J. Phys. Chem. A, 105, 6808. Wu, G., Rovnyak, P.C., Huang, P.C. & Griffin, R.G. (1997) Chem. Phys. Lett., 277, 79. Xu, Z. & Stebbins, J.F. (1998) Geochim. Cosmochim. Acta, 62, 1803. Xu, Z. & Stebbins, J.F. (1998) Solid State Nucl. Mag. Reson., 11, 243. Xu, Z., Maekawa, H., Oglesby, J.V. & Stebbins, J.F. (1998) J. Amer. Chem. Soc., 120, 9894. Xue, X., Stebbins, J.F. & Kanzaki, M. (1994) Amer. Mineralogist, 79, 31. Xue, X. & Kanzaki, M. (1998) Phys. Chem. Minerals, 26, 14. Xue, X. & Kanzaki, M. (1999) J. Phys. Chem. B, 103, 10816. Xue, X. & Kanzaki, M. (2000) Solid State Nucl. Mag. Reson., 16, 245. Yang, S., Park K.D. & Oldfield, E. (1989) J. Amer. Chem. Soc., 111, 7278. Youngman, R.E., Haubrich, S.T., Zwanziger, J.W. Janicke, M.T. & Chmelka, B.F. (1995) Science, 269, 1416. Zhao, P., Kroeker, S. & Stebbins, J.F. (2000) J. Non-Cryst. Solids, 276, 122. Zhao, P., Neuhoff, P.S. & Stebbins, J.F. (2001) Chem. Phys. Lett., 344, 325.
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Chapter 7
NMR of Other Commonly Studied Nuclei 23Na NMR 7.1.1 General Considerations 7.1.2 23NaNMR Spectra of Sodium Compounds 7.1.3 Relationships between the 23Na Chemical Shift and Structural Parameters 7.1.4 23NaNMR of Crystalline Materials 7.1.5 23NaNMR Studies of Thermal Reactions 7.1.6 23NaNMR of Glasses 7.1.6.1 Silicate and Aluminosilicate Glasses 7.1.6.2 Sodium Borosilicate Glasses 7.1.6.3 Sodium Borate, Germanate and Tellurite Glasses and Melts 7.1.6.4 Phosphate Glasses 7.1.6.5 Miscellaneous Glass Studies 7.1.7 23NaNMR of Zeolites 7.2. liB NMR 7.2.1 General Considerations 7.2.2 11B NMR of Crystalline Compounds 7.2.3 I~B NMR of Glasses 7.2.4 I~B NMR of Zeolites 7.3. 31p NMR 7.3.1 Relationships between 3~p NMR Parameters and Structure 7.3.2 3~p NMR of Glasses 7.3.2.1 Binary Phosphate Glasses 7.3.2.2 Phosphosilicate Glasses 7.3.2.3 Alkali Borophosphate Glasses 7.3.2.4 Borosilicophosphate Glasses 7.3.2.5 Phosphoaluminosilicate Glasses 7.3.2.6 Alkali Phosphoaluminoborosilicate Glasses 7.3.2.7 Phosphorus Chalcogenide Glasses 7.3.3 31p NMR of A1PO4 Molecular Sieves 7.3.4 3lp NMR of Biomaterials References 7.1.
399 399 399 403 406 412 413 413 414 415 415 416 418 420 420 421 424 431 432 438 441 441 443 445 445 446 447 447 448 450 452
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Chapter 7
NMR of Other Commonly Studied Nuclei The most important nuclei from the point of view of materials science are 29Si, 27A1 and 170. These have been dealt with individually in chapters 4, 5 and 6 respectively. There are, however, several other nuclei whose importance to materials science has led to a significant number of published NMR studies. These are the quadrupolar nuclei 23Na and 11B and the spin I = 1/2 nucleus 31p, and are covered in this chapter.
7.1. 23NaNMR
7.1.1 General considerations Sodium is an important element commonly occurring in many inorganic materials, especially glasses and minerals. 23Na is a quadrupolar nucleus with a non-integer spin (I = 3/2) and is thus subject to quadrupole effects displacing resonances from their isotropic chemical shift (undistorted) position. 23Na is similar to 27A1 in having a large quadrupole moment, 100% natural abundance and a similar Larmor frequency but because its nuclear spin is smaller than that of 27A1, the second-order quadrupolar width of 23Na is larger for the same site distortion. As with 2VA1these drawbacks can often be minimised by working at higher magnetic fields and by the use of magic angle spinning, which may, however, narrow the second-order quadrupolar broadening less than for 27A1 under the same conditions. Other special techniques such as double rotation (DOR), dynamic angle spinning (DAS) and multiple-quantum (MQ) experiments (see Chapter 3) can all be used to improve the resolution of the quadrupolar 23Na spectra. Cross-polarisation (CP) experiments between 1H and 23Na using NaBH4 as a convenient material for establishing the Hartmann-Hahn condition have been demonstrated for Na2B2OT.10H20 (borax) and the layer lattice mineral Na20.22SiO2.10H20 (kenyaite) (Harris and Nesbitt 1988). Solid NaC1 is a convenient secondary shift reference at 7.2 ppm relative to dilute aqueous NaC1.
7.1.2 23Na NMR spectra of sodium compounds The 23Na MAS NMR spectra of a number of simple sodium compounds show narrow resonances resulting from the very symmetrical Na environments with XQ values which are virtually zero (Table 7.1). Other compounds with |ess symmetrical Na environments show well-defined 23Na second-order quadrupolar lineshapes which can readily be simulated to give the quadrupolar interaction parameters (Table 7.1). The 399
400
Multinuclear Solid-State NMR of Inorganic Materials
Table 7.1. 23Na interaction parameters for sodium compounds. Compound
~iso*
Na20 Na202 Na3OC1 NaOH NaOH.H20 NaF
55.1 6.9 ND 12.2, 21 5.0 7.9, 7.2
NaCI NaBr NaI
-
NaC104 NaC104.H20 site 1 site 2 NaIO4 NaA102 NazSO4 Na2CrO4 site 1 site 2 NaNO3 NaHCO3 NaA1CO3(OH)2 dawsonite Na3P309 site 1 site 2 Na4PeO7.10H20 Na2HPO4 site 1 site 2 site 3 NaH2PO4.2HeO Na H2PO4.H20 NaN3 NazS o~-NazSi205 [3-Na2Si205 site 1 site 2 Na2SiO3
xl
Reference
---0 0.47 11.34 3.50, 3.6 2.20 ND
ND ND 0 0, 0.07 0.70 ND
7.9, 7.2
ND
ND
6.0, 5.2
ND
ND
2.7,
ND
ND
-25.5
0.8
0.35
K16sters & Jansen (2000) Bastow (1994) K16sters & Jansen (2000) Koller et al. (1994), Dec et al. (1990) Koller et al. (1994) Tabeta & Saito (1984), Dec et al. (1990) Tabeta & Saito (1984), Dec et al. (1990) Tabeta & Saito (1984), Dec et al. (1990) Tabeta & Saito (1984), Dec et al. (1990) Koller et al. (1994)
- 11.7 - 12.4 - 12.5 19.00 -8.5 -20.00 - 13.90 -7.3 -5.4 2 - 14.80 -5.60 1.5
1.71 1.48 0.0368 2.15 2.60 2.78 0.5** ND ND 3.64 2.20 1.57 ND
0.20 0.10 ND 0.60 0.58 0.57 0"* ND ND 0.56 0.70 0.55 ND
Koller et al. (1994) Koller et al. (1994) Tabeta & Saito (1984) Koller et al. (1994) Koller et al. (1994) Koller et al. (1994) Koller et al. (1994) Tabeta & Saito (1984) Tabeta & Saito (1984) Bastow et al. (1995) Koller et al. (1994) Koller et al. (1994) Tabeta & Saito (1984)
0.21, 0.18 0.69, 0.7 0.27, 0.26 -4.8 - 10.69 -3.5 49.8 16.9, 17.40
-0.9, -1.4 - 1.6, -2.5 0.14, -1.1 1.19 ND ND ND 1.79, 1.82
0.258, 0.42 0.71, 0.65 1.234, 1.178 0.46 ND ND ND 1, 1
Baldus et al. (1995), Lim & Grey (1998) Baldus et al. (1995), Lim & Grey (1998) Baldus et al. (1995), Lim & Grey (1998) Koller et al. (1994) Koller et al. (1994) Tabeta & Saito (1984) Dec et al. (1990) Xue & Stebbins (1993), Koller et al. (1994)
15.6, 20.4 9.4, 8.30 23, 15.45
2.29, 2.50 2.2, 2.22 ND, 1.46
0.85, 0
Xue & Stebbins (1993), Koller et al. (1994) Xue & Stebbins (1993), Koller et al. (1994) Xue & Stebbins (1993), Koller et al. (1994)
-
•
3.1
(MHz)
0.55, 0.55 ND, 0.88
401
NMR of Other Commonly Studied Nuclei
Table 7.1. (Continued) Compound
~iso*
Na2SiO3.9H20 3.7 Na2SiO2(OH)2.8H20 -3.67 NazS iO2(OH)2.7H20 site 1 - 0.94 site 2 - 0.74 NazSiOz(OH)z.5H20 site 1 - 1.50 site 2 - 7.20 Na2SiOz(OH)z.4H20 site 1 1.80 site 2 2.30 NazO.4SiOz.5H20 site 1 0 site 2 1 site 3 8 NaZrO3 site 1 15.0 site 2 27.0 site 3 19.5 o~-NaVO3 site 1 - 15.6 site 2 - 4.8 [3-NaVO3 - 10.3 NazWO4 4.5 NazWO4.2H20 site 1 - 0.9 site 2 6.3 Na2MoO4 3.2 NazMoO4.2H20 site 1 - 1.4 site 2 4.0 NazGeO3 22.6 Na4Ge902o - 2.1 NazGe409 - 6.4 NaBO2 1.8 NaBOz.2H20 10.6 Na4B205 site 1 19.6 site 2 14.4 NazO.4B203 site 1 3.4 site 2 - 11.4 NazB405.(OH)4.8H20 (borax single crystal) site 1 ND site 2 ND NazSnO3.3H20 12.8 NazTe409 site 1 - 3 site 2 5 Na2TeO3 site 1 5.8 site 2 17.0
XQ (MHz)
qq
Reference
1.11 1.14
0.63 0.5
Hayashi (1994) Koller et al. (1994) Koller et al. (1994)
2.56 0.81
0.59 0.77
1.35 2.01
0.45 0.84
1.80 2.83 1.3 1.5 1.4 2.52 2.08 4.20
0.75 0.17 0.6 0.4 0.6 0.67 0.05 0.27
Koller et al. (1994) Hanaya & Harris (1997) Hanaya & Harris (1997) Hanaya & Harris (1997) Bastow et al. (1994) Bastow et al. (1994) Bastow et al. (1994)
1.50 0.765 1.42 2.49
0.58 0.06 0.27 0
Skibsted et al. (1993) Skibsted et al. (1993) Skibsted et al. (1993) Skibsted & Jakobsen (1994)
0.88 2.7 2.59
0.35 0.09 0
Skibsted & Jakobsen (1994) Skibsted & Jakobsen (1994) Skibsted & Jakobsen (1994)
0.875 2.68 1.3 2.7 2.4 1.2 1.53 2.2 3.0 4.1 2.0
0.23 0.08 0.8 0.54 0.7 0.09 0.80 1.0 0.5 0.18 0.35
Skibsted & Jakobsen (1994) Skibsted & Jakobsen (1994) George et al. (1997) George et al. (1997) George et al. (1997) George et al. (1997) Hayashi (1994) George et al. (1997) George et al. (1997) George et al. (1997) George et al. (1997)
0.541 0.849 1.76
0.449 0.143 0
Cuthbert & Petch (1963) Cuthbert & Petch (1963) Hayashi (1994)
4.4 3.6
0.08 0.12
Tagg et al. (1994) Tagg et al. (1994)
1.84 1.36
0.8 0.9
Tagg et al. (1994) Tagg et al. (1994)
Koller et al. (1994) Koller et al. (1994) Koller et al. (1994) Koller et al. (1994)
402
Multinuclear Solid-State NMR of Inorganic Materials
Table 7.1. (Continued) Compound
~iso*
XQ (MHz)
xl
Reference
2.24 5.90 2.00 1.55 3.30 2.69, 2.60
0.37 0.10 0.19 0.16 0.25 0.25
Hayashi (1994) Koller et al. (1994) Koller et al. (1994) Koller et al. (1994) Xue & Stebbins (1993) George & Stebbins (1995), Kundla et al. ( 1981), Kirkpatrick et al. (1985), Phillips et al. (1988)
25.0 5.4
2. l0 2.96
0.75 0.10
Xue & Stebbins (1993) Xue & Stebbins (1993)
- 4.5 - 3.0
1.65 4.70
0.25 0
Bastow et al. (1996) Bastow et al. (1996)
0 2.0
1.50 2.8
1.0 0.85
Bastow et al. (1996) Bastow et al. (1996)
- 7.0 - 1.0
2.05 2.65
0.50 0.75
Bastow et al. (1996) Bastow et al. (1996)
- 7.0 9.0 7.66
2.30 3.10 1.41
0.85 0.30 0.44
Bastow et al. (1996) Bastow et al. (1996) Skibsted et al. (1995)
0 - 4.3 - 9.8
2.67 2.57 2.72
0.34 0.47 0.59
Abrahams et al. (2000) Abrahams et al. (2000) Abrahams et al. (2000)
0 - 4.1 - 9.9
2.50 2.66 2.67
0.38 0.51 0.59
Abrahams et al. (2000) Abrahams et al. (2000) Abrahams et al. (2000)
ND ND - 0.2 - 0.2 - 0.6 - 9 - 13 - 9
2.3 2.5 ND ND ND ND ND ND
0 0 ND ND ND ND ND ND
Clayden & Pugh (1998) Clayden & Pugh (1998) Tabeta & Saito (1984) Tabeta & Saito (1984) Tabeta & Saito (1984) Tabeta & Saito (1984) Tabeta & Saito (1984) Tabeta & Saito (1984)
- 9.6 - 6.10
1.30 0.77
0.82 10.77
Koller et al. (1994) Koller et al. (1994)
Na2TeO4.2H20 12.5 Na8[A1SiO4]6 3.00 Na8[A1SiO4]6(OH)2 - 4.00 Na8[A1SiO4]6(OH)z.2H20 - 8.40 NaA1Si206 11.0 NaA1Si308 (albite) - 7.1, - 6.8, 7.3
Na2BaSi206 site 1 site 2 Na2ZrSi4Ol l site 1 site 2 Na 2ZrSi207 site 1 site 2 NazZrSi6015.3H20 site 1 site 2 NaaZr2Si3Ol2 site 1 site 2 Na4A1BeSi401 zC1 NaMg(PO3)3 site 1 site 2 site 3 NaZn(PO3)3 site 1 site 2 site 3 NaSn2(PO4)3 site 1 site 2 Na oxalate 3H20 Na formate Na tartrate Na citrate 2H20 Na citrate Na glutamate Na maleate H20 site 1 site 2
* chemical shifts with respect to aqueous NaCI ** Gaussian lineshape
NMR of Other Commonly Studied Nuclei
403
values of ~iso for 23Na compounds cover a reasonably wide range, suggesting that they should be capable of yielding structural information. A study of a small number of model sodium compounds by Dec et al. (1990) indicates that the isotropic chemical shifts (8i~o) of compounds with Na in octahedral coordination fall in the range - 3.0 to 7.2 ppm with respect to aqueous NaC1, while a tetrahedral Na compound (Na2S) had a giso value of 49.8 ppm. The five-fold coordinated sodium in NaOH has a Siso value of 21.2 ppm.
7.1.3 Relationships between the 23Na chemical shift and structural parameters An objective of several 23Na NMR studies has been to derive useful relationships between a measured NMR parameter (chemical shift or XQ) and some parameter related to the structure. Such relationships can potentially provide unique information about the local bonding environment of the sodium. Possible trends have been noted between the 23Na chemical shift and the Na coordination number (Xue and Stebbins 1993, Dec et al. 1990, Dirken et al. 1992), the Na-O distance or the number of non-bridging oxygens per tetrahedral cation (Xue and Stebbins 1993). Good correlations have been found between ~iso and the mean Na-O bond length for groups of similar compounds, including crystalline silicates and aluminosilicates (Xue and Stebbins 1993, George and Stebbins 1995), crystalline germanates, borates and carbonates (Georetale et al. 1997) and the sodium aluminium fluorides cryolite and chiolite (Stebbins et al. 1992). Although these relationships, shown in Figure 7.1A, do not A
B
- - - i
. . . .
i
. . . .
|
. . . .
i
. . . .
u . . . .
|
. . . .
J . . . .
~ . . . .
|
. . . .
~borates
20
20 ~o
m,
o 0
\ \
",~
..,.
:: . \ - . .
germanates g O n g o
c,O
z
e~ e,i
-20
x fluorides ~
"carbonates silicates~ "
e,i
-20 ,
2.2
2.6
3.0
Mean Na-O length (A)
0.5
r
,
0.7
,
019
1.1
Shift parameter A
Figure 7.1. A. Relationships between the 23Na ~iso values for a number of sodium compound groups and the mean Na-O bond length, from Stebbins (1998) by permission of Elsevier Science. B. Relationship between 3iso for a range of sodium oxy-compounds and the 23Na shift parameter A, defined by Equation 7.8. From K611er et al. (1994) by permission of the American Chemical Society.
404
Multinuclear Solid-State NMR of Inorganic Materials
coincide, they indicate that for each ligand there is an obvious decrease in ~iso with increasing site size. The regression lines for each group of compounds are: ~iso(ppm) = - 67 (Na-O A) + 179 (silicates)
(7.1)
8iso(ppm) = - 47 (Na-O/k) + 130 (germanates)
(7.2)
8iso(ppm) = - 144 (Na-O ,~) + 366 (borates)
(7.3)
8iso(ppm) = - 66 (Na-O .~) + 159 (carbonates)
(7.4)
8iso(ppm) = - 70 (Na-O ,~) + 163 (fluorides)
(7.5)
Correlations between 23Na 8iso values and the ratio of non-bridging oxygens to tetrahedral network-forming cations (NBO/T ratio) have been noted in oxide glasses and liquids containing only Na as the network modifier. These measurements, which are made at high temperatures at which the rapid cation motion simplifies the measurement of ~iso by completely averaging the NMR peak shapes, include silicate and aluminosilicate liquids (Xue and Stebbins 1993, George and Stebbins 1996) and germanate liquids (George et al. 1997). Similar relationships are maintained in binary silicate glasses at room temperature, for which the values of ~iso were extracted from analysis of the second-order quadrupolar lineshapes obtained at multiple magnetic fields (Gee et al. 1997). In a series of sodium tellurite glasses, for which the values of ~iso were obtained from dynamic angle spinning (DAS) experiments, a similar relationship between 8iso and the NBO/T ratio was observed (Tagg et al. 1995). The relationship between 23Na 8iso and the average size of the Na site becomes more complicated in mixed alkali silicate and borate glasses, in which the 23Na 8iso value becomes less positive when Li is substituted for Na, and more positive on substitution of K for Na (Gee et al. 1997, Ratai et al. 1998). Based on the behaviour of the binary glasses, these results would seem to suggest that the average size of the sites occupied by a given cation increases with partial substitution by a smaller cation and decreases with partial substitution by a larger cation; other factors must therefore be operating, e.g. competition among unlike cations for negatively charged oxygen sites (Stebbins 1998). Koller et al. (1994) have developed an empirical chemical-shift parameter which takes into account the bond length of the Na-O coordination sphere and the bond valence Sij between two atoms i and j, defined as Sij = exp[ro - r0/B]
(7.6)
405
NMR of Other Commonly Studied Nuclei
where rij is the bond length between the Na (atom j) and O (atom i), ro is the length of a bond with unit valence (Brown and Altermatt 1985) and B = 0.37 is a constant. The bond valences around an O atom are summed to give the atomic valence Wi and a shift parameter A is then defined as the sum of all the contributions of oxygens within 3.4A of the Na atom: A-
X(Wi/ri 3)
(7.7)
The assumption in Equation 7.7 that the decrease in the effect of the oxygens on the 23Na chemical shift shows a 1/r3 dependence with the Na-O bondlength was made on the basis of the best fit to the experimental data for a number of Na compounds (Koller et al. 1994) (Figure 7.1B). The resulting linear relationship between A and the chemical shift 6 is given by --
133.6A + 107.6
(7.8)
This relationship has been derived for a variety of oxyanion ligands, and provides a useful approximation for a wide range range of compounds, but is a relatively poor predictor of detailed behaviour within smaller compound groups, such as the anhydrous silicates. Relationship (7.8) has also proved unable to explain the effect of bond A
B ~~]
-7
,k"l
T (~
....
~
I ....
II'"l
....
I ....
I ....
I ''~
/KSlleretal.
',/"~,,
50
I
/Xue & Stebbins
e,i
258
/
~~310 , ~ ~ 20
-20
/
352 -60
23Na shift (ppm) w.r.t. NaCl soln.
-9
,/,~l ....
0
I,,,tl,,,,I,,,,
I~,,,I
,
200
9
400
Temperature (~
Figure 7.2. A. 23Na MAS NMR spectra of NaA1Si308 (albite) at various temperatures. B. 23Na isotropic chemical shifts of albite derived from the second-order quadrupolar lineshapes and plotted as a function of temperature. The solid line is the shift predicted from the bond length relationship of Xue and Stebbins (1993), the broken line is the shift predicted by the bond valence relationship of K611er et al. (1994). From George and Stebbins (1995), by permission of the Mineralogical Society of America.
406
Multinuclear Solid-State NMR of lnorganic Materials
changes with temperature in crystalline albite (NaA1Si3Os) (George and Stebbins 1995). In this compound (Figure 7.2), the increase in the Na-O bond lengths in samples heated from 25 to 352~ is accompanied by a decrease in ~iso consistent with the simple bond-length relationship of Xue and Stebbins (1993). However, Equation (7.8) wrongly predicts a change in ~iso of several ppm in the opposite direction (Figure 7.2B). This suggests that Equation 7.8 should be used with caution. A linear relationship has been demonstrated by Clayden and Pugh (1998) between the quadrupole coupling constant • and the ionic radius r (in nm) of the M ions in the small group of sodium metal(IV) phosphates NaM2(PO4)3 where M = Sn, Ge, Ti and Zr. The two resolvable Na sites of NaSn2(PO4)3 straddle the line for these compounds, defined by: NQ ~--- - - 4 3 . 9 r
+ 5.50
(7.9)
7.1.4 23Na N M R of crystalline materials Frequent questions arising in studies of crystalline materials concern the number of atomic sites within the structure and the details of the atomic environment. 23Na NMR is useful for answering such questions in sodium compounds, but the results can be complicated by the overlap of the quadrupolar lineshapes often encountered in compounds with more than one Na site. Thus, the 23Na MAS NMR spectrum of Na2ZrO3 (Figure 7.3) shows that the central transition has a complex lineshape composed of the overlapping second-order lineshapes from the three Na sites present. Because such a complex spectrum can be simulated in a variety of ways, the spectra were acquired at three different magnetic fields, allowing the correct set of simulation parameters for each site (those giving the best fit for all fields) to be identified (Bastow et al. 1994). As a further check on the correctness of this simulation, the combination of the constituent lineshapes arising from the three Na sites was in the intensity ratio 2:1:1, consistent with the known Na site occupancies in this structure. The simulations of the individual sites enables their NMR interaction parameters to be extracted (Table 7.1). Although all the Na in this structure is in octahedral coordination with oxygen, the larger • value of site 3 (with the widest separation of the lineshape) shows that it is considerably more distorted than sites 1 and 2, allowing its identification with the structural site known to have the greatest difference in Na-O bond lengths (Bastow et al. 1994). Similar techniques have been used to distinguish and assign some of the sodium sites in crystalline Na2ZrSiO5 and the related minerals vlasovite (Na2ZrSi4Oll), parakeldyshite (Na2ZrSi207) and elpidite (Na2ZrSi6015.3H20) (Bastow et al. 1996). The anhydrous and hydrated phases of Na2MoO4 and Na2WO4 have been studied by 23Na MAS NMR (Skibsted and Jakobsen 1994). The hydrates contain two distinguish-
NMR of Other Commonly Studied Nuclei
407
Na2ZrO3 11.7 T
~ t ~
simulated
site 1 _ site2 site3 |
50
I
I
0
I
I
I.
-50
23Na shift (ppm) w.r.t. NaCl soln. Figure 7.3. 23NaMAS NMR spectrum of NaZrO3 acquired at 11.7 T, with the simulated spectrum composed of three constituent second-order quadrupolar lineshapes combined in the ratio 2:1:1. Note that the relative intensities of the individual components as shown are not those used in the final simulation. From Bastow et al. (1994), by permission of the copyright owner. able Na sites which were resolved and their quadrupolar parameters determined by analysing both the central and satellite sideband transitions, the correctness of the simulations being checked by making the measurements at two different magnetic fields. Simulation of the NMR spectra can be used quantitatively to evaluate the relative proportions of anhydrous and hydrated phases in a mixture, and thus provides a means of monitoring the hydration or dehydration process (Skibsted and Jakobsen 1994). The crystal structures of the sodium tellurites Na2Te409 and Na2TeO3 are of importance to the understanding of the technically useful sodium tellurite glasses. Both compounds contain two crystallographically distinct Na sites which have been resolved by variable-angle spinning and double angle spinning (DAS) experiments (Tagg et al. 1994). The resulting quadrupolar parameters have been used to provide confirmation of some of the details of the crystal structures. Cross-polarisation (CPMAS) experiments have been used to study and simulate the quadrupolar lineshapes of several hydrated compounds, including Na2TeO4.2H20, Na2SnO3.3H20, Na2SiO3.9H20 and NaBO2.2H20 (Hayashi 1994). 23Na MAS NMR has provided useful information about the coordination of the sodium ions in the various structural modifications of sodium phyllosilicate, Na2Si2Os,
408
Multinuclear Solid-State NMR of lnorganic Materials
allowing two non-identical Na sites to be identified in the interlayer space of the ~-polymorphic form (Heidemann et al. 1992). The NMR parameters of these two sites suggest that in one the sodium is 6-coordinated to oxygen, and is 5-coordinated in the other. The 23Na MAS NMR spectrum of the hydrous layered silicate compound Na20.4SiOz.5H20 (makatite) is broadened and distorted due to the superposition of the signals from Na in three different structural environments, but these have readily been resolved and their quadrupolar parameters determined using MQMAS NMR (Figure 7.4) (Hanaya and Harris 1997). The resolution in the isotropic dimension was found to be significantly improved by using ~H decoupling (Figure 7.4B). 23Na MQMAS NMR has been used to refine the structures of the crystalline compounds NaMg(PO3)3 and NaZn(PO3)3 (Abrahams et al. 2000). The X-ray scattering of Na and Mg is too similar to allow these sites to be distinguished by X-ray diffraction, but the structure of the compound NaZn(PO3)3 was successfully determined by MQMAS NMR. Comparison of the 23Na MQMAS NMR spectra of both compounds (Figure 7.5) shows that each compound contains three similar sodium sites. This information allows the complex overlapping 1-dimensional MAS NMR spectra to be A
9
-
..~
'00i I
9
o
.m
50 0 -50 MAS dimension (t~) j / %
~
-100
~
4
.e I..
""
observed simulated
100-~ ~=2~,, .... 50 0 -50 MAS dimension (tO
200~ t.
t
....
r .
.
.
.
.
.
.
.
.
!
30 50 (ppm)
10 (ppm)
Figure 7.4. Two-dimensional triple-quantum 23NaMAS NMR spectra of makatite, Na20.4SiO2.5H20, showing the triple-quantum-filtered single-quantum MAS cross-sections (at right) projected on to the isotropic axis. A. Spectrum without 1H decoupling, B. Spectrum with 1H decoupling, showing the significant improvement in resolution allowing simulation of the crosssection lineshapes. From Hanaya and Harris (1997), by permission of copyright owner.
409
NMR of Other Commonly Studied Nuclei
A
B
NaMg(PO3)3
C),,,
\\
10
0
NaZn(PO3)3
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F i g u r e 7.5. 23Na MQMAS NMR spectra of A. NaMg(PO3)3 and B. NaZn(PO3)3, showing the resolution of the three similar Na sites in each compound. The broken line indicates the axis of the quadrupole-induced shift. From Abrahams et al. (2000), by permission of the Royal Society of Chemistry.
simulated and the NMR interaction parameters obtained, confirming that the compounds are isostructural, thereby establishing the cation distribution in the Na-Mg compound (Abrahams et al. 2000). Anhydrous NazHPO4 has been the subject of a number of 23Na studies because its three non-equivalent Na sites provide a good test of the resolving ability of various NMR techniques. 23Na MQMAS NMR experiments using Rotation-Induced Coherence Transfer (RIACT) have proved useful for determining the quadrupolar parameters of all three sites in both rotor-synchronised and non-rotor-synchronised experiments (Lim and Grey 1998). The compound NaA19014 is of both theoretical and practical interest because it is isostructural with the technically important engineering aluminosilicate ceramic mullite (A16Si2013) yet does not contain silicon. Some of the structural questions posed by the existence of this compound have been answered by its 23Na MAS and DOR spectra acquired at several magnetic fields from 8.45 to 16.9 T (Figure 7.6A,B) (MacKenzie et al. 2001). The combination of these NMR techniques shows the presence of two approximately equally populated non-identical Na sites (Figure 7.6C), in disagreement with the average structure deduced from a Rietveld X-ray analysis in which the two possible Na sites are identical and indistinguishable. This result illustrates the ability of NMR techniques to distinguish small variations in the environment of atoms which are not resolved in an averaged X-ray structure (MacKenzie et al. 2001). A similar experimental strategy involving a combination of 23Na MAS, DOR and MQMAS NMR at five different magnetic fields has been used to determine the NMR parameters of the four inequivalent Na sites in Na4P207 (Engelhardt et al. 1999). These
410
Multinuclear Solid-State NMR of Inorganic Materials
A MAS
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Figure 7.6. A. 23NaMAS NMR spectrum of the mullite-structured compound NaAl9014 acquired at 8.45 T. B. 23NaDOR spectrum of the same compound acquired at 8.45 T. C. Observed and simulated 8.45 T MAS NMR spectrum of NaA19O14showing the separate simulated lineshapes of the two crystalline Na sites. The Gaussian lineshape on the left is from the residual amorphous precursor gel phase. From MacKenzie et al. (2001), by permission of the copyright owner. experiments showed that all the NMR interaction parameters for the four sites could be obtained from a single MQMAS measurement using the Double Frequency Sweep (DFS) method, but the most reliable results were obtained from the combination of five DOR and three MQMAS spectra (or even from two DOR and one MAS spectrum) (Engelhardt et al. 1999). The geologically important alkali feldspar minerals occur in a range of compositions from the fully sodium end-member albite (NaA1Si308) to the fully potassium compound microcline (KA1Si3Os). 23Na MAS NMR has provided information about changes in the distribution of Na and K in the single alkali ion site across the composition range (Phillips et al. 1988). As the composition becomes more K-rich, the 23Na quadrupolar lineshape becomes less well resolved due to inhomogeneous broadening (Figure 7.7A). At higher fields, the 23Na NMR spectra are less influenced by secondorder quadrupolar effects, suggesting that the substantial broadening still evident (Figure 7.7B) reflects a range of isotropic chemical shifts resulting from inhomogeneous distribution of K and Na throughout the samples. Changes in the shape of the broadened 23Na NMR spectra in heated samples also suggest that spinodal decomposition is occurring, and allow the spinodal temperature to be determined (Phillips et al. 1988).
411
N M R of Other Commonly Studied Nuclei
A
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B
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increasing K content
I
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23Na shift (ppm) w.r.t. NaCl soln.
Figure 7.7. A. 8.45 T 23NaMAS NMR spectra of albite (NaA1Si3Os) with increasing substitution of K for Na. B. Spectra of the same samples acquired at 11.7 T, revealing broadening due to regions of inhomogeneous Na and K distribution. Adapted from Phillips et al. (l 988).
Makatite (Na20.4SiO2.5H20), kanemite (Na20.4SiO2.7H20), octosilicate (NazO.8SiOz.9H20), magadiite (NazO.14SiOz.10H20) and kenyaite (Na20.22 SiO2.10H20) form an interesting series of polysilicate layer structures containing hydrated Na + in the interlayer spaces. The facile Na exchange properties of these materials suggest potential applications as catalysts and detergents. The 23Na NMR spectra of these materials indicate significant differences in the interlayer Na configurations, with a sodium CN of 5 tentatively proposed for kanemite and a CN of 6 in octosilicate, magadiite and kenyaite. The Na CN in makatite can be both 5 and 6 (Almond et al. 1996). The NMR results have led to the proposal of new structures for these materials based on the structure of anhydrous KHSi205 rather than on the known layer structure of makatite (Almond et al. 1997). 23Na MAS NMR has provided useful information about the interaction between NaC1 and the calcium silicate hydrate phases typically occurring in hydrated cement pastes (Viallis et al. 1999). The results suggest that the Na + is absorbed together with its hydration sphere on the surface of dry calcium silicate hydrate, whereas in the hydrated material, the cations are located in a diffuse ion swarm on the calcium silicate surface.
412
Multinuclear Solid-State NMR of Inorganic Materials
7.1.5 23Na N M R studies o f thermal reactions
Thermal reactions (dehydration, thermal decomposition, solid state reactions) play an important part in the production of inorganic materials from minerals, gels and related raw materials. Solid state NMR can provide useful insights into the progress of these reactions and their mechanistic details, as well as information on phase transformations and other high-temperature phenomena. 23Na MAS NMR at three magnetic fields has been used to determine the dehydration kinetics and mechanism of sodium aluminate trihydrate (Dec et al. 1990). The dehydrated product is of commercial importance as a source of aluminate ions and as a coagulating agent. The NMR results suggest that the trihydrate principally contains two pseudo-octahedral sodium sites, whereas in the dehydrated form the principal sodium site is pseudo-trigonal bipyramidal (5-fold coordinated). This dehydrated structure, containing monomeric A104 tetrahedra linked by six-coordinated Na ions, differs from the known structure of NaA102 in which both the A1 and Na are tetrahedrally coordinated, suggesting that the stable dehydrated form cannot be obtained below 600~ (Dec et al. 1990). Dehydration of the small-pore zeolite analcime (NaA1Si206) has also been studied by high-temperature 23Na NMR measurements up to 500~ (Kim and Kirkpatrick 1998) in which changes in the 23Na peak positions and widths with temperature were interpreted in terms of exchange between the Na sites and the effect of motional averaging of the intensity of the (_+ 1/2,_ + 3/2) satellite transitions. A number of other thermal reaction studies made by exploiting the availability of NMR probes which can operate at high temperatures have been reviewed by Stebbins (1991). The in situ heating technique has been used in a high-temperature 23Na NMR study of the compound LiNaSO4 which undergoes a phase transition at 518~ from trigonal to cubic symmetry, accompanied by the appearance of fast ionic conductivity behaviour (Massiot et al. 1990). At temperatures below the phase transition, the 23Na line progressively narrows, but is abruptly replaced by a new, narrower line at the transition temperature. Two-dimensional nutation spectra show that this new line contains the collapsed satellite transitions, indicative of nearly liquid-like motion of the cations (Massiot et al. 1990). High-temperature 23Na MAS NMR has been used to study the exchange of Na + among the alkali sites of nepheline ([Na,K]A1SiO4) at 500~ (Stebbins et al. 1989). These materials are of interest because they display ionic conductivity. The Na + exchange rates estimated from the temperature-dependent changes in the 23Na NMR spectra are consistent with the correlation times derived from cation diffusivity measurements. When the clay mineral kaolinite is heated to about 980~ it transforms to an intimate mixture of amorphous silica and an aluminium-rich spinel phase, the latter having potentially useful catalytic properties if the silica can be removed. Selective leaching
NMR of Other Commonly Studied Nuclei
413
with NaOH solution has been used for this purpose, but this can introduce Na into the system. 23Na MAS NMR has been used to provide information about the incorporation of Na during leaching, showing that the amount increases almost linearly with leaching time up to 40 minutes soaking (MacKenzie et al. 1996). The 23Na resonance positions of the Na thus incorporated in the structure are closer to that of hydrated Na + than to the peak position in hydrothermally altered glasses, indicating a high degree of hydration. When these materials are re-heated at higher temperatures, the hydration water is lost and 23Na resonance shifts to a value more typical of unhydrated sodium aluminosilicate glass (about - 22 ppm) (MacKenzie et al. 1996). 23Na MAS NMR has also been used to study the structure and sodium environment in amorphous sodium aluminosilicate geopolymers, showing that the charge-balancing Na + is present in a highly hydrated form (Barbosa et al. 2000). When the geopolymer is heated to > 1200~ the sodium ions lose their hydration water, as evidenced by a shift in the position of the 23Na resonance from- 5.5 ppm to - 19 ppm, but the amorphous nature of the material is retained. The Bayer Process produces a highly alkaline spent liquor which when treated with gaseous CO2 precipitates dawsonite, NaA1CO3(OH)2 which can be recovered as a useful by-product or recycled into the Bayer Process. 23Na and 27A1MAS NMR, CP and DOR have been used to provide detailed structural information on dawsonite and on the X-ray amorphous phase which occurs as an intermediate in its thermal decomposition to NaA102 (Bastow et al. 1995). The 23Na and 27A1 NMR spectra of dawsonite from Bayer liquor revealed the presence of the minor impurity phases NaHCO3 and AI(OH)3 which were too amorphous to be detected by X-ray powder diffraction. The NMR spectra of dawsonite heated at 350~ for 16 h suggested the amorphous intermediate material contains a mixture of disordered phases related to NaHCO3, NaA102 and an amorphous alumina (Bastow et al. 1995). 7.1.6 23Na N M R of glasses 7.1.6.1 Silicate and aluminosilicate glasses. Since solid state NMR does not depend on the presence of long-range atomic order, it is an ideal technique for studying the atomic environments in glasses and liquids. 23Na NMR has been used to study the local coordination environment of Na in sodium silicate and aluminosilicate glasses and melts (Xue and Stebbins 1992). The spectra of Na2Si205 and Na2Si409 glasses quenched from high-pressure melts are broad, and do not show characteristic secondorder quadrupolar lineshapes, suggesting a range of isotropic chemical shifts and/or XQ values, as would be expected from a disordered phase. The spectra also show relatively small changes in the 23Na peak position (i.e. average sodium coordination) with glass composition or pressure, but the introduction of other alkali metal cations such as K or Rb has a greater effect on the local Na coordination environment as evidenced by
414
Multinuclear Solid-State NMR of Inorganic Materials
the 23Na resonance position (Xue and Stebbins 1992). 23Na and 6'7Li MAS NMR studies of single and mixed Na-Li silicate glasses have shown monotonic changes in the 23Na chemical shift with the overall alkali content, suggesting the presence of intimate mixing of the Na and Li in these glasses, with no evidence of spatial cation segregation (Ali et al. 1995, Gee et al. 1997). The absence of like-cation clustering in these glasses is also supported by 29Si{23Na} and 29Si{7Li} Rotational Echo DOuble Resonance (REDOR) experiments (Gee et al. 1997). The 23Na chemical shifts of a number of framework aluminosilicate glasses have been reported to become slightly less shielded (less negative) with decreasing Si/(Si + A1) values, showing similar behaviour to the 27A1 and 29Si NMR shifts of these glasses. The 23Na shifts also become less shielded with decreasing Na/(Na + K) values, opposite to the trend found for the crystalline alkali feldspars (Oestrike et al. 1987). This result may reflect the predominant occurrence of 6-membered rings in these glasses by contrast with the 4-membered rings occurring in crystalline feldspars. The effect of substituting Ge for Si and Ga for A1 in a series of sodium aluminosilicate glasses of albite (NaA1Si3Os), jadeite (NaA1Si206) and nepheline (NaA1SiO4) compositions has been studied by 23Na, 27A1 and 29Si NMR (Sherriff and Fleet 1990). The NMR spectra of all three nuclei are shifted systematically to lower fields by the substitution of both Ge and Ga into the glasses. The effect of tetrahedral substitution of Ge can be accounted for in terms of changes in the cation-oxygen bond lengths resulting from the random replacement of neighbouring cations. The effect on the 23Na spectra of replacing A1 by Ga is less well understood but may be related to changes in the bond angle of the bridging oxygen, reflecting an increase in the number of smallmembered tetrahedral rings (Sherriff and Fleet 1990). The effect of dissolved water in aluminosilicate glasses has important mineralogical implications. A multinuclear MAS NMR study including 23Na NMR has been made of the hydration of a series of sodium and potassium aluminosilicate glasses with compositions related to the ternary system quartz-albite-orthoclase (Schmidt et al. 2001). The hydration reactions were marked by a significant decrease of 23Na ~iso with increasing water content, consistent with the formation of a hydration shell around the sodium, reducing the mean Na-O distance by comparison with the anhydrous aluminosilicate network. 7.1.6.2 S o d i u m borosilicate glasses. Glasses in this system have been extensively studied by 23Na NMR. In principle, the spectra should be capable of distinguishing between sodium ions associated with non-bridging oxygen atoms in the structure and the sodium acting as charge compensators for the tetrahedral boron sites, but in practice the 23Na resonances were found to be too broadened for the two types of sodium to be distinguished (Bunker et al. 1990). However, the 23Na resonance position which typically varies from - 4 to - 18 ppm has been suggested to reflect the relative contributions of
NMR of Other Commonly Studied Nuclei
415
the two types of Na, the less negative values resulting from a higher proportion of sodium associated with non-bridging oxygens and the more negative values associated with charge-compensating Na + (Bunker et al. 1990). The formation of sodium borosilicate glasses from sol-gel precursors has also been studied by 23Na NMR, which indicated relatively unchanged Na coordination but increased asymmetry of the Na environment as the gel water was lost during heating (Villegas et al. 1990). A 23Na NMR study of a wider range of sodium borosilicate compositions has confirmed the presence in all the samples of asymmetrically broadened resonances reflecting a wide distribution of electric field gradients (Martens and Mtiller-Warmuth 2000). Although the lineshapes do not change with composition, the positions of the peak maxima were found to show a good linear dependence on the mole fraction of Na20 (except for glasses in the immiscibility region which were demonstrably phase-separated). 23Na MQMAS NMR has been used to investigate the Na-O bond distance in several glass compositions including sodium borosilicate glasses (Angeli et al. 2000). By inverting the MQMAS spectrum, the distributions of the isotropic chemical shift have been determined for the various glasses, confirming that the addition of network formers such as A1 or B results in a shift towards greater Na-O distances. 7.1.6.3 Sodium borate, germanate and tellurite glasses and melts. These glasses and melts have also been studied by 23Na NMR (George et al. 1997). The 23Na chemical shifts indicate an apparent decrease in the average Na-O bond length with increasing Na20 content in these systems. The 23Na isotropic chemical shifts of single and mixed alkali borate glasses, extracted from their MAS NMR spectra acquired at two fields, show an almost linear dependence on the alkali ion content (Ratai et al. 1998). This result, together with the results of 23Na spin-echo decay measurements, have been taken as a strong argument against previously-proposed clustering models for the glass structure. 23Na dynamic angle spinning (DAS) NMR at two magnetic fields has been used to determine the chemical shift and quadrupole coupling parameters of a series of sodium tellurite glasses (Figure 7.8A) (Tagg et al. 1995). By contrast with the findings for sodium borate glasses (Ratai et al. 1998), the isotropic shift (and the quadrupolar product PQ) does not change monotonically with the sodium content of the glass (Figure 7.8B). Above about 18 mole % Na20 the relationships with sodium content become approximately linear, corresponding to a decrease in the sodium coordination number probably as a result of the formation of a substantial concentration of nonbridging oxygen atoms (Tagg et al. 1995).
7.1.6.4 Phosphate glasses These have glass transition temperatures typically < 1000~
and thermal expansion coefficients which make them suitable for applications such as glass-to-metal seals and biocompatible materials. The 23Na MAS NMR spectra reported
416
Multinuclear Solid-State NMR of lnorganic Materials A
B
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Figure 7.8. A. 23Na two-dimensional DAS spectrum of glassy Na2Te409 acquired at a magnetic field of 8.46 T at spinning speeds of 5.5-6.0 kHz and rotor angles of 37.38~ and 79.19~ to the magnetic field. The maximum of the peak projection onto the isotropic shift axis is taken as the most probable isotropic shift. Note that the shifts are referenced to solid NaC1. B. Variation with glass composition of the most probable isotropic chemical shifts at two magnetic fields deduced from the DAS spectra of sodium tellurite glasses (referenced to solid NaC1). From Tagg et al. (1995) by permission of the American Chemical Society. for a series of sodium phosphate glasses are typically broad and featureless, but their peak maxima, which range from - 5.4 to - 9.3 ppm, show a similar trend to that of sodium silicate glasses, becoming systematically more positive (less shielded) with increasing Na20 content of the glass (Brow et al. 1990). The association of the Na with specific phosphorus sites in sodium phosphate glasses has been investigated using onedimensional CPMAS NMR in which the polarisation transfer was from the quadrupolar 23Na to the spin-1/2 31p nuclei (Prabakar et al. 2000). The CP spectra indicate that 23Na nuclei are present in the next-nearest neighbour coordination sphere of each of the three types of phosphorus sites (Q~, Q2 and Q3) in these glasses. Although the Q1 and Q2 sites would have been expected to be associated with Na since these contain negativelycharged non-bridging oxygens (NBOs) which are charge-compensated by Na +, the correlation of Na with the Q3 phosphorus sites in ultraphosphate glasses (where Na/P = 0.25) is not predicted by current theory. Variable contact time CPMAS experiments have suggested a more complex coordination role than previously thought for the double bonded oxygens (DBOs) associated with the Q3 sites (Prabakar et al. 2000).
7.1.6.5 Miscellaneous glass studies. The reaction of minerals and glasses with aqueous fluids at low temperatures (hydrothermal alteration) is one of the most important geochemical processes occurring in the earth's crust. Hydrothermal alteration of
NMR of Other Commonly Studied Nuclei
417
sodium aluminosilicate glass has been studied by 23Na, 27A1 and 29Si NMR (Yang and Kirkpatrick 1989). As the hydrothermal reaction proceeds, the 23Na NMR resonance becomes narrower and more positive (less shielded), moving from - 1 9 . 6 to - 12.9 ppm. These spectral changes are thought to result primarily from a decrease in the average 23Na quadrupole coupling constant reflecting an increasingly more isotropic Na environment as the alkali ion becomes hydrated. The hydration shell which forms around the Na during hydrothermal reaction reduces the interaction with the oxygens of the aluminosilicate structure, decreasing the electric field gradient at the Na nucleus. The hydrothermal dissolution mechanism therefore involves incorporation of water molecules into the bulk glass, followed by exchange of Na by protons and depolymerisation of the aluminosilicate framework (Yang and Kirkpatrick 1989). Although major changes in the 23Na NMR spectra of albite glasses were found by Kohn et al. (1989) to accompany water uptake, the corresponding 27A1 and 298i NMR spectra were relatively insensitive to the water content, suggesting the operation of the equilibrium NaA1Si308 + H20 ~ HA1Si308 + NaOH This mechanistic conclusion, that the water in the glass does not break the T-O-T bonds to produce terminal T-OH groups, is supported by more recent 23Na MAS NMR spectra obtained at three magnetic fields, and by 23Na off-resonance nutation spectroscopy which show the mean value of 8iso to be strongly dependent on the concentration of dissolved water in the glasses, varying from - 13.4 ppm in the dry glass to - 4.4 ppm in a glass containing 56 mol % water. By contrast with the earlier 23Na results, the mean value of • (2.1-2.2 MHz) was found to be essentially constant up to water contents of 60 mol % (Kohn et al. 1998). B ioglass| is a soda-lime phosphosilicate glass with bioactive affinity for bone. A structural study of Bioglasses by multinuclear NMR including 23Na NMR has shown that both Na + and Ca 2+ are associated with the phosphate rather than the silicate species. Where cation association with the silicate units occurs, the sodium appears to associate preferentially with the Q3 structural units, by contrast with Ca 2+ which associates preferentially with the Q2 units. In the bioactive composition range the Ca-Q 2 regions moderate the dissolution of the Na-Q 3 regions, allowing gel formation to occur, followed by the appearance of the biocompatible calcium phosphate layer (Lockyer et al. 1995). Another glassy system of interest for its possible bioactive properties, CaO-NazO-PaOs-AI203, has been studied by 23Na, 27A1 and 31p NMR (Abrahams et al. 1997). The 23Na MAS NMR spectra of the glasses are broad and featureless, but on thermal recrystallisation, the spectral lineshapes showed structure corresponding to two sodium sites which could be simulated using parameters refined from a 2D MQMAS NMR spectrum. The NMR parameters of these sites (~iso = - 1.6 and - 10.7 ppm, • = 1.4-1.6 and 2.3-2.4 MHz) are similar to those of cyclic sodium
418
Multinuclear Solid-State NMR of Inorganic Materials
trimetaphosphate Na3P309, in which there are two equally-populated five-coordinate Na sites (Abrahams et al. 1997). 7.1.7 23Na N M R o f zeolites Zeolites are aluminosilicates consisting of tetrahedral frameworks in which the charges resulting from the four-coordinated A1 in the structure are balanced by the presence of extra-framework cations such as Na + located in the structural cavities and channels. These cations play an important role in the absorption and catalytic properties of the zeolite, and have been extensively investigated by solid-state NMR. The 23Na NMR spectra of hydrated zeolites have lineshapes and chemical shifts which are influenced by the sodium mobility which in turn reflects the state of hydration of the Na + (Challoner and Harris 199 l, Beyer et al. 1993). The linewidths of hydrated Na + in hydrated zeolites is relatively narrow, but on dehydration they broaden and shift to more negative values. The broad lineshapes of dehydrated zeolites result from overlapping second-order quadrupolar patterns of sodium ions localised at crystallographically distinct sites, and if they could be resolved they have the potential to provide useful structural information. For this reason, a number of advanced NMR techniques (multiple-field high-speed MAS, DOR, MQMAS NMR, 2D nutation) have been used to resolve the 23Na NMR spectra of dehydrated zeolites and determine the quadrupolar parameters of the various sodium sites. An earlier 23Na NMR study of the dehydrated zeolite faujasite NaY used a combination of MAS at two magnetic fields, DOR and 2D nutation to resolve two Na lineshapes, one with ~iso = - 11 ppm, XQ = 0 MHz, the other with ~iso = - 7 ppm, XQ = 4.2 MHz and xl = 0.25 (Hunger et al. 1993). The 2D nutation experiments confirmed the presence of two lines characterised by very small strong quadrupole interactions. On the basis of EFG calculations using a point charge model, the line characterised by XQ = 0 MHz was assigned to the SI sites in the centre of the hexagonal prism, whereas the line with XQ -- 4 MHz was identified with SI' and SII sites close to the centre of the six-ring windows in the sodalite cages and supercages respectively (Hunger et al. 1993). 23Na MQMAS NMR spectra have been acquired for several dehydrated zeolites including NaY, and have proved capable of resolving sites with quadrupole parameters up to --- 4 MHz. The MQMAS spectrum of the dehydrated zeolite NaY showed two resolvable signals within this range, one of which is associated with the SI sites (Hunger et al. 1997). However, the quadrupolar product determined by MQMAS NMR for the SI site in dehydrated NaY is significantly larger than previously estimated from 2D nutation studies, for reasons which have not been discussed. The location and migration of Na + in a lanthanum-exchanged faujasite zeolite NaY has also been studied by 23Na MAS, DOR and 2D nutation spectroscopy (Hunger et al. 1995). The sodium ions in dehydrated faujasite zeolite NaX are located in as many as seven
419
NMR of Other Commonly Studied Nuclei
sites, posing a stringent test for the resolving powers of NMR techniques. As in the related dehydrated zeolite NaY, the Na cations are located in the centre of the hexagonal prism (I) and close to the centres of the six-ring windows of the sodalite cages (I') and supercages (II). NaX has further Na-occupied sites (III') at the four rings in the supercage. Site I' is further split into two and III' into three closely related positions, giving seven Na sites. Although the use of 23Na NMR to distinguish between cubic (I) and non-cubic (I',II, II' and III') sites should be straightforward, further resolution of the non-cubic sites has been considered intractible. However, by using a combination of MAS N M R at three magnetic fields, DOR and 2D nutation experiments (Figure 7.9) six of the possible sites were resolved and their quadrupolar parameters determined (Feuerstein et al. 1996). The MAS NMR spectra (Figure 7.9A) could be consistently simulated at three magnetic fields by fitting five sites using quadrupolar parameters calculated from the known atomic coordinates of the structure using a point-charge model (Feuerstein et al. 1996). The resonances of the second I' site and the third III' site could not be resolved by MAS NMR, but the second I' site was resolved by DOR using an outer rotor speed of 1.5 kHz (Figure 7.9B). Since the DOR spectra contain numerous spinning side bands, their correct simulation requires the spinning side bands to be identified by making measurements at several spinning speeds. The results were qualitatively confirmed by 2D nutation experiments (Figure 7.9C) which is useful for resolving resonances with different quadrupole interactions. In A
MAS
B
DOR
C
~
d
2D nutation
. _~.
site Ill'
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-"-site I'site 1 -
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Figure 7.9. A. 23Na MAS NMR spectum of dehydrated zeolite NaX at a magnetic field of 14.1 T, with the simulated spectrum and the five components used in the simulation. B. 23Na DOR spectrum acquired at a magnetic field of 9.4 T with an outer rotor speed of 1.5 kHz, with simulated spectrum and the components used in the simulation. C. 2 3 N a 2D nutation spectrum acquired at a magnetic field of 9.4 T. The asterisk denotes spinning side bands. From Feuerstein et al. (1996), by permission of copyright owner.
420
Multinuclear Solid-State NMR of lnorganic Materials
these spectra, cations in site I show up as a weak line, but sites I' and II are not well resolved, appearing as a broad feature. The three sites III' are likewise not resolved; two have similar XQ values (2.6 MHz) and appear as an intense feature, but the third site, with a smaller Xq, is not obvious. Similar experiments on other dehydrated fajusite zeolites with different A1/Si ratios have allowed the changes in the occupancies of the various Na sites to be determined as a function of zeolite composition (Feuerstein et al. 1996). The catalytic efficiency of ~/-alumina, an important supporting material for industrial hydrodesulphurisation catalysts, can be improved by the deposition of alkali metal ions on the surface. The surface adsorption of Na + on ~/-alumina has been studied by 23Na MAS NMR, revealing the presence of two distinct adsorbed species (Deng et al. 1993). At lower Na loadings, the Na + is associated with the surface oxygen atoms of the ~/-alumina in a single site with a chemical shift of - 5.1 ppm, but at higher Na loadings (15%) a second Na species with a chemical shift of about - 4.1 ppm has been detected by 23Na two-dimensional nutation experiments. The new site corresponds to a surface salt which deposits over the original Na + monolayer with an accompanying increase in the number of cations associated with the Br#nsted acid sites. The latter give rise to the improved catalytic efficiency of the Na+-A1203 system (Deng et al. 1993).
7.2. liB NMR
7.2.1 General considerations liB is a quadrupolar nucleus with a spin of 3/2 which can give rise to significant broadening of the NMR resonances of compounds with asymmetrical B environments. However, the natural abundance (80.42%) and sensitivity of lib is good and its relaxation rate generally sufficiently fast for useful spectra to be acquired quickly. Thus, liB NMR provides a facile method for examining a common element in concentrations as low as 10 ppm which is difficult to analyse by chemical methods. The chemical shifts of liB are referenced to boron trifluoride etherate (BF3.Et20). NaBH4 is a useful solid secondary reference compound at 3.2 ppm. Boron-containing compounds can typically contain tetrahedral BO4 or trigonal BO3 configurations. Where the resonances are narrowed in MAS experiments at higher magnetic fields, tetrahedral BO4 has a relatively narrow single resonance with ~iso of 2 to - 4 ppm and XQ of 0 to 0.5 MHz. By contrast, trigonal BO3 has larger nuclear quadrupolar coupling constants (2.3 to 2.5 MHz) which give rise to a typical secondorder quadrupolar lineshape and g~o values ranging from 12 to 19 ppm. These chemical shift ranges of the two boron coordinations are relatively small, and overlap can sometimes occur, but normally the lines are sufficiently well resolved to allow estimates to be made of the relative amounts of each species, especially at magnetic fields of 14.1 T when the sites can be completely resolved without the need for spectral simulation.
421
N M R o f Other C o m m o n l y S t u d i e d N u c l e i 7.2.2 t t B N M R
of crystalline compounds
The pioneering liB N M R studies of crystalline and glassy boron compounds were made using wide-line continuous-wave techniques to distinguish between BO3 and BO4 units (Bray et al. 1982). With the advent of pulsed NMR and magic-angle spinning techniques, the liB MAS N M R spectra of a series of crystalline boron compounds and borate minerals were determined by Turner et al. (1986). Since many of the natural boron minerals are hydrated, the best-resolved spectra were obtained by spinning at > 6 kHz or by using proton decoupling to eliminate broadening due to 11B-1H dipolar interactions. Boron-boron dipolar interactions are weak, and are effectively removed by MAS spinning of > 1-2 kHz. The N M R interaction parameters for ~B in boron compounds are collected in Table 7.2. Table 7.2. 11B NMR interaction parameters for boron compounds. Compound
giso(ppm)*
XQ (MHz)
"q
B(OH)3 Na2B204.10H20 (borax) Naz[BzOs(OOH)412H20 Naz[BzO4(OH)4].6H20 KzB1oO16.8H20 K[BsO6(OH)a].2H20 Tl[BsO6(OH)4].2H20 Tlz[B406(OH)2].2H20 Ag6[BlzO18(OH)6].3H20 PbBaO7.3H20 MgB303(OH)5.4H20 (inderite) CaB303(OH)5.4H20 (inyoite) NaCaBsO6(OH)6.5H20 (ulexite) NazBzO6(OH)z.3H20 (kernite) CaB3Oa(OH)3.H20 (colemanite) NaBSi3Og(reedmergnite) CaBzSizOs (danburite) CaB(SiO4)(OH) (datolite) Mg3B7OI3C1 (boracite) tourmaline grandidierite LizB207 SrB407
18.8 19.0, 2.0 20.5, 6.0 3.5 18.9, 1.4 17.0, 0 18.0, 0 18.0, - 1.0 18.0, - 0.6 19.2, 0.9 18.1, 1.0
2.55 2.4, -0.3 2.55, 0 ND 2.5, --0.3 2.55, 0 2.55, 0 2.55, 0 2.65, 0 2.55, 0 2.4, -0.2
ND ND ND ND ND ND ND ND ND ND ND
Muller et al. Turner et al. Muller et al. Muller et al. Turner et al. Muller et al. Muller et al. Muller et al. Muller et al. Muller et al. Turner et al.
17.4, 1.5 18.2, 1.2
2.3, -0.2 2.45, --0.3
ND ND
Turner et al. (1986) Turner et al. (1986)
18.5, 0.9
2.4, --0.2
ND
Turner et al. (1986)
17.0, 1.4
2.4, -0.3
ND
Turner et al. (1986)
- 1.9 -0.7 1.0 16.0, 1.0 12.7 17.0 17.9, 1.7 1.0 ND - 7.4 - 6.8 - 7.9 3.3
-0 -0 -0 2.6, --0.3 ND 2.55 2.5, -0.2 ND 2.5735 ND 0.74 ND 0
ND ND ND ND ND 0.2 ND ND 0.511 ND 0.16 ND ND
Turner et al. (1986) Turner et al. (1986) Turner et al. (1986) Turner et al. (1986) Turner et al. (1986) Smith & Steuernagel (1992) Turner et al. (1986) Muller et al. (1993) Mao & Bray (1992) Couto et al. (1994) Couto et al. (1994) Couto et al. (1994) Turner et al. (1986)
CaB204 oL-Ks[BWI204o].I1H20 K9[BW11039].l 1H20 K6[BWllO39Co(HzO].15H20 BPO4
-
Reference (1993) (1986) (1993) (1993)
(1986) (1993) (1993) (1993)
(1993) (1993) (1986)
422
M u l t i n u c l e a r Solid-State N M R o f l n o r g a n i c M a t e r i a l s
Table 7.2. (Continued)
Compound
~iso(ppm)*
XQ(MHz)
xl
Reference
B4C
- 4.6, 130
ND, 5.58
ND, 0
Kirkpatrick et al. (1991)
B13C2 B9C BN (cubic) BN (hexagonal) Si3B3N7 SiBN3C
0 1.3 1.6 30.4 30.2 30, 29.8
ND ND Kirkpatricket al. (1991) ND ND Kirkpatricket al. (1991) < 0.05 ND Jeschkeet al. (1998) 2.936 ND Jeschkeet al. (1998) 1.4-1.55 0-0.1 Jeschkeet al. (1999) 2.9, 1.4-1.55 0.1, (M).I Van Wfillen & Jansen (2001), Jeschke et al. (2001)
* chemical shifts referenced to BF3.Et20
The liB MAS and static NMR spectra of a series of boron carbides show a broad major resonance at about 1.3 to - 4.6 ppm, the peak position varying almost linearly with carbon content (Figure 7.10A). This resonance has been assigned to boron in the B-rich icosahedral units which are bonded together both directly and via three-atom chains (Kirkpatrick et al. 1991). A small additional shoulder on the major resonance of the static ~lB spectra (Figure 7.10B) which increases in intensity with decreasing C content and can be simulated as a second-order quadrupolar lineshape has been assigned to the boron site in the centre of the various possible C-B-C chains (Kirkpatrick et al. 1991). Boron nitride occurs in cubic and hexagonal forms. The symmetric B site in cubic BN gives a single liB resonance at 1.6 ppm with a XQ value of--~0 MHz (Figure 7.11A), with slight broadening due to homonuclear and heteronuclear dipole-dipole interactions (Jeschke et al. 1998). The presence of dipole-dipole interactions in cubic B N has been confirmed by comparison of the static l~ and liB NMR spectra. The linewidth of the former should be substantially greater if quadrupolar broadening is present, but this was found not to be the case (Jeschke et al. 1998). The boron site in hexagonal BN lacks the symmetry of the cubic form, and exhibits a second-order quadrupolar lineshape (Figure 7.11B) with XQ = 2.936 MHz and ~iso = 30.4 ppm (Jeschke et al. 1998, Marchetti et al. 1991). This value of XQ was determined from measurements of the satellite transition in the static spectrum (Figure 7.11C), while the ~iso value was determined from a DOR measurement (Figure 7.11D), since the MAS spectrum is broadened by the anisotropic component of the second-order quadrupole effect and therefore gives a less precise result (Jeschke et al. 1998). Hexagonal boron nitride can be conveniently prepared by pyrolysis in an inert atmosphere of polymeric precursors such as polyborazilene, the structure of which has been investigated by liB and ~SN NMR (Gervais et al. 1998, 2001). Signals at 31 and 26 ppm in the 1~B MAS NMR spectrum of the polymer are in the region of tricoordinated boron, and are attributed to BN2H and BN3 sites. Since the central transition in these
423
N M R o f Other C o m m o n l y Studied Nuclei
B
A
B4C s t a t i c
o B9C
0 -m,lq
_
-2
_
\ simulated
o
m -4
~ r o n --------~
-6_ 8
12 ' 16 Atom % C
20'
site
chain site
500
0-500
~IB shift (ppm) w.r.t. BF 3 in E t 2 0
Figure 7.10. A. Change in the I IB isotropic chemical shift of boron carbides with carbon content. B. Observed and simulated static I1B NMR spectra of B4C, with the simulated components. Note the small shoulder at about 130 ppm, simulated by a quadrupolar lineshape arising from B in the chain sites. From Kirkpatrick et al. (1991), by permission of the copyright owner.
A Cubic BN
MAS
C
SATRAS
~ II ]/ observed
~_____~_~/
L.____
,,
/
800
B
400
v
Hexagonal BN / ~
D
0
(knz)
DOR ~b
Hexagonal BN ~ ] ~ ~
80
0
-80
~
t
I
t
100
50
0
-50
~ -100
nB s h i f t ( p p m ) w.r.t. BF3.Et20 Figure 7.11. 11B MAS NMR spectra of A. cubic BN, B. hexagonal BN (observed and simulated spectra). From Marchetti et al. (1991) by permission of the American Chemical Society. C. lIB satellite transition spectrum of hexagonal BN, from Jeschke et al. (1998). The asterisk denotes the central transition. D. lIB DOR spectrum of hexagonal BN, from Jeschke et al. (1998), by permission of the copyright owner. Inner and outer rotors spun at 5.5 and 1.1 kHz respectively. The asterisk denotes the isotropic line.
424
Multinuclear Solid-State NMR of lnorganic Materials
spectra is perturbed only to second-order by the quadrupolar interaction, and is inversely proportional to the applied magnetic field, the 11B spectrum acquired at 18.8 T is sufficiently narrowed to allow the two sites to be distinguished readily. These sites were also resolved by ~IB MQMAS NMR at 9.4 T, allowing the 1D MAS NMR spectra to be simulated and the NMR parameters to be obtained (Gervais et al. 2001). The related materials boron silicon nitride (Si3B3N7) and boron silicon carbonitride (SiBN3C) have potentially useful properties (hardness, high temperature durability, low density and cheapness). They are typically synthesised by thermal degradation of amorphous polymeric precursors and consist of covalent amorphous networks. The ~B MAS NMR and DOR spectra of these compounds are virtually indistinguishable from those of hexagonal BN, and the MQMAS spectra could not be obtained, but satellite transition spectroscopy (SATRAS) has provided a degree of resolution of the broadened liB spectra (Jeschke et al. 1999). The results indicate that the boron in both compounds is coordinated exclusively to nitrogen with almost trigonal planar geometry and nearly equal bond lengths. A 29Si{ liB} REDOR experiment on the pyrolysed carbonitride product indicates that the boron and silicon are homogeneously distributed in the network even at nanoscale levels (Jeschke et al. 1999).
7.2.3 UB N M R o f glasses Because of its ability to readily discriminate between BO3 and BO4, liB MAS NMR has been widely used to determine the relative numbers of these units in a variety of glassy borate systems, allowing their local structures to be deduced. However, in some glasses the distinction between the different borate units may be hampered by the small differences in their chemical shifts and the inability of MAS techniques to completely narrow overlapping l~B resonances. In such cases, short excitation pulses combined with high-speed magic angle spinning and satellite transition spectroscopy has been shown to give good separation of the overlapping resonances, allowing the determination of reliable isotropic chemical shift values and providing information about the nearest and next-nearest neighbour environments of the boron (van Wtillen and Mtiller-Warmuth 1993). B203 is a network-forming oxide, participating in many important glass systems, including borosilicates, boroaluminates and borosilicophosphates. The simplest boron glass-forming system is B203 itself, which melts at 577~ to form a three-dimensional network of planar BO3 units. The melting and glass-forming processes have been studied by liB NMR up to 1200~ in a high-temperature NMR probe (Maekawa et al. 1995). The room-temperature spectrum (Figure 7.12A) shows a characteristic second-order quadrupolar lineshape giving on simulation a XQ value of 2.6 MHz. Above the glass transition temperature Tg (260~ where the system becomes a supercooled liquid, the linewidth decreases steadily up to the melting point, above which it increases again due
425
NMR of Other Commonly Studied Nuclei A
T (~
700 9
J~.
500
~
20 O0
i......
500
,
.......... | . . . . . . . . . .
0
O(D
o,II
oOoO
0
-500
nB shift (ppm) w.r.t. BF3.Et20
. 600 ....
1 0 00
' 1400
Temperature (~
Figure 7.12. A. Static liB NMR spectra of B203 at various temperatures. B. liB chemical shift of B203 as a function of heating temperaure. After Maekawa et al. (1995), by permission of the copyright owner.
to multiexponentional quadrupolar relaxation effects (Figure 7.12A). The l i B chemical shift increases slightly with increasing temperature reflecting slight changes in the connectivity of the BO3 units (Figure 7.12B), but remains in the shift range of BO3 at all temperatures. The gradual change in chemical shift with temperature does not support a suggestion that the boroxol ring fragments into unionised bipyramidal molecules above 800~ (Maekawa et al. 1995). An interesting comparison has been made of a number of ~IB NMR measurements of B203 and K20-B203 glasses to determine the strengths and weaknesses of the MAS, DOR, DAS and MQMAS techniques for resolving the spectra of these disordered systems (Youngman et al. 1996). MAS narrows the static powder pattern significantly (Figure 7.13), but an even greater improvement in resolution of the boroxyl ring sites and the non-ring BO3 sites is obtained by using DOR, DAS and MQMAS (Figure 7.13). Of these, DOR is less favourable because of the spinning side bands introduced at the low spinning speeds associated with this technique (Youngman et al. 1996). The DAS and DOR measurements give giso values for the two resolved sites (4.6 and 1.0 ppm from DAS and 4.6 and 1.5 ppm from DOR), but the values from the MQMAS spectrum are significantly different (54.7 and 39.5 ppm) due to different peak dispersion behaviour in the MQMAS experiment (Youngman et al. 1996). Borosilicate glasses are the most widely useful family of glass compositions for applications ranging from laboratory ware to optical glasses and nuclear waste encapsulation. In the low-Na composition range, all the boron is in either symmetric trigonal or tetrahedral sites, with the concentration of tetrahedral B directly proportional to the Na content. When the Na content becomes greater than that required to form diborate, the excess Na enters the silicate network to form NaBSi4Olo units containing tetrahedral B with four Si next-nearest neighbours. 11B MAS NMR has been used to study these changes in boron coordination with composition in a series of sodium
Multinuclear Solid-State NMR of Inorganic Materials
426
Static / 150
~ 50
-50
0
A ,/',B~,
40
0
-150
MAS 60
DOR
-60
MQMAS
-40
A
A DAS
AB 160
80
0
-80
liB shift (ppm) w.r.t. BF3.Et20 40
0
-40
11Bshift (ppm) w.r.t. BF3.Et~O Figure 7.13. One-dimensional ~lB NMR spectra of B203 glass acquired using several different techniques for narrowing the spectra and improving resolution. Asterisks denote spinning sidebands. Peaks marked A and B arise from the boroxol ring site and the non-ring BO3 site respectively. Adapted from Youngman et al. (1996).
borosilicate glasses (Bunker et al. 1990). The spectra of the boron-rich compositions indicate phase separation into sodium borate and silicate-rich phases. The spectra of the sodium-rich compositions indicate a homogeneous borosilicate network structure containing tetrahedral boron with non-bridging oxygens providing a charge compensation mechanism. A liB MAS NMR study of a larger number of sodium borosilicate compositions (Martens and Miiller-Warmuth 2000) suggests an even greater degree of homogeneity between the borate and silica networks than is usually assumed in borosilicate glass models, with the exception of the known immiscibility region. The isotropic liB chemical shift of BO3 in a series of binary B203-SIO2 glasses follows a linear relationship with the silica content M (in mole %) (Figure 7.14A). This reflects the shielding effect of Si in the second coordination sphere of the B, and is described by the relationship: ~(BO3) - - 0.057M + 18.6
(7.10)
A similar plot of NBO3) for the series of ternary sodium borosilicate glasses shows a similar trend with the mole fraction of SiO2 (x) but with much greater scatter even when the compositions in the immiscibility region are omitted (Martens and MtillerWarmuth 2000) (Figure 7.14B). The fitted line to these data is given by: ~(BO3)--
12.8x + 19
(7.11)
427
NMR of Other Commonly Studied Nuclei
A 18 20 o
9
16
o
16 o
o o "12 t~
14 0
2;
40
60'
80'
80
Mole % SiO2
ooo
o o
' 0.2
' 0.4
0"6.
Mole fraction SiO2
F i g u r e 7.14. A. Changes in the liB chemical shift of the BO3 units in B203-SIO2 glasses as a function of SiO2 content. B. Similar plot for sodium borosilicate glasses, excluding compositions in the immiscibility region. In view of the scatter in this plot, the regression line would best be treated as a guide to the eye only. From Martens and Mtiller-Warmuth (2000), by permission of Elsevier
Science.
The similar trends in the plots of Figure 7.14 have been taken as an indication that the three-coordinated boron in these glasses is mixing with the silica network (Martens and Mtiller-Warmuth 2000). liB MAS NMR has been used to monitor the thermal evolution of borosilicate glasses from precursors prepared by a sol-gel procedure (Villegas et al. 1990). The spectra indicate that the boron is mostly tricoordinated below 500~ reflecting a relatively low degree of B incorporation in the glass network. Above 500~ the B becomes increasingly incorporated in the silica network, with a corresponding increase in the intensity of the tetrahedral B resonance. The presence of Na20 in the B203-SIO2 system increases the proportion of tetrahedral B in the spectra at all temperatures. High-resolution liB MAS NMR at a magnetic field of 14.1 T has been used to study the boron species present in fast-cooled and slow-cooled borate, borosilicate and boroaluminate glasses (Sen et al. 1998). The spectrum of a sodium borate glass containing 0.05 mol % Na20 (Figure 7.15A) can be simulated with a small BO4 single resonance and two BO3 second-order quadrupolar lineshapes corresponding to ring and non-ring BO3 units. The ~iso values of the ring and non-ring BO3 units are 17.4 and 13.7 ppm respectively, but both units have similar values of XQ (2.5 MHz) and +1 (0.2). The two types of BO3 units can also readily be resolved by ~IB MQMAS NMR, yielding quadrupolar parameters in good agreement with the MAS NMR values (Sen et al. 1998). Similar liB MAS NMR studies of a sodium borosilicate glass containing 44.5 mol % Na20 have resolved a sharp BO4 component and two BO3 lineshapes (Figure 7.15B), one corresponding to asymmetric BO3 sites with one non-bridging oxygen
428
Multinuclear Solid-State NMR of Inorganic Materials A
(Na20)o.s(U203)o.95
B
(Na20)44.s(B203),, (SIO2)44.5
ed
ed
BOs ring
~.. ~"-~.~BO
~
A _.o4 i.
30
I
i
20
1
I
10
i
1
0
_J L_ ~
3 non-ring
|
!
-10
liB shift (ppm) w.r.t. BF3.Et20
--
B03 symmetric ~
20
~
803
0
asymmetric
20
~IB shift (ppm) w.r.t. BF3.Et20
Figure 7.15. Observed and simulated liB MAS NMR spectra of A. (Na20)o.os(B203)o.95 and B. (Na20)na.5(B203)ll(SiO2)44.5 slow-cooled glasses showing the individual simulated BO3 and BO4 sites. The spectra of the corresponding faster-cooled glasses show similar details. From Sen et al. (1998), by permission of Elsevier Science.
(nbo) and the other corresponding to symmetric BO3 groups associated with all bridging oxygens. The liB MAS NMR spectrum of a sodium borosilicate glass with lower NazO content (14.0 mol%) showed that the asymmetric BO3 content is negligible. Boroaluminate glasses were found to contain almost all BO3 units (Sen et al. 1998). A series of sodium boroaluminosilicate glasses with compositions between NaBSi308 and NaA1Si308 have been studied by 11B NMR (Geisinger et al. 1988). The spectra indicate that BO3~O4 ratio increases as the ratio of A1/B increases, and that for low-boron glasses (analogous to the magmatic compositions of interest to geochemists) most of the boron is present as BO3 located in borate-rich regions, co-existing with separate boroaluminosilicate regions containing BO4 (Geisinger et al. 1988). ~IB MAS NMR has been used to determine the fraction of tetrahedral boron in a series of alkaline earth (Mg, Ca, Sr, Ba) boroaluminate glasses as a function of composition. The spectra provide good resolution of the BO3 and BO4 units (Figure 7.16A), and confirm that the proportion of BO4 decreases with increasing A1203 content (Figure 7.16B), this trend being essentially independent of the alkaline earth cation (Bunker et al. 1991). The changes in the tetrahedral BO4 content with the content of
429
N M R of Other Commonly Studied Nuclei
A
"~
04
05 " L\
9 MgO 9 CaO 9 SrO
I \
t BO3
50
10
..~. 15U4
.
O aO
"~a O ~ 0.1
-30
11B shift (ppm) w.r.t. BF3.Et~O
0
011
012
0.3
Mole fraction A1203
Figure 7.16. A. Typical liB MAS spectra of alkaline earth boroaluminate glasses showing resolution of the BO4 and BO3 structural units. Asterisks denote spinning side bands. B. Change in the fraction of BO4 units in alkaline earth boroaluminate glasses as a function of A1203 content. The open symbols indicate data from wide-line 11B NMR spectroscopy. From Bunker et al. 1991, by permission of the American Ceramic Society. alkaline earth metal also determined in this study were taken to indicate that the entry of both boron and aluminium into tetrahedral sites is similarly facilitated by the presence of the alkaline earth cations (Bunker et al. 1991). A series of barium borosilicate and borotitanate glasses with Ba:B compositions of approximately BaB204 stoichiometry have been studied by 11B MAS NMR (Clayden et al. 1999). The borosilicate glasses show the expected monotonic increase in the number of BO4 groups with increasing SiO2 content (Figure 7.17), but the behaviour of the borotitanate glasses is more complex. At low TiO2 concentrations, the Ti behaves as a glass former, converting BO3 groups to BO4, but at higher Ti contents it enters the network in 6-fold coordination. This requires some of the Ba 2+ in a charge-balancing function, resulting in the conversion of BO4 groups back to BO3 (Figure 7.17) (Clayden et al. 1999). 11B MAS NMR has been used in conjunction with 29Si and 31p NMR to determine the nature of the borate and other species in a series of sodium borosilicophosphate glasses (Yamashita et al. 1999). In glasses with molar boron fractions less than about 0.5, the borate species is solely BO4, but at about this composition, the peak broadens, suggesting the presence of two types of tetrahedral boron corresponding to the appearance of two different sodium phosphate units. At higher B contents the proportion of BO4 decreases with the formation of BO3 and the peak position progressively moves to a higher frequency, indicating that the 11B is becoming less shielded. These results are consistent with calculations made on the basis that P205 reacts with both Na20 and B203 (Yamashita et al. 1999). An interesting property of boron phosphate and
430
Multinuclear Solid-State N M R of Inorganic Materials
oJ.i
0.35 Si
9 o o
op,,N
0.25
0.15 0
t 4
I 8
I 12
I 16
20
Glass composition x Figure 7.17. Change in the fraction of BO4 units determined by ~]B MAS NMR with composition of glasses near barium metaborate stoichiometry in the systems BaO-B203-SiO2 and BaO-BzO3TiO2. After Clayden et al. (1999), by permission of Elsevier Science.
borosilicophosphate glasses is their ability to form gas ceramics when heat-treated. The resulting material is a microfoamed composite consisting of uniformly sized bubbles of hydrogen encapsulated in a glass or glass ceramic matrix. ~]B and 3~p MAS NMR has been used to study the changes in the glass-forming species when these gas ceramic foams are formed by heat treatment (Youngman et al. 2000). The results indicate that nucleation of the hydrogen-filled voids is accompanied by the crystallisation of BPO4 and a change of boron speciation from BO3 to BOa, and that the hydrogen is dissolved in the glass as molecular H2 with little evidence for the participation of the hydroxyl groups associated with the structural cations (Youngman et al. 2000). ~]B and 19F NMR have been used to investigate the structural details of a boron oxyfluoride glass, BO1.3F0.4 (Boussard-P16del et al. 1997). The ~IB spectrum indicates that the most significant structural unit is BO3 (XQ = 2.928 MHz, "q = 0.14), with trigonal BO2F units (XQ = 2.724 MHz, xl = 0.22) or trigonal BOF2 units (XQ = 2.422 MHz, qq = 0.35) also present. NMR evidence was also found for the presence of a small number of tetrahedral BO2F2 units (Boussard-P16del et al. 1997). ~IB and 19F NMR was also used to study the effect of heating this glass to above its glass transition temperature (333 K), above which it decomposes with the evolution of gaseous BF3 (Le Floch et al. 1998). The synthesis of a range of silicon boron carbonitride ceramics can conveniently be carried out by thermal treatment of suitable amorphous precursor molecules in which the desired atomic linkages are already in place. A series of new amorphous precursor molecules has been investigated by ~B MAS NMR and double resonance techniques such as 29Si-{ liB} Rotational Echo Adiabatic Passage DOuble Resonance (REAPDOR) and ~B-{ 29Si } Rotational Echo DOuble Resonance (REDOR) (van Wiillen and Jansen 2001). The results indicate the presence of hexagonal BN and 13-Si3N4 in some
NMR of Other Commonly Studied Nuclei
431
of the pyrolysed precursors, consistent with undesired cleavage of the Si-C bonds in these compounds. The double resonance experiments show that in the most successful precursors, the intermediate-range order is characterised by an agglomeration of B-N-B and Si-N-Si linkages forming B-rich and Si-rich domains into which carbon enters in the cationic positions, replacing both Si and B (van Wtillen and Jansen 2001). The synthesis of silicon boron oxycarbide glasses from gel precursors has also been studied by I~B and 29Si MAS NMR (Soraru et al. 1998). The 11B NMR spectrum of the as-prepared gel consists of a second-order quadrupolar lineshape with quadrupolar parameters (~iso = 19.4 ppm, XQ = 2.4 MHz, Xl = 0) characteristic of trigonal BO3 units such as in B203 or B(OH)3, indicating that the silicate gel network contains boron-rich regions containing only B-O-B or B-OH bonds. Heating at 1000~ produces an ~B NMR spectrum with two distinct trigonal BO3 second-order quadrupolar lineshapes, one with parameters corresponding to the gel precursor and the other (~iso = 12.7 ppm, XQ = 2.6 MHz, rl - 0) containing B-O-Si bonds as found in Pyrex glass. These results indicate that the product at 1000~ consists of an amorphous silicon boron oxycarbide network in which silicon oxycarbide units are interspersed with trigonal boron units. Heating to 1500~ completely removes the gel precursor 11B resonance but the quadrupolar lineshape attributed to the silicon boron oxycarbide network remains unchanged. The 298i NMR spectrum of the glass heated at 1500~ suggests it to be a borosilicate matrix containing a dispersion of [3-SIC nanocrystals (Soraru et al. 1998).
7.2.4 t~B N M R of zeolites The technically important zeolite ZSM-5 has an open framework structure in which some of the tetrahedral silicon is replaced by aluminium, with charge balance provided by cations located in the structural cavities. Boralite is the boron analogue of ZSM-5 in which boron atoms rather than aluminium atoms are substituted for silicon. However, the catalytic activity of boralite is less than the aluminosilicate ZSM-5 due to the lower strength of the Br~nsted acid sites. The coordination state of the framework boron in H-boralite has been determined by ~B MAS NMR (Scholle and Veeman 1985). The single resonance at - 3 ppm indicates that the boron is present in highly symmetrical tetrahedral BO4 sites (Figure 7.18A). On dehydration, the intensity of this single ~IB line decreases and a new second-order quadrupolar lineshape appears (Figure 7.18B), with parameters ~i~o = 10 ppm, XQ = 2.55 MHz, ~q = 0, indicating the conversion of the BO4 units into planar BO3 units. ~B MAS NMR also indicated that this conversion is reversed on rehydration (Scholle and Veeman 1985). These results were subsequently confirmed by Axon and Klinowski (1994) for samples prepared in a fluoride medium. The reversibility of the dehydration process was also found to depend on the presence of protons; exposure to protonated organic solvents restored
432
A
Multinuclear Solid-State NMR of lnorganic Materials
Hydrated
B
Dehydrated
C
Partially dehydrated
BOs ~ 30 10 -10 -30
30 10 -10 -30
I~B shift (ppm) w.r.t. BFs.Et20
M
~
BO4 ~
H
z
toa F1 2r
F2
Figure 7.18. A. liB MAS NMR spectrum of as-prepared hydrated boron-ZSM-5 (boralite). B. I~B MAS NMR spectrum of fully hydrated boralite. C. Two-dimensional liB nutation spectrum of partially dehydrated boralite showing the weak presence of BO4 groups. The corresponding ~lB MAS NMR spectrum is shown above. From Axon and Klinowski (1994) by permission of the American Chemical Society. the single BO4 resonance but aprotic solvents did not have this effect, suggesting that solvation by the absorbed molecules of the proton associated with the framework results in a lengthening of the O-H bond allowing a more symmetric arrangement of the B atoms. The simultaneous existence of both BO4 and BO3 units in partially dehydrated boralite has also been demonstrated by a nutation experiment (Sec. 3.6.1). The ~B nutation spectrum of partially hydrated boralite (Figure 7.18C) shows a strong line from BO3 groups overlapping a very weak line from BO4, which would not have been detected by conventional MAS NMR (Axon and Klinowski 1994).
7.3. 31p NMR 31p is a spin-l/2 nucleus with 100% natural abundance and a high resonance frequency, all factors which make it readily observable. Its chemical shifts, which are usually quoted with respect to 85% H3PO4, occur over a reasonably large range of values, and, as with 29Si, change with structure and composition. NH4H2PO4 is a good secondary shift reference at 0.9 ppm and can be used to set up 1H ~ 31p CP. The 31p MAS NMR spectra of a number of orthophosphates reported by Turner et al. (1986a) show that some compounds display small or zero chemical shift anisotropy (CSA) and their spectra are dominated by the main isotropic resonance, as in A1PO4 and Li3PO4 (Figure 7.19A). Other phosphates such as NaH2PO4.H20 and NaNH4HPO4.4H20 have
433
N M R of Other Commonly Studied Nuclei
A A I P O ~
10.0
Li3PO4 a-CaZnz(PO4)2
50
0
-50
NaH2PO4.H20
50
0
. . . . - ~ , ~--~,-=-=,,. . . . 50 0 -50 NaNH4HPO4.4H20
-50
50
0
31p shift (ppm) w.r.t. H3PO 4
-50
509 ' '20
'-lb ' '-4'0
31p shift (ppm) w.r.t. HsPO 4
Figure 7.19. A. Typical 31p MAS NMR spectra of phosphates, showing the complex side band patterns ocurring in some compounds as a result of significant CSA. From Turner et al. (1986a) by permission of the copyright owner. B. 31p MAS NMR spectrum of oL-CaZnz(PO4)2showing the isotropic resonances at 10 and 2.2 ppm corresponding to the two P sites. Note the differences in the intensities of the spinning side bands resulting from differences in the CSA of the two sites. From Cheetham et al. (1986) by permission of the Royal Society of Chemistry.
a large CSA, resulting in complex sideband patterns (Figure 7.19A). A compilation of the 3~p CSA parameters for various phosphate units has been published by Duncan and Douglas (1984). The 3~p MAS NMR spectra of a number of double phosphates have been acquired by Cheetham et al. (1986), many of which have more than one site, each with its own set of spinning sidebands from which the CSA can be derived (see Chapters 2 and 3). An example is provided by the 3~p MAS NMR spectrum of oL-CaZnz(PO4)2 (Figure 7.19B), which contains two P sites with ~iso values of 10.0 and 2.2 ppm and associated CSA values of - 35.4 and - 64.2 ppm respectively (Cheetham et al. 1986). A selection of 31p NMR parameters for phosphorus compounds is presented in Table 7.3. The chemical shifts for the crystalline phosphates follow the broad trends shown schematically in Figure 7.20, but the overlaps in such a diagram make it suitable for determining only the broader aspects of phosphate structure rather than the structural details. The structures of some of the solid inorganic phosphates such as Cd3(PO4)2 can be complex, containing in this case six crystallographically independent P sites and nine independent Cd sites. The 3~p MAS NMR spectrum of Cd3(PO4)2 (Figure 7.21A) shows resonances from all six P sites (Dusold et al. 1997) which have been related to those in the known structure by a homonuclear 3~p_3~p dipolar coupling experiment (Dollase et al. 1997). Since the magnitude of the dipolar coupling constants strongly depends on the internuclear distances, a two-dimensional single-quantum/double-quantum spectrum
434
Multinuclear
Solid-State NMR
of Inorganic Materials
Table 7.3. 31p NMR parameters for phosphorus compounds. Compound
giso (ppm)*
Reference
A1PO4 GaPO4 YPO4 Li3PO4 Na3PO4 Na3PO4.12H20 K3PO4 Be3(PO4)2 Ca3(PO4)2 Mg3(PO4)2 oL-Zn3(PO4)2 [3-Zn3(PO4)2 site 1 site 2 Cd3(PO4)2 site 1 site 2 site 3 site 4 site 5 site 6 BPO4 LaPO4.nH20 Cd2P207 site 1 site 2 Na4P207 site 1 site 2 Na5P3Olo middle unit end unit NavP5016 middle unit end unit K4P207 ot-Ca2P207 site 1 site 2 [3-Ca2P207 site 1 site 2 site 3 site 4 c~-Mg2P207 site 1 site 2 Mg2P4012 site 1 site 2
- 24.5, - 25.9 - 9.8 -0.9 10.8, 10.0 13, 14.1 7.8 11.7 - 9.2 3.0 4 . 6 , - 1.9 3.7
Turner et al. (1986a), Cheetham et al. (1986) Turner et al. (1986a) Turner et al. (1986a) Turner et al. (1986a), Cheetham et al. (1986) Dupree et al. (1988), Ducel et al. (1994) Turner et al. (1986a) Grimmer & Haubenreisser (1983) Turner et al. (1986a) Turner et al. (1986a) Turner et al. (1986a), Cheetham et al. (1986) Cheetham et al. (1986)
2.5 7.2
Cheetham et al. (1986) Cheetham et al. (1986)
22.0 21.5 19.5 12.0 10.6 7.9 - 29.5, - 31.2 -3.3
Dusold et al. (1997) Dusold et al. (1997) Dusold et al. (1997) Dusold et al. (1997) Dusold et al. (1997) Dusold et al. (1997) Turner et al. (1986a), Ducel et al. (1994) Turner et al. (1986a)
-4.7
Cheetham et al. (1986) Cheetham et al. (1986)
1.6, 1.3 2.5 -8.9 0.2 16.6 1.0 0.9
Cheetham et al. (1986), Ducel et al. (1994) Cheetham et al. (1986) Ducel et al. (1994) Ducel et al. (1994) Griffiths et al. (1986) Griffiths et al. (1986) Dupree et al. (1988)
-7.8 10.7
Cheetham et al. (1986) Cheetham et al. (1986)
-6.3 -7.8 -9.1
Cheetham Cheetham Cheetham Cheetham
- 1 . 9
-
-
10.0
-
13.1
et et et et
al. al. al. al.
(1986) (1986) (1986) (1986)
- 20.0
Cheetham et al. (1986) Cheetham et al. (1986)
-31.6 - 35.3
Cheetham et al. (1986) Cheetham et al. (1986)
NMR of Other Commonly Studied Nuclei
Table 7.3. (Continued) Compound
8iso (ppm)*
Reference
site 1 site 2
-6.3 -9.1
Cheetham et al. (1986) Cheetham et al. (1986)
site 1 site 2 site 3
- 15.0 - 17.8 - 20.6
Cheetham et al. (1986) Cheetham et al. (1986) Cheetham et al. (1986)
site 1 site 2
- 20.0 - 28.4
Cheetham et al. (1986) Cheetham et al. (1986)
site 1 site 2
- 23.4 - 29.0
Cheetham et al. (1986) Cheetham et al. (1986)
2.5 10.0 3.7 -24.0 0.9 1.5 6.6 2.3 4.3 2.1 -0.6 1.6
Cheetham et al. (1986) Cheetham et al. (1986) Cheetham et al. (1986) Cheetham et al. (1986) Turner et al. (1986a) Turner et al. (1986a) Turner et al. (1986a) Turner et al. (1986a) Turner et al. (1986a) Turner et al. (1986a) Turner et al. (1986a) Cheetham et al. (1986)
-0.9 0.8
Cheetham et al. (1986) Cheetham et al. (1986)
- 5.3 -0.6
Cheetham et al. (1986) Cheetham et al. (1986)
-0.5 - 2.8 5.1
Turner et al. (1986a) Turner et al. (1986a) Turner et al. (1986a)
10.0 2.2 - 18.7
Cheetham et al. (1986) Cheetham et al. (1986) Clayden (1987)
-9.4 - 27.4 - 4 6 to - 5 4 - 15.1 1793, 1788 123
Clayden (1987) Clayden (1987) Mudrakovskii et al. (1985) Turner et al. (1986a) Fur6 et al. (1990) Fur6 et al. (1990)
oL-Sr2P207
oL-Zn2P207
NaA1P207
KA1P207
KZn2(PO4)3 site 1 site 2 MgZnz(PO4)2 NaZrz(PO4)3 NH4HzPO4 (NH4)zHPO4 NazHPO4 NaHzPOmH20 KHzPO4 KzHPO4.3H20 CaHPO4 CaHPO4.2H20 Ca(HzPO4)2 site 1 site 2 Ca(HzPO4)z.H20 site 1 site 2 BaHPO4 site 1 site 2 NaNHaHPO4.4H20 oL-CaZnz(PO4)2 site 1 site 2 oL-Zr(HPO4)z.2H20 ~/-Zr(HPO4)z.2H20 site 1 site 2 SiP207 P2Os.24WO3.nH20 Ni3P Cu3P
* chemical shifts reportedwith respectto 85% H3PO4.
435
436
Multinuclear Solid-State NMR of Inorganic Materials Q2 aluminophosphate M + & M 2+ aluminophosphate AI orthophosphate
, Q3 -
Q~ Q,
I !
Q0
1 L
i
i
-10
10
.
!
i
i
-30
-50
31p shift (ppm) w.r.t. H3PO 4 Figure 7.20. Schematic diagram of the range of 31p chemical shifts in crystalline phosphate and aluminophosphate phases. The Q0 range refers to the alkali and alkaline earth orthophosphates, Q~ denotes the end groups, Q2 the middle and ring groups and Q3 the branching groups in these compounds. The upper three bands refer to aluminophosphates, including those of the alkali and alkaline earth metals. From data of Turner et al. (1986a).
A
B
AB C
ABC
D E F
D E F .....m./
~
20 "_. ;
..
,.
g" .
.
~..._ '
*.
I
30
.
~
i
_ t
I
25 20 15 10 5 31p shift (ppm) w.r.t. H3PO 4
.~
~
20
15
10
5
O
Single quantum dimension (ppm) ~1 Figure 7.21. A. 31p MAS NMR spectrum of Cd3(PO4)2 showing the six resolved P resonances corresponding to the six independent structural sites. B. Two-dimensional 31p double-quantum spectrum of Cd3(PO4)2 correlated to the single-quantum dimension indicating the double-quantum coherences between the six different 31p resonances. The strongest correlation is between A and B, indicating that A/B is associated with the shortest P-P distance. Similar considerations allow the other resonances to be assigned to the other crystallographic sites on the basis of their X-ray P-P distances. From Dollase et al. (1997) by permission of the American Chemical Society.
437
NMR of Other Commonly Studied Nuclei
(Figure 7.21B) reveals distance connectivities which can be compared with the known distances in the X-ray crystal structure, thereby allowing the various 31p resonances to be assigned to their correct crystallographic sites (Dollase et al. 1997). These assignments have been confirmed by a two-dimensional 31p spin diffusion MAS NMR measurement, allowing the assignment of the nine Cd sites in the 113Cd NMR spectrum to then be made from one-dimensional selective 31P-113Cd CP measurements (Dusold et al. 1997). Thorium phosphate-diphosphate, Th4(PO4)4P207, is a promising material for the long-term immobilisation of radioactive wastes containing tetravalent actinides. Details of its crystal structure have been determined by 3~p MAS NMR and 2D double quantum experiments to probe the dipolar interaction between connected 3Jp nuclei (Pichot et al. 2001). The 31p MAS NMR spectrum is characterised by two peak maxima which can be deconvoluted into five components (Figure 7.22A). Since the two phosphorus atoms of the P207 group were shown by the 2D connectivity experiment to be equivalent (Figure 7.22B), the four remaining components of the deconvoluted MAS spectrum of approximately equal intensity were assigned to the two inequivalent PO4 groups, each of which is split by its proximity to two configurations of the P207 group. The resulting structure refinement is in good agreement with the general structural principles of phosphate compounds (Pichot et al. 2001). Compounds with structures related to ZrP207 show unique negative isotropic thermal expansion over a broad temperature range up to at least 950~ The 31p MAS NMR
A 3 /
4
-40
m
2
-35
-15.6
-19.0
-22.4
31p shift (ppm) w.r.t. H3PO4
i
-16
i
i
-18
-20
'! ....
-22
~"~2(ppm)
Figure 7.22. A. 31p MAS NMR spectrum of Th4(PO4)4P207 deconvoluted into five peaks. Peak three corresponds to the equivalent P atoms of the P207 unit, the other four peaks correspond to the two inequivalent PO4 units, each split by its proximity to two configurations of the P207 unit which differ in the position of the bridging oxygen. B. 2D double-quantum 31p spectrum demonstrating the connectivity between two equivalent coupled P nuclei (P207 unit) with the same chemical shift ( - 18.6 ppm). The ~12 dimension is the 2D spectrum filtered by the double quantum, the f~ dimension is the double quantum spectrum. From Pichot et al. (2001) by permission of Elsevier Science.
438
Multinuclear Solid-State N M R of Inorganic Materials
spectrum of ZrP207 has been fitted to 11 mainly Lorentzian peaks consistent with the space group Pa-3, and one Gaussian peak ascribed to an impurity phase (Korthuis et al. 1995). More recently the structure of the low-temperature phase of ZrPzO 7 has been refined using 3~p MAS NMR, 2D exchange MAS NMR and 2D MAS double quantum spectroscopy to provide information about the P connectivity and phase-purity of the sample (King et al. 2001). The results indicate that the 13 observed 31p resonances are related to 13 different P sites in a single phase, ruling out the Pa-3 space group previously proposed for the ZrP207 superstructure for which only 11 sites would be expected. The connectivity data for this phase indicate that the correct space group is Pbca (King et al. 2001). The room-temperature 3~p NMR spectrum of the related phase ZrVPO7 shows a line at 30 ppm arising from V-O-P bonds and a weaker line at 44 ppm corresponding to P-O-P bonds. The relative intensities of these two peaks indicate that the sites are not randomly occupied, but that PVO7 groups are favoured over a mixture of P207 and V207 groups in this compound (Korthuis et al. 1995). The cation distributions in solid solutions Zn3-xMgx(PO4)2 for x ranging from 0 to 3.0 have been studied by 3~p MAS NMR (Jakeman et al. 1985). For values of x > 1, four phosphorus sites could be distinguished, the relative intensities of which allowed the distribution of the cations to be calculated at each composition. Structural details of two related magnesium phosphates phosphoellenbergerite and holtedahlite have been determined by 31p MAS and CP MAS NMR, together with 1H NMR (Brunet and Schaller 1996). Both minerals contain protonated and unprotonated PO4 tetrahedra, identified with 3~p resonances at - 7 . 0 to - 8 . 7 ppm and 1 to - 4 ppm respectively. These assignments were confirmed by CP NMR measurements from which the relative intensities of the protonated and non-protonated tetrahedra were also determined. The NMR measurements were thus able to detect and characterise sites containing too small a proportion of the total protons to study by X-ray diffraction techniques (Brunet and Schaller 1996). Accurate measurements of the small 3~p Knight shifts in crystalline Ni3P and Cu3P alloys indicate that the effect is much smaller in Ni3P, although spin-lattice relaxation time measurements indicate the operation of a typically metallic Korringa-type relaxation mechanism in both materials (Fur6 et al. 1990). Heteronuclear J-coupling between nearest-neighbour 31p and ~3In in undoped InP has been studied by ll3In-31p polarisation transfer using rapid MAS (Tomaselli et al. 1998).
7.3.1 Relationships between 31p NMR parameters and structure Direct relationships between the 3~p NMR parameters and structure are complicated by the fact that the chemical shifts are determined in phosphorus compounds predominantly by the paramagnetic rather than the diamagnetic contribution to the shielding tensor. This paramagnetic term is not necessarily directly related to the bonding electron
NMR of Other Commonly Studied Nuclei
439
distribution, and hence any relationships with structural parameters are most likely to be fortuitous. Nevertheless, empirical relationships have been found for small groups of phosphorus compounds. Starting from the concept that the 31p chemical shift is influenced by the number and electronegativity of the nearest-neighbour ligands, the bond angles around the P atom and the occupation of the v-bonding orbitals on the P atom, Turner et al. (1986a) have reasoned that for a restricted group of orthophosphates, the neighbouring ligands and bond angles should be similar, making w-bonding the most important factor. The expected linear relationship between 8iso and n~ (the number of w-electrons per P atom) has been demonstrated for a group of anhydrous orthophosphates with only one cation and no other anions present (Figure 7.23A). This relationship is described by ~iso (ppm) -- - 155n~ + 192
(7.12)
On the basis that the effects of nearest neighbour cations are generally of use in deducing structures, relationships have also been sought between 8iso and the Pauling electronegativity (EN) or simple functions of the charge (Z) and ionic radius (r) of the neighbouring cations present (Figure 7.23B) (Turner et al. 1986a). For the same restricted group of orthophosphates, the following relationships were deduced: giso(ppm)- - 35.5(EN) + 41.4
(7.13)
giso(ppm) = - 7.7(Z/r) + 18.6
(7.14)
A
B
20
20
o
r -~
-20
-20
!
1.2
1.3
1.4
No. of e-s in ~r-orbitals (n~)
!
3
J
6
Z/r
Figure 7.23. A. Relationship between 31p ~iso values and the number of p-electrons (rip) for six simple orthophosphates. B. Relationship between 31p ~iso and the cation charge and radius for the same group of simple orthophosphates. Plotted from the data of Turner et al. (1986a).
440
Multinuclear Solid-State NMR of Inorganic Materials
Cheetham et al. (1986) noted that the paramagnetic factor which determines the chemical shift in phosphorus compounds is inversely related to the bond strength of the coordinating oxygen atoms calculated by the method of Brown and Shannon (1973). Their plot of ~iso as a function of the summed oxygen bond strengths S(O 2-) (in valence units) of 22 phosphates, including representative orthophosphates, higher phosphates, hydrogen phosphates and double phosphates, shows a high degree of scatter (Figure 7.24A), but is fitted by the line: ~iso (ppm) - 33.9 S(O 2-) - 253
(7.15)
An alternative approach is based on the concept that the CSA in phosphorus compounds is directly related to w-bonding effects, which are in turn inversely related to the P-O bond length. Grimmer (1978) demonstrated a good linear relationship between the 3~p CSA and the P-O bond lengths along the 3-fold axis for a small series of molecular phosphoryl compounds (POF3, POC13, POBr3, POMe3) and three other compounds (P401o, K3PO4, K4P207), described by the line: 31p CSA (ppm) - - 1585(P-O) (,~) + 2553
(7.16)
A similar plot for a restricted group of orthophosphates (Turner et al. 1986a) also gives a straight line, but with a different equation, indicating that these relationships are specific to related groups of compounds, and cannot be applied generally" 31p CSA (ppm) - 5073(P-O) (,~) - 7754
A
(7.17)
B
-40
.o
"
~" 105
-20 ._~ c,O
~
9 ~
0
e~
~ 20
~ ~
o o~
o~
~
J i
7.6
70
35 I
I
I
7.9
I
!
!
I
8.2
S ( 0 2") (valence units)
I
!
8.5
1
2
3
4
Distortion index (DI)
Figure 7.24. A. Relationship between 31p ~iso values and the summed bond strengths of the oxygen atoms for a number of phosphates. Open symbols denote orthophosphates, closed symbols denote higher phosphates. From Cheetham et al. (1986), by permission of the Royal Society of Chemistry. B. Relationship between the 3~p CSA for several orthophosphates and the geometrical distortion index (DI) of the phosphate group. From data of Turner et al. (1986a).
NMR of Other Commonly Studied Nuclei
441
A better correlation has been demonstrated for the same restricted group of phosphates between the CSA and the distortion index (DI) describing the mean deviation of the PO4 bond angles from ideal tetrahedral symmetry: DI = 10i- 0ol/n
(7.18)
Where 0i is the ideal tetrahedral angle (109.5 ~ and 0o is the actual O-P-O angle. The resulting linear relationship between the CSA and DI (Figure 7.24B) is: 3~p CSA (ppm) = 30.90(DI) - 1.22 7.3.2 31p N M R
(7.19)
of glasses
7.3.2.1 Binary phosphate glasses. Phosphate glasses are technically important because their thermal expansions are generally greater and their transition temperatures are generally lower than silicate or borate glasses. Metaphosphate glasses are composed of long phosphate chains which can be cross-linked by decreasing the concentration of network modifiers or adding oxides such as A1203.31p MAS NMR has been used to identify the phosphate species present in a series of simple binary sodium phosphate glasses and investigate the changes in the distribution of these species with glass composition (Brow et al. 1990). The phosphate units present in these glasses (defined in terms of Qn where n is the number of bridging oxygens per phosphate tetrahedron) were identified by comparing their isotropic 31p shifts with those of known phosphate structures (Figure 7.25A). The 31p NMR spectra show that Q2 tetrahedra are dominant over the whole range of glass composition, with Q~ tetrahedra also present in glasses with < 50 mol % P205 and Q3 tetrahedra also present in glasses with > 50 mol % P205 (Figure 7.25A). These results provide evidence for a polymerisation model of the glass structure which takes into account the effect of H20 especially in the high phosphate compositions (Brow et al. 1990). 3~p MAS NMR has been used to provide qualitative chemical bonding information about the environment of the phosphate tetrahedra in a series of metaphosphate glasses (Brow et al. 1991). The effect of adding a second alkali ion (Li +) to sodium phosphate glasses has also been studied by 31p MAS NMR, showing that the 31p shifts become less shielded (less negative) as the Na-content increases (Figure 7.25B, Sato et al. 1992). This result has been explained in terms of an increase in the average electron density of the P-O bonds giving rise to increased paramagnetic deshielding in samples with greater [Na]/[NaLi] ratios. A 31p MAS NMR study of a series of binary alkaline earth phosphate glasses in the highphosphate composition region (Losso et al. 1992) has shown a linear change in the ~iso and CSA values of both the Q2 and Q3 units with composition. By contrast with Brow et al. (1990) who discussed their results in terms of changes in the bond order of the
442
Multinuclear Solid-State NMR of Inorganic Materials A
U
QO Q1
Q2
9
Q3
.
_
_20 f
~
t
9 /
9
9
9
9
increasing Na 9
20
I
;
31p s h i f t
,
,j
-20 (ppm)
~
-22
9 9
~
O I
-40
w.r.t. H3PO 4
~,
-60
-24
0.2
0.6
1.0
[Na] / [Na + Li]
Figure 7.25. A. 3ip chemical shifts for a series of sodium phosphate glass compositions compared with those of known tetrahedral phosphate units. From Brow et al. (1990). B. 31p chemical shifts for a series of sodium lithium phosphate glasses as a function of the mixed alkali ratio. From Sato et al. (1992). Both figures used by permission of Elsevier Science. non-bridging oxygens, Losso et al. (1992) advanced an explanation in terms of systematic changes of bridging and dihedral angles between the phosphate tetrahedra and changes in the cation positions with composition. One-dimensional 31p NMR is only able to probe the the local structure of phosphate glasses. To gain information about the manner in which the different Qn units or domains in phosphate glasses are connected, two-dimensional MAS exchange experiments have been used in which radio-frequency dipolar recoupling (RFDR) techniques allow the reintroduction of the through-space 3~p 31p dipolar-dipolar interaction even in the presence of fast magic angle spinning. The utility of this technique as a tool for studying phosphate glasses was first demonstrated for a binary sodium phosphate glass by J~iger et al. (1994). The technique has also been used in a study of anhydrous lithium ultraphosphate glasses to determine the connectivity distributions of the various Qn species from the exchange cross-peak intensities which are governed by the fraction and type of linkage between the various species (Alam and Brow 1998). The resulting connectivity distributions can be described by a statistical model consistent with a mechanism of random depolymerisation in the glasses. A high-resolution double-quantum method for determining structural connectivity relationships in glasses has been applied to binary sodium phosphate glasses, allowing more extended structural features to be probed (Feike et al. 1998). The technique provides more selective information about the connectivities of different polyhedra than the simpler 2D exchange method since it takes into account the chemical shift information concerning the local geometry (bond lengths and angles) of the connected Qn groups. At least three Q2 groups have been identified in sodium phosphate glasses using this method (Feike et al. 1998).
NMR of Other Commonly Studied Nuclei
443
31p MAS NMR and 2D double-quantum NMR have been used to study a range of binary magnesium phosphate glasses (Fayon et al. 2001). Simulation of the MAS spectra indicated a binary distribution of the Qn phosphate units, consistent with the action of Mg 2+ as a network modifier. The intermediate-range order in these glasses, deduced from the double-quantum experiments, suggests a change from phosphate rings to chains in glasses near the metaphosphate composition (Fayon et al. 2001). A 31p MAS NMR study of a series of calcium phosphate glasses has been undertaken because of the possible biocompatibility of these materials (Fletcher et al. 1993). The 3~p chemical shift of the glasses becomes less shielded (less negative) with increasing Ca content (a trend also noted in the Na phosphate glasses) but the chemical shifts of the Ca glasses are consistently more negative by about 8 ppm than in the corresponding Na glasses, as expected from the relative electronegativities of Na and Ca (Fletcher et al. 1993). Zinc ultraphosphate glasses containing significant concentrations of hydroxyl groups (up to 14 mol%) have been studied by 3~p MAS NMR and ~H-3~P CP MAS NMR (Mercier et al. 1998). Measurements of spin-lattice and spin-spin relaxation times indicate that the water molecules are strongly absorbed and that the protons are homogeneously distributed in the glass matrix. The CP experiments are capable of distinguishing P sites bonded to H and Zn from those connected to other PO4 groups (Mercier et al. 1998). A 31p NMR study of a wider range of zinc phosphate glass compositions has shown that the ultraphosphates contain mainly ring structures whereas only chain units are present in the zinc metaphosphate glasses (Wiench et al. 2000). This conclusion was based on simulation of the solid state NMR lineshapes and the measured values of the CSA which is larger in ring structures than in chains. Since the zinc phosphate glasses are soluble in water, the solid state NMR was augmented by solution NMR measurements to provide more quantitative information about the glass structure (Wiench et al. 2000). 3~p MAS NMR has been used to study a series of rare-earth phosphate glasses as a function of the rare-earth atomic number and glass composition (Cole et al. 2001). The results illustrate the high sensitivity of the Qn phosphate speciation to changes in the composition, showing a preponderence of Q2 species in the ultraphosphate composition range, with an increasing concentration of Q3 species as the rare-earth concentration decreases. The application of 31p NMR to structural studies of alkali and alkaline earth phosphate and aluminophosphate glasses, fluorophosphate glasses and phosphorus oxynitride glasses has been reviewed by Kirkpatrick and Brow (1995). 7.3.2.2 Phosphosilicate glasses. The formation of phosphosilicate glasses by thermal treatment of gel precursors has been studied by 31p MAS NMR (Clayden et al. 2001). The dried gels were found to consist of siloxane frameworks containing trapped
444
Multinuclear Solid-State N M R o f Inorganic Materials
molecules of orthophosphoric and pyrophosphoric acid. The temperature at which copolymerisation of the phosphate and silicate tetrahedra occurs was found to depend on the gel composition, decreasing with higher phosphate content. The higher-P samples were found however to remain amorphous to higher temperatures. Chemical heterogeneity on a nanometre scale has been studied in phosphosilicate gels by 2D NMR magnetisation exchange spectroscopy (Clayden et al. 2001a). Monomeric phosphorus was found near to singly cross-linked P-P but the location of other types of phosphorus species was more remote. The close association observed between QO and Q1 phosphorus units is thought to involve hydrogen bonding. 31p MAS NMR has shown that the addition of small concentrations of P205 to alkali disilicate glasses results in scavenging of the modifier cations by the phosphorus, producing orthophosphate and pyrophosphate units with the elimination of non-bridging oxygens in the silicate network and the conversion of Q3 silicon units to Q4 (Dupree et al. 1988). This is illustrated in Figure 7.26 for sodium silicate glass, where increasing the P205 content to 5 mol % results in a progressive replacement of the 31p MAS NMR orthophosphate resonance at ---15 ppm by a resonance at ---2.5 ppm with a CSA of---125 ppm corresponding to Na4P207. As the P205 content is increased to 30 mol %, the phosphate units increasingly polymerise, first to pyrophosphate dimers then to metaphosphate chains with a single characteristic resonance at - 16 ppm (Dupree et al. 1987). At still higher P205 contents, a new 31p resonance appears SIP207 pyro
P2Os
or,ho
,
I
100
I
I
0
'm7%'
i
f
-100
P20s
(mole %)
metal _
I
200
,
I
,
I
0
i
I
~
!
-200
3~p shift ( p p m ) w.r.t. H3PO4
Figure 7.26. Changes in the 3Ip MAS NMR spectrum of sodium disilicate glass with increasing additions of P205. At the lower concentrations the orthophosphate resonance at 15 ppm is progressively replaced by pyrophosphate at 2.5 ppm. At higher P205 concentrations the metaphosphate chain units ( - 16 ppm) are replaced by SiP207 units ( - 33 to - 40 ppm). All peaks other than the marked isotropic peaks are spinning side bands. From Dupree et al. (1987, 1988) by permission of MacMillan Magazines Ltd and the Society of Glass Technology.
445
NMR of Other Commonly Studied Nuclei
at - 32.7 to - 39.5 ppm, attributed to phosphorus in an environment similar to that of crystalline SiP207. The identification of SiP207 units in these glasses is supported by the presence of a 29Si peak at - 213 ppm, similar to the spectrum of 6-coordinated Si in cubic SiP207 ( - 214 ppm) and Si50(PO4)6 ( - 217 ppm) (Dupree et al. 1987). Similar results have also been found for the addition of P205 to K20-SiO2 glasses of potassium tetrasilicate composition (Lockyer et al. 1995a). 31p NMR structural studies of glasses containing both phosphorus and silicon have been reviewed by Kirkpatrick and Brow (1995). 7.3.2.3 Alkali borophosphate glasses. 31p MAS NMR has been used to study the phosphorus environment in a series of sodium borophosphate glasses in the system (1-x)NaPO3-xNazB207 (Ducel et al. 1994). The 31p spectra of these glasses were interpreted by comparing their isotropic chemical shifts with the known shifts for the various types of polymerised borophosphate species known to occur in such systems (Figure 7.27A). The appearance and disappearance of the various 31p resonances and the changes in their chemical shifts with the glass composition (Figure 7.27B) provided a structural hypothesis for these glasses as a function of their composition (Ducel et al. 1994). Other 31p NMR studies of borophosphate glasses have been reviewed by Kirkpatrick and Brow (1995). 7.3.2.4 Borosilicophosphate glasses. A 31p MAS NMR study of a series of sodium borosilicophosphate glasses of low alkali content has been made to investigate the
A
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Figure 7.27. A. Schematic representation of the 31p ~iso ranges for the various structural units occurring in borophosphate glasses, re-drawn from Ducel et al. (1994). B. Changes in the 31p isotropic chemical shifts of the main structural units in sodium borophosphate glasses as a function of composition. From Ducel et al. (1994), by permission of the copyright owner.
446
Multinuclear Solid-State NMR of Inorganic Materials
competition between the three acidic oxides SiO2, B203 and P205 for the available basic Na20 component (Yamashita et al. 1999). The NMR spectra were deconvoluted to provide an indication of the different phosphate species as a function of composition, suggesting that the P205 reacts preferentially with the Na20 and B203. When heated in the presence of a reducing agent such as NH4HzP204 or elemental Si, glasses in the borosilicophosphate system form hydrogen gas-ceramics consisting of a glass foam in which uniformly sized bubbles of hydrogen gas are encapsulated. 3~p MAS NMR (Youngman et al. 2000) shows that heat treatment of the glass in the absence of the reducing agent results in the appearance of a dominant resonance at - 30.5 ppm corresponding to the formation of phosphorus units with all boron next-nearest neighbours, as in crystalline BPO4. The reduced glasses show a similar 3~p speciation, but in addition contain spectral features in the region - 400 to - 600 ppm. A more shielded 31p peak at - 532 ppm which shifts to - 464 ppm during crystallisation of BPO4 accompanied by the nucleation of molecular hydrogen bubbles is consistent with the presence of gaseous P4 ( - 552 and - 461 ppm), but the concentration of this species is too small to account totally for the foaming. 3~p CP MAS NMR spectra of the reduced samples suggest that the resonance at - 30 ppm is possibly due to interactions between molecular hydrogen and the phosphorus in the B PO4 units (Youngman et al. 2000).
7.3.2.5 Phosphoaluminosilicate glasses. The solution of phosphorus in aluminosili-
cate glasses is of considerable importance to the development of technology for disposing of hazardous wastes by vitrification. 3~p MAS NMR spectra of alkali phosphosilicate glasses in which the P205 is progressively replaced by A1203 are characterised by an isotropic resonance at - 3 3 ppm attributed to an SiP20: environment (Dupree et al. 1989). These spectra show little change up to about 18% A1203 substitution, but at A1203 substitutions up to 39% the 3~p resonance occurs at - 21.5 ppm, characteristic of an A1PO4 environment. These results have been taken to indicate that the metaphosphate structure of the glass is destabilised when half the phosphate content is replaced by alumina (Dupree et al. 1989). Another model for the solution of phosphorus in aluminosilicate glasses has been proposed on the basis of 3ip MAS NMR studies (Toplis and Schaller 1998) which suggest that peralkaline glasses contain, in addition to M3PO4 and M4P207 units, PO4 tetrahedra coordinated to between one and three A1 atoms in the aluminosilicate network. Direct evidence of such P-to-A1 and P-to-Na coupling has been provided by TRansfer of Population in DOuble Resonance (TRAPDOR) experiments which reveal the shortrange dipolar couplings between the two nuclei by observing the effects on the 31p MAS echo spectrum caused by simultaneous irradiation of the 27A1 or 23Na resonance frequency (Schaller et al. 1999). These experiments, which have the advantage of allowing the identification of species with different heteronuclear couplings even in glassy
NMR of Other Commonly Studied Nuclei
447
systems where the chemical shift distribution produces typically broad resonances, confirm that glasses in the composition range Na/(Na + A1) = 0.6 to 0.7 contain PO4 tetrahedra connected to (2Na + 1A1) and (2A1 + 1Na) (Schaller et al. 1999).
7.3.2.6 Alkali phosphoaluminoborosilicate glasses. The interest in glasses of this complexity stems from the need to dispose of waste radioactive materials. One method involves vitrification in a borosilicate matrix. In such systems, aluminium, silicon and boron components are included to promote glass formation, whereas other components such as phosphorus enter the glass as part of the waste stream. Because of the difficulties associated with the uptake of phosphorus, studies of model P-containing glasses have been made by multinuclear NMR, including 3~p MAS NMR and TRAPDOR experiments exploring the connectivities of 3~p to 23Na, 27A1 and ~IB (Rong et al. 1998). At least six distinct phosphorus environments were discovered in these glasses, some of them corresponding to separate dispersed orthophosphate and pyrophosphate phases.
7.3.2.7 Phosphorus chalcogenide glasses. The binary systems P-S, P-Se and P-Te form glasses which have been studied by 31p spin-echo and MAS NMR techniques as reviewed by Mutolo et al. (1999). The spin-echo experiments are sensitive to the extent of P-P bonding, whereas the MAS NMR experiments differentiate between three and four-coordinated P sites. 31p chemical shift information and data on the dipole-dipole coupling in these glasses has allowed the competition between homopolar P-P and heteropolar P-S and P-Se bonding to be quantified and has revealed a hierarchy in the competitive formation of P-S-, P-Se- and P-Te- bonds (Mutolo et al. 1999). None of the glasses studied so far has shown any evidence for the formation of P-Te bonds. Glass formation and local structure in the related system P-Se-A1 has also been studied by 31p and 27A1NMR which indicates that the local environments of both the P and A1 are dominated by the selenium (Huda|la et al. 1998). Although there is evidence of interaction between the A1 and P (the formation of four-coordinated P is facilitated by the presence of A1), a more quantitative assessment by 27A1{31p } spin echo double resonance indicates that direct A1-Se-P links occur with less than statistical probability. These results suggest a glass structure which is partially segregated into phosphorus selenide and aluminium selenide-rich regions (Hudalla et al. 1998). The addition of phosphorus to glasses in the system Ge-As-S decreases their refractive index, making them possible cladding materials for GeAsS optical fibres. A 3~p NMR study of a series of GeAsPS glasses indicates that the dominant P species at the stoichiometric composition are tetrahedral S = PS3/2 groups (Aitken et al. 2001). These, together with trigonal PS3/2 groups, are replaced by P-P species, and ultimately
448
Multinuclear Solid-State NMR of lnorganic Materials
by P-(Ge,As) bonds as the sulphur content of the glasses decreases. The results suggest that P-P bonds form in preference to metal-metal bonds containing either As or Ge, and that the phosphorus tends to associate with As nearest neighbours rather than Ge (Aitken et al. 2001).
7.3.3 31p N M R o f A l P 0 4 molecular sieves
Aluminophosphates (A1PO4s) are a class of materials which, like the aluminosilicate zeolites, assume open framework structures containing channels of molecular dimensions with molecular sieve properties. Since the ALP04 structures contain equal numbers of A104 and P04 units there is no necessity for charge-balancing extraframework cations and consequently no sites to provide acid catalytic properties. The compound VPI-5 is a crystalline microporous AIPO4 with a large pore opening resulting from its 18-membered ring structure. As prepared, it contains hydration water molecules which can be reversibly removed stepwise to give an anhydrous form. Upon calcination it transforms irreversibly into another structure, designated A1PO4-8.31p and 27A1MAS NMR has been used to investigate the structural details of the various forms of VPI-5 and A1PO4-8 and to monitor the details of the dehydration-rehydration reactions (Martens et al. 1991). The 31p MAS NMR spectrum of fully hydrated VPI-5 contains three strong peaks, at - 34, - 28 and - 24 ppm corresponding to 3 crystallographically distinct phosphorus sites. Double-resonance heteronuclear correlation experiments between the 31p and 27A1 nuclei in this compound (dipolar-dephasing, CP, REDOR and TEDOR, Fyfe et al. 1992, 1992a, 1993) have shown that the 3 P sites are all connected to the 2 resolved (tetrahedral and octahedral) A1 sites (Figure 7.28A). On dehydration of VPI-5 to the fully anhydrous form, the 31p MAS NMR spectrum shows 3 31p resonances, at - 3 2 , - 2 7 and - 17 ppm (Martens et al. 1991), but in a more recent MAS NMR study, de Ofiate Martinez et al. (2000) have fitted the spectrum with a total of 7 peaks at - 15.0, - 17.5, - 22.3, - 27.1, - 30.6, - 32.4 and - 34.0 ppm, indicating that the symmetry is much lower than the expected topochemical space group of P63cm. The final product of thermal treatment of VPI-5 is the compound A1PO4-8, which itself exists in both hydrated and dehydrated forms. The dehydrated form has a broad, featureless 31p MAS NMR resonance centred at about - 29 ppm (Martens et al. 1991). The 31p MAS NMR spectrum of hydrated A1PO4-8 contains a major resonance at 31 ppm, with smaller features at about - 26 and - 23 ppm (Martens et al. 1991). The 3 distinct P environments in hydrated A1PO4-8 have been shown by two-dimensional cross-polarisation experiments between the 31p and 27A1 (Figure 7.28B) to be connected to both the tetrahedral and octahedral A1 sites (Fyfe et al. 1993). Two-dimensional correlation experiments have also been used to provide structural information about the compounds A1PO4-5 (Fyfe et al. 1996) and A1PO4-14 (Fyfe et al. 1997). The 31p MAS NMR spectrum of A1PO4-5 shows a single broad asymmetric -
449
NMR of Other Commonly Studied Nuclei
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Figure 7.28. A. Two-dimensional 2VA131p TEDOR spectrum of the A1PO4 VPI-5, showing the connectivities between the three P resonances and the two resolved A1 resonances (the six resonances within the dashed box). The asterisks denote spinning side bands within the projections. From Fyfe et al. (1992) by permission of Elsevier Science. B. Two-dimensional 27A1-31P CP heteronuclear correlation spectrum of A1PO4-8 showing the connectivities between all three of the resolved 31p sites with both the octahedral and tetrahedra127A1 peaks. Asterisks denote spinning side bands. From Fyfe et al. (1993) by permission of the American Chemical Society.
resonance centred at - 2 6 ppm with a small shoulder at about - 10 to - 2 0 ppm. Two-dimensional 27A1-31P cross-polarisation and TEDOR experiments show that the intense phosphorus resonance is associated with the tetrahedral A1 in the structure, while the downfield 31p shoulder is associated with octahedral A1. The phosphorus resonance also shows a correlation with a small A1(v~ peak in the 27A1 spectrum at 8 ppm, suggesting that this resonance arises from the coordination of one water molecule to a framework A1. When the sample was hydrated with D20 rather than H20, the efficiency of the coherence transfer from octahedral 27A1 to 31p was significantly improved, suggesting that the presence of an abundant third nucleus such as 1H may interfere with such coherence transfer experiments (Fyfe et al. 1996). The 31p MAS NMR spectrum of A1PO4-14 shows three resonances, at - 5.7, - 20.6 and - 24.5 ppm, the resonance at - 20.6 ppm containing contributions from 2 of the 4 P sites known to be present in this structure (Fyfe et al. 1997). Two dimensional cross-polarisation, TEDOR and INEPT experiments have allowed the connectivities to be unambiguously established between the 4 P sites and the 4 A1 sites (2 tetrahedral, 1 pentacoordinated and 1 octahedral) identified in this structure by 27A1MAS and DOR experiments at 3 magnetic fields (Fyfe et al. 1997). Silicon can also be incorporated into A1PO4 frameworks, altering the charge balance and introducing acid sites with their associated catalytic properties. An example of
450
Multinuclear Solid-State NMR of Inorganic Materials
such materials (called SAPOs) is SAPO-37, with a structure related to faujasite. The 3~p MAS NMR spectrum of SAPO-37 contains a single sharp resonance at - 26.1 ppm arising from phosphorus surrounded by four A1 next-nearest neighbours (Fyfe et al. 1995). Two-dimensional TEDOR experiments show that the phosphorus in this structure is strongly correlated with the tetrahedral A1 sites but not with the octahedral A1 known also to be present from 27A1 NMR. The latter is therefore thought to be not incorporated in the SAPO framework, but in an external amorphous alumina phase (Fyfe et al. 1995). Mixtures of transition metal (Mo or W) sulphides dispersed on ~/-alumina supports are used in hydrotreatment processes to remove sulphur, nitrogen, oxygen and metals from oil fractions. The addition of phosphorus to these catalysts enhances the solubility and stability of molybdate and improves the thermal stability of the alumina support. Solid state 31p double-resonance NMR experiments (3~P-27A1 REDOR and TRAPDOR) have been used to investigate the interaction between the impregnating phosphorus and the support surface (van Eck et al. 1995). The results showed that most of the phosphorus is in close contact with the aluminium, and that the layer of A1PO4 formed on the surface is not completely amorphous, but is slightly more ordered. 3~p MAS NMR spectroscopy has proved useful in determining the structures of several new A1PO4 compounds. The 3~p MAS NMR spectrum of a compound designated APO-CJ3 shows two principal signals at - 16 and - 32.7 ppm corresponding to 2 distinguishable P sites, the former associated with 3 A1(vI) and 1 A1(~v) atoms and the latter associated with 1 A1(v~) and 3 A1(~v) atoms (Wang et al. 2000). On removal of the template by heating at > 450~ the signal at - 16 ppm disappears as the structure transforms to a known compound, A1PO4-D (Wang et al. 2000). A new layered aluminophosphate designated Mu-4 shows 5 equally intense 3~p MAS NMR resonances at - 12.5, - 21.7, - 22.7, - 28.5 and - 31.8 ppm corresponding to the 5 distinguishable P sites in the structure (Marichal et al. 2000). A two-dimensional heteronuclear correlation experiment between 27A1 and 3~p was used to determine the association of the P atoms with the 3 distinct A1(Iv) and 1 A1(v) sites in the structure, showing that the 3~p resonance at - 32 ppm is the only 1 strongly correlated to every aluminium site. Similar considerations for the other P sites confirm the X-ray structure of this compound (Marichal et al. 2000). The structure of a new microporous gallophosphate with an 8membered ring opening, designated Mu-8, has been confirmed by 31p MAS NMR (Reinert et al. 2000). Three 3Jp signals, at - 9 . 5 , - 10.9 and - 15.0 ppm, fall in the characteristic range for P(OGa)4 groups and correspond to 3 distinguishable P sites which were assigned on the basis of bond valence sum calculations (Reinert et al. 2000). 7.3.4 31p N M R o f biomaterials
Bioglass| is a material in the Na20-CaO-SiO2-P205 system with the useful property of bonding to bone in vivo. 31p MAS NMR has been used to investigate the structure
NMR of Other Commonly Studied Nuclei
451
of a number of materials with Bioglass compositions (Lockyer et al. 1995). The results indicate the presence of PO43- associated with both Ca 2+ and Na +. The 31p chemical shift of the phosphate units decreases linearly with the Ca content of the Bioglass due to the slight difference in electonegativity between Na + and Ca 2+ giving rise to a slight displacement of charge from the oxygen (and hence from the P-O bond) with increasing substitution of Ca for Na. The linear dependence of the 31p chemical shift on the Ca content indicates that the average phosphorus environment is quite disordered, with no preferential association of P with either Na or Ca ions (Lockyer et al. 1995). Other glasses of possible interest as possible biocompatible materials include those in the system SiO2-P2Os-CaO-MgO, the structural aspects of which have been studied by 3~p MAS NMR (Oliveira et al. 2000). The NMR spectra show that the phosphorus is present in an orthophosphate environment and that replacement of Ca 2+ by Mg 2+ in the structure is accompanied by a shift of the 3~p resonance to lower frequencies. This replacement is reflected by the composition of the glass recrystallisation products, which contain mainly hydroxyapatite Calo(PO4)6(OH)2 at lower MgO contents, but whitlockite (CaTMgzP6024) in glasses of higher MgO content (Oliveira et al. 2000). 31p and 27A1 two-dimensional correlation NMR experiments have been used to study the structure and crystallisation of another potential bio-compatible glass system, PzO5-NazO-CaO containing A1203 as a deliberately introduced impurity to simulate the effect of fortuitous contamination from alumina crucibles used to melt these glasses (Hartmann et al. 2000). The results indicate the existence of a chain-like structure containing both amorphous diphosphate units and longer chains, with the A1203 apparently acting as a network modifier in these glasses. Crystallisation results in the formation of a number of different crystalline phosphates and a decrease in the average degree of phosphate unit condensation with simultaneous changes in the A1 coordination associated with the formation of a stable crystalline aluminium calcium sodium diphosphate in the resulting glass-ceramic material (Hartmann et al. 2000). Extruded calcium phosphate glasses with a small amount of added SiO2 to prevent crystallisation during the extrusion process have been studied by two-dimensional rotor-synchronised 3~p NMR (Braun et al. 1998). The rotor synchronised experiments provide information about possible local structural alignment of the phosphate chains arising from the extrusion process, since if there is no orientational order in the sample, all the free induction decays (FIDs) of the various tl slices will be equal irrespective of the rotor position and only the centre slice in the 2D plot will display a spectrum. This is demonstrated in Figure 7.29A for a powdered sample of the glass in which structural ordering is averaged out. However, the presence of order in the sample will result in additional spectral intensity outside the centre slice, as is seen for the extruded glass sample (Figure 7.29B). These results reveal a partial alignment of the tetrahedral chains along the extrusion axis which would be almost impossible to verify by ordinary static lineshape analysis (Braun et al. 1998).
452
Multinuclear Solid-State NMR of Inorganic Materials A
.
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Figure 7.29. Two-dimensional rotor-synchronised 31p NMR spectra of calcium metaphosphate glass with a small amount of added SiO2 to prevent crystallisation. A. Spectrum of powdered sample showing a spectrum only in the central slice. B. Spectrum of extruded glass showing additional spectral intensity outside the central slice arising from structural order induced during the extrusion process. From Braun et al. (1998), by permission of Elsevier Science.
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454
Multinuclear Solid-State NMR of Inorganic Materials
Fayon, F., Massiot, D., Suzuya, K. & Price, D.L. (2001) J. Non-Cryst. Solids, 283, 88. Feike, M., J~iger, C. & Spiess, H.W. (1998) J. Non-Cryst. Solids, 223, 200. Feuerstein, M., Hunger, M., Engelhardt, G. & Amoureux, J.P. (1996) Solid State Nucl. Mag. Reson., 7, 95. Fletcher, J.P., Kirkpatrick, R.J., Howell, D. & Risbud, S.H. (1993) J. Chem. Soc. Faraday Trans., 89, 3297. Fur6, I., Bakonyi, I., Tompa, K., Zsoldos, E., Heinmaa, I., Alla, M. & Lippmaa, E. (1990) J. Phys. Condens. Matter, 2, 4217. Fyfe, C.A., Mueller, K.T., Grondey, H. & Wong-Moon, K.C. (1992) Chem. Phys. Lett., 199, 198. Fyfe, C.A., Grondey, H., Mueller, K.T., Wong-Moon, K.C. & Markus, T. (1992a) J. Amer. Chem. Soc., 114, 5876. Fyfe, C.A., Mueller, K.T., Grondey, H. & Wong-Moon, K.C. (1993) J. Phys. Chem., 97, 13484. Fyfe, C.A., Wong-Moon, K.C., Huang, Y. & Grondey, H. (1995) Microporous Mater., 5, 29. Fyfe, C.A., Wong-Moon, K.C. & Huang, Y. (1996) Zeolites, 16, 50. Fyfe, C.A., Meyer zu Altenschildesche, H., Wong-Moon, K.C., Grondey, H. & Chezeau, J.M. (1997) Solid State Nucl. Mag. Reson., 9, 97. Gee, B., Janssen, M. & Eckert, H. (1997) J. Non-Cryst. Solids, 215, 41. Geisinger, K.L., Oestrike, R., Navrostky, A., Turner, G.L. & Kirkpatrick, R.J. (1988) Geochim. Cosmochim. Acta, 52, 2405. George, A.M. & Stebbins, J.F. (1995) Amer. Mineralogist, 80, 878. George, A.M., Sen, S. & Stebbins, J.F. (1997) Solid State Nucl. Mag. Reson., 10, 9. Gervais, C., Babonneau, F., Maquet, J., Bonhomme, C., Massiot, D., Framery, E. & Vaultier, M. (1998) Mag. Reson. Chem., 36, 407. Gervais, C., Maquet, J., Babonneau, F., Duriez, C., Framery, E., Vaultier, M., Florian, P. & Massiot, D. (2001) Chem. Mater., 13, 1700. Griffiths, L., Root, A., Harris, R.K., Packer, K.J., Chippentale, A.M. & Tromans, F.R. (1986) J. Chem. Soc. Dalton Trans., 2247. Grimmer, A-R. (1978) Spectrochim. Acta, 34A, 941. Grimmer, A-R. & Haubenreisser, U. (1983) Chem. Phys. Lett., 99, 487. Hanaya, M. & Harris, R.K. (1997) Solid State Nucl. Mag. Reson., 8, 147. Harris, R.K. & Nesbitt, G.J. (1988) J. Mag. Reson., 78, 245. Hartmann, P., Vogel, J., Friedrich, U. & J~iger, C. (2000) J. Non-Cryst. Solids, 263&264, 94. Hayashi, S. (1994) Solid State Nucl. Mag. Reson., 3, 93. Heidemann, D., Hiibert, C., Schwieger, W., Grabner, P., Bergk, K-H. & Sarv, P. (1992) Z. Anorg. Allgemeine Chem., 617, 169. Hudalla, C., Weber, B. & Eckert, H. (1998) J. Non-Cryst. Solids, 224, 69. Hunger, M., Engelhardt, G., Koller, H. & Weitkamp, J. (1993) Solid State Nucl. Mag. Reson., 2, 111. Hunger, M., Engelhardt, G. & Weitkamp, J. (1995) Microporous Mater., 3,497. Hunger, M., Sarv, P. & Samoson, A. (1997) Solid State Nucl. Mag. Reson., 9, 115. J~iger, C., Feike, M., Born, R. & Spiess, H.W. (1984) J. Non-Cryst. Solids, 180, 91. Jakeman, R.J.B., Cheetham, A.K., Clayden, N.J. & Dobson, C.M. (1985) J. Amer. Chem. Soc., 107, 6249. Jeschke, G., Hoffbauer, W. & Jansen, M. (1998) Solid State Nucl. Mag. Reson., 12, 1.
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Chapter 8
NMR of Low-y Nuclides 8.1.
General Considerations 8.1.1 Problems Associated with Low-y Nuclei 8.2. NMR of Spin-~/2 Nuclei 8.2.1 89y NMR 8.2.2 l~ and l~ NMR 8.2.3 183W N M R 8.3. Quadrupolar Nuclei 8.3.1 14N NMR 8.3.2 25Mg NMR 8.3.3 33S NMR 8.3.4 35C1and 37C1 NMR 8.3.5 39K NMR 8.3.6 43CaNMR 8.3.7 47Ti and 49Ti NMR 8.3.8 67ZnNMR 8.3.9 91ZrNMR 8.3.10 95Mo and 97Mo NMR 8.3.11 135Baand 137BaNMR 8.3.12 Other Miscellaneous Low- 7 Nuclei References
461 461 462 462 469 473 475 475 479 488 491 495 502 505 511 514 516 522 525 526
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Chapter 8
NMR of Low-y Nuclides 8.1. GENERAL CONSIDERATIONS At a first glance, the Periodic Table of NMR-active nuclei suggests that essentially all the elements are amenable to NMR spectroscopy. Although many of these nuclei, which include some of considerable interest in materials science (e.g. 47' 49Ti, 89y, 25Mg and 91Zr) are of significant natural abundance, and appear initially attractive, they are low-~, nuclides with small magnetic moments which offset the advantages of natural abundance. Reports of solution NMR using low-~/nuclides are more numerous than solidstate NMR studies, since the former are often made not by simple direct excitation but indirectly, by exploiting polarisation transfer techniques such as INEPT and DEPT and/or by using reverse detection methods. Many of these indirect methods for overcoming the problems inherent with low-y nuclides are not available in the solid state, imposing limitations on the solid-state NMR of these nuclides. Any definition of a low-~/nucleus will be somewhat arbitrary, but a convenient practical definition is based on the tuning frequency ranges of commercial MAS probes. Since standard probes normally tune down to 15N, nuclei which resonate below this frequency (40 MHz on a spectrometer equipped with a 9.4 T magnet) will be defined as low-'/for the purpose of this Chapter. The lower limit of this frequency range is taken to be 10 MHz, since this is the lowest frequency at which most commercial spectrometers will operate.
8.1.1 Problems associated with low-?nuclei The intrinsic sensitivity per spin of a nucleus is proportional to @. The practical effect of the factors which degrade the sensitivity of low-~/nuclei can be illustrated by taking as an example 89y. The sensitivity of 89y compared to ~3C (i.e. per magnetic nucleus) is only 7.5 • l0 -3. Thus, despite being 100% naturally abundant compared to 1.1% for ~3C, 89y has a relative receptivity of only 69.6% by comparison with 13C. In practice, the situation can be even worse for low-~/nuclei because of relaxation effects. All other things being equal, relaxation processes are usually less efficient for low- 7 nuclei with small magnetic moments (Chapter 2). This results in a greatly increased experiment time needed to acquire a spectrum with an adequate signal/noise ratio. There are, however, exceptions to this situation, as in the case of ~~ NMR of silver-containing glasses, where the high ionic mobility of the silver ion shortens the relaxation time and facilitates the observation of this nucleus. 89y is another low-y nucleus whose 461
462
Multinuclear Solid-State NMR of Inorganic Materials
relaxation time in the high-temperature superconductors YBa2Cu307 is less than a few seconds, making the NMR study of these systems much easier than for other Y-containing materials which can have relaxation times of many minutes. Experimental strategies for overcoming the common problems of sensitivity, probe deadtime and ringing, and broad resonance lines are discussed in Chapter 3.
8.2. NMR OF SPIN-I/2 NUCLEI
8.2.1 89y N M R An extensive range of shifts is found for 89y, even in diamagnetic materials (Table 8.1). The 89y spectrum of Y203 shows two resonances with a shift difference of ca. 40 ppm corresponding to the two octahedral YO6 environments known to be present in this crystal structure (Sebald 1994, Thompson and Oldfield 1987, Dupree and Smith 1988, Harazono and Watanabe 1997). The intensity ratio of these two peaks is 3:1, in good agreement with the known site occupancies, and the T~ values for both these sites are reported to be just under four hours (Harazono and Watanabe 1997). Measurements of the static spectrum of Y203 indicate a chemical shift anisotropy (CSA) of 109 and 102 ppm for the two sites (Harazono and Watanabe 1997). By contrast, the oxysulphur compound Y2028 has a significantly larger CSA (161 ppm) and a longer Tz(6.6 hours) (Harazono and Watanabe 1997). The room-temperature MAS NMR spectra of YC13 and Y2C13 show evidence of very strong deshielding in the latter due to d-bonding states arising from direct Y-Y bonding (Table 8.1) (Kremer et al. 1992). In a cross-polarisation study of a number of Y compounds (Merwin and Sebald 1990), Y(NO3)3.6H20 was found to be a good compound for setting up the Hartmann-Hahn condition despite its broad match characteristics. Extremely narrow resonances (10-20 Hz) were obtained in CP experiments on a number of yttrium salt hydrates but the optimum contact time in these compounds was quite long (8-30 ms). The CSA of these compounds was estimated from their static spectra to be 27-75 ppm (Merwin and Sebald 1990). In other 89y CP studies of a range of yttrium alkoxide compounds the contact time was found to be typically 20 ms (Wu et al. 1993). These experiments were able to distinguish a number of yttrium environments with different 89y resonances, allowing the technique to be used as a fingerprinting tool for the ligand environments and hence to determine the purity of the bulk samples (Wu et al. 1993). The effect of the nearest neighbour (nn) and next-nearest neighbour (nnn) coordination on the 89y shift has been investigated in a series of crystalline yttrium aluminates and silicates (Dupree and Smith 1988). The 89y shift was found to increase with an increasing number of nearest neighbour oxygen atoms, and to become less paramagnetic as the electronegativity of the coordinating group increases. It is clear, however, that the various effects of nn and nnn groups overlap, suggesting the need for caution in
NMR of Low- y Nuclides
463
Table 8.1. 89y interaction parameters in yttrium compounds. Compound
~iso* (ppm)
Reference
Y203 (monoclinic) Y:O3 (cubic) Y203
336 314, 273 314-315,330*
Florian et al. (1995) Florian et al. (1995) Sebald (1994), Thompson & Oldfield (1987), Dupree & Smith (1988), Harazono & Watanabe (1997) Sebald (1994) Battle et al. (1988) Harazono & Watanabe (1997) Dupree & Smith (1988), Florian et al. (2001) Dupree & Smith (1988), Harazono & Watanabe (1997), Florian et al. (2001) Florian et al. (2001) Dupree & Smith (1988) Dupree & Smith (1988) Dupree & Smith (1988) Dupree & Smith (1988) Dupree & Smith (1988) Grey et al. (1990) Grey et al. (1990) Sebald (1994) Balakrishnan et al. (1988) Balakrishnan et al. (1988) Kremer et al. (1992) Kremer et al. (1992) Wu et al. (1993) Thompson & Oldfield (1987) Wu et al. (1993) Thompson & Oldfield (1987), Merwin & Sebald (1990) Thompson & Oldfield (1987), Merwin & Sebald (1990) Thompson & Oldfield (1987), Merwin & Sebald (1990) Thompson & Oldfield (1987), Merwin & Sebald (1990) Meinhold & MacKenzie (1995) Meinhold & MacKenzie (1995) Ekstrom et al. (1997) Ekstrom et al. (1997) Ekstrom et al. (1997) Barnes et al. (1989) Barnes et al. (1989) Barnes et al. (1989) Helluy et al. (1999) Helluy et al. (1999)
270-273, 289* Y(OH)3 (Bi203)o.6(Y203)0.a YA103
66 275 293' 214.5,215
Y3A13012
222, 237', 222
Y4A1209 YzSiO5
Y2Ti207 YzSiBe207 YBazCu307 Y2BaCuO5 Y2C13 YC13 YF3 YC13.6H20 YBr3.6H20 Y(NO3)3.6H20
184, 216, 195, 231 237, 148 144 208 198 122 149 65 163 - 103.2 - 1250 762, 506 - 230 - 112 58 80 - 55, - 53.2
Yz(SO4)3.8H20
-46, - 50
Y(Oac)3.4H20
47, 45
Y(acac)2.3H20
27, 21.8
Y2Si303N4 Y4SizOvN2 YSi3N5 YzSi3N6 Y3Si3N6 oL-YHo.12 ~-Y(H,D)l.92 8-Y(H,D)l.98 ~-YH1.99 YH1.99 + 0.1
185-160.5 202-214 391,506 394, 510 497 3590 910, 760 920 946 905
Y202S
oL-YzSi207 ~-Y28i207 ~-Y28i207 ~/-Y2Si207
YzSn207
* Shift relativeto diluteaqueousYC13exceptfor *which are relativeto 1.5 M aqueousY(NO3)3.
464
Multinuclear Solid-State NMR of lnorganic Materials
making detailed structural assignments based solely on the chemical shift. The yttrium aluminates and silicates also show wide variation in their linewidths, ranging from 10 to 170 Hz (Dupree and Smith 1988). The value of T~ for Y3A15012 (yttrium aluminium garnet or YAG) is just over 60 minutes, considerably shorter than for Y203 due to the additional dipolar coupling to the 27A1 (Harazono and Watanabe 1997). The CSA of Y3A150~2 has been determined from its static spectrum to be 103 ppm (Harazono and Watanabe 1997). Y3A150~2 is an important high-technology ceramic with excellent mechanical properties (high-temperature strength, low creep and compatibility with alumina) making it a candidate both as a monolithic ceramic and in fibre-reinforced oxide-oxide composites. Since the formation of YAG by solid-state reaction of the component oxides requires high processing temperatures, the possibility of synthesising it at lower temperatures by sol-gel methods from hybrid (inorganic/organic) precursors has been investigated by 89y in conjunction with 27A1NMR (MacKenzie and Kemmitt 1999). The results indicate that the amorphous gels crystallise abruptly to YAG at ca. 900~ with the comparatively less mobile yttrium moving into its final lattice sites only after the aluminate structure of the YAG has been established. An important practical use of Y203 is as a sintering aid to assist the densification of non-oxide ceramic materials based on Si3N4 and sialons. The oxide forms glassy grainboundary phases which have been studied by 89y NMR. The disorder of these intergranular compounds can give rise to broad 89y resonances with long T~ times making them difficult to detect. The addition of small amounts of paramagnetic rare earth ions, particularly Eu 3+, has been found significantly to improve the detection of 89y in non-oxide compounds by decreasing the T~ time without producing significant broadening of the resonance (Meinhold and MacKenzie 1995). The 89y NMR spectra of Y-Si-N compounds show, as would be expected, a positive shift from the position of the oxides resulting from the presence of nearest neighbour nitrogen, allowing the compounds YSi3Ns, Y2Si3N6 and Y3Si6Nll to be distinguished (Figure 8.1) (Ekstr6m et al. 1997). The formation of intergranular Y-containing phases during sintering of Si3N4 at 1500-1750~ in the presence of Y203 has been studied by a combination of 89y, 27A1 and 29Si NMR, which indicates the initial appearance of a composition similar to Y4Si207N2 (J-phase) (MacKenzie and Meinhold 1994). As the sintering temperature is increased, the centre of gravity position of the broad 89y resonance moves progressively towards the position of Y2Si303N4 (N-melilite), indicating that the Y-rich intergranular phases formed initially become progressively Si-rich due to reaction between the Y203 and Si3N4 (MacKenzie and Meinhold 1994). The sintering processes in these materials are facilitated by liquid-phase transport through the Y-containing glass. An 89y NMR study has also been made of the effect of Y203 on the sintering of the silicon aluminium oxynitride [3-sialon, a compound structurally related to [3-Si3N4 but
N M R of Low-7 Nuclides
465
A
C 505.8
394
~
497.3
1311"1
.....
t_
800
|
9
400
_1
l
0
_
t
800
i
i
400
~
i
_ _ _
.,,,-~t,m.~.
0
.-, , ' . . . . b . . , . ~ . . , ~ . . . . ; . . . . ~ . . . . : . . . .
800
400
;.... 5.~ -
0
s9y shift ( p p m ) w.r.t. Y C I 3 soln.
Figure 8.1. 89y MAS NMR spectra of representative compounds in the system Y-Si-N. A. YSi3N5 with a small amount of Y3Si6Nll impurity, B. YzSi3N6with a small amount of YSi3N5 impurity, C. Y3Si6Nll with a small amount of Y4SizO7N2(J-phase) impurity. From Ekstr6m et al. (1997), by permission of the copyright owner. with substituted A1 and O. The results indicate the initial formation of Y3A15012 (YAG), followed by the appearance at higher temperatures of a Y-rich intergranular glass (MacKenzie and Meinhold 1996). X-sialon is a related phase which can be visualised as a solid solution between mullite (A16Si2013) and Si3N4. Its formation by a silicothermal process in the presence of 10 wt% Y203 has been studied by 89y NMR which shows evidence of the early formation of a composition resembling N-melilite rather than yttrium aluminates or silicates. Throughout the reaction the yttrium remains in the amorphous oxynitride phase containing predominantly SiOzN2 units, through which liquid-phase sintering is also enhanced (MacKenzie et al. 2000). 89y NMR has proved to be extremely useful for providing information about the positions and distribution of the atoms in solid solutions such as the oxide ion conductor (BizO3)o.6(Y203)o.4 (Battle et al. 1988). The position of the 89y resonance in this compound (275 ppm) is very close to that of the < 111 > yttrium site in Y203 (Figure 8.2), suggesting that the y3+ ions in the solid solution are predominantly surrounded by < 111 > anion vacancy pairs, with the Bi 3+ ions surrounded by the < 110 > vacancy pairs (Battle et al. 1988). In an 89y NMR study of the rare-earth pyrochlores of the type Yz-yLnyM207, where Ln is a lanthanide such as Ce, Pr, Nd, Sm, Eu or Yb and M can be Sn or Ti, the presence of the paramagnetic ions significantly shortened the T1 time, and also gave rise to additional 89y signals (Figure 8.3) attributed to substitutions of a Ln ion for yttrium in the next-nearest neighbour coordination sphere (Grey et al. 1990). The pyrochlore structure of these compounds gives rise to a number of atomic environments of the form Y(OY)6-x(OLn)x which can be distinguished on the basis of their 89y shifts. These are
466
Multinuclear Solid-State NMR of lnorganic Materials
275
(Bi 203)0.6(Y203)0.4
313
Y203
.i
....
,
....
X ~ ' ' j ' ' ' '
400 300 200 s9y shift (ppm) w.r.t. YCI3 soln. Figure 8.2. S9y MAS NMR spectrum of (Bi203)o.6(Y203)o.4 showing the similarity in its centreof-gravity position with the sharp resonance from the < 111 > site of Y203. From Battle et al. (1988), by permission of the Royal Society of Chemistry.
YTi207
Y1.9ProaTi207 Y(OY)s(OPr)
Y(OY)6
Y(OY)4(OPr)2
r162 qr
, ......
200
o
-260
400
~. . . . . . . . . . . . . . . . .
200
S9y shift (ppm) w.r.t. YCI 3 soln. Figure 8.3. 89y MAS NMR spectra of the pyrochlore compound Y2Ti207 (left) showing the single resonance from the Y(OY)6 and from the Pr-doped compound Y1.9Pro.lTi207 (right) showing the additional resonances due to Y(OY)5(OPr) and Y(OY)4(OPr)2 units. Asterisks denote the spinning side bands. From Grey et al. (1990), by permission of the American Chemical Society.
different for each lanthanide atom, but each shows a cumulative change with x (Figure 8.3). Although these shifts probably arise through a combination of effects, the detection of the additional resonances allows the atomic distribution and hence the solubility in the solid solution to be determined (Grey et al. 1990).
467
N M R o f Low- y Nuclides
The sensitivity of 89y NMR spectra to the atomic distribution in rare-earth compounds has also been exploited in studies of laser and phosphor materials such as Eu-doped Y203 (a red phosphor) (Harazono et al. 1998), Y2028 (Harazono et al. 1999), Eu-doped Y202S (a red phosphor) (Harazono et al. 2001) and Tb-doped Y3A15012 (a green phosphor) (Harazono et al. 1998a). Additional 89y resonances are observed in all these materials, arising from nnn substitution by the rare-earth ion. Measurements of the linewidths and intensities of the various 89y resonances indicated that the brightness of the phosphor is correlated with the distribution of the rare-earth ions, the brightest results being achieved in materials in which this distribution is the most homogeneous. One of the most extensive uses made of 89y NMR has been in studies of the hightemperature superconducting compounds of the type YBa2Cu307 and YBazCu408 in which the yttrium resides between the CuO2 planes, playing a central role in the superconductivity of these materials. Determinations of the 89y Knight shift provide a direct means of gaining information at the atomic level about the temperature dependence of the spin susceptibility of these materials, which reflects both the spin and orbital character of the couplings in these highly correlated systems. The first accurate measurements of the 89y shifts reported in the late 1980s provided information about the contributions from the s and d bands. Changes in the 89y shift of YBazCu307-~ with oxygen content (Figure 8.4) result from a lowering of the density of the d-states at the Fermi level (Balakrishnan et al. 1988), and measurements of the 89y static linewidth provide an estimate of the penetration depth of the magnetic field (Markert et al. 1987,
A
B 0
50 ~-"
o
t
~
~
-so
~
0=6.6
,~
0=6.2
~ -100
I '0-6.2
O=6.61 0=6.66 0=6.8
"~ ~
o
=
7
r~ t
,
_
l
. . . .
l
9
- -
o -8oo s9y shift (ppm) w.r.t. YCI3 soln.
" 200
'
3;0
400
oo
Temperature (K)
Figure 8.4. A. 89y MAS NMR spectra of YBa2Cu307 superconductors showing the effect of the oxygen content on the 89y shift. B. Temperature dependence of the 89y shifts for YBazCu307 superconductors with different oxygen contents. From Balakrishnan et al. (1988), by permission of the copyright owner.
468
Multinuclear Solid-State N M R of Inorganic Materials
Alloul et al. 1988 and 1991, Ohno et al. 1990 and 1991). The NMR shifts from 89y, 170 and 63Cu NMR measurements have been found to display similar temperature dependences, and since the 89y shift correlates directly with the spin susceptibility, this result has been interpreted as evidence that the properties of the system are determined by a single spin fluid. For a recent review of the primary literature on these topics see Rigamonti et al. 1998. A wide range of substitutions can occur in the different sites of these superconductor compounds, particularly in the plane and chain copper sites of YBaz(CUl-xMx)307 (Balakrishnan et al. 1989, Dupree et al. 1992, Riseman et al. 1994, Tunstall and Webster 1991). Recently 89y NMR has been used to show that the density of states in YBazCu307-~ has a peak that is pinned at the Fermi level and grows with overdoping (Williams et al. 1998). 89y T~ measurements have also been used to indicate that the energy of the spin dynamics near q (wave vector) = 0 must be strongly temperaturedependent to be consistent with other data supporting the one-band model of the system (Auler et al. 1999). A recent 89y NMR experiment on YBa2Cu408 isotopically exchanged with ~6/~80 was able to examine the role of phonons in the spins and pseudo-gaps, and show that the pseudo-gap correlations are independent of the superconducting pairing correlations (Williams et al. 1998a). 89y NMR measurements have provided evidence of the internal magnetic field distribution in these materials below Tc in which state a flux lattice exists giving rise to variations in the internal field (Brom and Alloul 1991). The motion of this flux lattice could also be monitored by measurements of the 89y resonance in slightly underdoped YBazCu307 (Carretta and Costi 1992). Analysis of the relaxation data for these systems is very complex because of the many different sources of relaxation effects, including fluctuations of the copper moments (Suh et al. 1993), but in the case of YBa2Cu408 measurements of the magnetic field dependence of the temperature-dependent 89y T1 were able to show that the activation energy of thermal depinning is proportional to the applied magnetic field (Borsa et al. 1997). These recent 89y NMR studies are not intended to provide an exhaustive review of the application of 89y to superconductors, but to give an idea of the extent to which the technique is now being used in this area, and of the breadth and variety of the results. Yttrium forms a range of compounds with hydrogen. These have potential applications to hydrogen storage technology, and have been extensively studied by 89y NMR. Significant Knight shifts have been found in many of these compounds (e.g. 0.11% in yttrium dihydride) (Barnes et al. 1989). The lack of a detectable 89y signal in c~-YHo.~8 below 350 K has been attributed to insufficient hydrogen pair ordering, but at higher temperatures, averaging due to hydrogen pair diffusion produces a single 89y resonance with axial symmetry and a typical shift of 0.359%. Dihydride phases containing deuterium show two 89y signals associated with hydrogen ordering, which merge at higher temperatures (Barnes et al. 1989). Cross-polarisation (CP) experiments
NMR of Low- y Nuclides
469
between 1H and 89y made on the hydrides 8-YH1.99 and YH1.99 + o.1 (Helluy et al. 1999) show a single resonance in both compounds, but with very different CP characteristics attributed to differences in the room-temperature hydrogen motion. Separate 89y lines associated with hydrogen ordering become apparent as the hydrogen motion is reduced by cooling (Helluy et al. 1999). 8.2.2
1~
and 1~
NMR
Although there are two silver nuclei suitable for NMR spectroscopy, the sensitivity of l~ is higher, and most of the silver NMR studies reported to date, ranging from fast ion conducting glasses to organometallic complexes of biological significance, have been made with this nucleus. Being a heavy metal, the chemica! shift range of silver is large (---1000 ppm), making it a sensitive probe for discriminating inequivalent crystallographic sites. Silver occurs in a range of coordinations (typically two to four), with the more asymmetric coordination types likely to exhibit significant CSA. Cross-polarisation (CP) l~ studies have proved to be extremely useful for acquiring spectra of materials with long T1 values and which contain strong sources of magnetisation, although the contact times for most silver compounds are long (typically 10-50 ms) (Merwin and Sebald 1992). The most useful silver compound for setting up the Hartmann-Hahn CP condition has proved to be silver lactate. The compounds most extensively studied by l~ NMR are the silver halides and related compounds (Looser and Brinkman 1985, Beeker and von Goldammer 1990, Villa et al. 1986, Mustarelli et al. 1998). Silver iodide, which is of particular interest because of its importance in silver-based fast ion conducting glasses, occurs in a number of polytypes with isotropic chemical shifts spanning a range of ---100 ppm (Villa et al. 1986). The chemical shifts of the silver halides are reported to vary inversely with the halogen (Looser and Brinkman 1985), and are also composition dependent, as in the solid solutions of the type Ag• where the shift decreases with increasing values of x. Ab initio calculations for the tetrahedral unit AgI43- have shown that changes in the chemical shift are related to changes in the d-hole and p-electron densities giving rise to decreases in the tetrahedral bondlength (Ida et al. 1999). In materials where there is significant silver ion motion even at room temperature, the l~ NMR spectra may show a single peak at a position which is the weighted average of the different structural environments, as in the ot and [3 polymorphs of RbAgaI5 and KAg415 which show a single resonance. The site symmetry of [3-RbAgaIs is lower than in the or-phase, giving rise to a measurable CSA (Looser and Brinkman 1985). When this compound is cooled further to form the "y-phase the value of T1 is lengthened as a consequence of the greatly reduced silver motion, resulting in significant linebroadening and the complete loss of the l~ NMR signal. The chemical shifts of Ag-S compounds have been recorded in studies of a series of silver thiolates and related compounds (Fijolek et al. 1996) which indicate that the
470
Multinuclear Solid-State N M R o f Inorganic Materials
l~ shifts corresponding to AgS2 groups (790-890 ppm) are readily distinguishable from those of AgS3 groups (950-1250 ppm). The l~ NMR spectrum of the compound [(C6H5)aP]2[Aga(SCH2C6H4CH2S)3].6CH3OH contains 3 closely-spaced resonances between 1203 and 1230 ppm, corresponding to 3 similar but crystallographically inequivalent AgS3 sites in this compound. A further resonance at 1083 ppm arises from a more distorted AgS3 site on the three-fold axis. The silver sites in these thiolate compounds also show significant CSA ranging from 570 to 2120 ppm (Fijolek et al. 1996). The l~ solid state NMR shifts of the various silver compounds are collected in Table 8.2. l~ NMR has provided valuable information about a number of silver-containing glass systems. The l~ shift in glasses of the type AgI.Ag20.B203 indicates that the silver is coordinated to both iodide and oxygen atoms in the structure rather than being of 2 types, associated with the borate network and and the iodide domains (Villa et al. 1986, Chung et al. 1990). l~ NMR has also proved useful for analysing phase separation phenomena and compositional fluctuations in these glasses (Villa et al. 1986) while relaxation rate measurements have shown that 2 sites are present which have widely differing rates of motion (Chung et al. 1990). Table 8.2. l~
chemical shifts in silver compounds.
Compound
~iso* (ppm)
Reference
Plischke et al. (1992) Looser & Brinkmann (1985), Beeker & von Goldammer (1990), Mustarelli et al. (1998) Looser & Brinkmann (1985), Beeker &von AgC1 370, 353.5,409 Goldammer (1990) Looser & Brinkmann (1985), Beeker & von AgBr 350, 350 Goldammer (1990) Looser & Brinkmann (1985), Beeker &von AgF - 110, - 125 Goldammer (1990) Plischke et al. (1992) AgNO3 - 87 Olsen & Zwanziger (1995) AgPO3 96 Olsen & Zwanziger (1995) Ag4P207 383 Mustarelli et al. (1998) Ag2MoO4 130 Jansen (1987) AgloSi4013 320 Looser & Brinkmann (1985) Ag26Ii8W4O16 580 Looser & Brinkmann (1985) Ag415 810 Looser & Brinkmann (1985) RbAgaI5 790 Merwin & Sebald (1992) Silver lactate 345.9, 320.2, 219.7, 210.7 Merwin & Sebald (1992) Silver acetate 401.2, 382.7 Merwin & Sebald (1992) Ag(CH3C6HaSO3) 44.1 Merwin & Sebald (1992) Ag(CsH702) 471.6 Merwin & Sebald (1992) AgN(SOzMe)2 210.3 Merwin & Sebald (1992) AgN(SO2Me)z.0.25H20 182.8, 130.7, 32.5 Silver metal AgI
* R e l a t i v e to dilute a q u e o u s silver nitrate.
- 5253 710, 728,680
471
N M R o f Low- y Nuclides
Glasses in the system AgI.xAg2MoO4 show l~ NMR resonances corresponding to the iodide and molybdate end members, as well as an intermediate peak at 548 ppm arising from the glassy phase (Mustarelli et al. 1998, Mustarelli et al. 1998a). Crystallisation of these glasses is accompanied by the appearance of a powder pattern indicating an axial CSA of --~70 ppm (Mustarelli et al. 1998). Other silver-containing glassy systems studied by l~ NMR include (AgI)• (Ag2 S.GeS2)I_• for which T1 and T2 measurements indicate that on the NMR timescale all the silver ions are mobile (Roos et al. 1988), and tellurite-AgI based fast ion conducting glasses in which the silver mobility was shown by l~ NMR to increase with the AgI content (Sanz et al. 1995). These glasses show several ~~ NMR signals corresponding to oxyiodide and iodide environments. The l~ NMR spectra of a series of glasses in the system (AgI)x(AgzO)y (PzOs)l-x-y show a single silver resonance which becomes rapidly narrower over the temperature range - 8 5 to 50~ (Figure 8.5A). The linewidth and shift of this resonance depend strongly on both temperature and composition of the glass (Figure 8.5B) (Olsen and Zwanziger 1995). Accurate quantification of these spectra clearly indicates that the iodine is uniformly distributed throughout the structure, in which the silver hops rapidly (on the NMR timescale) between the iodine and oxygen ligands (Olsen and Zwanziger 1995). A ~~ NMR study of fast ion conducting glasses in the system AgzS-B2S3-SiS2 showed 3 well-resolved lines corresponding to 3 Ag species distinguishable both by their chemical environment and their ion dynamics. The NMR data determined as a A
B
700/-" ** ., 5001
~******** ***
l
(Agl)3o(Ag~O~7~"'''"
300
l~
'7:20 ' 600 ; 480i-~ shift (ppm) w.r.t. AgNO 3 soln.
(Agl)6o(AgPO3)4o
<
-15o
(AgI)lo(AgPO3)9omlhlw
-5o 50 Temperature (~
15o
Figure 8.5. A. Static l~ NMR spectra of (AgI)6o(AgPO3)4oglass at various temperatures. B. Changes in the l~ chemical shift of a series of glasses in the system (AgI)l-•215 as a function of temperature. From Olsen and Zwanziger (1995), by permission of copyright owner.
472
Multinuclear Solid-State NMR of Inorganic Materials
function of temperature confirmed that the ionic conduction process is associated with fast diffusion within separate clusters of similar chemical environment where ionic motions are more rapid than between the clusters of different chemical environment (Akai et al. 2000). Both the l~ and l~ spectra of Ag9GaSe6 have been recorded as a function of temperature and the applied magnetic field (Tansho et al. 1996). Of the 3 different phases of this compound, the cubic high-temperature phase has a T1 which is independent of the applied magnetic field strength, while the ratio of the relaxation times of the 2 silver isotopes suggests that in the fast motion limit the principal relaxation mechanism involves scalar coupling of the first kind via an exchange interaction (Tansho et al. 1996). The low-temperature phase of this compound shows a very broad (--~ 1000 ppm) silver resonance related to the CSA. Silver has been used for many years as a catalyst for the reaction between ethylene and molecular oxygen, but the precise role of the silver in the catalytic reaction is still unclear, l~ NMR has been used to shed light on the various stages in which the metal is deposited on oxide supports (SiO2, xI-A1203 and zeolite A) (Plishke et al. 1992). Impregnation of the support material with AgNO3 results in a single l~ resonance at - 11 ppm (Figure 8.6A) corresponding to the signal from aqueous AgNO3. On drying at 373 K (Figure 8.6B) the signal appears at the position of crystalline AgNO3 ( - 82 ppm), but after reduction in hydrogen at 673 K the typical Knight-shifted resonance of metallic Ag appears at 5263 ppm (Figure 8.6 C). This metallic resonance arises from particles larger than 50 nm, since surface interactions with the conduction A
.~-11 -82 -97
D
5273
5400 ' 5iOO l~
100 ' -100
shift (ppm) w.r.t. AgNO3 soln.
Figure 8.6. l~ MAS NMR spectra of SiO2 impregnated with AgNO3 solution. A. After impregnation, B. after drying at 373 K, C. after reduction in hydrogen at 673K, D. after subsequent exposure to oxygen at 443 K. The small feature at - 97 ppm in spectrum B. is described as an artifact due to misadjustment of the magic angle. From Plishke et al. (1992), by permission of the American Chemical Society.
473
NMR of Low- 7 Nuclides
electrons broaden the resonances in the smaller particles beyond detection. Adsorption of oxygen on this material caused the metal peak to weaken and broaden (Figure 8.6 D) but no other resonance arising from Ag20 or Ag202 could be detected in the nonmetallic region of the spectrum. Since no unique resonance from the surface Ag atoms could be detected, the absorption of oxygen, chlorine or hydrogen chloride on these surfaces had no effect on the l~ NMR spectrum, militating against the use of this technique to study the specifics of the catalytic processes (Plishke et al. 1992).
8.2.3 183W N M R
Very few solid state 183WNMR studies have been reported, no doubt due to the fact that tungsten has a lower receptivity than yttrium and silver and shows extremely long T1 values. Furthermore, being a heavy metal, it would be expected to exhibit CSA which will spread the intensity over a broad spectral range except for tungsten atoms in the most symmetric environments. As expected, very narrow ~83W resonances are found in tungsten compounds in which the W is located in very symmetric environments, e.g. the alkali tungstates (Figure 8.7A) and W(CO)6 (Figure 8.7B). The linewidth in the latter is 6 Hz, and no spinning sidebands are observed in these compounds (Knight et al. 1986).
A
i
""'
I
~"''
I
. . . .
I
. . . .
B
CaWO 4
~
. . . .
"~
,
.",'",
I
,
,
,
I
'
"
'
W(CO)6
"""!"""'"
'
1
'
'l',
-200
,
'
,
!
'
'
'
'
I
'
I
"'
-400
"
'
!
'
'
'"""!
'
-600
'"'
'
1
~~-
I
'
'
i
!
",""'~','
,
l"
.
,
-3700
H3P(W1204o).nH20
D
' "
. . . .
-200 -3300 -3500 lS3W shift (ppm) w.r.t. Na2WO, soln.
200
8~07 -174 ppm
-'
500
'
9
1
''
0
'
"
''
i
'
'
'"
'
'-I
''
-500
l~W shift ~ p m ) w.r.t. Na2WO4 soln. Figure 8.7. A selection of representative 183WMAS NMR spectra of tungsten compounds, from Knight et al. (1986), by permission of the American Chemical Society.
474
Multinuclear Solid-State NMR of Inorganic Materials
The 183W NMR spectrum of WO3 (Figure 8.7C) contains 2 resonances corresponding to the 2 crystallographically inequivalent but equally populated WO6 sites at - 414 and - 4 3 8 ppm (measured with respect to aqueous Na2WO4) (Knight et al. 1986), while the compound H3[P(W1204o)].nH20 (Figure 8.7D) shows a single resonance with a very large CSA. All these ~83W spectra required very long total acquisition times (7-43 hours). The ~83W chemical shifts for tungsten compounds are summarised in Table 8.3. The sensitivity can be improved and the long recycle delay times shortened in compounds containing protons by cross polarisation (CP) techniques, although the contact times in such experiments have been found to be extremely long (typically - 100 ms) (Merwin and Sebald 1992a). Such long contact times place the high-power amplifier of the spectrometer under considerable strain at the low operating frequency of tungsten. The most useful compound for setting up the Hartmann-Hahn condition is found to be (NHa)2WS4, which has a usefully shorter contact time (5-10 ms) and shows seven isotropic resonances which have not, however, been assigned to specific W sites because of the lack of a crystal structure for this compound (Merwin and Sebald 1992a). It has also been noted that structural water molecules do not provide a suitable source of ~H for cross polarisation in these W compounds, and even in favourable cases the CP experiments are slow, often requiring more than 15 hours. These results indicate that although ~83W NMR is possible, it is difficult and time-consuming, and still far from routine.
Table 8.3. ~83Wchemical shifts in tungsten compounds. Compound
gi~o* (ppm)
WO3 H2WO4
Li2WO4 NazWO4 K2WO4 Cs2W04
(NH4)zWO4
CaWO4 SrWO4
BaWO4 W(CO)6 H3[P(WIzOao)].nH20 (NH4)2WS4
-
-414, - 438 - 251.1 - 40 - 63 - 18 - 36 38.6, - 55.8, -63.6, - 80.0, - 95.6, - 107.4, - 115.9 29 - 33 - 78 - 3470 - 174 3639.6
* relativeto saturatedaqueousNa2WO4
Reference Knight et al. (1986) Merwin & Sebald (1992a) Knight et al. (1986) Knight et al. (1986) Knight et al. (1986) Knight et al. (1986) Merwin& Sebald (1992a) Knight et al. (1986) Knight et al. (1986) Knight et al. (1986) Knight et al. (1986) Knight et al. (1986) Merwin & Sebald (1992a)
NMR of Low- y Nuclides
475
8.3. QUADRUPOLARNUCLEI 8.3.1 14N N M R 14N is one of the few quadrupolar nuclei with integer spin. Since these nuclei have no (1/2, - 1/2) state, all the transitions suffer first-order quadrupolar broadening, with all the intensity spread over a range of about 2VQ. Thus, the experimental conditions for observing such broad lines make similar demands on the spectrometer as those used to detect the satellite transitions of nuclei with non-integer spins. The values of XQ for 14N compounds fall in the range 0-8 MHz, with typical values 3--4 MHz (Yesinowski and Hill 1999). Although 14N is 99.63% abundant, by contrast with the spin-l/2 isotope 15N (0.37 % abundance), most of the nitrogen studies reported to date have used 15N even though the latter usually involves the preparation of isotopically enriched samples. Nevertheless, the prospect of making natural abundance studies is attractive, and several ingenious methods have been developed to improve the resolution of 14N NMR and overcome some of the drawbacks with this nucleus. These are: (i) Overtone spectroscopy, in which the sample is irradiated at approximately twice the Larmor frequency (Tycko and Opella 1987). In cases where the quadrupole interaction is sufficiently large for second-order quadrupolar effects to be significant, the ( - 1 6-~ 1) transition becomes weakly allowed under these irradiation conditions. The resulting powder spectrum is narrower by a factor of 8vo/XQ times, but retains its structure from which the NMR interactions can be deduced. (ii) Use of two and three-pulse sequences to generate double coherence (Ylinen et al. 1997). (iii) Dipolar coupling of the 14N to a spin-l/2 nucleus, allowing the NMR interaction parameters to be deduced from the modulation of the response of the spin-l/2 nucleus. This can be achieved by spin-locking the 14N for differing periods of time (Grey et al. 1995) or by applying short pulses to the 14N as in the REAPDOR experiment (see Chapter 3) in which the magnetisation is transferred through quasi-adiabatic processes (Ba et al. 1998). To overcome difficulties associated with recording the very broad lineshapes encountered in 14N NMR spectroscopy, a stepped-frequency approach can be used, in which the lineshape is constructed from observations made at a number of smaller frequency intervals. Figure 8.8A shows the lineshape determined in this manner for KNO3, together with the simulated lineshape (Hill and Yesinowski 1996). This approach is, however, very time consuming and inefficient, since only a small fraction of the spins is excited at any particular instant. To increase the sensitivity and reduce the recycle time, a technique known as Reorientation-Induced Redistribution of Isochromats (RotisseRie) has been developed, in which the powder sample is slowly rotated at --~1 rpm about an axis perpendicular to the magnetic field (Yesinowski and
476
Multinuclear Solid-State NMR of Inorganic Materials
A
.-~"--'--
400
0
-400
Frequency (kHz)
2000
---..,,..I-..,,
1000
0
Frequency (kHz)
Figure 8.8. A. 14N stepped-frequency NMR spectrum of KNO3 and the simulated spectrum (broken line). From Hill and Yesinowski (1996). B. One half of the axially-symmetric 14N steppedfrequency NMR spectrum of Si3N4 acquired under RotisseRie conditions with a rotation speed of 1 rpm. From Yesinowski and Hill (1999). Both spectra used by permission of the American Chemical Society. Hill 1999). This has the effect of exposing a new group of spins which are for the most part unsaturated, allowing faster pulsing and increased sensitivity. Figure 8.8B shows one half of the axially-symmetric 14N NMR powder pattern of Si3N4 acquired under RotisseRie conditions. Although the 14N T1 value for Si3N4 is several minutes, this RotisseRie experiment was conducted with a recycle delay of 5.4 seconds. An extension of this technique called STEAMER (Slow Turning for Echo Amplitude Modulation and Echo Reduction) has been developed to measure the modulation of an echo from which information about the NMR interaction parameters can be extracted (Yesinowski and Hill 1999). Two further problems with 14N NMR are evident from the literature. The first is a lack of agreement about a suitable spectral reference material with at least 5 different reference compounds having been used. NH4C1 appears to be a good choice as a secondary reference for solid state NMR, since its narrow MAS signal at 342.4 ppm from the primary reference (nitromethane) can be observed with a good signal/noise ratio in a single scan (Khitrin and Fung 1999). The second problem with 14N arises from the very long T1 values encountered, ranging from 41 seconds in NH4C104 (Bastow and Stuart 1989) to 1080 seconds in A1N (Bastow et al. 1998). A delay of 30 minutes between pulses was used to acquire the 14N NMR spectrum of hexagonal BN (Jeschke et al. 1998). Improvements in control of the MAS spinning speed and stability will make 14N MAS NMR possible even in samples with large quadrupole interactions. The reported solid state 14N interaction parameters in nitrogen compounds are summarised in Table 8.4. The 14N NMR spectra of A1N have been determined under both static and MAS conditions (Figure 8.9A) (Bastow et al. 1998). The MAS spectrum shows a narrowing of the approximately Gaussian static lineshape, with the appearance of sidebands. The
NMR of Low- 7 Nuclides
477
Table 8.4. 14N interaction parameters in nitrogen compounds. Compound
~iso* (ppm)
Si3N4 Cubic BN Hexagonal BN
N.D. - 17.6 612
A1N TiN Li3N KNO3
•
(MHz)
2.1 --~0 0.140 + 0.01, 0.142
N.D. 359.5 t, 338 * N.D. 373.2, 342.7 NaNO3 337.8 NH4NO3 (phase IV) 335.3 NH4NO3 (phase III) 338.9 Ba(NO3)2 N.D., 340.5 Sr(NO3)2 338.0 Pb(NO3)2 350.2, 334.5 NH4C1 0 NH4Br 2.2 (NH4)2804 site I N.D. site II N.D. NH4AI(SO4)2.12H20 N.D. NH4IO4 N.D. NH4ReO4 N.D. NIH4SCNII site I N.D. site II N.D. ND4C104 N.D.
qq
Reference
0
Yesinowski & Hill (1999) Jeschke & Jansen (1998) Bastow et al. (1998), Jeschke & Jansen (1998) Bastow et al. (1998) MacKenzie et al. (1995) Differt & Messer (1980) Bastow & Stuart (1990), Jakobsen et al. (2001) Jakobsen et al. (2001) Jakobsen et al. (2001) Jakobsen et al. (2001) Weiden & Weiss (1974), Jakobsen et al. (2001) Jakobsen et al. (2001) Ermolaev & Fung (1999), Jakobsen et al. (2001) Ermolaev & Fung (1999) Smith, M.E. (unpublished) Blinc et al. (1990) Blinc et al. (1990) Bailey & Story (1974) Segel (1981) Segel (1978) Batchelder & Ragle (1980) Batchelder & Ragle (1980) Bastow & Stuart (1989)
0
< 0.01
0
0.505 0.751, 0.746 0.740 0.620 0.662 0.6998, 0.650 0.614 N.D., 0.539 -~ 0 N.D. 0.15443 0.11571 0.196 0.013 0.050 0.093 2.28 0.052530
0 0.022, 0.02 0.01 0.25 0.01 0, 0.01 0.01 N.D., 0 -~ 0 N.D. 0.684 0.749 --~0 0 0 1 --~0.04 0.1257
* chemicalshiftrelativeto solidNH4C1exceptt whichare reportedrelativeto liquidammonia. surprisingly small value of the nuclear quadrupole coupling constant derived from these spectra has been ascribed to fortuitous cancellation of the various contributions, since a larger value would be expected to result from the appreciable electric field gradient (efg) which must exist at this non-cubic nitrogen site (Bastow et al. 1998). The 14N spectra of the cubic and hexagonal modifications of BN show considerable differences (Figure 8.9B). The sideband manifold in the spectrum of hexagonal BN can be simulated only by taking into account both the quadrupolar interactions and the CSA, which is found to be 160 ___ 20 ppm (Jeschke and Jansen 1998, Jeschke et al. 1998). The simpler sideband structure of cubic BN (Figure 8.9B) has been explained in terms of the dipolar interaction. The 14N isotropic chemical shift of the 3-coordinate NB3 units in hexagonal BN differs from the 4-coordinate NB4 units of cubic BN by --~70 ppm, providing a means of distinguishing between these environments (Jeschke and Jansen 1998). The static ~4N NMR spectrum of TiN (Figure 8.9C) shows a broad resonance with
478
Multinuclear Solid-State N M R o f Inorganic Materials
A
B
AIN
338
1
100
0
l
t
l
l
r
/ -10
150
Frequency (kHz)
,
l
i'
t
-150
LL tLL]iLLIJLLLLLL,._
simulated 0
,
0
hexagonal Ill
/
10
37 90
,
20
l
-100
150 MAS
Ti-C-N
[
cubic
static
C
BN
II
.Ill, Ill/ 0
Frequency (kHz)
I.
2000
-150 14N
l
0
I
,
,
,
I.
--
-2000
shift (ppm) w.r.t. NH 3 (liq).
14N
Figure 8.9. Representative NMR spectra of inorganic nitrides. A. A1N under static (upper) and MAS (lower) conditions, from Bastow et al. (1998). B. MAS spectra of cubic BN (upper), hexagonal BN (middle) and the simulation of hexagonal BN (lower), from Jeschke and Jansen (1998). C. Static spectra of TiN and a series of titanium carbonitrides, from MacKenzie et al. (1995). All spectra used by permission of the copyright owners.
a metal-like Knight shift (MacKenzie et al. 1995). When the cubic TiN forms a series of carbonitride solid solutions with cubic TiC, the 14N spectra become much broader and the positions of their maxima change, possibly due to a change in the density of states. Although no discernible quadrupolar structure occurred in the broadened carbonitride spectra, the reduction in the local site symmetry could produce a significant electric field gradient. Thus it is possible that these lines are really much broader, and consequently are only being partially excited (MacKenzie et al. 1995). Very high quality 14N NMR spectra have been obtained by Jakobsen et al. (2001) for a number of metal nitrates using high-quality MAS probes to determine the complete manifold of spinning sidebands from which extremely reliable 14N NMR interaction parameters for these compounds (Table 8.4) were extracted by spectral simulation. Single-crystal 14N NMR studies have allowed the XQ values to be obtained for NH4C104 (Bastow and Stuart 1989), NH4Al(SO4)z.12H20 (Bailey and Story 1974), Ba(NO3)2 (Weiden and Weiss 1974) and KNO3 (Bastow and Stuart 1990). Detailed analysis of the single crystal pattern from KNO3 allowed the elements of the chemical shift tensor to be determined, the largest component being 149.6 ppm (Bastow and Stuart 1991). A single-crystal laN NMR study of AgNO3, Ba(NO3)2 and Pb(NOs)2 has been used to resolve a controversy in the crystallography of these compounds by showing that the unit cell in all three has a centre of symmetry (Santos et al. 1990).
NMR of Low-7 Nuclides
479
The 14N single crystal rotation pattern of Li3N has also been determined, showing evidence of CSA (Differt and Messer 1980). A recent series of 14N MAS NMR studies has been based on the principle that the first-order quadrupole interaction is inhomogeneous, allowing narrow MAS resonances to be observed, providing the magic angle is set accurately. The signal/noise ratio can be enhanced by adding sidebands, and the centrebands can be determined by using variable spinning speeds (Ermolaev and Fung 1999). By using this approach, a mixture of KNO3, Pb(NO3)2 and NH4C1 could be quantitatively deconvoluted (Ermolaev and Fung 1999). Residual broadening of the lineshape is caused by slight deviations from the magic angle, but if the true angle is known, the broadened lineshape can be used to deduce the interaction (Khitrin and Fung 1999).
8.3.2 25Mg NMR Magnesium is an important element in various areas of materials science, including technologically important high-temperature ceramics, minerals and glasses. It would be particularly helpful if 25Mg NMR was able to provide similar structural information to that obtained from 27A1 NMR, or even if octahedral and tetrahedral Mg could be distinguished on the basis of the chemical shift. However, the second-order quadrupole broadening of the central transition (~/2 --~ - ~/2)of 25Mg is about 9 times greater than for 27A1 in sites with the same electric field gradient in the same applied magnetic field. An initial report of 25Mg MAS NMR spectra of simple inorganic compounds showed that cubic materials in which the Mg sites are symmetrical (MgO, MgS and Mg2Si) give very narrow MAS resonances (<- 50 Hz at a magnetic field of 8.45 T) (Dupree and Smith 1988a). Less symmetric compounds give much broader resonances, some of which show secondorder quadrupole structure from which ~i~ocan be estimated, but the spectra of others are too broad to be narrowed by MAS, often preventing their isotropic chemical shift values from being deduced. For compounds where the chemical shift can be determined, the difference between MgO6 (as in MgO) and MgO4 (as in MgA1204) is ca. 25 ppm, but for the 25Mg-enriched silicate minerals diopside (containing octahedral Mg) and akermanite (containing tetrahedral Mg) a larger difference in the shift (ca. 40 ppm) has been reported (Fiske and Stebbins 1994). The only MgO5 spectrum reported so far is that of the unusual mineral grandidierite (Figure 8.10G), which shows a broad resonance with a quadrupolar lineshape yielding values of 55 ppm and 3.8 MHz for ~i~o and XQ respectively (MacKenzie and Meinhold 1997). The reason why this ~i~ovalue is more positive than might have been expected from the values for fourfold and sixfold coordinated Mg in other minerals is not clear, but it appears to indicate an overlap in the shift ranges for fourfold and fivefold coordinated Mg possibly reflecting the influence of the species in the first and second coordination spheres. The solid state interaction parameters reported for 25Mg are summarised in Table 8.5.
480
Multinuclear Solid-State NMR of Inorganic Materials
A
C
Peric~
Dol~
G
200 0 -200
Hydrotalcite ~ d
200 ~ d 0-200
BBraeite I/~observed D A Magnesite simulated
Hydromagnesite
? ~ Hahnecho
~[,i~ ed
/
?~chdecay
~ated ~._._._julated_~_ , '1- siteM2
50 -50 -150
Grandierite
F
E
t
.
.
.
.
.
i
,
.
.
100
U
/
/
,
l~ated
--x
-100 -300 40 -40 -120 100 -100 -300 2SMg shift (ppm) w.r.t. MgSO4soln.
50 -5O -150
of
Figure 8.10. A selection 25Mg MAS NMR spectra of magnesium compounds with simulated lineshapes where appropriate. A. MgO (periclase). B. Observed and simulated lineshapes of Mg(OH)2 (brucite). The feature to the left of the main lineshape is from a small amount of MgO impurity. C. Ca,Mg(CO3)2 (dolomite). D. Observed and simulated lineshapes of MgCO3 (magnesite). E. Observed and simulated lineshapes of Mg6A12(OH)16CO3.4H20(hydrotalcite), F. Observed and simulated lineshapes of Mgs(CO3)n(OH)e.4H20 (hydromagnesite) showing the individual simulated lineshapes of the two octahedral sites M 1 and M2 (lower). G. (Mg,Fe)A13 BSiO3 (grandidierite), showing the experimental spectrum acquired using a Hahn spin-echo pulse sequence (upper) and by Bloch decay (middle) with simulated spectrum (lower). Spectra A, C, E and F from MacKenzie and Meinhold (1994a), spectrum B from MacKenzie and Meinhold (1993), spectrum D from MacKenzie and Meinhold (1993a), spectrum G from MacKenzie and Meinhold (1997). All spectra used by permission of the copyright owner.
T1
MgO shows a single sharp resonance at 25-27 ppm with a long value (ca. 50 s) (Figure 8.10A). The spectrum has been measured from room temperature to 1300~ and shows very little variation in 8iso and T~ over this temperature range (Fiske et al. 1994). The small change in 8iso with temperature was in the opposite direction to that expected from simple bondlength correlations, necessitating an explanation in terms of the orbital overlap being increased by the thermal vibration (Fiske et al. 1994). The 25Mg linewidth of nanocrystalline MgO prepared by hydrolysis of Mg(OCH3)2 decreased from 2 kHz to 450 Hz and the peak position changed from 18 to 25.3 ppm as the crystallite particle size increased from 3 to 35 nm (Chadwick et al. 1998). No 25Mg resonance was observed from particles < 3 nm, due to broadening effects related to crystallite disorder. The 25Mg MAS spectrum of Mg(OH)2 (brucite) is broad (Figure 8.10B), but shows a quadrupolar lineshape from which 8iso and XQ values of 14.1 ppm and 3.15 MHz were determined (MacKenzie and Meinhold 1993). These values are in reasonable agreement with a single crystal study of Mg(OH)2 in which the linewidth was found to
481
N M R o f L o w - 7 Nuclides
Table 8.5. 25Mg interaction parameters in magnesium compounds. ~q
Reference
0
0
10, 14.1, 13.5
2.9, 3.056, 3.15
0
52 ~--0"* 27 N.D. N.D. - 15 -3 61 --- 10"* 79, 45, - 57** 45, - 59** --~0
0 N.D. 2.4 4.996 4.313 1.88 0 N.D.
0 N.D. 0.4 0.963 0.396 0.10 0 0
Dupree & Smith (1988a), Fiske et al. (1994), MacKenzie & Meinhold (1994a) Dupree & Smith (1988a), Bastow (1991 ), MacKenzie & Meinhold (1994a) Dupree & Smith (1988a) Dupree & Smith (1988a) MacKenzie & Meinhold (1994a) Derighetti et al. (1978)
Compound
~iso (ppm)*
MgO
26, 26.5, 25
Mg(OH)2 MgA1204 Mg2Si308 MgSiO3 (enstatite) Mg2SiO4 (forsterite) MgTiO3 MgS Mg2Si Mg3N2 MgSiN2 MgA1SiN3 Mg3A12(SiO4)3 (pyrope) Mg6SisOzo(OH)4 (tale) Mg3SizOs(OH)4 (antigorite) Hectorite Montmorillonite Phlogopite Palygorskite Sepiolite CazMgSi207 (akermanite) CaMgSi206 (diopside) (Mg,Fe)A13BSiO3 (grandidierite) MgCO3 (magnesite) MgCa(CO3)2 (dolomite) Hydromagnesite
Hydrotalcite MgC12.nH20 MgC12.6H20 (77K) MgSO4.6H20 Mg diacetate.4H20 Mg formate.2H20 (site 1) (site 2)
•
(MHz)
N.D.
N.D.
N.D. N.D. N.D.
N.D. N.D. N.D.
Padro et al. (2000) Dupree & Smith (1988a) Dupree & Smith (1988a) Dupree & Smith (1988a) MacKenzie & Meinhold (1994) MacKenzie & Meinhold (1994) Dupree & Smith (1988a)
48
2.4
0.7
MacKenzie & Meinhold (1994a)
50
2.9
0.7
MacKenzie & Meinhold (1994a)
2.2
1.0
2.8, 1.5
0
68 80 51 49
3.0 3.2 2.7 2.8
0.65 0.5 0.7 0.25
MacKenzie & Meinhold (1994a) MacKenzie & Meinhold (1994a) MacKenzie & Meinhold (1994a) MacKenzie & Meinhold (1994a) MacKenzie & Meinhold (1994a) Fiske & Stebbins (1994)
8
20
0.95
Fiske & Stebbins (1994)
55
3.8
0.6
MacKenzie & Meinhold (1997)
4.8-8.8 - 4.3
2.5-2.6 0.9
0--0.3
MacKenzie & Meinhold (1994a) MacKenzie & Meinhold (1994a)
-4 16 10
3.6 3.1 4.4
0 0 0
- 2
N.D.
N.D.
N.D. 0 27
1.477 1.6 1.90
N.D. 0 0.82
MacKenzie & Meinhold (1994a) Dupree & Smith (1988a) Hiyama et al. (1987) Dupree & Smith (1988a) Sham & Wu (2000)
10 10
1.70 1.40
1.0 0.8
Sham & Wu (2000) Sham & Wu (2000)
43 -4,
11
0
MacKenzie & Meinhold (1994a)
482
Multinuclear Solid-State NMR of Inorganic Materials
Table 8.5. (Continued). Compound
~iso (ppm)*
XQ(MHz)
xI
Reference
1.95 0.266
0.8 0
1163
0.266
0
Sham & Wu (2000) Bastow & Celotto (1999), Bastow (1991a) Bastow & Smith (1995)
990 1445 1705
2.20 0 1.2(e)
0.95 0 --~0
Bastow & Smith (1995) Bastow & Smith (1995) Bastow & Smith (1995)
Mg methylmalonate.4H20 12 Mg metal 1179 Mg6%A1 MglvA112 (site 24g) (site 2a) (site 8c)
* chemical shifts quoted with respect to aqueous MgSO4 ** peak positions (e) = estimated
be strongly related to the rotation angle of the crystal, suggested to be due to mosaic spread of the crystal (Bastow 1991). Although the Mg-O bond is quite ionic, a small CSA contribution of ca. 6 ppm was deduced. 25Mg NMR has been used to investigate the role of Mg(OH)2 used as a fire retardant in ethylene-vinyl acetate copolymers (Pecoul et al. 1997). The spectra acquired before and after combustion indicate that the retarding action results from the formation of a glassy phase. The 25Mg NMR interaction parameters have been reported for a series of 18 minerals at 11.7 T and a spinning speed of 20 kHz using single pulse excitation (Table 8.5) (MacKenzie and Meinhold 1994a). The spectra were processed by linear back prediction (see Chapter 3) to recover data which were not acquired during the first 100 txs after excitation in an attempt to alleviate spurious effects introduced by probe ringing, etc. The T~ values for these minerals vary from 0.2 s for talc to 15.9 s for MgO (MacKenzie and Meinhold 1994a). Very accurate 25Mg NMR single crystal data have been obtained for the M1 and M2 Mg sites in forsterite (Mg2SiO4) using dynamic nuclear polarisation (Derighetti et al. 1978) and the quadrupole interaction in this mineral has also been calculated by superposition analysis of the structural distortion of the oxygen coordination tetrahedra (Such and Lehmann 1988). It was found not to be possible to obtain an experimental 25Mg MAS NMR spectrum of forsterite corresponding to that simulated from the data of Derighetti et al. (1978); a spectrum from heated chrysotile containing predominantly crystalline forsterite showed a resonance which could have arisen from the simulated M2 site, but may have arisen from an impurity phase (MacKenzie and Meinhold 1994a). The inability to observe the expected forsterite spectrum may be due to efg broadening effects, or to long T~ times in forsterite. The Mg in the silicate minerals reflects an octahedral site preference, being located predominantly in MgO6 sites. Various correlations between the XQ values and geometrical distortion factors of the Mg sites in these minerals have been sought
NMR of Low- g Nuclides
483
(MacKenzie and Meinhold 1994a), the most promising being between XQ and the distortion index (DI), defined for octahedral bond angles as 12 OI- ZlOi -0ol/12 00 i-1
(8.~)
where 0i is the actual bond angle and 0o is the undistorted octahedral bond angle. The linear relationship between XQ and DI for 15 Mg minerals (Figure 8.11A) is described by the equation X Q - 41.1 DI + 1.05
(8.2)
Another promising relationship with XQ is also related to the octahedral bond angles and involves the shear strain ~ within the coordination polyhedra, defined as 12
~-Zltan(oi-Oo)l
(8.3)
i=l
The resulting linear relationship (Figure 8.11B) has the equation XQ -- 2.18~ + 0.832
(8.4)
Although the efg effects in these minerals are likely to be more complex than suggested by these simple relationships, they could provide a simple means of tentatively assigning octahedral resonances to specific sites. No satisfactory relationships have been found between the 25Mg NMR parameters and functions involving the octahedral bondlength (MacKenzie and Meinhold 1994a). 25Mg NMR measurements at temperatures up to 1400~ have been used to investigate the mechanism of magnesium exchange between different sites in crystals and melts. The high-temperature 25Mg spectra of a single crystal of forsterite (MgzSiO4) have allowed the magnesium exchange between the different sites to be measured, giving results which are in agreement with estimates of the hopping frequency based on diffusion data (Stebbins 1996). The behaviour of Mg in molten silicates has also been studied by 25MgNMR (Figure 8.12A) which indicates that melting is accompanied by an increase in the shielding of ca. 4 ppm, suggesting an increase in the average coordination of the Mg which apparently prefers an octahedral coordination state in the melt (George and Stebbins 1998). Measurements ofT1 in Mg silicate melts suggest that the motion of the Mg is strongly coupled to that of the molten network, as reflected in the viscosity of the system (Figure 8.12B). Because the linewidth is related to T1, the
484
Multinuclear Solid-State NMR of Inorganic Materials A 5
5
N
N
i
9 ~ 3
c~
~
r
R 2 = 0.85 v~
!
0-
'
.
.
.
.
0.04 .
'
0. ;s
Distortion Index (DI)
Octahedral shear strain
Figure 8.11. Relationship between the 25Mgnuclear quadrupole coupling constant XQ for a number of magnesium compounds and minerals and A. the octahedral distortion index DI defined in Equation (8.1). Regression line fitted only to the most reliable data points (full circles), less reliable data points (full squares) are included for interest only. B. the octahedral shear strain defined in Equation (8.2). Regression line fitted to all data points. From MacKenzie and Meinhold (1994a) by permission of the copyright owner.
melt viscosity will therefore control the fundamental limit at which Mg can no longer be observed by 25Mg NMR in the melt (George and Stebbins 1998). Many technically important processes involve the thermal decomposition of magnesium minerals. 25Mg NMR has been used to study the thermal decomposition of brucite, Mg(OH)2 to MgO, providing evidence that the mechanism involves coherent growth of small domains of MgO which subsequently coalesce into crystallites (MacKenzie and Meinhold 1993). 25Mg NMR has also been used to show that the formation of MgO by heat treatment of magnesite, MgCO3, can be detected by NMR well before it becomes detectable by X-ray diffraction (MacKenzie and Meinhold 1993a). A similar result was found for the thermal decomposition of the double magnesium calcium carbonate dolomite, which was also shown by 25Mg NMR to form a mixture of MgO and CaO without the intermediate formation of MgCO3, either as a discrete phase or in solid solution (MacKenzie and Meinhold 1993b). The thermal decomposition reactions of more complex minerals have also been studied using 25Mg NMR. Hydrotalcite, Mg6A12(OH)]6CO3.4H20 is a member of a large family of double hydroxides with layer structures between which charge-balancing anionic groups such as CO3"- are located. These materials have a variety of uses, including catalyst supports, flame retardants, absorbents and anion exchangers. Thermal decomposition to MgO and MgA1204 occurs by a series of overlapping reactions which have been shown by 25Mg and 27A1 NMR to involve the formation of an assemblage of intergrown microdomains of poorly crystalline MgO, possibly containing substituent A 13+ and vacancies, and a spinel-based alumina phase (MacKenzie et al. 1993). These
NMR of Low-~ Nuclides I11
o
A
485
B
M -9.0
N1500~ .................
~1200~
oC o
-9.4
~"
-9.8
-10.2 -10.2
1440 l",
,
i
,
i
,
2O0
,
,
,
i
,
,
,
,
f",
0
,
,
,
i
,
,
,
,
~
,
,
.
.
i
-200
2SMg shift (ppm) w.r.t. Mg(NO3)2 soln.
~176
-9,8
'/a
t
, ........... -9.4 -9.0
log ~c (NMR relaxation) o
Figure 8.12. A. Static 25MgNMR spectra of a magnesium calcium aluminosilicate silicate melt of c o m p o s i t i o n (MgO)0.14(CaO)0.41(A1203)o.o6(SiO2)0.39at various temperatures. Note the narrowing of the resonance line with increasing temperature. B. Relationship between the shear viscosity of several magnesium silicate melts and the 25MgNMR relaxation time. Circles denote sodium magnesium silicate, squares and diamonds denote two different magnesium calcium aluminosilicate compositions, triangles denote magnesium calcium silicate. From George and Stebbins (1998) by permission of the Mineralogical Society of America. changes are reflected in the 25Mg NMR spectra by the loss of the distinctive secondorder quadrupolar lineshape of hydrotalcite (Figure 8.10E) and a downfield shift of the resulting single peak which achieves the position of MgO by 600~ (Figure 8.13). Heating to higher temperatures further narrows this line, but the tetrahedral 25Mg signature of MgAI204 was surprisingly not detected, even though its presence in these samples was confirmed by X-ray powder diffraction (MacKenzie et al. 1993). The thermal decomposition sequence of the asbestos mineral chrysotile, Mg3SizOs(OH)4, has also been shown by a combination of 25Mg and 2 9 8 i NMR to be more complex than previously thought. Loss of the hydroxyl water is accompanied by the formation of a Mg-rich amorphous phase in which the Mg is octahedrally coordinated and which transforms to forsterite, MgzSiO4 at 670-700~ (MacKenzie and Meinhold 1994b). A significant reduction in the intensity of the 25Mg signal observed in samples heated at 800~ is probably due to quadrupolar broadening and is a general phenomenon in dehydroxylated minerals, being also found for 27A1 NMR spectra (see Chapter 5). 25Mg and 298i NMR has also been used to study the thermal decomposition reactions of the related layer-lattice mineral talc, Mg6Si8Ozo(OH)4, which was shown to form orthoenstatite, MgSiO3 by a simple reaction in which the Mg remains in octahedral coordination throughout (MacKenzie and Meinhold 1994c). A similar thermal reaction sequence was shown by a 25Mg, 29Si and 7Li NMR study of the structurally related synthetic mineral hectorite, in which some of the octahedral Mg is replaced by Li with charge balance being provided by interlayer cations such as Na+.
486
Multinuclear Solid-State NMR of lnorganic Materials
unheated
14
-39 3
600~
6/~ "17
.16
27
_
'
400
'
i
0
*
.
[
_
-400
4;0
"
O
' -400
.
400
_
0
_
_
-400
25Mg shift (ppm) w.r.t. M g S 0 4 soln.
Figure 8.13. 25MgMAS NMR spectra of hydrotalcite (Mg6AI2(OH)16CO3.4H20)taken at various stages of its thermal decomposition sequence. Note the loss of the hydrotalcite quadrupolar lineshape and the progressive shift of the resonance line towards the position of MgO. From MacKenzie et al. (1993), by permission of the copyright owner. As the silicate structure is disrupted by the loss of hydroxyl water, the octahedral Mg sites become progressively distorted, as reflected by broadening and intensity loss of the 25Mg signal, but the Mg remains in octahedral coordination throughout (MacKenzie and Meinhold 1994d). MgO finds an important use in the processing of non-oxide technical ceramics such as Si3N4 and sialon compounds, to which it is added to promote densification during sintering. 25Mg NMR has been used to study the effect of 10 wt % additions of MgO to Si3N4 sintered at 1500-1700~ The spectra of these samples show intensity in 2 broad bands (Figure 8.14A) and their similarity to the 25Mg spectra of MgSiN2 and MgA1SiN3 (Figure 8.14D,E) suggests that the magnesium finds its way into a glassy intergranular phase containing both 6-fold and 4-fold coordinated Mg and stabilised by nitrogen. This glass, which is molten above 1550~ promotes liquid-phase sintering and densification of the Si3N4. The 25Mg NMR spectra provide evidence that it is formed by reaction of the MgO with both the oxidised surface layer on the nitride grains and with the Si3N4 itself, forming a Mg-Si-O-N glass which is not readily recrystallised, and at higher MgO concentrations contains appreciable proportions of 4-coordinated Mg (MacKenzie and Meinhold 1994). Similar 25Mg NMR spectra are found in 13-sialon samples sintered with additions of MgO (Figure 8.14B) (MacKenzie and Meinhold 1996), and in X-sialon synthesised by a silicothermal process and densified in the presence of 10 wt % MgO (Figure 8.14C) (MacKenzie et al. 2000). All these results suggest that the formation of a nitrogen-containing glassy phase containing Mg in both octahedral ( - 35 to - 46 ppm) and tetrahedral (38 to 51 ppm) sites is a common feature of MgO-assisted sintering of silicon nitride and the closely related sialon compounds.
NMR of Low- 7 Nuclides
487
25Mg NMR studies of metallic Mg are facilitated by its short T1 time resulting from a relaxation mechanism involving the contact interaction with the conduction electrons. Early NMR studies reported the Knight shift in Mg powder (Dickson and Seymour 1970) and in single crystals (Dougan et al. 1969). A more recent study of Mg metal at a higher magnetic field (9.4 T) has revealed a small temperature dependence of the Knight shift, which then increased sharply in the vicinity of the melting point as expected for a first-order phase transition (Bastow and Celotto 1999). A determination of the XQ value of metallic Mg from measurements of the satellite transitions has shown a T 15 dependence (Bastow 1991a). Comparison of the static and MAS 25Mg spectra of Mg metal (Figure 8.15A) shows that MAS produces only partial narrowing, indicating that a significant contribution is being made to the linewidth from the second-order quadrupole interaction. Work-hardening of the Mg metal produces a small increase in the linewidth of the central transition, but the non-central transitions are significantly smeared out due to the distribution of the electric field gradients (Bastow and Smith 1995). The increasing use of lightweight dilute magnesium-aluminium alloys in the automotive industry has led to 25Mg NMR studies of some of these materials. The 25Mg NMR spectrum of Mg-6 % A1 is very similar to that of pure Mg apart from a small variation in the chemical shift and a smearing out of the satellite transitions resulting from a distribution of interactions as the A1 substitutes into the framework (Bastow and A
Si3N 4
B
[3-sialon
C
X-sialon D
49 -37 33~-85
33~
43
79~ 45
1 .5r
j 45
4
1
36
~
~
~
1
M
/
9
400
0
-400
400
0
400
100
-300
!
400
,,
,
~-3__~
0
,~
I
9
_1
-400
2SMg shift (ppm) w.r.t. MgSO 4 soln.
Figure 8.14. 25MgMAS NMR spectra of nitride-based ceramic bodies sintered with 10 wt % MgO and related magnesium silicon nitride compounds. A. Si3N4,from MacKenzie and Meinhold (1994), B. [3-sialon, from MacKenzie and Meinhold (1996), C. X-sialon, from MacKenzie et al. (2000), D. Crystalline MgSiN2 (upper) and MgA1SiN3 (lower), from MacKenzie and Meinhold (1994). All spectra used by permission of the copyright owner.
488
Multinuclear Solid-State NMR of Inorganic Materials
Smith 1995). The static 25Mg NMR spectrum of the alloy Mgl7All2 shows 3 sites with distinct Knight shifts (Figure 8.15B) and with intensity ratios determined by spectral integration as 1.1:3.6:12, in very close agreement with the ratio of 1:4:12 expected from the structure (Bastow and Smith 1995). The 25Mg NMR spectra of several magnesium salts of organic acids have been obtained from samples isotopically enriched in 25Mg (Sham and Wu 2000). The spectrum of magnesium formate is particularly interesting, since it contains a complex lineshape arising from the overlap of two inequivalent Mg sites (Figure 8.16A). These sites could be clearly resolved in the two-dimensional MQMAS NMR spectrum (Figure 8.16B) (Sham and Wu 2000), demonstrating the possibility of applying Mg MQMAS techniques to resolve overlapping Mg sites in inorganic materials and silicates.
8.3.3
33S N M R
Sulphur-containing compounds are of considerable practical importance in a range of applications including mineral processing, high-technology materials (semiconductors, optoelectronics) and polymer processing (rubber vulcanisation). However, reported solid state 33S NMR studies are sparse, due to the difficulties of observing this low- 7 nucleus with a low natural abundance (0.76%). The usual primary reference compound for 33S is CS2, but recent high-field MAS experiments suggest that CaS and NH4AI(SO4)2.12H20 would be useful secondary solid reference compounds in view of their narrow MAS linewidths l0 and 18 Hz respectively (Daunch and Rinaldi 1996). Of these two compounds, the alum appears preferable, since it has the shorter T1 value (0.23 s, compared with 23 s for CaS). Most of the 335 NMR studies in the literature have been made on ZnS (Eckert and Yesinowski 1986, Retcofsky and Friedel 1972, Karr and Schultz 1968, Haller et al. 1980, Lutz 1983, Bastow and Stuart 1988, Daunch and Rinaldi 1996). The static 33S NMR spectra of cubic and hexagonal ZnS show differences between the two forms (Figure 8.17A). Hexagonal ZnS shows an apparently second-order quadrupolar powder pattern from which values of XQ and the CSA were determined (Bastow and Stuart 1988). Analysis of the lineshape from this polymorph in terms of a mixture of interactions has yielded a value for the CSA of 23 _+ 1 ppm (Eckert and Yesinowski 1986). The cubic form has a much more symmetric spectrum (Figure 8.17A), but the two polymorphs have isotropic shifts which differ by only --~ 3 ppm. Other 33S NMR investigations of sulphides include an early study of paramagnetic ct-MnS which provided information about the transferred hyperfine field (Lee 1968), and ferromagnetic EuS, in which the internal magnetic field at the sulphur site was determined to be 1.71 T extrapolated to 0 K (Suzuki et al. 1971). An extensive static 33S NMR natural abundance study of 26 sulphides and sulphates has been made by Eckert and Yesinowski (1986). In all these spectra only the central
NMR of Low- 7 Nuclides
A
Mg metal
B
Mg17AI12 sRe24g ! 4 - / J ~ ~ mulated
static ~
1250
489
site 8c lslte
J~_~erved
1150 3000 1000 2SMg shift (ppm) w.r.t. MgSO 4 soln.
-1000
Figure 8.15. A. Static and MAS 25Mg NMR spectra of pure annealed magnesium metal, B. 25Mg NMR spectrum of Mg17A112showing the (1/2, - 1/2)lines from the three inequivalent Mg sites. The line at 24 ppm (extreme right) is from MgO. The inset shows the expanded lineshape from the 24 g site (lower) with the simulated spectrum (upper). From Bastow and Smith (1995) by permission of the copyright owner. A
B
~
obs
r
-20 0 simulated J ;o 'o ' 2SMg shift (ppm) w.r.t. MgS04 soln.
20 40
' oo~0
'
40
9i
"~
0
-40 ppm
'".
-80
Figure 8.16. A. Experimental and simulated 25Mg MAS NMR spectrum of magnesium formate dihydrate (simulation based on data from the MQMAS spectrum). B. Two-dimensional 25Mg MQMAS spectrum of magnesium formate dihydrate showing the two inequivalent Mg sites resolved. From Sham and Wu (2000) by permission of the American Chemical Society. transition appears to be excited. In general, the sulphate resonances tend to be broader than the sulphides, and show more evidence of second-order quadrupolar lineshape (Figure 8.17B), although in many cases the details of these lineshapes are not distinct and allow only a value of the quadrupolar product (PQ) to be estimated. The shifts of the sulphates occur in a narrow range (320 to 338 ppm) but the values of Xe show a wider
490
Multinuclear Solid-State NMR of Inorganic Materials A
ZnS
hexagonal
i -160
I
B
Cs2SO 4
_
I -200
I
I -240
1
i -280
J
i -320
1
0
i
2 0
0
_.__1_
600
400
_..
33S shift (ppm) w.r.t. CS2
33S shift (ppm) w.r.t. CS2
Figure 8.17. Static 338 NMR spectra of representative sulphide and sulphate compounds. A. Cubic ZnS (upper) and hexagonal ZnS (lower) adapted from Bastow and Stuart (1988). B. Cs2SO4, from Eckert and Yesinowski (1986) by permission of the American Chemical Society. variation (0.53 to 2.3 MHz). All the sulphates which crystallise with the K2SO4 structure (except (NH4)2804) have identical second-order quadrupolar powder patterns, with no correlation being apparent between • and the counterion (Eckert and Yesinowski 1986). By contrast, the sulphides studied by Eckert and Yesinowski (1986) have smaller XQ values, reflected in their narrower linewidths, and a wider range of chemical shifts ( - 347 to 291 ppm). The temperature dependence of some of these shifts is 0.007 to
~ PbS CaS
.... l I
400
. . . .
AINH4(_SO4)2"12I-I20 I
' ' " ' 1
. . . .
200
I
. . . .
I
~ ' ' '
I ~ ' ' ' 1
0
'
-200
~'
'
1
''
'
'
I
-400
33S shift (ppm) w.r.t. CS~ Figure 8.18. 33S MAS NMR spectra of a selection of sulphides and sulphates acquired at 14.1 T with spinning speeds of 15-18 kHz. From Daunch and Rinaldi (1996) by permission of the copyright owner.
NMR of Low- y Nuclides
491
0.116 ppm/K. The trends in the chemical shifts of these sulphides can be explained in terms of electronegativity considerations involving ionicity and bond overlap effects (Eckert and Yesinowski 1986). More recently MAS NMR techniques have been applied to 33S NMR studies of a few sulphides and sulphates using a high magnetic field (14.1 T) and fast spinning speeds (15-18 kHz) (Figure 8.18) (Daunch and Rinaldi 1996). Magic angle spinning typically narrowed the spectra by a factor of --~10, suggesting that the static linewidth is broadened by a number of contributions. The marked benefits of working with this nucleus at the higher field and MAS conditions were well illustrated by a static experiment at 4.7 T, which, despite using 10 times as much sample, took 40 times longer to achieve a comparable signal/noise ratio (Daunch and Rinaldi 1996). Of the 10 compounds included in this high-field study, magic angle spinning removed sufficient of the broadening of 4 of the sulphates (KAl(SO4)z.12H20, Cs2SO4, NazSO4 and BaSO4) to reveal their second-order quadrupolar lineshapes. Although the 33S NMR spectra of only a relatively small number of compounds have so far been reported (Table 8.6), the most recent results should encourage further 33S NMR studies.
8.3.4
35C1and 37C1N M R
The magnetic moments of both these chlorine nuclei are similar and their Larmor frequencies differ by only about 20%. The second-order quadrupolar broadening of 37C1 is 75% that of 35C1, but this is offset by the greater natural abundance of the latter, giving it a receptivity 5.34 times greater than 37C1. For this reason, 35C1 is the nucleus most favoured for NMR studies. The covalent bonding in chlorine compounds is typically very strong, leading to high Xo values; up to 80 MHz is not uncommon (Lucken 1969). The practical consequence of these high nuclear quadrupole coupling constants is that in many cases the normal high field limit is not reached, placing NMR experiments at high magnetic fields in the complex regime where the energy level separation is being determined by both interactions, and the quadrupole interaction is not a simple perturbation of the Zeeman levels. For this reason, much of the magnetic resonance data for chlorine has been from nuclear quadrupole resonance (NQR). The 35'37C1 MAS NMR spectra of the alkali chlorides and ammonium choride in a magnetic field of 11.7 T all show narrow resonances (Figure 8.19A), with a range of shifts of --~150 ppm (Weeding and Veeman 1989). The chemical shifts of the 2 isotopes were found to be identical, indicating the absence of quadrupole effects. These shift values have been confirmed by another study at 9.4 T (Hayashi and Hayamizu 1990) and correlate well with both the interionic distances (Figure 8.19B) and cation electronegativities. KC1 shows a very narrow MAS NMR signal (--~20 Hz wide) appearing at 3.07 ppm from the normal reference of dilute Cl-, suggesting its suitability as a secondary
492
Multinuclear Solid-State NMR of lnorganic Materials
Table 8.6.
335
interaction parameters in sulphur compounds.
Compound
~iso* (ppm)
XQt(MHz)
Reference
ZnS (wurzite)
- 231, - 234, - 235, - 229
0.43 a
ZnS (zincblende)
- 228, - 230
N.D.
LieS NazS MgS CaS SrS BaS PbS
284 - 347 - 338 - 174 - 28.5 42.8 291 - 297, - 293, - 300
N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D
NaSO4
330 (338,333)
0.82
CaSO4
CaSO4.2H20 BaSO4
326 337 N.D., (353,326)
1.0 0.77 2.3
K2504 Rb2SO4 Cs2SO4
334 329 335, (331,326)
1.13 1.01 0.97
T12SO4 Li2SO4 (NH4)2504
320 321 328, 331
1.03 N.D. 0.59
NaCa(SO4)2 Na2Mg(SO4)2.4H20 KAI(SOn)2.12H20
338 321 327, (330, 325)
0.71 2.2 0.79
RbAI(SOn)2.12H20 CsAI(SO4)2.12H20 NHnAI(SO4)2.12H20
334 331 333, 331
0.56 0.53 0.53
T1AI(SO4)2.12H20
332
0.56
Eckert & Yesinowski (1986), Bastow & Stuart (1988b), Daunch & Rinaldi (1986), Haller et al. (1980) Eckert & Yesinowski (1986), Bastow & Stuart (1988b) Eckert & Yesinowski (1986) Eckert & Yesinowski (1986) Eckert & Yesinowski (1986) Eckert & Yesinowski (1986) Eckert & Yesinowski (1986) Eckert & Yesinowski (1986) Eckert & Yesinowski (1986) Eckert & Yesinowski (1986), Daunch & Rinaldi (1996) Eckert & Yesinowski (1986), Daunch & Rinaldi (1996) Eckert & Yesinowski (1986) Eckert & Yesinowski (1986) Eckert & Yesinowski (1986), Daunch & Rinaldi (1996) Eckert & Yesinowski (1986) Eckert & Yesinowski (1986) Eckert & Yesinowski (1986), Daunch & Rinaldi (1996) Eckert & Yesinowski (1986) Eckert & Yesinowski (1986) Eckert & Yesinowski (1986), Daunch & Rinaldi (1996) Eckert & Yesinowski (1986) Eckert & Yesinowski (1986) Eckert & Yesinowski (1986), Daunch & Rinaldi (1996) Eckert & Yesinowski (1986) Eckert & Yesinowski (1986) Eckert & Yesinowski (1986), Daunch & Rinaldi (1996) Eckert & Yesinowski (1986)
-
* chemical shift relative to liquid CS2; some values are peak positions and not the isotropic shifts. Values in parenthesis refer to singularity positions of MAS powder patterns. t estimated upper limit of the quadrupole effect parameter, except value marked a which is based on lineshape analysis.
reference. A 35C1 N M R study of CuC1 determined the shift to change by 50 ppm over the temperature range - 150~
to the melting point at about 400~
(Becker 1978).
The 35C1 M A S N M R 14.1 T spectra of 13 inorganic perchlorates have recently been reported (Figure 8.20A,B) (Skibsted and Jakobsen 1999). Spectral simulation of all the
NMR of Low- T Nuclides
493
B
A
NH4CI
, ..................
150 . . . . . . . . . . . .
Z ~/ 5o m.
KCI
-50 ~'I
,
'
'
200
'
u
,
100
,
,
,
i
,
'~"I
0
i
i",
,"
2.8
-100
3sC1 shift (ppm) w.r.t. NaCI soln.
3.2
3.6
Internuclear separation R (A)
Figure 8.19. A. 35C1MAS NMR spectra of NH4C1 (upper) and KC1 (lower). Asterisks denote spinning side bands. The broadening of the upper spectrum is due to the protons which were not decoupled in this experiment. B. Relationship between the 35C1chemical shift of the alkali chlorides and the internuclear separation R. From Weeding and Veeman (1989), by permission of the Royal Society of Chemistry.
A
Mg(CIO4)2
9
1150
n
1050
!
|
950
..... t
B
_ _t
Ba(CIO4) 2
C
LiCIO 4
:
850
1150
1050
35C1 shift (ppm) w.r.t. NaCI
950
850
1050
1046
1042
37Cl shift (ppm) w.r.t. NaCl
Figure 8.20. A. 35C1 MAS NMR spectrum of the observed and simulated central transition of Mg(C104)2. The sharp resonance in the experimental spectrum marked by an asterisk is due to a small amount of hexahydrate impurity. B. 35C1MAS NMR spectrum of the observed and simulated central transition of Ba(C104)2. C. 37C1MAS NMR spectra of the central transitions of LiC104 (upper) and Mg(C104)2.6H20 (lower). From Skibsted and Jakobsen (1999), by permission of the American Chemical Society. transitions including the satellite transitions allowed the N M R interactions to be determined accurately. The perchlorate shifts are displaced from the chloride region by --~ 1000 ppm, but the variation in 6iso between all the perchlorates is only - 20 ppm. A wider variation is found in the Xo values, which can therefore be used to provide
494
Multinuclear Solid-State N M R o f Inorganic Materials
information about the number and symmetry of the perchlorate sites, and to monitor the state of hydration (Skibsted and Jakobsen 1999). The 37C1 MAS NMR spectra have also been determined for a number of these perchlorates (Figure 8.20C) and have giso values in good agreement with the 35C1 values. Furthermore, the ratio of 35C1 XQ / 37C1 • for these compounds occurs in the range 1.23-1.28, in excellent agreement with the theoretically expected value of 1.269. Although the smaller degree of quadrupolar broadening of 37C1 may make this a preferable nucleus in cases where MAS narrowing is borderline, these comparative experiments clearly show that its lower intrinsic sensitivity makes the production of good quality spectra significantly more difficult than from 35C1 (Skibsted and Jakobsen 1999). Other perchlorates investigated by 35C1 NMR include a single-crystal roomtemperature study of NH4C104, from which the quadrupolar and chemical shielding tensors were deduced, the extreme principal components of the CSA tensor being found to differ by 18.1 ppm (Bastow and Stuart 1989). 35C1 NMR studies have also been reported of a range of n-methylammonium perchlorates (Jurga et al. 1986), piperdinium perchlorate (Ono et al. 1997) and pyrroldinium perchlorate (Ono et al. 1999). The 35C1 NMR parameters of chlorine compounds are collected in Table 8.7.
Table 8.7. 35C1NMR interaction parameters in chlorine compounds. Compound
giso*(ppm)
• (MHz)
~1
Reference Weeding & Veeman (1989) Hayashi & Hayamizu (1990) Weeding & Veeman (1989), Hayashi & Hayamizu (1990) Weeding & Veeman (1989), Hayashi & Hayamizu (1990) Weeding & Veeman (1989), Hayashi & Hayamizu (1990) Weeding & Veeman (1989), Hayashi & Hayamizu (1990) Jelinek et al. (1993) Bastow et al. (1994) Kirkpatrick et al. (1999) Skibsted & Jakobsen (1999) Skibsted & Jakobsen (1999) Skibsted & Jakobsen (1999) Skibsted & Jakobsen (1999) Skinsted & Jakobsen (1999) Skinsted & Jakobsen (1999) Skinsted & Jakobsen (1999) Skibsted & Jakobsen (1999) Skibsted & Jakobsen (1999)
NH4C1 LiC1 NaC1
74.0 5.2 - 46.1, - 45.8,
0 0 0
0 0 0
KC1
3.07, 3.9
0
0
RbC1
44.7, 45.0
0
0
CsC1
109.4, 109.9
0
0
AgC1 BaFC1 Ca2AI(OH)6C1.2H20 LiC104 LiC1Oa.3H20 NaC104 NaC1Oa.H20 KC104 RbC104 Mg(C104)2 Mg(C104)z.6H20 site 1 site 2
- 7 N.D. 30 988.2 999.9 998.3 993.9 1003.2 1003.4 990.2 1000.6 999.5
0 2.38 2.87 1.282 0.695 0.887 0.566 0.440 0.537 2.981 0.309 0.475
0 0 --~0 0.34 0.00 0.92 0.90 0.88 0.87 0.57 0.00 0.00
495
NMR of Low- 7 Nuclides
Table 8.7. (Continued). Compound
8iso>~(ppm)
XQ (MHz)
~q
Reference
Ba(C104)2 Ba(C104)2.3H20 Ba(C|O4)z.6H20 NH4C104 (CH3)4NC104
983.6 994.6 998.4 991.5 1003.3
2.256 0.383 0.328 0.6949 0.307
0.58 0.00 0.00 0.755 0.00
Skibsted & Jakobsen (1999) Skibsted & Jakobsen (1999) Skinsted & Jakobsen (1999) Bastow & Stuart (1989) Skibsted & Jakobsen (1999)
* - chemical shift with respect to aqueous NaC1.
A 35C1 NMR study of the X-ray storage phosphor material BaFC1 has revealed a well defined second-order quadrupolar lineshape from which the quadrupolar parameters were deduced (Bastow et al. 1994). 35C1 NMR has also been used to study the anion dynamics, interlayer structure and phase transitions in the mixed-metal layered hydroxide compounds hydrotalcite [Mgo.764Alo.z36(OH)z](CO3)o.ooyClo.zzl.rlH20 and hydrocalumite CazAI(OH)6C1.2H20 (Kirkpatrick et al. 1999). Hydrocalumite shows a well-defined 35C1 lineshape over a range of temperatures and relative humidities, whereas hydrotalcite spectra are more poorly defined. Differences in the atomic ordering of the interlayers of these two materials are reflected in the 3SC1NMR evidence for an order-disorder structural phase transition, which occurs over a much wider temperature range in hydrotalcite (Kirkpatrick et al. ! 999). Materials with useful semiconducting properties can be produced by encapsulating sodium silver halides in the pore structure of zeolites such as sodalite (Si6A16012). Typically these compounds are prepared by progressively exchanging silver for sodium in the sodalite cavities, leading to the eventual formation of an expanded Ag4C1 supralattice. 35C1 NMR has been used to study this process, initially showing a sharp resonance at - 22 ppm from Na4C1 clusters (Figure 8.21A) (Jelinek et al. 1993). On the addition of silver, a weak line at - 6 ppm corresponding to surface AgC1 appears (Figure 8.21B), but at very high silver exchange levels a new resonance appears at - 3 1 0 ppm (Figure 8.21D), attributed to Ag4C1 clusters. Changes in the position of the 35C1NMR resonance observed when Iis exchanged into the system provide evidence for interactions between the clusters (Jelinek et al. 1993). 8.3.5 39K N M R
With its natural abundance of 93.1% and a relative receptivity of its central transition comparable to that of 13C, 39K appears to be a suitable nucleus for solid state NMR. These factors are to some extent offset by its small magnetic moment and quadrupolar characteristics which have limited the number of solid-state NMR studies to date. An early study demonstrated the success of the nuclear quadrupole double resonance
496
Multinuclear Solid-State NMR of lnorganic Materials
A
[
NaCI
-44 -122
B
-6
C D
-310
-100 -2 0 -300 35C1 shift (ppm) w.r.t. NaCI soln.
Figure 8.21. 35C1MAS NMR spectra monitoring the progressive replacement of Na + by Ag + in the cavities of sodalite. A. Sodalite containing NasC12 groups. Inset-resonances of bulk NaC1 and AgC1. B. After treatment with Ag to form Na7AgCI2 groups. The small resonance at - 6 ppm is from surface AgC1. C. After formation of Na4Ag4C12groups. D. Fully-exchanged Ag8C12sodalite. Note the characteristic tails to the negative sides of the resonances arising from a distribution of XQ values. From Jelinek et al. (1993), by permission of the American Chemical Society.
technique in determining the 39K resonance from 11 potassium salts at room temperature (Poplett and Smith 1981). More recently, the static 39K NMR spectra of 16 potassium compounds have been obtained by Bastow (1991) using a solid pulse echo sequence. Most of these compounds have relatively small dipolar coupling, resulting in sharp powder pattern features of the observed central transition. Accurate simulations allowed the values of XQ and TI to be determined and also indicated very small CSA contributions. Most of these model compounds contain only 1 potassium site, but even those compounds with 2 inequivalent K sites could be sufficiently resolved to allow accurate and unambiguous simulation (Bastow 1991). The 39K NMR interaction parameters for potassium compounds are collected in Table 8.8. Potassium superoxide KO2 exists in 2 polymorphic forms (cubic and tetragonal) which co-exist over the temperature range 353-423 K. The 39K MAS NMR spectra of these polymorphs (Figure 8.22A) show shifts which vary with temperature (Figure 8.22B), due to simple paramagnetism which is reflected in a temperaturedependent Curie-like susceptibility (Krawietz et al. 1998). The 39K NMR spectrum of KOH shows a typical quadrupolar powder pattern which changes with temperature as the compound is cooled through the temperature at which an antiferroelectric structural transition occurs (Figure 8.23A). Simulation of these
497
NMR of Low- y Nuclides
Table 8.8. 39K NMR interaction parameters of potassium compounds. Compound
giso* (ppm)
XQ(MHz)
~
Reference
KF KC1 KBr KI KOH
22.6 47.8 55.1 59.3 N.D.
N.D. N.D. N.D. N.D. 1.680, 1.682
N.D. N.D. N.D. N.D. 0.104
KNO3
N.D.
1.322, 1.326
0.173
N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. N.D. - 100 - 17 N.D. N.D. 90, - 30
0.056 0.958, 0.864 0.614,1.220 1.06 1.45 1.536, 1.546 1.490 0.978, 0.995 0.952, 0.968 0.738 0.107, 0.300 1.956 0.904 1.694, 1.680 1.148 N.D. N.D. 0.780 0.518 N.D.
--~0 0.043, 0.90 0.356, - 1 0.58 0.85 0.860, 0.63 0.239 0.689 0 0.274 0 0 0 0 0 N.D. N.D. 0.69 0 N.D.
Hayashi & Hayamizu (1990) Hayashi & Hayamizu (1990) Hayashi & Hayamizu (1990) Hayashi & Hayamizu (1990) Bastow (199 lb), Bastow et al. ( 1991) Bastow & Stuart (1989), Bastow (1991b) Bastow (199lb) Bastow (1991b) Bastow (1991b) Lim et al. (2001) Lira et al. (2001) Bastow (1991b) Bastow (1991b) Bastow (1991b) Bastow (1991b) Bastow (1991b) Segel (1981), Bastow (1991b) Bastow (1991b) Bastow (1991b) Bastow (199 lb) Segel (1978) Markgrabe & Engelhardt (1999) Markgrabe & Engelhardt (1999) Bastow (1991b) Bastow (199lb) Alloul et al. (1994)
KNO2 K2804 KHSO4 KHSO4 (site 1) (site 2) KCO3 KHCO3 KC103 KBrO3 KC104 KIO4 KAI(SO4)2.12H20 KH2AsO4 KH2PO4 KReO4 KNiF3 K2SiF6 KBF4 KN3 K3C6o -
* shiftwithrespectto diluteaqueousKC1.
powder patterns indicates that the values of XQ changes only slowly as the structure passes through the transition temperature, but ~q changes very rapidly (Figure 8.23B), consistent with the first-order nature of the transition (Bastow et al. 1991). The potassium halides show static 39K N M R spectra with linewidths of 8.4-14.7 ppm and isotropic shifts which vary by about 37 ppm throughout the series (Hayashi and Hayamizu 1990). A single crystal 39K N M R study of KNO3 has confirmed that the CSA in this compound is very small and that the Xe value decreases by ~--10% over the temperature range 2 9 5 - 3 7 5 K (Bastow and Stuart 1990, 1991). Other potassium compounds investigated by 39K N M R include KNO2, in which relaxation in the low-temperature plastic crystal was found to be due to the motion of the NO2(Kenmotsu et al. 1994), and KSCN which undergoes an order-disorder antiferromagnetic phase transition at 415K, the temperature of a change from orthorhombic to tetragonal
498
Multinuclear Solid-State N M R o f Inorganic Materials
A
o 450 ' -,,~Tet
ragonal (13)
..........
~d 250
~
~Cubic
s0 800 400
0 -400-800
,,,l:l
% qB. 2.0
39K shift (ppm) w.r.t. KCI soln.
(~)
~u,t
.....
I''~"
I ....
3.0
I .... " " l "
4.0
'~''
5.0
Reciprocal temperature (1000/K)
Figure 8.22. A. 39K MAS NMR spectra of KO2 at temperatures in the vicinity of the phase transition between the c~ (cubic) and 13(tetragonal) forms. The a-form is solely present below 388K, but between 353 and 423K the two polymorphs coexist. Above 423 K only the [3-form is present. B. Temperature dependence of the 39K chemical shifts of the e~ and 13-forms of KO2. From Krawietz et al. (1998), by permission of the American Chemical Society. A
B
1050
A
~
~A
1.0
A A
N
o
=-,=~
950 9 9
24~__J ~ ~~_ ..........
40
, ........
0
-40
Frequency (ldtz)
0.5 1"1
9
85o
A A
750 150
tinp OooO
200
9
250
o
o 300
Temperature (K)
Figure 8.23. A. Typical 39K NMR powder patterns of KOH at temperatures near the antiferroelectric transition. B. Variation with temperature of the 39K nuclear quadrupole coupling constant XQ and the asymmetry parameter TI for KOH. Note the sharp change in ~1at the transition temperature by contrast with the sluggish change in XQ. From Bastow et al. (1991) by permission of Elsevier Science.
symmetry. The homogeneous and inhomogeneous contributions to the 39K N M R spectrum were separated by a two-dimensional N M R experiment (Figure 8.24), showing the line broadening at about the temperature of the transition is purely homogeneous, and that the disordering process is thus a dynamic one (Blinc et al. 1995).
499
N M R of Low- 7 Nuclides A
T(K)
4o6
-4000
L__
0
~ ~ ~ . , ~ . 419 - 4000
~ ~Ll
i
2000
,
!
'-2000 ~..... O)fl2~: ( H z )
i
:6000
4 i
_l
2 i
1 L
_~A. ~
=.
2 -2 -6 4 f02/2~ (kHz) r inhomogeneous
~ i
=:l
~
L
0 -4 (kHz)
homogeneous
Figure 8.24. A. Two dimensional 39K spectrum of KSCN. The to1 and o~2dimensions represent the homogeneous and inhomogeneous interactions respectively. The three peaks in the 002 dimension are due to the partially overlapping site and domain splittings. B. The separate inhomogeneous and homogeneous 39K spectra at various temperatures about the temperature of the order-disorder transition of KSCN showing the gradual merging of the three-line homogeneous spectra as the temperature is lowered to Tc and the broadening of the homogeneous spectra below To. From Blinc et al. (1995), by permission of Elsevier Science.
Solid solutions of potassium and rubidium bromides or iodides have been studied by 39K NMR. In the course of this work, the chemical shifts of all the well-defined compounds in the system (e.g. K3RbI4, K2Rb2Br4) were documented, allowing the atomic distribution in mixed crystals of other compositions to be deduced. Values of XQ estimated from the residual MAS linewidths of these compounds fall in the range 0.35-0.53 MHz (Endo et al. 1996). The 39K MAS NMR spectrum of KNiF3 has been measured over the temperature range 180-450 K (Markgraber and Engelhardt 1999). The changes in the shift of this compound with temperature show a correlation with the magnetic susceptibility, indicating that the isotropic hyperfine interaction is non-zero, contrary to previous predictions from molecular orbital (MO) theory. This compound undergoes a transition from antiferromagnetic to paramagnetic at 180 K. The 39K NMR signal which can be observed in the antiferromagnetic state rapidly broadens beyond detection below the transition temperature (Markgraber and Engelhardt 1999). A 39K NMR study of single-crystal KHSO4 has resolved two sets of crystallographically inequivalent K § ions and allowed their quadrupole interaction parameters to be determined (Lim et al. 2001). In both types of site the K is surrounded by nine
500
Multinuclear Solid-State NMR of Inorganic Materials
oxygen atoms, but the symmetry of site 2 is lower than that of site 1, as evidenced by the larger XQ value of the former (Table 8.8). The geological and agronomical significance of potassium suggests that greater future use could be made of 39K NMR by mineralogists, even though the spectra of minerals tend to be broad and rather featureless. By measuring the position of the centre-of-gravity (cog) of the 39K resonances, Lambert et al. (1992) have shown that the tectosilicate orthoclase (--~205 ppm) can be distinguished from the phyllosilicates in which the cog position is below 40 ppm (Figure 8.25A). 39K has also been used to distinguish between structural and exchangeable potassium in the clay mineral montmorillonite as it undergoes wetting and drying. Dry montmorillonite shows only a broad resonance, but when the material is wetted, a superimposed narrow resonance appears, attributed to the exchangeable potassium that becomes mobile on hydration (Figure 8.25B). The integrated area of the narrow residual 39K signal remaining after subtraction of the spectra of a wetted and dry sample correlates with the exchangeable K determined by chemical analysis (Lambert et al. 1992). Geopolymers are useful inorganic aluminosilicate framework compounds which form and harden at room temperature. Both the aluminium and silicon atoms in their structure occupy tetrahedral sites, with charge balance achieved by the presence of hydrated monovalent ions. 39K NMR has been used to study the changes occurring when a potassium sialate geopolymer is heated to high temperatures. The cog positions
A
B montmor~
?~t,,',-~'v-v.,m~
orthoc
montmorillonitewet
vermiculite ~
100%
~ : / # . , a ~ 1000
-1000
2000
-2000
39K shift (ppm) w.r.t. KCI soln. Figure 8.25. A. 39KNMR spectra of nominally single crystal orthoclase (upper), muscovite (middle) and vermiculite (lower). B. Use of 39KNMR to estimate the amount of exchangeable potassium in montmorillonite by subtracting the spectra of the dry and wetted material to give the narrow residual spectrum corresponding to the exchangeable K. From Lambert et al. (1992), by permission of the copyright owner.
N M R o f Low- 7 Nuclides
501
A Unheated
.
.
-4o
& ~
t,.
i
I
1000
i
.
i
0
I,
i
i
......
i
-1000
39K shift (ppm) w.r.t. KCI soln.
,~ @ ~,4
-80
~.,
-120
~
r~
,
0
!
500
.
_
_
|
1000
. . . . . .
i
1500
Temperature (~
Figure 8.26. A. 39K NMR spectra of a potassium sialate geopolymer after curing at room temperature (upper) and after heating at 1300~ (lower). B. Change of the 39K peak position of potassium polysialate geopolymer as a function of heating temperature. Note the progressive shift in the peak position towards that of the anhydrous potassium aluminosilicates. From Barbosa and MacKenzie (unpublished). of the spectra shift progressively from - 47 ppm (more typical of a hydrated potassium phase) in the unheated geopolymer, to - 120 ppm in the material heated at 1300~ (Figure 8.26). The latter cog position is approaching that of an anhydrous feldspar, consistent with the known formation of leucite (KA1Si206) and kalsilite (KA1SiO4) in these heated geopolymers (Barbosa and MacKenzie, unpublished). The discovery that buckminsterfullerine (C6o) containing K or Rb in the composition K3C6o shows several structural phase changes and becomes superconducting at lower temperatures has prompted several 39K NMR studies of this phenomenon (Alloul et al. 1994, Yoshinari et al. 1996, Apostol 1996, Apostol et al. 1996, Sasaki et al. 1998). The potassium ions which occupy the two tetrahedral and one octahedral void per C6o molecule can be distinguished by 39K NMR, and occur in the expected intensity ratio of 2"1 (Figure 8.27A). These peaks are symmetrical and show no evidence of quadrupolar structure. At 210 K, K3C6o undergoes a phase transition, accompanied by the appearance of a second tetrahedral 39K NMR peak. This splitting of the tetrahedral resonance has been analysed in terms of alkali cation-vacancy interactions, providing a satisfactory explanation of the temperature dependence of the tetrahedral splitting (Apostol 1996). The temperature dependencies of the tetrahedral and octahedral shifts (Figure 8.27B) have been shown to be similar to that of the 13C shift in this compound, and estimates of the spin relaxation time suggest that the conduction electron density at the potassium site is very similar to that of the carbon site indicating a uniform Pauli susceptibility (Sasaki et al. 1998).
502
Multinuclear Solid-State NMR of Inorganic Materials
A 100 oetahedral (0)
tetrahedral (T)
~'
300K
,~
te
T' site
0
o~
230K / ' ~ tetrahedral (T')
..... -200
J ..... -100
t . 0
.
.
39K shift ~ p m ) w.r.t. KCI soln.
~ -100
. . 100
.
200 0
100
200
300
Temperature (K)
Figure 8.27. A. 39K spectra of K3C6oshowing the splitting of the tetrahedral peak as the temperature is lowered. From Yoshinari et al. (1996), by permission of the copyright owner. B. Temperature dependence of the octahedral and tetrahedral 39K shifts in K3C6o. From Sasaki et al. (1998), by permission of Elsevier Science. 8.3.6
43CaN M R
Calcium is an important element in the chemistry of many compounds (minerals, cements, bone, etc.) but NMR studies of calcium compounds have been severely hampered by the low natural abundance of this nucleus (0.14%) and the high cost of isotopically-enriched 43Ca compounds. In spite of these drawbacks, most of the more recent 43Ca NMR work has been on isotopically-enriched samples. IH ~-> 43Ca cross-polarisation (CP) has also been investigated as a means of increasing the sensitivity, using a 46 kHz spin-locking field on the protons, matched to the central transition of the Ca nucleus. Measurements made on 50 %-enriched 43Ca acetate indicate an optimum contact time of 20-30 ms, placing severe strain on the rf circuits of the spectrometer. Two signals were detected, one at about 60 ppm with a narrow linewidth (24 Hz), and a broader line covering the range - 50 to l0 ppm. It is unclear whether the elements of structure which may be present in the broad line are from a second-order quadrupolar lineshape or from overlapping lines (Bryant et al. 1987). The relatively ionic nature of the Ca-O bond results in generally small values of the nuclear quadrupolar coupling constant XQ, making it possible to narrow the NMR signal by the application of even relatively modest magic angle spinning rates. The problem of low sensitivity may then be addressed by using large diameter sample rotors (9.514 mm) spun at 1-2 kHz. Under these conditions it has proved possible to acquire natural abundance 43Ca MAS NMR spectra of 12 inorganic compounds including silicates, carbonates and sulphates, with reasonable signal/noise characteristics in about
N M R o f Low- y Nuclides
A
B
503
t
calcite
g
dd~,,,
JL,.J,,k.,~
,
.
,
.
J
.
,
.
i
.
40 0 -40 shift (ppm) w.r.t. CaCI2
43Ca la
C
C ~ j ~-simulated . . . , ,
200
1O0
0
-100
. . . . . . . . . .
100
,
. . . .
0
,
. . . .
, , ~ - . ~
-100
43Ca shift (ppm) w.r.t. CaCl 2 soln.
43Ca (CaAlz(OH)z[SizO7]H20,
Figure 8.28. A selection of natural abundance MAS NMR spectra obtained using a 14 mm rotor with a sample volume of 2.1 ml. and MAS speeds of about 2 kHz. A. Spectra from bottom: CaO, pectolite (CazNaH[SiO3]3), lawsonite apatite (CasPO4)3(OH,F)) and gypsum B. MAS spectra of the two forms of CaCO3, Upper spectrum calcite, observed and simulated, lower, aragonite. From Dupree et al. (1997), by permission of Elsevier Science. C. MAS spectra of calcium hexaluminate (upper) and calcium dialuminate, observed and simulated (lower). From MacKenzie et al. (2000a), by permission of the copyright owner.
(CaSO4.2H20).
24 hours (Figure 8.28A) (Dupree et al. 1997). These compounds show a range of shifts of about 160 ppm, but at a field strength of 8.45 T the resolution was not sufficiently good to distinguish between sites in samples containing more than 1 Ca environment. The 43Ca NMR interaction parameters of the various Ca compounds are collected in Table 8.9. The 2 polymorphs of CaCO3, aragonite and calcite, can readily be distinguished on the basis of their 43Ca MAS spectra (Figure 8.28B). Calcite shows a characteristic second-order quadrupolar lineshape from which the NMR parameters can be extracted by spectral simulation. The narrower 43Ca MAS resonance from aragonite shows no discernible structure, but the corresponding static aragonite spectrum is about 20 times broader than under MAS conditions. This suggests that CSA is a major contributor to the static linewidth, which is confirmed by satisfactory simulation of the spectrum
504
Multinuclear Solid-State NMR of Inorganic Materials
Table 8.9.
43Ca
NMR interaction parameters in calcium compounds.
Compound
~p*(ppm)
Av (Hz)**
Reference
CaO CaCO3 (calcite) CaCO3 (aragonite) CaSiO3 Ca silicate gel CaSOa.2H20 CaTiO3
128 14i - 34 -9 50 - 28 13 45.7 i - 52.6 - 21 45 7 - 6 - 22 - 2 60, - 20
20 250 80 1400 2-3* kHz 500 1000 1300 80 700 500 200 470 900 360 24,600
Dupree et al. (1997) Dupree et al. (1997) Dupree et al. (1997) Dupree et al. (1997) Nieto et al. (1995) Dupree et al. (1997) Padro et al. (2000) MacKenzieet al. (2000a) MacKenzieet al. (2000a) Dupree et al. (1997) Dupree et al. (1997) Dupree et al. (1997) Dupree et al. (1997) Dupree et al. (1997) Dupree et al. (1997) Bryant et al. (1987)
CaA1407 CaAll2019
CazMgSizO7 CazNaH(SiO3)3 KFCaa(SigOz0).8HzO CaAIz(OH)z(SizOv).H20 Ca5(PO4)3.(OH,F) Ca6[Al(OH)612(SO4)3.26H20
Ca(CH3COO)2
* MAS peak position with respect to saturated aqueous CaC12 solution, except i which indicates an isotropic chemical shift. ** MAS linewidth, except $ - which represents the static linewidth.
using CSA parameters alone (Dupree et al. 1997). The chemical shifts of the various types of calcium compounds in this study show linear relationships with the Ca-O distance in the first coordination sphere (Figure 8.29). The lines for the groups of oxides, silicates and carbonates all show a similar slope of 280 ppm/A (Dupree et al. 1997). The natural abundance 43Ca NMR spectra of calcium hexaluminate, CaAll2019, and calcium dialuminate, CaA1407, have also been reported (MacKenzie et al. 2000a). The resonance line of the hexaluminate (Figure 8.28C) is much narrower than that of the dialuminate, which shows quadrupolar features allowing it to be simulated. The 43Ca shifts for both these aluminates lie closer to the silicate line in Figure 8.29 than, as would be expected, to the oxide line. This suggests either that aluminium exerts a similar effect to silicon on the 43Ca isotropic chemical shift, or that the influence of the next-nearest neighbour in these compounds is less than structural or geometrical factors of the Ca coordination polyhedron (MacKenzie et al. 2000a). The 43Ca MAS NMR spectra of model calcium compounds related to cement and concrete formation (CaO, CaCO3, Ca(OH)z) have also been reported by Zanni et al. (1996). 43Ca NMR has been used in a study of calcium silicate formation by sol-gel synthesis (Nieto et al. 1995). The initial sol shows a broad symmetric peak at about 50 ppm which, immediately after gelling, becomes too broad to observe. Aging of the gel causes the reappearance of a broad 43Ca peak near the initial position. The disappearance of the 43Ca peak on gelling and its reappearance in the same position on aging suggest that the Ca environment remains the same throughout the process, but the
N M R o f Low- y Nuclides 150 I I ~ I
505
+' I I I I I+"'"I I I I I I I I
100 r~
r~
5t1
_
_
0
-511 2.3
2.5
2.7
Mean Ca-O distance (,~)
Figure 8.29. Relationship between the 43Cashifts of some inorganic calcium compounds and the mean Ca-O distance of the coordination polyhedron. Full circles denote oxides, full squares denote silicates and full diamonds denote carbonates. The open symbols denote sulphates and phosphate. From Dupree et al. (1997), by permission of Elsevier Science. NMR parameters suffer a large distribution on gelling which is slowly reduced on aging (Nieto et al. 1995). Calcium compounds such as (Bi,Pb)2Sr2CaCu2Os+• (Bi,Pb)zSr2CazCu3Olo+x and (Cao.sLao.5)(Bal.zsLao.75)Cu3Oy have attracted considerable interest as hightemperature superconductors. 43Ca NMR of isotopically-enriched samples has been used to monitor the effect of temperature and doping levels on the spin susceptibility at the Ca site, which is usually located between the CuO2 planes (Trokiner et al. 1994, 1994a, 1994b, Bellot et al. 1998). The 43Ca NMR linewidth also provides information about the magnetic field distribution below the transition temperature caused by the vortex lattice (Bellot et al. 1998). The 43Ca quadrupolar and shift parameters have been deduced for the main Ca site in the (Bi,Pb) compounds (Trokiner et al. 1994b), while the relatively high symmetry of the 43Ca resonance in the Ca-La-Ba cuprates has been taken as evidence of a relatively symmetrical cation environment in the central plane (Trokiner et al. 1994a).
8.3.7
47Ti a n d
49Ti N M R
Titanium is an element of considerable interest in materials science because of its role in technical electroceramics such as barium titanate and lead zirconium titanate, and in engineering ceramics (TIN, TiC) and glasses. Titanium compounds also play a role in establishing catalytic activity in microporous materials. Despite the practical interest in Ti compounds, there have been relatively few NMR studies of this nucleus because of experimental difficulties, some of which are associated with the properties of its two NMR-active nuclei, which both have moderately large quadrupole moments Q in the
506
Multinuclear Solid-State NMR of lnorganic Materials
ratio 49Q/47Q = 0.8179. B e c a u s e the 2 nuclei h a v e very similar values of ~/, their resonance frequencies are similar, differing by only about 9 k H z at the high magnetic field of 14.1 T. Normally, the s e c o n d - o r d e r - b r o a d e n e d central ( 1 / 2 , - 1/2) transition is observed, for which the relative broadening Al147/Al) 49 is 3.522. The result of this combination of circumstances is that most Ti spectra will consist of the completely overlapping b r o a d e n e d resonances from the two isotopes, except in the case of Ti in very symmetrical sites. The conventional standard for Ti N M R is liquid TIC14, but a convenient solid secondary standard is cubic SrTiO3 which produces a sharp M A S N M R line shifted by - 843 p p m against 49Ti in the liquid primary standard (Dec e t al. 1993). Hence, although the 49Ti N M R data for titanium c o m p o u n d s collected in Table 8.10 are referenced to SrTiO3 they can readily be related to the primary shift reference.
Table 8.10. 49Ti NMR interaction parameters for titanium compounds. Compound
~i~o* (ppm)
TiO2 (anatase)
- 195
TiO2 (rutile) TiO2 (brookite) Ti203 SrTiO3 FeTiO3 CaTiO3 CdTiO3 (ilmenite) CdTiO3 (perovskite) MgTiO3 PbTiO3 BaTiO3 YzTi207 LiTi204 Ti metal TiAg Ti3A1 TiAI TiA12 TiA13 TiB2 TiN TiC Till2 (cubic)
•
(MHz)
xI
Reference
4.6, 4.7, 4.79
0
--~0 - 100 1100 0 4500 - 10.5 + 1
13.9 6.04 N.D. 0 N.D. 2.75, < 3.7
0.19 0.55 0 0 > 0 0.70
30 40 - 150 153 112 - 30 - 300 2750 3155 3600 5000 3300 27500 2750 - 748** - 317** 3293, 2600 +
11.1 4.07 15.52 9.98 3.7 24.0 17.6 9.25 --~0 13.9 14.0 8.5 14.39 12.35 0 0 N.D.
0.10 0.40 0.0 0.0 0.0 0.0 --~0 0 --- 0 0.1 0 0.70 0 0 0 0 N.D.
Kanert & Kolem (1998), Labouriau & Earl (1997), Bastow et al. (1998a) Bastow et al. (1998a) Bastow et al. (2000) Bastow et al. (1998a) Dec et al. (1993) Bastow et al. (1998a) Padro et al. (2000), Bastow et al. (1998a) Padro et al. (2000) Padro et al. (2000) Padro et al. (2000) Padro et al. (2000) Padro et al. (2000) Padro et al. (unpublished) Tunstall et al. (1994) Bastow et al. (1998a) Bastow et al. (1998a) Bastow et al. (1998a) Bastow et al. (1998a) Bastow et al. (1998a) Bastow et al. (1998a) Bastow et al. (1998a) MacKenzie et al. (1995) MacKenzie et al. (1995) Nowak et al. (1992), Frisch & Forman (1968)
* chemicalshifts quotedrelativeto solidSrTiO3, except+ whichwas measureddirectlyas a magneticfield. ** with respectto TiCla
507
N M R o f L o w - 7 Nuclides
TiO2 is a commercially important material, with uses as a white pigment, an opacifier in ceramic glazes and as paper filler. The 3 TiO2 polymorphs anatase, rutile and brookite all contain TiO6 structural units but are distinguished by different connectivities and orientations of these octahedral units. The 49Ti NMR spectrum of anatase obtained from a static sample shows that the Ti site in this polymorph is the most symmetric, with the smallest XQ valHe. These anatase spectra contain sufficient detail for the lineshapes from both isotopes to be distinguished and accurately recorded (Figure 8.30A) (Bastow et al. 1998a, Labouriau and Earl 1997). MAS has allowed the 49Ti lineshape of anatase to be clearly seen, but the spinning speeds used (4-6 kHz) were not sufficient to disentangle the 47Ti centreband from the 49Ti sidebands (Dec et al. 1993). The static 49Ti resonance of rutile has been recorded and simulated (Figure 8.30B) (Bastow et al. 1998a), giving parameters which agree with a single crystal rutile study (Kanert and Kolem 1998). The 49Ti shift and quadrupolar parameters of brookite (Figure 8.30C) have values intermediate to those of the other 2 polymorphs (Bastow et al. 1998a, Labouriau and Earl 1997, Bastow et al. 2000, Bastow 2000). The resonance widths of natural and synthetic brookite differ considerably, the broader linewidth of the natural sample being due to the
A
B
Anatase
C
Rutile
Brookite
1
simulated
/
synthetic
simulated
lOO -lOO -3oo Frequency (kHz) observed
. . 40. . .
0
Frequency (kHz)
80
D
200 0 -200 Frequency (ldtz)
'' '200 ' {) '-20O'Frequency (kHz)
Figure 8.30. Static 47'49TiNMR spectra of titanium oxides. A. TiO2 (anatase), showing the resolved quadrupolar lineshape of 49Ti (inner) enclosed by the 47Ti lineshape for which a simulation is also shown. B. TiO2 (rutile), observed and simulated. Note the significantly broader lineshape corresponding to the 49Ti spectrum only. C. TiO2 (brookite) showing a superposition of the unresolved 47Ti and 49Ti lineshapes of the single Ti site in the synthetic material (lower) and additional broadening of the natural mineral (upper). D. Ti203 consisting of superposed 47Ti and 49Ti central transition lineshapes. Spectra A, B and D from Bastow et al. (1998a), by permission of the copyright owner. Spectrum C from Bastow et al. (2000), by permission of the American Chemical Society.
508
Multinuclear Solid-State N M R of Inorganic Materials
presence of paramagnetic impurities. The Ti NMR spectrum of the suboxide Ti203 (Figure 8.30D) reveals no well-defined lineshape from either isotope and its large paramagnetic shift (1100 ppm) arises from the presence of Ti 3+ (Bastow et al. 1998a). Static 47'49TiNMR has been used to study the thermal evolution of rutile from TiO2 gels formed by hydrolysis of titanium isopropoxide (Bastow and Whitfield 1999).The amorphous gel heated at 200~ shows a surprisingly narrow resonance, suggesting either that even at this early stage the TiO6 units are quite symmetric, or that partial averaging of the quadrupolar parameters is occurring. The spectra of samples heated at 500 and 600~ contain complex lineshapes, which can be shown to contain both anatase and rutile resonances. The coexistence of these 2 polymorphs over a temperature range of at least 100~ has been taken to indicate that the development of crystallinity in these gels is spatially inhomogeneous, while the presence of crystalline anatase and rutile NMR lineshapes in samples showing little X-ray crystallinity implies that small but well crystallised TiO2 particles are already present at these low temperatures (Bastow and Whitfield 1999). BaTiO3, an important electroceramic with useful piezoelectric and ferroelectric properties, has been extensively studied by Ti NMR spectroscopy. The static 47'49Ti NMR spectrum of the cubic phase above the Curie point shows narrow resonances from both isotopes which disappear when the tetragonal phase is formed on cooling below the Curie point (Forbes et al. 1987). Three static 49Ti NMR studies of singlecrystal BaTiO3 have given values for 49XQof 3.65, 3.78 and 4.03 MHz (Sommer et al. 1990, Bastow 1989, Kanert et al. 1994), in agreement with a static powder study of three different BaTiO3 samples which all showed spectra with XQ = 3.7 + 0.1 MHz (Padro et al. 2000). The rotation pattern of Bastow (1989) also indicated a CSA contribution of about 40 ppm in that BaTiO3 sample. Solid state titanium NMR could be much more usefully applied to a range of technologically significant problems if it had the ability to distinguish between different local coordination states of titanium (e.g. TiO4, TiOs, TiO6). Although this is not yet possible, it is the goal of much ongoing research effort. The effect of the second cation on the 49Ti NMR parameters has been studied by Dec et al. (1993) who showed the existence of such an influence on the peak positions in a limited range of complex oxides and related compounds. A more extensive static and MAS study at 14.1 T of a series of ATiO3 titanates with perovskite and ilmenite structure has examined this matter in greater detail (Padro et al. 2000). The NMR interaction parameters were accurately deduced from the very clean lineshapes recorded for these compounds, and show a linear relationship between • and the shear strain t~ as defined in equation (8.3). This relationship, shown in Figure 8.31A, has the equation
XQ (MHz) = 7.28qJ + 1.53
(8.5)
509
NMR of Low-~[ Nuclides
A 2O
5/
150
100 ~
~1o
~"
50
om~
~"
0 . . . . . . . . . . . . .
0
.
0
.
.
.
i
.
.
.
.
.
i
.
.
.
.
.
1.0
i
,
9
-
-
Mean shear strain
i
.......
I
I__A
-
J
2.0
1.;6
1.98
2.00
2.02
Mean Ti-O distance (/l.)
Figure 8.31. A. Relationship between the 49Ti nuclear quadrupole coupling constants of ATiO3-type titanates and their structural shear strain q~ defined in equation (8.3). B. Relationship between the 49Ti isotropic chemical shifts (ppm) and the mean Ti-O bond lengths (A) for a series of ATiO3 compounds with the perovskite structure. Note that compounds with the ilmenite structure do not fit this relationship and are not included here. From Padro et al. (2000), by permission of the copyright owner.
The perovskites also show a reasonable linear correlation between ~iso (in ppm) and the mean Ti-O bondlength (in A) (Figure 8.31B) given by the equation (3iso = 2370(Ti-O) - 4632
(8.6)
but the compounds with ilmenite structure do not fit well to this line (Padro et al. 2000). The compound FeTiO3 is paramagnetic at room temperature, with a broad featureless Ti resonance and a large positive shift due to the transferred hyperfine field from the Fe 3+ (Bastow et al. 1998a). Another titanate of interest because of its superconducting properties is the spinel Lil +xTi2-xO4 which has a critical temperature of about 12 K for the composition range 0 < x < 0.10. The static 47'49Ti NMR spectra of stoichiometric LiTi204 have been recorded as a function of temperature and as a function of composition of the non-stoichiometric compounds. A negative Knight shift indicates the probable dominance of core polarisation in these compounds, in which Xe decreases monotonically with increasing temperature (Tunstall et al. 1994). The NMR parameters for titanium metal deduced in earlier studies by field sweeping techniques (Narath 1967, Ebert et al. 1986) have been confirmed by more recent room temperature FT NMR (Bastow et al. 1998a). The value of XQ deduced from the (+_ 3/2, +_ 1/2) satellite transitions was used in an accurate simulation of the central transition, which required an axial Knight shift of 70 + 10 ppm. The Ti NMR spectra of a number of titanium aluminide alloys and TiAg have also been reported
510
Multinuclear Solid-State N M R o f lnorganic Materials
(Bastow et al. 1998a). These alloys show Knight shifts of 2750-5000 ppm and have values of XQ up to 14.4 MHz (Table 8.10). The axial component of the Knight shift in TiA13 has a value of 800 + 50 ppm. The electric field gradient at the Ti site in TiAg is unexpectedly small for this tetragonal structure, giving a Ti spectrum of 2 narrow lines separated by 6 kHz (Figure 8.32A) corresponding to the resonances of 47Ti and 49Ti as seen in perovskite and other compounds in which the Ti is located in much more symmetrical sites (Bastow et al. 1998a). Other titanium compounds which have been studied by 47'49Ti NMR include TiB2, a hexagonal metal in which the 47Ti and 49Ti lineshapes can be observed (Figure 8.32B). The reason for the very small Knight shift in this compound is probably the fortuitous cancellation of positive and negative shift contributions (Bastow et al. 1998a). The cubic and tetragonal phases of Till2, has also been studied by 47'49TiNMR and the temperature variation of the Ti Knight shift measured (Nowak et al. 1992, Frisch and Forman 1968). A series of Ti-V alloys has also been studied by 4749Ti NMR, which shows a decrease in the Knight shift with increasing vanadium content (Nowak et al. 1992). The 47'49Ti spectra of the cubic phases TiN and TiC (Figure 8.32C) show peaks from both isotopes, with very little change in the resonance positions measured at field
A
TiAg
C
Ti-C-N
TiC "~..__._._.~-860 50 2oo' '5o' Frequency (kHz)
~-~1160
/
9
'260''
0
L
i
i
,
500
-200
Frequency (kHz)
|
-11o0
Ti N ~
-500 -1500 shi~ (ppm) w.r.t. 49TIC]4
Figure 8.32. Static 47'49TiNMR spectra of A. TiAg, showing the narrow resonances of 47Tiand 49Ti separated by 6 kHz, B. TiB2, showing the lineshape of the central transition of 49Ti (lower) with the 47Ticentral transition lineshape superimposed (upper). From Bastow et al. (1998a), by permission of the copyright owner. C. TiC and TiN, showing resonances from both 47Tiand 49Tiin the single cubic sites, and a series of titanium carbonitride solid solutions formed between these two compounds. From MacKenzie et al. (1995), by permission of the copyright owner.
NMR of Low- y Nuclides
511
strengths of 11.7 T and 14.1 T reflecting the small electric field gradients in these compounds (MacKenzie et al. 1995). The formation of cubic carbonitride solid solutions between TiN and TiC results in considerable broadening and loss of detail from the 47'49Ti spectra, which, however, retain characteristics of the nitride and carbide end-members (Figure 8.32C) (MacKenzie et al. 1995).
8.3.8 67Zn NMR Zinc is of interest to a range of practical applications from electroceramics to metalloproteins, for which 67ZnNMR studies could potentially provide valuable information. However, 67ZnNMR suffers from the various problems associated with its low natural abundance and low magnetic moment, in addition to its nuclear quadropole interaction which presents linewidth problems for samples containing Zn in a non-cubic environment. Consequently, the number of reported 67Zn studies of solids is limited. The primary reference for 67Zn NMR is a dilute aqueous Zn 2+ solution, but cubic ZnSe would make a useful solid secondary reference material since its MAS linewidth is only 16 Hz (Wu 1998). ZnO occurs in a wurtzite-type crystal structure with hexagonal symmetry containing tetrahedral ZnO4 with the zinc site axially symmetric, giving a 67Zn MAS NMR spectrum with a quadrupolar lineshape and qq = 0 (Figure 8.33A) (Dec et al. 1993). The rotation pattern of single-crystal ZnO has been recorded as a function of temperature over the range 250-400 K (Bastow and Stuart 1988). The static spectrum of ZnO is determined by both the quadrupolar and CSA effects. Since MAS removes the CSA but leaves the quadrupolar lineshape, the 67ZnMAS NMR spectrum of ZnO has been used to determine the quadrupolar parameters, which were then combined with a CSA contribution to simulate the static spectrum (Wu 1998). The value of the chemical shift parameters derived in this indirect manner agree well with the results of an earlier single crystal determination. Heavily doped ZnO is of technical importance as a transparent electrical conductor with applications in advanced optical displays. The effect of adding 0.03-3 at % A1 or Ga metal to ZnO has been studied by monitoring the changes in the 67Zn NMR spectrum (Roberts et al. 1998). The addition of Ga produces a blurring of the distinct second-order quadrupolar lineshape of the ZnO spectrum (Figure 8.33B) due to the appearance of a distribution of nuclear quadrupole coupling constants (but with little change in the linewidth), and the 67Zn shift also increases with increasing dopant concentration (Roberts et al. 1998). ZnS occurs in 2 polymorphic forms, cubic wurtzite and hexagonal zincblende, which often occur together in commercial ZnS powders. The 67Zn NMR resonances from the 2 forms have different shifts (Figure 8.33C) (Bastow and Stuart 1988), both of which can be resolved in the 67Zn MAS NMR spectra of commercial powders
512
Multinuclear Solid-State N M R o f lnorganic Materials
C A
ZnO
B
ZnS
ZnO
MAS
observed
600
400
200
si
0%~Ga
1, . . . . . .
!
_
T ~ 7 - - - _ .J_
400 200 0 67Zn shift (ppm) w.r.t. ZnCI2 soln.
al,
-10
,
,
0
,I,
10
i
I
20
Frequency (kHz)
__••.•exagonal i
~
i
l
i
i __J...._....l_......~
4 0 360 3OO 67Zn shift (ppm) w.r.t.
ZnSO 4 soln.
Figure 8.33. A. Observed and simulated 67ZnMAS NMR 11.7 T spectrum of ZnO, from Dec et al. (1993), by permission of the American Chemical Society. B. Effect of additions of Ga on the 67Zn NMR spectrum of ZnO, adapted from Roberts et al. (1998). C. 67ZnMAS NMR spectrum of commercial ZnS (upper) showing the Zn resonance from the cubic and hexagonal polymorphs, from Dec et al. (1993). Below: static 67ZnNMR spectra of the two ZnS polymorphs, adapted from Bastow and Stuart (1988).
(Dec et al. 1993, Wu 1998). The 67Zn NMR spectra of all the zinc chalcogenides (ZnS, ZnSe and ZnTe) have been determined, showing that static 67Zn lineshapes of the hexagonal forms contain a CSA contribution (Bastow and Stuart 1988). All the NMR interaction parameters for zinc metal have been determined from the central and satellite transitions of the 67Zn spectrum which indicates a value of - 124 ppm for the axial component of the Knight shift. The isotropic component of the Knight shift and the nuclear quadrupole coupling constant have also been determined as a funtion of temperature over the range 149-432 K (Bastow 1996). Other zinc compounds for which the 67Zn NMR spectra have been reported include ZnSO4, in which the zinc is in octahedral coordination with oxygen. Comparison of this spectrum with that of tetrahedral Zn in ZnO indicates that the two coordination states are separated by about 200 ppm (Wu 1998). The 67Zn NMR spectra of zinc acetate and its hydrate show that on hydration the zinc coordination changes from four to six, with a large change in the isotropic chemical shift. (Figure 8.34). Distortions in the octahedral units of the hydrate are reflected by a large increase in the XQ value
513
NMR of Low- y Nuclides -'--
ZnS4 9 ZnC4, ZnN 4 -
ZnSe 4 --
ZnO 4 -
ZnTe4
ZnO 6 I
400
I
,I
I
300
I
200
67Zn i s o t r o p i c
I
I
,,,
I
100
I
_
O
shift (ppm) w.r.t. Zn(NO3)2 soln.
Figure 8.34. Range of 67Znisotropic shifts in various zinc compounds, from data of Sham and Wu (1999).
(Wu et al. 1998). A detailed single-crystal 67Zn study of zinc acetate has indicated that although the CSA effects in this compound are small, they significantly affect the value of XQ (Vosegaard et al. 1999). The 2 inequivalent octahedral Zn sites in zinc formate dihydrate have been studied by 67Zn NMR, which indicates that although their isotropic chemical shifts differ by only 10 ppm, their XQ values differ by more than 50% (Larsen et al. 1999). The 67Zn MAS NMR spectrum of Kz[Zn(CN)4] has also been recorded as part of a study of potassium tetracyanometallates in which the CN ligand was partially enriched in 13C (Wu et al. 1995). Information about the 67Zn NMR characteristics of ZnN4 and ZnS4 groups (Figure 8.34) has been provided by a study of zinc complexes with imidazole and thiourea respectively (Sham and Wu 1999). The XQ values of the compounds in which the zinc is tetrahedrally coordinated to a variety of ligands show no systematic relationship with the distortion index of the tetrahedral bond angles (the degree of deviation from the ideal tetrahedral angle of 109.5~ indicating that the nature of the coordinating ligand is exercising as powerful an effect on XQ as the structural geometrical factors, and must be taken into account. For the limited number of compounds investigated containing octahedrally coordinated Zn-O (Sham and Wu 1999) there is evidence of some degree of correlation between XQ and the octahedral distortion index DI which can be expressed by
XQ(MHz) = 0.229DI + 0.0542 The 67Zn interaction parameters for zinc compounds are collected in Table 8.11.
(8.7)
514
M u l t i n u c l e a r Solid-State N M R o f I n o r g a n i c M a t e r i a l s
Table 8.11. 67Zn interaction parameters in zinc compounds. Compound
~iso~ (ppm)
ZnO
238,240.1,240
ZnS (cubic) (hexagonal) ZnSe
Reference
XQ (MHz)
Dec et al. (1993), Bastow & Stuart (1988b), Wu (1998) -,0,0 Haller et al. (1980), Bastow & Stuart (1988b), Wu(1998) Bastow & Stuart (1988b), Wu (1998) Bastow & Stuart (1988b) 0 Bastow (1996) N.D. Bastow (1996) 0.2 Wu (1998) 0, 0.1 Wu (1998), Vosegaard et al. (1999) Wu (1998) 0.87, 0.819 Larsen et al. (1999) 0.99 0.39 Wu et al. (1995) N.D. Wu et al. (1995) 0.4 Wu et al. (1995) 1.0 Wu et al. (1995)
2.4, 2.4065, 2.40, 2.38 378, 380.5, 381.9 --~0 360, 365 --~0, < 0.5, < 0.4 276.3,276 --~0
ZnTe Zn CUl.O1Zno.99 ZnSOn.7H20 Zn(CH3COO)2
87.6 1776 1879 10 260, 245
--~0 11.983 N.D. 1.70 2.42, 2.42
Zn(CH3COO)2.2H20 Zn(OOCH)2.2H20 I II Zn(C104)2.6H20 Zn[ImH]4(C104)2 Zn[SC(NH2)2]4(NO3)2 K2[Zn(CN)4]
0, - 123+ ----10 0 - 3 291 359 291
5.3, 5.34 6.05 9.52 < 0.2 2.80 3.15 0
0, 0
* shifts relativeto diluteaqueousZn2+, exceptvaluemarkedt whichis relativeto 1MZnC12.
8 . 3 . 9 9~Z r N M R
Zirconium plays an important role in a number of materials applications including engineering ceramics, toughened ceramic materials, fuel cells, and as an additive in the synthesis and sintering of non-oxide ceramics. Although narrow 91Zr N M R resonances can be obtained from a limited number of materials such as cubic BaZrO3 in which the Zr is in an extremely symmetrical site (Dec et al. 1993, Hartman et al. 1991), the majority of Zr-containing materials of practical interest have very broad 91Zr N M R spectra, as in ZrSiO4 (zircon) in which the width of the central transition is about 350 kHz at 9.4 T (Bastow 1990). Unfortunately this width is much greater than can be narrowed by any accessible MAS speed, and is also too great to be recorded without distortion by direct pulse methods, including echo techniques. For this reason stepped frequency measurements have been most commonly used for solid materials with this nucleus, but the t i m e - c o n s u m i n g and laborious nature of these experiments has severely limited the number of reported 91Zr N M R studies of solid materials. 91Zr N M R spectra are normally referenced to a saturated solution of bis(cyclopentadienyl)zirconium dibromide in tetrahydrofuran, but the narrow line of BaZrO3 makes it an excellent solid secondary reference resonating at 208.1 ppm from the primary liquid reference (Hartman et al. 1991). The pointwise stepped frequency approach has been used to determine the undistorted 91Zr lineshapes in the various forms of ZrO2 (Figure 8.35A) (Bastow and Smith 1992).
515
N M R of Low- y Nuclides
Pure zirconia exhibits a monoclinic-to-tetragonal phase transition at 1000~ which involves a large volume change and makes it impractical as an engineering material. The addition of elements such as Ca, Mg or Y forms stable cubic solid solutions, the technically important stabilised zirconias. Analysis of the stepped-frequency 91Zr lineshapes of the tetragonal, monoclinic, orthorhombic and cubic forms of ZrO2 indicates that they cover a remarkably small is 9 shift range (--~ 10 ppm), but the quadrupolar parameters provide a means of discriminating between the phases and estimating the phase compositions in transformation-toughened zirconias (Bastow and Smith 1992). The tetragonal and cubic phases of ZrO2 are only stable at room temperature when stabilised by Ca, Mg or Y, the highest level of doping being required for cubic ZrO2. This is reflected in the featureless 91Zr lineshape of cubic ZrO2 (Figure 8.35A) in which atomic disorder produces a range of electric field gradients. A 91Zr NMR single crystal study of ZrSiO4 (zircon) showed a considerable variation of the linewidth as the crystal was rotated, possibly resulting from a range of electric field gradients associated with defects in the natural mineral sample. The fitted rotation pattern revealed a sizeable axial CSA of about 183 ppm (Bastow 1990).
D A
Zr02
Zr metal
B simulated
-. tetragonai
p
BaZrO3 [
"""'"" "
~
~~,
observed
/
J
"~-.
SrZr03
/ "~,.~
%
9
"
monoclinic " 9
9
9
9149
cubic
9
9
9 9 9
o 9149
2010 . . . . 0'. . . . -200' ~-91Zr shift (ppm) w.r.t.BaZrO3
9
9
400
i
I
_0
i
I
-4o0
E AI3Zr
..
.
t
o
Frequency (ir~z)
Na2ZrSiOs .: "" "... 9 9
"
~ _ ~ . .
460
i
l
i.
-400
Frequency (kHz)
i
,
:
:
~
,,
9 .'
. 9
200 -100 -400
Frequency (kltz)
9
L
. . I. . .
t
I
0 -50 Frequency (kHz) 50
Figure 8.35. A selection of 91Zr NMR spectra. A. Stepped-frequency spin-echo spectra of 3 phases of ZrO2, adapted from Bastow (1994). B. 14.1 T MAS NMR spectra of Ba and Sr zirconates. Asterisks denote spinning side bands. Note the broader lineshape with possible quadrupolar structure of the more distorted Zr site in SrZrO3. From Dec et al. (1993), by permission of the American Chemical Society. C. Static stepped-frequency NMR spectrum of NazZrSiOs. 1996. D. Stepped-frequency NMR spectrum of Zr metal (lower) with simulated spectrum (upper). 1992. D. Static 91Zr NMR spectrum of the central transition of A13Zr. From Bastow et al. (1992, 1996, 1998b), by permission of the copyright owners.
516
Multinuclear Solid-State N M R o f Inorganic Materials
Although the 91Zr resonance of cubic BaZrO3 (Figure 8.35B) is sufficiently narrow to be detected by MAS NMR, the zirconium site in the orthorhombic analogue SrZrO3 is not at the centre of the octahedron defined by the six nearest-neighbour oxygen atoms. This greater distortion is reflected in its broader MAS NMR resonance (Figure 8.35B) which shows signs of second-order quadrupolar structure (Dec et al. 1993). The stepped frequency method has been used to determine the broad central transitions of several more complex zirconium compounds including NaZrO3 (Bastow et al. 1994a) and Na2ZrSiOs, (Figure 8.35C) in which the two crystallographically inequivalent Zr sites could not be distinguished by 91Zr NMR (Bastow et al. 1996). A stepped echo approach has been used to determine the 91Zr NMR spectra of a range of zirconium-containing phosphate and fluoride compounds for which the values of XQ are much smaller, typically 1-2 MHz (Table 8.12) (Hartmann and Scheler 1995). Although the 91Zrcentral transition of ZrF4 extends over a range of 1.5 MHz, its static lineshape has been detemined by the stepped frequency method (Bastow 1994). Early 91Zr NMR studies of Zr metal reported its Knight shift at room temperature (Yamada and Asanuma 1965) and at 4 K (Hioki et al. 1975). More recently the values of XQ and xl for zirconium metal have been determined from simulation of the roomtemperature resonance lineshape recorded by the stepped-frequency method (Figure 8.35D) (Bastow et al. 1992). 91Zr NMR has been used to study a series of amorphous Zr-Cu alloys (Abart et al. 1982), to examine the internal field in ZrFe2 (Dumelow and Riedi 1987) and to determine the Knight shifts of the isostructural metallic compounds ZrV2, ZrC2 and ZrMo2 (Torgeson and Barnes 1967). The static 91Zr NMR spectrum of the central transition of A13Zr (Figure 8.35E) shows a Knight shift of 40 ppm, compared with the value of 3500 ppm for zirconium metal (Bastow et al. 1998b). This small Knight shift is thought to result from a fortuitous cancellation of the positive s-contact and d-orbital terms by the negative d-core polarisation term. Relatively narrow 91Zr NMR resonances (11-16 kHz) have been reported in cubic ZrCo and ZrC and tetragonal ZrH2 (Bastow et al. 1992). The spectra of the carbide and hydride show evidence of weak subsidiary structure which is probably due to structural defects. The values of the Knight shift and T1 for ZrH2 have been used in an analysis of the density of states in that material (Zogal et al. 1991). Reported values of the solid state 91ZrNMR interaction parameters for zirconium compounds are collected in Table 8.12.
8.3.10
95Moa n d 97MoNMR
The resonance frequencies of the isotopes 95Mo and 97Mo are very similar, differing by only 2%, but the sensitivity of 95M0 and its significantly smaller second-order quadrupolar broadening make it the preferred Mo NMR nucleus even though 97M0 has the advantage of relaxing much faster in cases where quadrupole relaxation is dominant.
NMR of Low- 7 Nuclides
517
Table 8.12. 91ZrNMR interaction parameters of zirconium compounds.
Compound
~iso* (ppm)
XQ (MHz)
"q
Reference
ZrO2 (monoclinic) ZrO2 (tetragonal) ZrO2 (orthorhombic) BaZrO3
7 12 N.D. 0
23.1 19.1 17 ~ 0-0.05
--~0.1 0 0.8 ~ 0
SrZrO3
- 30*, - 12 - 29 --~0 - 285 -- 260 --~0 - 374 - 341 - 375 - 359 N.D. - 399 - 454 3292 3377 40 127 2262, 2550 t
N.D., 0.678 N.D. 14.6 1.288 20.47 11.3 29.4 0.318 1.591 0.637 0.848 53.7 2.682 1.481 18.2 0 7.3 0 --~0
N.D., 0.6 N.D. 0.15 0.5 0 0 0.70 0.9 0.2 0 0 0.30 0 0 0 0 -
Bastow & Smith (1992) Bastow & Smith (1992) Bastow & Smith (1992) Dec et al. (1993), Hartman et al. ( 1991) Dec et al. (1993), Hartman et al. (1991) Hartman et al. (1991) Bastow et al. (1994a) Hartmann & Scheler (1995) Bastow (1990) Bastow (pers. comm). Bastow et al. (1996) Hartmann & Scheler (1995) Hartmann & Scheler (1995) Hartmann & Scheler (1995) Hartmann & Scheler (1995) Bastow (1994) Hartmann & Scheler (1995) Hartmann & Scheler (1995) Bastow et al. (1992) Bastow et al. (1992) Bastow et al. (1998b) Bastow et al. (1992) Bastow et al. (1992), Zogal et al. (1991)
Ba2ZrO4 NaZrO3 CaZrO3 ZrSiO4 Zr(WO4)2 Na2ZrSiO3 CuZrz(PO4)3 RbZrz(PO4)3 AgZr2(PO4)3 NaZr2(PO4)3 ZrF4 Cs2ZrF6 Li2ZrF4 Zr metal ZrCo A13Zr ZrC ZrH2
* shifts withrespectto BaZrO3whichis 208.1ppm fromCp2ZrBr2,* indicatesthe peakposition, , referencedto RbC1solutionand scaledfromthe magneticmomentof 85Rb.
This is borne out by relaxation m e a s u r e m e n t s which have been m a d e for both isotopes in solid Na2MoO4. Q u a d r u p o l e r e l a x a t i o n is d o m i n a n t in this c o m p o u n d , and the values of T1 for 95Mo and 97Mo are 132 s and 1.1 s respectively (Bastow 1998). This was also borne out in an early solid state N M R study of anhydrous cubic NazMoO4 for which the spectra of both 95Mo and 97Mo were collected. The similarity in the spectra indicated that the quadrupole effects are small, but the 97Mo data took a p p r o x i m a t e l y 6 times longer to acquire to a c o m p a r a b l e signal/noise level (Lynch and Segel 1972). The chemical shift reference for M o N M R is normally aqueous Na2MoO4 solution, but the sharp resonance from Mo(CO)6 w h i c h can be o b s e r v e d in a single scan w o u l d m a k e this a g o o d secondary reference c o m p o u n d for 95Mo. The static and M A S 95Mo N M R spectra of MoO3 have been reported by Bastow (1998) and by Edwards et al. (1990) who used an isotopically-enriched sample. The shape of the spectrum determined by Bastow indicates that C S A dominates over the q u a d r u p o l e interaction, while the M A S spectrum, f r o m w h i c h the C S A has b e e n
518
Multinuclear Solid-State NMR of lnorganic Materials
eliminated, shows a clear second-order quadrupolar lineshape capable of direct simulation (Figure 8.36A). The resulting quadrupolar parameters are different from those of Edwards et al. (1990), which were derived from a simulation of the static powder pattern, for which the principal values of the CSA tensor and the angular orientation parameters had to be supplied. This discrepancy illustrates some of the difficulties which can arise with spectral simulations. The static 95Mo NMR spectra of a number of inorganic molybdates (Mastikhin et al. 1988) show that the XQ values of the alkali molybdates are generally approximately zero, but that over the whole series of compounds, • increases with increasing structural distortion. However, it should be noted that in determining XQ from the lineshapes, Mastikhin et al. (1988) generally did not take account of the CSA which can be very large, as has subsequently been shown. In compounds with small quadrupole interactions for which ~iso could be most precisely determined, the 95Mo shifts tend to become more positive with increasing Mo-O bondlength. The three polymorphs of K2MoO4 can readily be distinguished on the basis of their • and 8iso values (Table 8.13). A 95Mo MAS NMR study of Na2MoO4.2H20 has shown that the dipolar coupling can be removed under MAS conditions, giving a well-defined second-order quadrupolar lineshape from which XQ and -q can be deduced (Eichele et al. 1997). Knowledge of these parameters then allows the span of the CSA (about 200 ppm) to be estimated from the static spectrum.
A
MoO3
B
C
M02C
MoSe2 ~
/~Jl simulated
imulated
J 5
,,
0 -5 -10 Frequency (kttz)
t
I
i
I
,
'
100 0 -100 Frequency (kttz)
f-
I
~served i
.i
,
I
,l
'
200 0 -200 Frequency (kttz)
Figure 8.36. A selection of 95MoNMR spectra of molybdenum compounds. A. Observed MAS spectrum of MoO3 (lower) with simulation (upper). B. Powder lineshapes of the central transition of MoSe2 (upper) and MoS2 (lower). C. Observed powder lineshape of the central transition of Mo2C (lower) with the simulation (upper). From Bastow (1998), by permission of Elsevier Science.
519
NMR of Low- y Nuclides
Table 8.13. 95Mo NMR interaction parameters of molybdenum compounds. Compound
~iso*(ppm)
XQ (MHz)
qq
Reference
MoO3
- 1 5 0 , - 114 *, - 30 +
LizMoO4 NazMoO4
- 72 - 35, 0 +, - 32.9
N.D., 3.49, 2.85 "~ 0 --~ 0
N.D., 0.99, 0.32 N.D. N.D.
NazMoO4.2H20
4 t, 8
ot-KzMoO4 ~-KzMoO4 ~-KzMoO4
- 24 12 2 - 25 - 122 35 - 45 108, 151 82, 8
N.D., 1.15 1.5 1.3 --~0 --~0 --~ 0 2.7 1.6 1.9, 2.05 --~0
N.D., 0.82 N.D. N.D. 0 N.D. N.D. N.D. N.D. N.D., 0.20 N.D.
Mastikhin et al. (1988), Edwards et al. (1990), Bastow (1998) Mastikhin et al. (1988) Mastikhin et al. (1988), Lynch & Segal (1972). Machida & Eckert (1998) Eichele et al. (1997), Mastikhin et al. (1988) Mastikhin et al. (1988) Mastikhin et al. (1988) Mastikhin et al. (1988) Mastikhin et al. (1988) Mastikhin et al. (1988) Mastikhin et al. (1988) Mastikhin et al. (1988) Mastikhin et al. (1988) Machida & Eckert (1998), Kautt et al. (1976) Edwards et al. (1990) Machida & Eckert (1998) Machida & Eckert (1998)
Cs2MoO4
CsLiMoO4 CaMoO4 BaMoO4 PbMoO4 AgzMoO4 A12(MoO4)3 Ag2Mo207 NazMo207 Td Oh (NH4)zMo207 Td Oh (NH4)6Mo7Oza.4H20
H3PMo1204o.xH20 Mo(CO)6
- 300 t 63 -74 - 177 N.D. N.D. 4t
N.D. N.D. N.D. N.D. 2.44 3.41 (2.98, 5.76, 3.73) 100, (468, - 29) N.D. - 1857, - 1852, N.D. 1854 0.141. 0.091, 0.0893 -
MoS2 MoSe2 Mo2Se4 MoSi MozC
- 940 + - 1000 + 5885 + - 2100 + 900 +
6.2 5.7 --~0 2.928 6.1
N.D. N.D. N.D. N.D. 0.47 0.07 (0.73, 0.42, 1.00) N.D. N.D., N.D., N.D., < 0.1, 0.142, 0.151
0 0 N.D. 0 0.98
Edwards et al. (1990) Edwards et al. (1990) Mastikhin et al. (1988) Eichele et al. (1997), Mastikhin et al. (1988), Edwards et al. (1990), Nolle (1977), Shirley (1987), Vosegaard et al. (1999a) Bastow (1998) Bastow (1998) Bastow (1998) Bastow (1998) Bastow (1998)
* shift with respect to aqueous NazMoO4 solution, except values marked + for which cubic anhydrous NazMoO4 was taken as zero. t denotes the peak position, $ denotes the position of the most intense singularity. Parenthesis indicate multiple sites.
A n important consideration in applying 95M0 N M R to practical p r o b l e m s is the ability o f the t e c h n i q u e to d i s t i n g u i s h b e t w e e n M o O 4 and M o O 6 units. 95M0 N M R studies of a series o f i s o t o p i c a l l y e n r i c h e d p o l y o x o m o l y b d a t e c o m p o u n d s similar to t h o s e u s e d as h y d r o d e s u l p h u r i s a t i o n catalysts s h o w e d that at a r e l a t i v e l y l o w field (9.4 T) and l o w
520
Multinuclear Solid-State N M R of Inorganic Materials
MAS speeds (3-4 kHz) unambiguous deconvolution is extremely difficult where the spectra consist of strongly overlapping resonances from different sites (Edwards et al. 1990). The corresponding static spectra are hampered by the comparable magnitude of the second-order quadrupole and CSA effects. Extension of this work to other polyoxomolybdate compounds suggests that where both octahedral and tetrahedral Mo-O species are present they can be distinguished by 95Mo NMR, since the tetrahedral sites give much narrower resonances than the octahedral (Edwards et al. 1990a). The complex lineshapes arising from the presence of inequivalent octahedral sites can be fitted by multiple components which often reveal differences in the quadrupole and asymmetry parameters for the different sites related to the structural distortion of those sites (Edwards et al. 1990a). Cross polarisation from ~H to 95Mo has been shown to produce enhancements of 66-86% of the theoretical maximum and reliable second-order powder lineshapes in isotopically-enriched compounds such as (NH4)6MovOz4.4H20 and (Bu4N)zMo207 which contain suitable proton sources (Edwards and Ellis 1990). The optimum contact times were found to be 20-30 ms and the value ofT~ 0 for molybdenum is long by comparison with the other relaxation processes, making this a suitable candidate for CP. An important application of molybdenum as a surface species in heterogeneous catalysis has led to a number of 95Mo NMR studies of model molybdenum catalyst compounds (Edwards et al. 1990, 1990a) and compounds in the series MoOx-Al203 (Edwards et al. 1990). 95Mo NMR suggests that freshly prepared uncalcined Mo-A1 catalysts contain both tetrahedral and octahedral Mo species adsorbed on the alumina surface, with [Mo7024] 6+ and AI2(MoO4)3 possibly also occurring at higher molybdenum concentrations. Calcination of the catalyst produces a "MoO3-1ike phase" consisting of polymerised tetrahedral and octahedral polyoxomolybdenum species formed by condensation reactions with the surface hydroxyl groups (Edwards et al. 1990). 95Mo solid-state NMR has also been used to study the effects of cobalt, caesium and potassium on MoO3-A1203 catalysts (Edwards and Ellis 1991). The molybdenum carbonyl Mo(CO)6 has been extensively studied by solid state 95Mo NMR (Eichele et al. 1997, Mastikhin et al. 1988, Edwards et al. 1990, Nolle 1977, Shirley 1987, Vosegaard et al. 1999). Because the quadrupolar interaction in this compound is so small, all the transitions are readily observed in a one-pulse experiment, allowing accurate determination of all the interaction parameters from the central and satellite transitions and making use of both static and MAS spectra. The principal components of the CSA tensor ( - 1843, - 1855 and - 1865 ppm) have been derived from the static central transition, while the satellite transitions have provided information about the quadrupolar parameters. The MAS spectra of this compound also show weak peaks from a 68 Hz ~J-coupling between 95Mo and 13C (Eichele et al. 1997). Even more precise information about the relative tensor orientation in Mo(CO)6 has been provided by a single-crystal study using a two-axis goniometer probe (Vosegaard et al. 1999a).
521
NMR of Low-7 Nuclides
Other Mo compounds studied by 95Mo NMR include MoSe2 and Mo3Se4 which can be distinguished from each other in mixtures of the 2 phases, since Mo3Se4 shows a large shift and relatively rapid relaxation (< 1 s) due to conduction electron effects (Bastow 1998). The compound MoS2 shows a static 95Mo lineshape with a similar shift to MoSe2 but dominated by quadrupolar effects (Figure 8.36B), as is also the case with the 95Mo NMR spectrum of MoC2 (Figure 8.36C) which is not narrowed by magic angle spinning (Bastow 1998). The satellite transitions in the 95Mo spectrum of MoSi have been used to deduce the value of • The small negative Knight shift in this compound by comparison with that of Mo metal (+ 6100 ppm) has been explained in terms of the possible cancellation of the conventional positive Knight shift by a negative shift arising from core-polarisation effects (Bastow 1998). 95Mo NMR has been used to study a series of ionically-conducting glasses in the system AgI-Ag20-MoO3. The spectra suggest that only tetrahedral monomeric orthomolybdate anions MoO42-, evidenced by their single sharp peak at about 70 ppm, are present in glasses of composition Ag20/MoO3 = 1, whereas glasses with compositions Ag20/MoO3 < 1 show an additional broader 95Mo resonance at about - 100 to - 120 ppm (Figure 8.37B) interpreted as a polymeric species containing linked MoO4 tetrahedra and MoO6 octahedra probably similar to the chain units present in crystalline Na2Mo207 (Figure 8.37A) (Machida and Eckert 1998). A
B
Ag2MoO4
(e)
Na2MoO4 @ ~0
(a) t ....
2000
I ....
I ....
i ....
I''
"' I '"'
'1 ....
I ' ;-~ i
| ....
'/"' ' ' ' i ' '
""1 ....
I'
~ ' ~'"|"' ' ' ' i . . . .
0 -2000 2000 0 95Mo shift ~ p m ) w.r.t. Na2MoO4 soln.
I' ;";i
-2000
Figure 8.37. 95MoMAS NMR spectra of A. a series of model molybdenum compounds containing Mo in various environments and B. a series of glasses in the system AgI-Ag20-MoO3 with Ag20/MoO3 < 1. Note the growth of the broad feature to the right of the sharp tetrahedral MoO4 resonance with increasing MoO3 content, possibly due to a polymeric chain unit as in Na2Mo207. From Machida and Eckert (1998), by permission of Elsevier Science.
522 8.3.11
Multinuclear Solid-State NMR of Inorganic Materials
135Baand 137BaN M R
Although under the definition of low- 7 nuclei adopted in this chapter only one of the Ba nuclei (135Ba) falls into this category, both will be discussed here. Both nuclei are spin I - q/2 and have small gyromagnetic ratios (see Chapter 1, Table 1.2). Although both nuclei have been used in NMR studies of isotopically-enriched samples, the slightly greater natural abundance of 137Ba (11.32% by comparison with 6.6% for ~35Ba) makes it more favourable for studying unenriched samples. Narrow 137Ba spectra have been recorded for compounds in which the Ba is in a highly symmetrical environment. Thus, the Ba site in cubic BaO has a local octahedral symmetry and is unaffected by second-order quadrupole effects and its MAS NMR spectrum shows a single sharp resonance (Figure 8.38) (Dec et al. 1993, MacKenzie and Meinhold 2000). The 137Ba MAS NMR spectrum of cubic BaZrO3 shows a similarly sharp resonance at 279 ppm with respect to aqueous BaCI2, arising from its symmetrical Ba site (Figure 8.38), suggesting that this compound would be a useful solid secondary reference material (Dec et al. 1993, MacKenzie and Meinhold 2000). By contrast, the Ba ion in BaTiO3 is displaced from the centre of its coordination polyhedron resulting in a relatively broadened 137Ba MAS NMR spectrum containing a resolvable quadrupolar lineshape (Figure 8.38). Static 137BaNMR spectra of BaTiO3 recorded over a temperature range (118-133~ show that as the temperature is lowered to the onset of the cubic-to-tetragonal phase transition at the Curie point, the intensity of the 137Ba NMR signal decays abruptly to zero. The lack of observable NMR intensity below the Curie point reflects the greater electric field gradient (efg) associated with increased Ba site distortions in the tetragonal phase (Forbes et al. 1987). A single-crystal ~37Ba study of BaTiO3 has yielded values of Xo and the axially anisotropic component of the chemical shift tensor. The change in the second-order shift with temperature up to the Curie point has also been shown to relate to the temperature-dependence of the spontaneous polarisation (Bastow 1989). Analysis of single-crystal 135Ba and 137Ba NMR data for tetragonal BaTiO3 in terms of a polarisable point multipole model has proved unsatisfactory, possibly because the quadrupole moment was not taken into account (Sommer et al. 1990). More recently the 137Ba NMR interaction parameters of the non-cubic phases of BaTiO3 have been determined for multidomain crystals (Taye et al. 1999) and strain-free polycrystalline specimens (Bastow and Whitfield 2001). Between 278 and 393 K the stable phase of BaTiO3 has tetragonal symmetry, becoming orthorhombic between 193 and 278 K. Below 193 K the stable phase is ferroelectric with rhombohedral symmetry. The recent measurements of the NMR parameters of these phases in multidomain crystals and polycrystalline samples (Table 8.14) are in reasonable agreement and clearly reflect the changes in symmetry in the various phases. In most of the other inorganic Ba compounds of interest as ceramic materials, the Ba occurs in irregular coordination polyhedra giving very broad lineshapes which are
523
NMR of Low- y Nuclides
Table 8.14. 137Bainteraction parameters of barium compounds. Compound
~iso*
BaO
748, 760** 279,279**
BaZrO3
XQ (MHz)
BaTiO3
394** 395,417
2.829
BaTiO3 (tetrag)
414"*
2.85, 2.86
(orthorhombic)
409**
2.30, 2.28
(rhombohedral)
409**
2.15, 2.06
Ba(OH)2.8H20 BaCO3 BaA1204 BazSiO4
N.D. 11.5 7.8 N.D.
xl
Reference
MacKenzie & Meinhold (2000), Dec et al. (1993) MacKenzie & Meinhold (2000), Dec et al. (1993) 0.3 Dec et al. (1993), Forbes et al. (1987), Bastow (1989), MacKenzie & Meinhold (2000) 0,0 Bastow & Whitfield (2001), Taye et al. (2000) 0.85, 0.98 Bastow & Whitfield (2001), Taye et al. (2000) 0,0 Bastow & Whitfield (2001), Taye et al. (2000) N.D. MacKenzie & Meinhold (2000) 0.3 MacKenzie & Meinhold (2000) 0.4 MacKenzie & Meinhold (2000) N.D. MacKenzie & Meinhold (2000)
BaAlzSi208 YBazCu307
- 260, - 7 82 1070 306 - 100, - 420 82 702, 401 82 2563
N.D. 14
N.D. 0.94
Ba acetate BaFC1 BaFBr
- 100 82 -
N.D. 1.7 17.0
N.D. 0 0
MacKenzie & Meinhold (2000) MacKenzie & Meinhold (2000), Shore et al. (! 992) MacKenzie & Meinhold (2000) Bastow & Stuart (1996) Bastow & Stuart (1996)
9shiftwithrespectto aqueousBaC12 9* originallyreportedwithrespectto BaZrO3 82 approximatepositionof the singularitiesof a possiblequadrupolarlineshape
in some cases too featureless to be simulated (as in Ba(OH)2, barium acetate, BaSi204 and BaAlzSi2Os). In other compounds (BaCO3, BaAI204), sufficient detail has been resolved to allow a spectral simulation to be made (Figure 8.38) (MacKenzie and Meinhold 2000). An insufficient number of values of XQ have been reported in Ba compounds to determine whether a relationship exists with geometrical parameters such as bond length or bond angle, but an apparent relationship has been observed in a limited number of cases between the centre-of-gravity (cog) of the Ba resonance and the coordination n u m b e r of the Ba polyhedron, reflecting the shielding at the Ba nucleus (MacKenzie and Meinhold 2000). 137Ba N M R has been used in conjunction with 27A1 and 29Si N M R to study the thermal evolution of crystalline celsian, BaAlzSi2Os, from gel precursors. The results indicate that although the characteristic elements of the tetrahedral aluminosilicate feldspar framework begin to form at quite low temperatures, migration of the larger Ba ion into the celsian sites is slower and requires higher temperatures (MacKenzie and Kemmitt 1999a).
524
Multinuclear Solid-State NMR of Inorganic Materials
B a ~ _ ~
BaAI204
BaTiO3 observed
rved
~~/
mulated
simulated , L
,
i
600
i
400
l
t_
!
2000
200
_
i
'
0
9
-2000
BaCO3/~A rved
.aa !
2000
t
0
j !..
~
i
I
9
,
t.
2000 -2000 2000 0 -2000 137Ba shift (ppm) w.r.t. BaCI2 soln.
ulatd 0
-2000
Figure 8.38. A selection of 137BaMAS NMR spectra of inorganic compounds, illustrating Ba in highly symmetric sites (as in BaO, BaZrO3 and BaTiO3) and in more distorted sites, some of which show sufficient second-order quadrupolar lineshape to be simulated (as in BaAI204 and BaCO3). From MacKenzie and Meinhold (2000), by permission of the copyright owner.
High-Tc superconducting ceramics such as YBa2Cu307 have been the subject of intense investigation since their discovery. Solid-state NMR studies of these compounds include determinations of the 135Ba and 137Ba NMR spectra of isotopicallyenriched samples, which have proved to be dominated by quadrupolar effects. The 135Ba and 137Ba spin-echo lineshapes of magnetically-aligned YBazCu307 in both the normal and superconducting states have been determined as a function of temperature and field dependence (Shore et al. 1992). In the superconducting state, the resonances broaden and shift to lower frequency and the 137Ba relaxation rates decrease abruptly, suggesting that the barium sites more closely reflect the behaviour of the plane rather than the chain copper sites (Shore et al. 1992). The temperature dependences of the 135Ba and 137Ba relaxation times have also been measured at lower fields for isotopically-enriched samples of differing oxygen content. Unlike the temperature dependence of the Y and O atoms in these compounds which show Korringa behaviour (relaxation rate increasing linearly with temperature), the temperature dependence of the Ba shows non-Korringa behaviour in the normal state, and is similar to that of
NMR of Low- y Nuclides
525
the Cu(2) sites but two orders of magnitude smaller (Yakubowskii et al. 1992). The quadrupole frequency determined for YBazCu307 by Shore et al. (1992) and confirmed by the NQR results of Yakubowskii et al. (1992) has been successfully used to simulate a reasonable approximation of the MAS lineshape (MacKenzie and Meinhold 2000). The barium fluorohalides BaFC1 and BaFBr are of practical interest as matrices for X-ray storage phosphors. The 137Ba NMR spectra of both compounds have been reported and their NMR interaction parameters determined (Bastow and Stuart 1996). The room temperature • value of BaFC1 is relatively small but increases rapidly with temperature, possibly due to variations in the thermally averaged structure about the Ba nucleus. The room temperature XQ value of BaFBr is larger by a factor of 10 than that of BaFC1, and its 137Ba linewidth was such that the spectrum had to be determined by the stepped-frequency method, by contrast with the static BaFC1 spectrum which was sufficiently narrow that the whole transition could be excited by ordinary pulse spectroscopy (Bastow and Stuart 1996).
8.3.12 Other miscellaneous low-), nuclei A small number of reports have appeared of the NMR spectra of other low-~/nuclei. These include 53Cr NMR which has been used in a study of the ferromagnetic materials CdCrzS4 and CuCr2Se4. Information about the magnetic hyperfine interactions in these compounds was derived from the echo formed by multiple quantum effects (Abelyashev et al. 1988). 61Ni NMR has been used to examine the approximately equiatomic alloy Ni49.6A15o.4, which showed a Knight shift of 1220 ppm at 9.4 T (Bastow et al. 1997), compared with a value determined at the lower field of 1.6 T of 1890 ppm (Drain and West 1965). The discrepancy between these two values illustrates the difficulty of making such measurements at lower fields and the critical dependence of the calculated diamagnetic position on the value assumed for the magnetic moment. 73Ge NMR is subject to great difficulty, and is seldom studied. Recently reported spectra from single crystal germanium with differing isotope contents show quadrupolar lineshapes arising from local lattice distortion related to isotopic disorder (Verkhovskii et al. 2000). 99Ru and l~ NMR spectra have been reported in RtlO2, SrRuO3 and SrzRuO4 which exhibit a variety of magnetic and superconducting properties. The NMR spectrum of ruthenium metal shows all the transitions, from which a value of 6700 ppm was deduced for the isotropic Knight shift. Values of XQ of 1.93 and 11.2 MHz were determined for 99Ruand l~ respectively in ruthenium metal, with -q = 0 for both nuclei (Mukuda et al. 1999). The ratio of these 2 XQvalues agrees exactly with the ratio of the quadrupole moments of the 2 isotopes. Only the central transition of 99Ru could be
526
Multinuclear Solid-State NMR of lnorganic Materials
detected in RuO2, from which it was determined that XQ = 21.1 MHz and xl = 0.74 (Mukuda et al. 1999).
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Multinuclear Solid-State NMR of lnorganic Materials
MacKenzie, K.J.D., Meinhold, R.H., McGavin, D.G., Ripmeester, J.A. & Moudrakovski, I. (1995) Solid State Nucl. Mag. Reson., 4, 193. MacKenzie, K.J.D. & Meinhold, R.H. (1996)J. Mater. Chem., 6, 821. MacKenzie, K.J.D. & Meinhold, R.H. (1997) Amer. Mineralogist, 82, 479. MacKenzie, K.J.D. & Kemmitt, T. (1999) Thermochim. Acta, 325, 13. MacKenzie, K.J.D. & Kemmitt, T. (1999a) Thermochim. Acta, 325, 5. MacKenzie, K.J.D. & Meinhold, R.H. (2000) Ceram. Int., 26, 87. MacKenzie, K.J.D., Sheppard, C.M. & McCammon, C. (2000) J. European Ceram. Soc., 20, 1975. MacKenzie, K.J.D., Schmticker, M., Smith, M.E., Poplett, I.J.F. & Kemmitt, T. (2000a) Thermochim. Acta, 363, 181. Markert, J.T., Noh, T.W., Russek, S.E. & Cotts, R.M. (1987) Solid State Commun., 63, 847. Markgrabe, D. & Engelhardt, G. (1999) Chem. Phys. Lett., 300, 701. Mastikhin, V.M., Lapina, O.B. & Maximovskaya, R.I. (1988) Chem. Phys. Lett., 148, 413. Meinhold, R.H. & MacKenzie, K.J.D. (1995) Solid State Nucl. Mag. Reson., 5, 151. Merwin, L.H. & Sebald, A. (1990) J. Mag. Reson., 88, 167. Merwin, L.H. & Sebald, A. (1992) J. Mag. Reson., 97, 628. Merwin, L.H. & Sebald, A. (1992a) Solid State Nucl. Mag. Reson., 1, 45. Mukuda, H., Ishida, K., Kitaoka, Y., Asayama, K., Kanno, R. & Takano, M. (1999) Phys. Rev. B, 60, 12279. Mustarelli, P., Tomasi, C., Quartarone, E., Magistris, A., Cutroni, M. & Mandanici, A. (1998) Phys. Rev. B, 58, 9054. Mustarelli, P., Tomasi, C., Magistris, A. & Cutroni, M. (1998a) J. Non-Cryst. Solids, 232-234, 532. Narath, A. (1967) Phys. Rev., 162, 320. Nieto, P., Dron, R., Thouvenot, R., Zanni, H. & Brivot, F. (1995) Comptes Rendu Acad. Sci. Serie H, 320, 485. Nolle, A. (1977) Z. Phys. A, 280, 231. Nowak, B., Zogal, O.J. & Niedzwiedz, K. (1992) J. Alloys Compounds, 189, 141. Ohno, T., Alloul, H., Mendels, P., Collin, G. & Marucco, J.F. (1990) J. Mag. Mag. Mater., 90-91, 657. Ohno, T., Mizuno, K., Kanashiro, T. & Alloul, H. (1991) Physica C, 185, 1067. Olsen, K.K. & Zwanziger, J.W. (1995) Solid State Nucl. Mag. Reson., 5, 123. Ono, H., Ishimaru, S., Ikeda, R. & Ishida, H. (1997) Chem. Phys. Lett., 275, 485. Ono, H., Ishimaru, S., Ikeda, R. & Ishida, H. (1999) Bull. Chem. Soc. Japan, 72, 2049. Padro, D., Howes, A.P., Smith, M.E. & Dupree, R. (2000) Solid State Nucl. Mag. Reson., 15,231. Pecoul, N., Bourbigot, S. & Revel, B. (1997) Macromol. Symp., 119, 309. Plishke, J.K., Benesi, A.J. & Vannice, M.A. (1992) J. Phys. Chem., 96, 3799. Poplett, I.J.F. & Smith, J.A.S. (1981) J. Chem. Soc. Faraday Trans. 2, 77, 1155. Poplett, I.J.F. & Smith, M.E. (1998) Solid State Nucl. Mag. Reson., 11, 211. Retcofsky, H.L. & Friedel, R.A. (1972) J. Amer. Chem. Soc., 94, 6579. Rigamonti, A., Borsa, F. & Carretta, P. (1998) Rep. Prog. Phys., 61, 1367. Riseman, T.M., Alloul, H., Mahajan, A.V., Mendels, P., Collin, G. & Marucco, J.F. (1994) Physica C, 235, 1593.
NMR of Low- T Nuclides
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Roberts, N., Wang, R-P., Sleight, A.W. & Warren, W.W. (1998) Phys. Rev. B, 57, 5734. Roos, J., Brinkmann, D., Mali, M., Pradel, A. & Ribes, M. (1988) Solid State Ionics, 28--30, 710. Santos, R.A., Tang, P., Chien, W-J., Kwan, S. & Harbison, G.S. (1990) J. Phys. Chem., 94, 2717. Sanz, J., Herrero, P., Rojas, J.M., Rossignol, S., Reau, J.M. & Tanguy, B. (1995) Solid State Ionics, 82, 129. Sasaki, S., Matsuda, A. & Chu, C.W. (1998) Physica C, 302, 319. Sebald, A. (1994) NMR Basic Princ. Prog., 31, 92. Segel,S.L. (1978)J. Chem. Phys., 68, 330. Segel, S.L. (1981) J. Chem. Phys., 75, 4746. Sham, S. & Wu, G. (1999) Can. J. Chem., 77, 1782. Sham, S. & Wu, G. (2000) Inorg. Chem., 39, 4. Shore, J., Yang, S., Haase, J., Schwartz, D. & Oldfield, E. (1992) Phys. Rev. B, 46, 595. Shirley, W.M. (1987) Z. Phys. Chem., 152, 41. Skibsted, J. & Jakobsen, H.J. (1999) Inorg. Chem., 38, 1806. Sommer, R., Maglione, M. & van der Klink, J.J. (1990) Ferroelectrics, 107, 307. Stebbins, J.F. (1996) Amer. Mineralogist, 81, 1315. Such, K.P. & Lehmann, G. (1988) Chem. Phys. Lett., 143, 463. Sub, J., Torgeson, D.R. & Borsa, F. (1993) Phys. Rev. Lett., 71, 3011. Suzuki, H., Komaru, T., Hihara, T. & Koi, Y. (1971) J. Phys. Soc. Japan, 30, 288 Tansho, M., Wada, H., Ishii, M. & Onoda, Y. (1996) Solid State Ionics, 86-88, 155. Taye, A., Klotzche, G., Michel, D., Mulla-Osman, S. & B6ttcher, R. (1999) J. Phys. Condensed Matter, 11,871. Thompson, A.R. & Oldfield, E. (1987) J. Chem. Soc., Chem. Commun., 27. Torgeson, D.R. & Barnes, R.G. (1967) Bull. Amer. Phys. Soc., 12, 313. Trokiner, A., Lenoc, L., Mikhalev, K., Yakubovskii, A., Lutgemeier, H., Heinmaa, I., Gippius, A., Verkhovskii, S., Goldschmidt, D. & Eckstein, Y. (1994) Physica C, 226, 43. Trokiner, A., Noc, L.L., Yakubovskii, A., Mykhalyov, K.N. & Verkhovskii, S.V. (1994a) J. Chim. Phys. Physico-Chem. Biol., 91, 862. Trokiner, A., Lenoc, L., Yakubovskii, A., Mykhalyov, K.N. & Verkhovskii, S.V. (1994b) Z. Naturforsch. A, 49, 373. Tunstall, D.P., Todd, J.R.M., Arumugam, S., Dai, G., Dalton, M. & Edwards, P.P. (1994) Phys. Rev. B, 50, 16541. Tunstall, D.P. & Webster, W.J. (1991) Supercond. Sci. Technol., 4, $406. Tycko, R. & Opella, S.J. (1987) J. Chem. Phys., 86, 1761. Verkhovskii, S.V., Malkin, B.Z., Trokiner, A., Yakubovskii, A., Haller, E., Ananyev, A., Gerashenko, A., Piskunov, Y., Saikin, S., Tikhomirov, A. & Ozhogin, V. (2000) Z. Naturforsch. A, J. Phys. Sci., 55, 105. Villa, M., Chiodelli, G., Magistris, A. & Licheri, G. (1986) J. Chem. Phys., 85, 2392. Vosegaard, Y., Andersen, U. & Jakobsen, H.J. (1999) J. Amer. Chem. Soc., 121, 1970. Vosegaard, T., Skibsted, J. & Jakobsen, H.J. (1999a) J. Phys. Chem. A, 103, 9144. Weeding, T.L. & Veeman, W.S. (1989) J. Chem. Soc. Chem. Commun., 946. Weiden, N. & Weiss, A. (1974) 18th AMPERE Congress, p.257. Williams, G.V.M., Tallon, J.L., Michalak, R. & Dupree, R. (1998) Phys. Rev. B, 57, 8696.
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Multinuclear Solid-State NMR of lnorganic Materials
Williams, G.V.M., Tallon, J.L., Quilty, J.W., Trodahl, H.J. & Flower, N.E. (1998a) Phys. Rev. Lett., 80, 377. Wu, J., Boyle, T.J., Shreeve, J.L., Ziller, J.W. & Evans, W.J. (1993) Inorg. Chem., 32, 1130. Wu, G., Kroeker, S. & Wasylishen, R.E. (1995) Inorg. Chem., 34, 1595. Wu, G. (1998) Chem. Phys. Lett., 298, 375. Yakubowskii, A., Egorov, A. & Ltitgemeier, H. (1992) Appl. Mag. Reson., 3,665. Yamada, T. & Asanuma, M. (1965) Phys. Rev. Lett., 15, 695. Yesinowski, J.P. & Hill, E.A. (1999) Solid State NMR Spectroscopy of lnorganic Materials American Chemical Society Symposium Series 717, Ed. Fitzgerald, J.J., p.358. Ylinen, E.E., Kaikkonen, A. & Punkkinen, M. (1997) Solid State Nucl. Mag. Reson., 10, 25. Yoshinari, Y., Alloul, H., Brouet, V., Kriza, G., Holczer, K. & Forro, L. (1996) Phys. Rev. B, 54, 6155. Zogal, O.J., Nowak, B. & Niedzwiedz, K. (1991) Solid State Commun., 80, 601.
Chapter 9
NMR of Other Spin- 1/2 Nuclei 9.1. 9.2.
9.3.
Introduction Abundant High-~/Nuclei 9.2.1 1HNMR 9.2.1.1 Background to Proton Studies in Inorganic Materials 9.2.1.2 Studies of Stoichiometric Protons in Crystalline Materials 9.2.1.3 Non-Stoichiometric Proton Environments in Crystalline and Glassy Materials 9.2.1.4 ~H NMR of Hydrous Glasses 9.2.1.5 Biomineral-Related Materials 9.2.2 ~9FNMR 9.2.2.1 Introduction 9.2.2.2 Simple Inorganic Fluorides 9.2.2.3 More Complex Fluorides 9.2.2.4 Applications to Fluoroapatite Studies 9.2.2.5 Fluorine in Aluminosilicate Minerals and Related Materials 9.2.2.6 Surface Interaction of Fluorine with Silica- and Alumina-Based Materials 9.2.2.7 Fluorine in Alumino- and Gallophosphates 9.2.2.8 Fluorine in Oxygen-Containing Glasses 9.2.2.9 Fluoride Glasses 9.2.2.10 Fluorine in Other Materials 9.2.2.11 Fluorine as a Source of Cross-Polarisation 9.2.2.12 Summary of 19F Shift Trends and Other NMR Properties Dilute or Medium-~ Nuclei 9.3.1 13C NMR 9.3.1.1 13C NMR of Elemental Carbon 9.3.1.2 Silicon Carbide 9.3.1.3 Other Binary Carbides 9.3.1.4 Ternary and Quaternary Carbides 9.3.1.5 Carbonates 9.3.2 lSN NMR
535 536 536 536 539 542 545 550 550 550 551 554 555 556 557 559 559 560 562 562 562 563 563 563 568 570 572 572 574
9.3.3 9.3.4 9.3.5
9.3.2.1 Nitrides 9.3.2.2 Silicon Aluminium Oxynitride Ceramics and Glasses 9.3.2.3 Nitride Ceramics from Polymeric Precursors 9.3.2.4 Nitrates and Nitrites 778e NMR l llCd and l l3Cd NMR l l5Sn, l lVSn and l l9Sn NMR
9.3.5.1 Crystalline Oxygen-Containing Materials 9.3.5.2 Oxide Solid Solutions and Glasses 9.3.5.3 Non-oxide Materials 9.3.6 J23Te and 125Te NMR 9.3.6.1 Crystalline Tellurides 9.3.6.2 Crystalline Tellurites and Tellurates 9.3.6.3 Glassy Tellurium-Containing Materials 9.3.7 129Xe NMR 9.3.8 195pt NMR 9.3.9 199Hg NMR 9.3.10 2~ and 2~ NMR NMR 9.3.11 2~ 9.3.11.1 Correlations between 2~ Chemical Shifts and Structure 9.3.11.2 2~ NMR of Crystalline Lead Compounds 9.3.11.3 2~ NMR of Lead-Containing Glasses 9.3.11.4 2~ in Sol-Gel Prepared Ceramics References
575 576 579 582 583 587 591 591 594 595 598 598 599 601 601 603 604 604 607 607 609 613 615 616
Chapter 9
NMR of Other Spin- 1/2 Nuclei 9.1. INTRODUCTION The most important spin-1/2 nucleus for studies of inorganic materials is undoubtedly 29Si, which is dealt with in Chapter 4. Spin-1/2 nuclei with magnetic moments below 15N were discussed in Chapter 8. In the present chapter solid state NMR studies of other spin-1/2 nuclei that are not dealt with elsewhere are discussed. Spin-l/2 nuclei can be usefully divided into two groups: (i) abundant high-~/nuclei and (ii) dilute or medium-~/nuclei. The group into which a nucleus falls often determines the experimental approach to be employed and also the information that can be extracted from the NMR spectrum. The abundant high-~/nuclei are 1H, 19F and also 31p (dealt with in Chapter 7). These nuclei usually have strong homonuclear dipolar coupling. The extent to which this dipolar coupling is homogeneous varies between the different nuclei. In 1H the small chemical shift range and large coupling often makes the interaction homogeneous. Significant homogeneous interaction can also exist for fluorine although this is often less strongly the case since fluorine experiences a larger chemical shift interaction. For 31p the interactions are rarely strongly homogeneous. The implication of these different interactions is that often MAS at moderate speeds (e.g. 20 kHz) is ineffective for 1H and 19F and a CRAMPS approach can be usefully applied. Although examples of 31p CRAMPS exist, the faster MAS rates now widely available and the typical dipolar couplings experienced by 31p has reduced the need for 3~p CRAMPS. Although the other 2 nuclei experience extremely strong dipolar couplings, these occur more often in organic polymeric materials which are outside the scope of this book. In inorganic materials the protons are usually relatively spatially dilute, decreasing the ~H-1H interaction to the extent that moderate MAS is often sufficient. This is also usually true for 19F although the 19F-19F distance can be quite short in some pure inorganic fluorides, requiring fast MAS for efficient narrowing. As well as cases where spatial dilution reduces the dipolar coupling, MAS narrowing will also be effective for isolated spin pairs and linear chains of spins. The signals from these nuclei are very strong, resulting in high sensitivity. The presence of a strong dipolar coupling also means that information about internuclear distances can potentially be extracted. The dilute or medium-~/spin-l/2 nuclei have relatively small dipolar coupling (especially homonuclear coupling) because of spatial dilution or the small magnetic moment. Thus, even slow MAS is often sufficient to narrow these resonances. The factors which reduce the dipolar coupling unfortunately also lie behind the low 535
536
Multinuclear Solid-State NMR of Inorganic Materials
sensitivity of these nuclei. It is often also true that in many such systems the relaxation is weak, classic examples being diamond and silicon carbide where the T~ can be many hours. For such nuclei the magnetisation often shows stretched exponential relaxation behaviour (Hartman et al. 1994).
9.2. ABUNDANT HIGH-~ NUCLEI
9.2.1 1HNMR 9.2.1.1 Background to proton studies in inorganic materials. An important problem in many hydrous solids is the determination of the speciation of the protons within the structure. As water is added it can (i) remain intact as a water molecule (as structural hydrate water, liquid inclusions or surface-adsorbed species), (ii) form hydroxyl groups, or (iii) take the form of structural acid protons. Water dissolution is a very important scientific problem in a number of branches of science but especially in mineralogy since it is the dominant low density fluid in the Earth's crust, playing a central role in geochemistry. For so-called "nominally anhydrous minerals" (NAMs) the presence of even minor amounts of water can have a significant influence on many of the properties and behaviour of these minerals. The interaction of water with other inorganic solids such as ceramics, glasses and catalysts is often a key scientific and technological problem. Hydrogen species in such cases are invariably bonded to oxygen and can occur in many different forms. Protons can be present in a crystal structure as stoichiometric OH groups, water molecules, or hydronium ions. Hydroxyl groups can also play an important role in substitutions such as OH- ~ F-,
A104- + H + ~ SiO4, H404 6--) SiO4 (hydrogarnet)
(9.1)
Water molecules can occupy vacant cation and anion sites, zeolitic sites, or interlayer regions. Water molecules can also be present on mineral surfaces, in microscopic cracks in the crystal, or in macroscopic fluid inclusions. If the proton content is significant and stoichiometric, diffraction, especially by neutrons, can elucidate the position of hydrogen. However when the hydrogen does not occur regularly throughout the structure and its content is low, diffraction does not usually provide much information. Infra-red (IR) spectroscopy can qualitatively distinguish the different sites but the question of quantitative integrity is often raised. 1H MAS NMR offers an alternative to these approaches. The main drawback anticipated with 1H MAS NMR is the strength of the homogeneous dipolar interaction between the protons, which would render it difficult to achieve significant line narrowing. An approach to overcoming this is to physically dilute the protons by deuterating the materials. It is fortunate, however, that in inorganic
537
NMR of Other Spin -1/2 Nuclei
materials the low hydrogen content often results in the hydrogen-bearing species being relatively isolated from each other so that inhomogeneous line broadening mechanisms dominate. An isolated water molecule can be considered as a special case of an isolated two-spin system where the homonuclear dipolar interaction between the two protons is inhomogeneous (Maricq and Waugh 1979). The interaction between two protons can only be regarded as truly inhomogeneous if each of the dipolar-coupled protons has the same chemical shift tensor in the same orientation. In rigid water molecules the chemical shift tensor orientations will be different for each proton but rapid (on the NMR timescale) 180 ~ flips about the bisector axis render these tensors the same and hence the dipolar coupling inhomogeneous (Yesinowski and Eckert 1987). Modest MAS can then average the interaction even if the residual sidebands are quite broad. A series of studies by Yesinowski and Eckert demonstrated that for crystalline hydrated minerals high-resolution 1H MAS-NMR spectra could be obtained using MAS of 8 kHz with results compared at 200 and 500 MHz. With OH groups there was a steady decrease in the linewidth of the centreband as the spinning speed increased from 2 to 8 kHz (Figure 9.1), suggesting that the residual linewidth has a significant homonuclear dipolar contribution. It was suggested that the density of protons was a crude measure of the proton homonuclear dipolar coupling, and shown that at a field of 4.7 T with MAS spinning speeds of 7-8 kHz, linewidths of -< 1.5 kHz could be obtained for proton densities < 15 atoms/nm 3. By contrast, for diaspore (A1OOH) in which the proton density is 33.9 atoms/nm 3 no narrow signal could be observed (Yesinowski et al. 1988).
N
800
D
D
rl
H20 []
.P....l
[]
600 9J,..l O
~
O
4o0
z
OH O O
2OO
I
0
I
2
I
I
4
I
I
6
I
I
8
Spinning speed (kHz) Figure 9.1. Influence of the MAS speed on the 1H NMR linewidth from OH groups and molecular water in hydroxyapatite. Note the marked effect on the OH protons reflecting the homonuclear dipolar contribution. After Yesinowski and Eckert (1987), by permission of the American Chemical Society.
538
Multinuclear Solid-State NMR of lnorganic Materials
Where the analysis of proton spectra includes wideline interactions, the contribution of the CSA should not be underestimated, especially where the work is carried out at the higher applied magnetic fields now available. Previous studies have determined the 1H CSA values for H20 in deuterium-diluted ice as 34 ppm (Emsley and Pines 1994) and --~ 10 ppm for BaC104.H20 (Tekely et al. 1994). Broadband homonuclear dipolar coupling using the MSHOT-3 sequence combined with both sample rotation and heteronuclear 31p decoupling was used to study the 1H CSA tensor in polycrystalline KHzPO4 (Rasmussen et al. 1999). The chemical shift tensor was characterised by ~iso = 14.9 ppm, a span of 40.8 ppm and a skew of -0.66. The system provides an isolated 31p-1H-31p fragment which can be analysed to determine the absolute orientation of the tensor in the molecular frame even in a powder sample. The large anisotropy reflects strong hydrogen bonding. Another important area studied by solid state ~H NMR is the determination of the surface speciation of adsorbed protons. For example, a number of peaks are observed in the 1H NMR spectra of the various forms of A1203 (Mastikhin et al. 1987). The strong peak observed at 3.4 ppm from o~-A1203 is believed to arise from bulk OH while a smaller peak at - 0 . 2 ppm corresponds to surface hydroxyls. Other peaks observed from the transition aluminas indicate the effect of the differing coordinations of the attached aluminium neighbours. Similar studies have been carried out for silica where the parent aerosil SiO2 gave 3 1H MAS NMR signals; 1, at 1.4 ppm is from isolated SiOH, while those at 2.5 and 3.2-3.5 ppm represent SiOH groups hydrogen-bonded to differing degrees (Mastikhin et al. 1995). It is the isolated and more weakly hydrogen-bonded OH groups that appear to interact with the supported metal oxide. The importance of protonation sites in microporous catalytic materials has led to 1H NMR studies of zeolites (Pfeifer et al. 1985, Pfeifer 1988). The 1H data provide information about the Br~nsted acidity (Pfeifer et al. 1991) and enable different hydroxyl groups readily to be distinguished (Freude et al. 1987). Detailed consideration has been given to the linebroadening mechanisms of the 1H spectra of zeolitic materials. In siliceous materials the resolution limit was reached at the relatively modest magnetic field of 7.05 T (Brunner 1990). However, for zeolites with higher aluminiumcontent this limit was reached only at a higher magnetic field of 11.7 T (Brunner 1993). The NMR data have been correlated with the stretching frequency of the hydroxyl groups in solids such as zeolites, corroborating results from vibrational spectroscopy (Brunner et al. 1992). Hunger (1996) has extensively reviewed the use of 1H NMR as a probe of the hydroxyl sites in microporous materials where there is a distinction between BrCnsted acid sites (e.g. between Si-O-A1) and silanol (SiOH) groups located at the surface. Table 2 in Hunger's review gives precise assignments to a range of different hydroxyl environments. Direct proton NMR not only reveals the different proton species present but can also be used to determine the characteristics of the attached framework site. For example,
539
NMR of Other Spin -lIe Nuclei
the quadrupole interaction has been suggested to increase on protonation of the oxygen attached to an aluminium atom. By using a 1H-detected TRAPDOR experiment (Section 3.8.3) the Xe of the 27A1 can be estimated. Examples of this approach include a study of the Br0nsted acid site in dehydrated HY (Grey and Vega 1995), the Lewis acid site in undehydrated HY (Kao and Grey 1996) and the dealuminated zeolites ultrastable HY, HZSM-5 and mordenite (Deng et al. 1998). Values of 27A1XQ as high as 15.3 MHz have been estimated using this approach.
9.2.1.2 Studies o f stoichiometric protons in crystalline materials. Studies of crystalline materials have examined well-defined hydrogen environments to determine the spectral characteristics of the different proton species. The shift range of protons is --~20 ppm with strong overlap between the resonances of hydroxyl groups and molecular water. The most obvious spectral distinction between OH and H20 is the extent of the spinning sideband manifolds. Hydroxyls tend to have only 1 or 2 pairs of sidebands whereas water molecules display a large manifold of intense spinning sidebands extending over a range of --~ 100 kHz (Figure 9.2). Even where H20 and OH groups cannot be distinguished on the basis of their chemical shifts, the important H20/OH ratio can readily be determined either from the sidebands or from the static spectrum. The very different dipolar couplings of OH and H20 groups give rise to static spectra containing a comparatively narrow OH resonance superimposed on a broader H20 resonance. Table 9.1 lists some typical proton shifts in inorganic materials.
A
B
tremolite
.
analcite
.
.
.
A
_..jk_.J ~. ~
,._..) I
100 0 -100 1H shift (ppm) w.r.t. TMS
200
i
I
-200 0 IH shift (ppm) w.r.t. TMS
Figure 9.2. 1H MAS NMR spectra of proton-containing minerals, illustrating the difference between the spectral characteristics of OH groups as in tremolite, Ca2MgsSisO22(OH)2 (spectrum A) and water molecules as in analcite, NaA1Si206.H20 (spectrum B). Note the typical manifold of spinning side bands associated with H20 but not OH. After Eckert et al. (1988), by permission of the American Chemical Society.
540
Multinuclear Solid-State NMR of Inorganic Materials
Table 9.1. Characteristic proton shifts in some inorganic materials containing stoichiometric OH and H20. Compound
1H ~iso(ppm)*
Nature of site
Reference
pyrophyllite tremolite analcite gypsum talc topaz elbaite datolite pectolite hydroxyapatite monetite brushite ilerite Mg5SizO8(OH)2 Hydrous Mg silicate B Superhydrous Mg silicate B MgvSizOs(OH)6 hydroxysodalite hydrate hyalite
2.3 0.7 3.1 5.3 1.1 3.0 4.7 4.3 15.8 0.2 13.6-16.2 6.4, 10.4 3.8, 16.3 1.1 4.7, 3.3 5.0, 3.4
OH OH H20 H20 OH OH OH OH OH OH Acid H H20, acid H, H20, acid H OH OH OH
Eckert et al. (1988) Eckert et al. (1988) Eckert et al. (1988) Eckert et al. (1988) Yesinowski et al. (1988) Yesinowski et al. (1988) Yesinowski et al. (1988) Yesinowski et al. (1988) Yesinowski et al. (1988) Yesinowski & Eckert (1987) Yesinowski & Eckert (1987) Yesinowski & Eckert (1987) Brenn et al. (2000) Phillips et al. (1997) Phillips et al. (1997) Phillips et al. (1997)
5, 3.7 16, - 1 7.1, 5.6, 3.9
KHSi205 NH4HzPO4 KH2PO4 NH4HSO4 LizSOa.H20 KAI3(SO4)z(OH)6 with H z O / H 3 0 + 6--) K + sorensenite makatite octosilicate phosphoellenbergerite holtedahlite Mg(OH)2 pargasite
15.6 7.2, 14.8 14.9 6.9, 11.7 5.6 4.5, 7.0, 11.4 5.1 5.8 3.6, 16.0 1.2, 4.5, 11.0 1.2, 4.5 0.5 1.2
OH Acid H, OH H20(1), H20(2), OH Acid H NH4 +, acid H acid H NH4 +, acid H H20 OH, H20, H30 + H20 OH/H20 H20, OH OH OH OH OH
Kagi et al. (2000) Engelhardt et al. (1992) Graetsch & Topalovicdierdorf (1996) Deng et al. (1995) Ratcliffe et al. (1985) Rasmussen et al. (1999) Ratcliffe et al. (1985) Ratcliffe et al. (1985) Ratcliffe et al. (1985) Sebald et al. (1990) Almond et al. (1997) Almond et al. (1997) Brunet & Schaller (1996) Brunet & Schaller (1996) Sears et al. (1988) Welch et al. (1994)
* chemicalshiftsquotedwithrespectto tetramethylsilane(TMS)
The work of Yesinowski and Eckert (1987) showed that high-resolution 1H MAS N M R spectra could be obtained from minerals containing stoichiometric hydroxyl groups as the sole proton species provided the proton density in the sample is _< 15 atoms/nm 3. This conclusion agrees with a detailed ~H N M R study of the dehydration processes of silica gel surfaces (Bronnimann et al. 1988). Models of structurally isolated water molecules provided by analcite, N a A 1 S i 2 0 6 . H 2 0 , and gypsum, CaSO4.2H20, yield characteristic 1H MAS N M R spectra with numerous spinning
NMR of Other Spin -1/2 Nuclei
541
sidebands reflecting the strong, largely inhomogeneous character of the homonuclear dipolar coupling. Tremolite (CazMgsSigOz2(OH)2)and pyrophyllite (AlzSi401o(OH)2) have similarly been used to model the OH groups. Lawsonite and hemimorphite, minerals containing stoichiometric amounts of both OH and H20 groups, yield spectra with numerous intense spinning sidebands from which it is difficult to discriminate between OH and H20. It has been suggested that hydroxyapatite, with a shift of 0.2 ppm would be a good secondary 1H reference. The number of model compounds containing stoichiometric H20 and OH groups was greatly extended in a study by Yesinowski et al. (1988). The 1H shifts in such units correlate well with the hydrogen bond strength which is also reflected by the distance d(O-H-O) (Berglund and Vaughan 1980). The experimental data (Yesinowski et al. 1988) suggest the relationship 6iso(ppm) = 79.05 - 0.255d(O-H-O)(pm)
(9.2)
By using this relationship, 1H shifts can reveal subtle differences between proton sites. A recent detailed review of the application of solid state NMR to probe hydrogenbonding by Brunner and Sternberg (1998) includes 1H data. Protons can occur in other environments such as NH4 groups, as in the aluminosilicate buddingtonite in which the ~H resonance occurs at 6.8 ppm (Yesinowski et al. 1988). Two hydrous magnesium phosphates give principal aH signals arising from OH groups, some of which contain protons attached to the apical oxygen, with others containing protons attached to the octahedral units of double chains. The different sites show very different proton couplings and are reflected in the sideband patterns. An additional signal in phosphoellenbergerite arises from protons associated with magnesium vacancies in the single chains of the structure (Brunet and Schaller 1996). Phillips et al. (1997) carried out an extensive ~H MAS NMR investigation of high pressure magnesium silicate phases. Of particular interest was the hydrated phase B and the closely related superhydrous phase B. The B phase showed 2 distinct peaks at 4.7 and 3.3 ppm from the paired but inequivalent hydroxyl pairs occurring in the structure. A proton separation of 1.86~ deduced from the powder pattern was different from the distance determined by X-ray methods, which are, however, not ideal for locating poorly scattering protons. The spin-echo data from the superhydrous B phase gave a slightly shorter H-H distance (1.83 A). In samples containing ~-MgzSiO4 coexisting with these hydrous phases, 1H-298i CP indicated that this nominally anhydrous phase contains a significant proton content. In these experiments it was noted that CSA effects had to be included in order to simulate the asymmetry of the static pattern and that their inclusion had an effect on the estimated H-H distance. General expressions have been derived for the frequency (and hence the lineshape) under a combination of CSA and dipolar effects (Zilm and Grant 1981, Harris et al. 1985). Based on these general
542
Multinuclear Solid-State NMR of lnorganic Materials
expressions an explicit and convenient formulation was derived by Phillips et al. (1997) for the particular case of pairs of coupled rigid hydroxyl groups. It was also noted that when there is strong coupling between nuclei the peak position does not correspond identically with 8iso. In the superhydrous B phase, as Vr increased from 11 to 14 kHz the peak separation increased by ---0.4 ppm. A detailed study of the effect of structure on the 1H chemical shift of both the hydroxyls and water molecules in smectite clay minerals has recently been made by Alba et al. (2000). Significant differences were found in the ~H shift of hydroxyls in trioctahedral clays (--~0.5 ppm) and dioctahedral clays (--- 2.0 ppm). This shift difference is related to the orientation of the hydroxyls relative to the layers, with more hydroxyl interaction possible in dioctahedral smectites. The greater variation possible in the dioctahedral clays gives rise to broader ~H resonances than in the trioctahedral minerals. Furthermore, within each group there is another level of variation related to the layer charge, which in the octahedral layer tends to increase the proton shielding but has a negligible effect in the tetrahedral layer. Substitution within layers causes broadening of the 1H resonance. Interlayer cations have a negligible effect on the hydroxyl resonance, but the same is not true for interlayer water. As the charge increases the proton acidity increases, changing the ~H shift from --- 4.57 through 4.36 to 4.10 ppm on going from trivalent to monovalent interlayer cations (Alba et al. 2000). The static 1H spectrum of an NH4+-exchanged mica after heat treatment showed a single sharp 1H resonance, which was taken to indicate rapid proton motion (Noma et al. 1997). ~H NMR signals have been detected in sepiolite, arising from Mg-OH (0.4 ppm) and interlayer water (4.4 ppm) (Aramendfa et al. 1997). On removal of water an Si-OH signal appeared at 1.9 ppm. Treatment with acid and heating caused the sepiolite structure to breakdown to form a fibrous silica retaining an Si-OH signal at 1.9 ppm.
9.2.1.3 Non-stoichiometric proton environments in crystalline and glassy materials. Non-stoichiometric hydrogen in nominally anhydrous minerals (e.g. feldspars, nepheline, quartz, and grossular garnet) is found to occur in a variety of forms including mobile H20 in fluid inclusions, anisotropically constrained isolated He0 molecules and clustered species such as H404. Even in such materials where the proton content is low, usable NMR signals can be obtained in a relatively short timescale, providing directly quantitative information and allowing fluid-like inclusions readily to be distinguished by their very narrow lines. 1H signals were detected in feldspars, arising both from fluid water inclusions undergoing rapid isotropic motion and from structural isolated water molecules, with the ratio between the different water environments varying significantly for different feldspar samples. Fluid inclusions in quartz were also observed as a ~H resonance at 4.7 ppm. The water molecules in nepheline show 2 distinct proton environments with peaks at 4.6 and 3.2 ppm. The clusters of 4 O H replacing SiO4- in garnets produced only a relatively broad line at MAS speeds
NMR of Other Spin -~/2 Nuclei
543
of 7.7 kHz (Yesinowski et al. 1988). In the 3 nominally anhydrous minerals clinopyroxene, enstatite and forsterite, fluid-like water inclusions are immediately apparent from the peak at 4.8 ppm (Kohn 1996) (Figure 9.3). The peak at 1.5 ppm in this spectrum typically arises from organic contamination and is common in such relatively weak spectra. These spectra of nominally anhydrous minerals contain at least 2 peaks arising from structural protons; the broader peak typically at 4 ppm is associated with clustered structural hydroxyls and the narrower peak typically at higher shifts arises from non-clustered hydroxyls (e.g. point defects). Peaks with particularly large shifts (5.9 and 7.9 ppm) in the spectrum of enstatite suggest the presence of sites with moderately strong hydrogen-bonding. The solubility of water in these minerals was estimated by Kohn (1996) by examining the NMR data in the light of several different partitioning schemes. The results indicate water solubility levels in enstatite down to 0.024 wt %. Hydrogen can be forced at high pressure into the structure of silica-germania glasses. It has been proposed that the photosensitivity of these materials is related to the formation of Si-OH, so that in defect-free glasses the photoactivity depends on the presence of hydrogen. The 1H NMR characteristics of the different hydrogen centres
clinopyroxene
clinopyroxene
f~
clinopyroxe .
.
cnstatite (x4).i..,.. 30 Figure 9.3.
.
.
.
J/ ]/ ,~3 ~ ' ~
10 -10 1H shift (ppm) w.r.t. TMS
1H MAS NMR spectra of the nominally anhydrous minerals enstatite, forsterite and three synthetic clinopyroxenes. Note the sharp resonance at 4.8 ppm from fluid-like water inclusions. The peak at about 1.5 ppm is attributed to organic contamination. From Kohn (1996), by permission of the Mineralogical Society of America.
544
Multinuclear Solid-State N M R o f lnorganic Materials
have been determined (Zeng et al. 1999a). These include H20 (4.8 ppm), SiOH (2.7 ppm), GeOH (3.3 ppm), Sill (4.5 ppm) and GeH (6.7 ppm). The NMR data were used to calibrate the infra-red spectra of these materials. Proton spectra have proved informative for studying the hydration kinetics of important cement phases (Rassem 1993) and distinguishing the different hydrates. The 1H MAS NMR spectra of polycrystalline paratungstates have been used to characterise the non-acid protons, and were able to distinguish the OH and water molecules (Fait et al. 1999). Incorporation of hydrogen into materials can have a profound effect on their optical and electrical properties, and is thus of considerable technological importance. The influence of the hydrogen on these properties is intimately associated with its distribution in the material. The ~H NMR spectra of materials such as amorphous hydrogenated silicon contain both broad and narrow components. A detailed understanding of the exact distribution of these components is of great importance. One of the most elegant approaches to solving this problem has been the application of multiple quantum NMR to the protons to determine the sizes of the clusters (Baum et al. 1986). This study concluded that in device-quality materials (and 1 sample not of device-quality) clusters of 6 hydrogens are formed, the clusters becoming physically closer as the hydrogen concentration is increased. This behaviour is in contrast to a polymeric sample in which a uniform proton distribution was observed. By modelling the hydrogen distribution it was concluded that the transition to a device-quality material occurs when the separation of the clusters matches that of the dilute monohydride group (Baum et al. 1986). Details of the growth of the 1H MQ coherence on the spatial distribution have been elucidated by comparing results from various phases where this distribution is well known, including calcium hydride, sodium bicarbonate and adamantane (Levy and Gleason 1992). The effect of the processing temperature on the detailed hydrogen distribution was investigated by the same methods, indicating that large hydrogen clusters tend to form in amorphous films at lower processing temperatures (Gleason et al. 1987). A strong relationship exists between the properties of the materials and the details of the hydrogen structure, rather than simply the hydrogen content. In porous silicon the hydrogen content has been determined by ~H NMR (Chang et al. 1996). The detection of low proton contents in materials demands careful probe design. A detailed description has been given of a probe from which polymeric materials have been eliminated and which is purged with dry nitrogen to remove ambient moisture (Levy and Gleason 1993). Techniques for cleaning the copper probe components to remove proton-containing contaminants are also described, and it is demonstrated that even at low proton contents the response of the system is highly linear, allowing accurate quantification. Proton studies using MQ techniques have been made to determine the proton distribution in systems such as hydrogenated amorphous silica films (Levy and Gleason 1993a) and silicon carbide (Petrich et al. 1987). Hydrogen doping of carbon-based
NMR of Other Spin -1/2 Nuclei
545
materials is also of considerable technological interest. ~H studies have included amorphous hydrogenated carbon (J~iger et al. 1994) and a series of studies on diamond (Levy and Gleason 1992, Mitra and Gleason 1993, McNamara et al. 1992, McNamara and Gleason 1994). Optimisation of the processing conditions for these materials has been greatly facilitated by the ability of ~H NMR to probe in detail the siting of the protons during the various stages of synthesis.
9.2.1.4 1H N M R of hydrous glasses. The effect of water in silicate and aluminosilicate glasses and the identification and quantification of the hydrous species present is a subject of long-standing interest to many fields, as far back as 1965 (Mtiller-Warmuth et al. 1965). Most NMR work has looked at glasses with comparatively high water content (> 10 mol%) but a recent study (Storek et al. 2000) of 3 alkali calcium silicate glasses containing 0.03 mols per litre showed 3 1H resonances at 6.3, 12.5 and 15.6 ppm. The relative population of the differently hydrogen-bonded sites changed with the nature of the alkali cation. The static 1H spectra of 2 essentially silicate glasses with low aluminium content, but with much higher water contents, revealed the presence of both hydroxyl and water, leading to the conclusion that significant alkali ion hydration produces mainly H20, with only a limited number of hydroxyls formed in these glasses (Bartholomew and Schreurs 1980). Another static 1H NMR study of hydrous glass has been made by Bray and Holubka (1984). The H20/OH ratios in a series of albite and orthoclase glasses were determined by fitting the spinning sidebands using model crystalline compounds as a template for the sideband intensity distribution (Figure 9.4) (Eckert et al. 1988). A solid-echo pulse sequence was used to overcome the effects of deadtime but care had to be taken to correct for differences in T2between the different species. Correcting for relaxation effects produced closer agreement between NMR and IR data but this agreement was not perfect. The residual broadening in glasses under MAS has been largely attributed to chemical shift dispersion. More recent work (Riemer et al. 2000) has concentrated on hydrous albite glasses using static 1H NMR to determine the HzO/OHratio. This study illustrates the need to use a probe (i) with a low proton background, (ii) which can generate hard rf pulses, and most crucially (iii) with very rapid recovery (-< 2 Ixs). This allows direct 1 pulse spectra to be acquired using short pulses without the use of echoes, thus eliminating the question of the differential T2 associated with echo techniques. It has also been pointed out that careful simulation of both components is necessary; in particular the Pake pattern from the water molecules requires correct simulation of the CSA. Furthermore, the spectrum in this case could only be simulated accurately using static interactions if the sample was cooled to 140 K. Multiple field studies on the hydrous glasses indicate the CSA for H20 ill these materials to be --~30 ppm. The HzO/OHratio has also been calculated for SiO2 glass with H20 contents varying from 0.12 to 8.7 wt % from their MAS spectra acquired using 1-pulse sequences with
546
Multinuclear Solid-State NMR of lnorganic Materials
t
_
~
w[:Oii~20
_
~" 21)0 '
0
' -21)0
~H shift (ppm) w.r.t. TMS Figure 9.4. 1H MAS NMR spectra of glasses with different water contents. Upper and middle spectra are of orthoclase glass, lower spectum is of anorthite-silica-wollastonite glass. Note the change in the intensity of the sidebands which is used to determine the H20/OH ratio. From Eckert et al. (1988), by permission of the American Chemical Society. relatively short deadtime delays (Figure 9.5) (Kohn et al. 1989). At low concentrations the 2 contributions can be simulated using 2 Gaussian peaks, 1 for H20 and 1 for OH. Initially the dissolution principally produces hydroxyls but at higher water contents more H20 begins to appear, as predicted by most dissolution models. In the 1H NMR spectrum of albite glass the 2 peaks corresponding to OH and H20 could not be distinguished in the centreband at 3.5-3.8 ppm, but the intensity distribution of the spinning sidebands indicated the presence of the 2 species. In SiO2 containing the highest concentration of water, 2 very narrow resonances, with an intensity ratio of 2:1, were attributed to an extremely well defined O H - : H 2 0 geometry, in the absence of a motional explanation capable of explaining the narrowness of the resonance and the lack of a narrow line in the static spectra. Two resonances have been observed in the depolymerised hydrous silicate glasses Na2Si2Os, BaSi205 and SrSi2Os. The resonances in the spectra of the sodium and barium are at --~ 4 ppm (arising from H20) and at 12 ppm (from a quite strongly hydrogen-bonded O H - group). In the strontium disilicate glass the hydroxyl was even more strongly hydrogen-bonded, as reflected by the resonance at 17 ppm, probably resulting from a Si-O- Si-OH interaction (Kohn et al. 1989).
N M R of Other Spin -1/2 Nuclei
A
547
wt% H 2 0
0.12
j
11
O
j
a
simulated
b 8.7
50
o
-50
~H shift (ppm) w.r.t. TMS
;o
1H shift (ppm) w.r.t. TMS
Figure 9.5. 1H MAS NMR spectra of hydrous silica glasses with different water contents. D is the simulation of spectrum C showing 4 separate proton resonances. Fitted peaks a and c are thought to be due to Si-OH groups in 2 different environments while peaks b and d are due to molecular water in 2 different environments. From Kohn et al. (1989), by permission of the copyright owner.
Other 1H NMR studies of glasses include those by Kummerlen et al. (1992) and Maekawa et al. (1998) employing increasingly sophisticated techniques for 1H observation. Kummerlen et al. (1992) applied CRAMPS to a hydrous Na2Si409 glass and resolved 2 lines separated by 7.5 ppm, taken to indicate quite strong hydrogen bonding at 1 of the sites. CRAMPS has the advantage that background signals from the probe are largely eliminated, and it removes broad sideband manifolds arising from the dipolar interaction. However, problems with the frequency offset dependence of CRAMPS data mean that although the sidebands are eliminated the quantitative certainty of the data is still questionable. It has also been shown that the CRAMPS linewidth is determined by the chemical shift dispersion. Simple MAS of a NazSi409 glass with 0.7 H20 per formula unit gave 2 well resolved signals at 4.5 and 13 ppm corresponding to molecular water and OH respectively (Schaller and Sebald 1995). This is in distinct contrast to aluminosilicate glasses in which resolution of the different sites cannot normally be achieved. The lines were also strongly asymmetric, probably due to the range of environments present with differing degrees of hydrogen-bonding. A non-rotor synchronised 2D ~H EXSY experiment was carried out to investigate
548
Multinuclear Solid-State NMR of Inorganic Materials
exchange between the different proton sites. Suppressing the homonuclear dipolar coupling removed the cross peaks, indicating that the exchange is a spin diffusion process resulting from intermolecular dipolar coupling. Common T1 proton relaxation and spin diffusion data provided an insight into the spatial arrangement and strongly suggest mixing of H20 and OH within the glass. Another 2D CRAMPS-MAS sequence was able to separate completely the sideband manifolds from the different sites (Schaller and Sebald 1995). The data (Figure 9.6) show the slices from the complete 2D data set, clearly revealing the different sideband distributions associated with OH and nonrigid water. 1H MAS, CRAMPS and static echo spectroscopy were employed to reveal relatively immobile SiOH and water molecules in several binary and ternary silicate electrode glasses (Herzog et al. 1994). This study showed systematic variations in the determinations of the OH/H20 ratio by the 3 NMR approaches. More recent work by Reimer et al. (2000) has revealed some of the reasons for these variations (see above). Zeng et al. (1999) employed IH-ZYA1 and IH-Z3Na TRAPDOR in combination with 1H MAS to investigate the proton distribution in 4 hydrous aluminosilicate glasses. Peaks at --~6 and 2.9 ppm correspond to 2 different water environments while others at 5-6, 3.5 and 1.5 ppm arise from 3 different hydroxyls. Rapid exchange occurs at room temperature between the 2 water environments. The data reveal the complexity of the proton speciation in these materials. All the resonances show a TRAPDOR effect to some extent. One of the groups identified is A1Q~3)-OH, implying that the aluminosilicate framework is depolymerised by the water. This conclusion does not agree B
A
o
00
00 18.0
,
,
,
,
90
,
i
,
,
,
,
.'',
,
v
.
.
30
.
.
'-30 ppm
,
,
.
._~
_
_
"
-'90
120 0 -120 ~H shift (ppm) w.r.t. TMS
Figure 9.6. A. ~H CRAMPS-MAS correlation spectrum of hydrated sodium disilicate glass showing projections in both dimensions. B. Slices through the CRAMPS dimension of spectrum (A) showing the separate spectra from the H20 resonance at 4.0 ppm (upper) and the OH resonance at 14.0 ppm (lower). Note that the different sideband distributions from the 2 protonated groups are clearly distinguishable. From Schaller and Sebald (1995), by permission of the copyright owner.
NMR of Other Spin -lIe Nuclei
549
with the work of Kohn et al. (1989) and is difficult to rationalise in terms of recent 170 NMR data (see Chapter 6). 1H NMR studies of water dissolution continue to hold strong interest; very recent work combined 1D spectroscopy with multiple quantum double resonance and 2D heteronuclear correlation experiments on dry and hydrous Na20.4SiO2 and the aluminosilicate phonolite (Robert et al. 2001). The contrast in resolution between proton environments in the pure silicate and aluminosilicate glasses was marked, 2 peaks being clearly observed in the silicate but only 1 peak in the aluminosilicate. Dipolar dephasing was used to improve the resolution by selectively dephasing some components of the line, allowing at least three distinct sites to be observed in phonolite with 1 site showing a strongly hydrogen-bonded environment. Since only double quantum coherence could be generated efficiently, significant proton clustering was ruled out. The position of the protons in relation to the other nuclei present was investigated by a combination of double resonance techniques including CP to 29Si, 1H-29SiHETCOR and {1H }-X REDOR to the other nuclei. Similar changes observed in the behaviour of the magnetisation accompanying hydration of sodium silicate and phonolite imply that both glasses depolymerise on hydration (Robert et al. 2001). It should be noted that both these glasses are already partially depolymerised differs from the findings for albite, which is initially fully polymerised. Interaction of water with inorganic materials at high pressure can result in sub-microscopic fluid water inclusions. In mineralogy it is important to know the pressure of the water to be able to determine its equation of state. If it is assumed that the molar magnetic susceptibility is a constant, the susceptibility is a function of the density (P) and hence the isotropic 1H chemical shift of the fluid in the inclusions can be used as an extremely accurate measure of its density (Withers et al. 2000). The relationship between these 2 parameters has been determined as p = 0.4921~iso - 1.340
(9.3)
This approach has greatly extended the pressure range that can be determined in fluid inclusions. Similar studies can be carried out on phosphate glasses. The presence of hydroxyls in ultraphosphate glasses strongly affect the thermal and optical properties and the high reactivity of the branching phosphate groups indicates that hydroxyls are nearly always present. Comparison of the spectrum of a zinc ultraphosphate glass with that of hydrated P205 showed that a resonance at 13 ppm is associated with QZ(H)-QZ(H) groups, with a more strongly hydrogen-bonded environment at 17 ppm due to QZ(H)-QZ(Zn) (Mercier et al. 1998). A similar assignment was made for lead-barium ultraphosphate glasses (Hosono et al. 1992). Glasses prepared by sol-gel methods may be regarded as examples of hydrous glasses since they can retain significant proton levels when the gel is dried and vitrified.
550
Multinuclear Solid-State NMR of Inorganic Materials
The intensities of the hydrogen-bonded and silanol groups can be monitored, providing insight into the dehydration process of the gel (Yang and Woo 1996). Since 1H NMR can be made quantitative it is very useful for determining the absolute proton content of sol-gel materials, providing important information needed for the analysis of scattering data (especially of neutrons) from such gels. 9.2.1.5 Biomineral-related materials. ~H NMR has been applied to studies of biomineralisation processes in the calcium phosphates apatite and fluoroapatite using crystalline calcium phosphates as standards. Several 1H peaks observed as fluorine was added to the system were attributed to changes in the hydrogen-bonding at different sites resulting from the presence of fluorine. The change in intensity of the peaks suggests that the O H - / F - mix is statistical rather than one which maximises the hydrogen-bonding between the different species (Yesinowski and Eckert 1987). In some of these phases a resonance from a surface-adsorbed water layer was clearly observed. It is possible for the motion in molecular water to cause significant averaging. Proton exchange can also average the IH-IH dipolar coupling but this is not sufficient to average the CSA; to accomplish this, the motion must change the orientation of the CSA tensor. 1H CRAMPS has also been used to examine model biomineral materials and some bone minerals (Santos et al. 1994). The ~H spectrum of actual bone mineral can be obscured by the contributions from the organic content of the attached collagen. To remove this unwanted part of the spectrum the collagen itself could be removed chemically, but this may change the nature of the sample. Alternatively IH-31p HETCOR can be used, in which the relatively large chemical shift range of the 31p provides a means of separating those ~H signals associated with the phosphate and therefore arising from the bone mineral phase.
9.2.2 19F NMR 9.2.2.1 Introduction. Two extremely comprehensive review articles summarise high resolution solid state 19F NMR, the first covering work up to 1990 (Harris and Jackson 1991) and the second extending this to 1996 (Miller 1996). 19F MAS studies can be classified according to the spinning speed below which only relatively dilute species can be observed; Harris and Jackson take this threshold MAS speed to be 7 kHz. The effect of MAS speed on 19F NMR spectra is illustrated by work on mixtures of fluorohydroxyapatite (FAP) and CaF2 (Kreinbrink et al. 1990). The effect of MAS on the linewidths differs greatly between dilute systems such as FAP and the more concentrated fluorides, the response being quite flat for FAP but decreasing much more steeply with increasing spinning speed in the simple inorganic fluorides (Figure 9.7A). Thus, at 15 kHz the residual linewidth of FAP is < 500 Hz, by comparison with linewidths of--~ 1.5 kHz for NaF and CaF2. Below 7 kHz only the relatively spatially
551
NMR of Other Spin -//2 Nuclei
B
A
,~, 4000 N ,I~ 3000 2000 ell ~ 1000 .p,,~
2.0
~
CaF2 FAP ~
MAS (kHz) $ 15.5
_
13.5
~
CaF2 i ~ ~
~ ~"~"~e.~
NaF
_ ~
FAP O ~ I ~ O - - - - - - O ~ 9 -O-O-O 6.0 10.0 14.0 Spinning speed (kHz) -
~
-
~
,
~
9
I
______~.~~~_~
_ 9"3 7.2
4.9
150 50 -5O 19F shift (ppm) w.r.t. C6F6
Figure 9.7. A. Effect of MAS spinning speed on the 19Fspectral linewidth of the spatially dilute compound fluorohydroxyapatite (FAP) and the more concentrated fluorides NaF and CaF2. B. 19F MAS NMR spectra of a binary FAP and CaF2 mixture showing acceptable resolution of the 2 phases at spinning speeds which are sufficiently fast to narrow the CaF2 signal. From Kreinbrink et al. (1990), by permission of the copyright owner. dilute FAP phase can be observed but as the speed increases the CaF2 signal narrows until spinning at > 15 kHz produces an acceptably quantitative spectrum reflecting the phase distribution (Figure 9.7B). The static linewidths of various simple fluorides are typically in the range 15-48 kHz (Aujla et al. 1987). Since fluorine is a nucleus for which various shift references have been used, care must be taken when comparing results from different workers. The standard reference is CFC13, the resonance of which is taken as 0 ppm, but other commonly used references are PTFE, C6F6 and 1 M NaF aqueous solution which have resonances at - 123.2 ppm, - 163.0 ppm and - 120 ppm respectively compared with CFC13. Spectra are usually readily obtained from fluorine-containing species but it may often be difficult to make completely unambiguous assignments of the spectra (Miller 1996).
9.2.2.2 Simple inorganic fluorides. There have been a number of reports of 19F MAS NMR of alkali metal fluorides (see Table 9.2). The analysis of these 19F shifts by Hayashi and Hayamizu (1990) revealed no simple relationship apart from a general trend downfield as the radius of the counterion increased. The advent of faster spinning has provided good quality high resolution NMR spectra from a wider range of inorganic fluorides, although Kreinbrink et al. (1990) found a wide variation in linewidths, suggesting the possibility that some of the broader observed resonances could be narrowed still further as the available spinning speed increases. In CaF2, which has
552
Multinuclear
Solid-State NMR
of Inorganic Materials
Table 9.2. Characteristic 19F shifts in some simple inorganic compounds. Compound LiF NaF
~iso(ppm)* 204 22 l, - 224 -
-
KF
- 130, - 130.2, - 123
RbF
-
CsF
-
88, - 9 0 80, - 79
KF.2H20 RbF.H20 CsF.2H20 KF-A1203 KF-SiO2 RbF-AI203 RbF-SiO2 CsF-CaF2 CsF-AI203 RbF-mont** CsF-mont** CaF2
- 133 - 113 - 97 - 159, - 115 - 129 - 109 - 122 - 79 - 116, - 88 - 123 - 113 - 104.8, - 107.7, - 107
SrF2 BaF2 CdF2 Hg2F2 HgF2 SnF2 SnF4 ot-PbF2
- 84.1 - 13 - 192.1 - 95.8 - 196.4 - 110.4 - 146.9 - 20.5, - 57.7 ( - 39.0)
CuF2-SiO2 ZnF2-SiO2 CdF2-SiO2 LaF3 A1F3
- 149 - 124 - 124 - 23.5, 17.1, 24.9 - 174, - 172
Reference Hayashi & Hayamizu (1990) Hayashi & Hayamizu (1990), Schaller et al. (1992) Hayashi & Hayamizu (1990), Kreinbrink et al. (1990), Clark et al. (1986) Hayashi & Hayamizu (1990), Clark et al. (1986) Hayashi & Hayamizu (1990), Clark et al. (1986) Schaller et al. (1992) Clark et al. (1986) Clark et al. (1986) Clark et al. 1986, Duke et al. (1990) Duke et al. (1990a) Duke et al. (1990) Asseid et al. (1990) Clark et al. (1986) Clark et al. 1986, Duke et al. (1990) Asseid et al. (1990) Asseid et al. (1990) Hayashi & Hayamizu (1990), Chan & Eckert (2001) Kreinbrink et al. (1990) C h a n & Eckert (2001) Kreinbrink et al. (1990) Kreinbrink et al. (1990) Kreinbrink et al. (1990) Kreinbrink et al. (1990) Kreinbrink et al. (1990) Wang & Grey (1995) (Sites F1, F2 and the exchange peak) Asseid et al. (1992) Asseid et al. (1992) Asseid et al. (1992) Wang & Grey (1997) Schaller et al. (1992), Chan & Eckert (2001)
* chemical shifts quoted with respect to CFCI3 ** adsorbed on to montmorillonite.
b e e n m u c h s t u d i e d b e c a u s e o f its i m p o r t a n c e in d e n t a l s c i e n c e , a s i g n i f i c a n t p a r t o f t h e l i n e w i d t h is d u e to h o m o n u c l e a r CRAMPS
dipolar coupling, necessitating fast spinning or
( S m i t h a n d B u r u m 1989).
19F N M R
h a s b e e n u s e d to s t u d y t h e i o n i c m o t i o n in e~-PbF2 in w h i c h t h e 2
c r y s t a l l o g r a p h i c a l l y i n e q u i v a l e n t f l u o r i n e sites h a v e shifts o f - 20.5 a n d - 5 7 . 7 p p m
553
NMR of Other Spin -1/2 Nuclei
corresponding to FPb4 and FPb5 coordinations respectively (Figure 9.8A) (Wang and Grey 1995). Bondlength data suggest that FPb4, with its shorter average bondlength, should show a larger 2~ J-coupling to the Pb. Lead decoupling removes the fine structure (Figure 9.8B). A peak with a shift of - 39.0 ppm, exactly half way between the above 2, is due to jumping of mobile fluorine atoms between the 2 possible sites. The data clearly show the existence of 2 populations of fluorine atoms in intermediate and slow motion regimes. The tysonite structure of LaF3 has 3 distinct fluorine sites, providing a mechanism for good conductivity via fluorine motion. Static 19F studies have demonstrated significant motion on the F1 sublattice at room temperature, with exchange occurring between the F1 and F2/F3 sublattices above - 247~ (Denecke et al. 1992). Doping with strontium lowers the temperature at which ionic mobility becomes significant. More recently fast MAS (23 kHz) has been applied to LaF3 and a sample doped with 1% strontium (Wang and Grey 1997). The improved resolution of the MAS spectrum allowed the different sites to be clearly resolved and the mobility of each site to be determined. The activation energies for fluoride hopping along each of the distinct pathways were shown to be very different, increasing in the order Fll < F13 < F12 (Wang and Grey 1997). High resolution 19F NMR spectra of samples from the solid solution Cal-xYxF2+• (0.03 --< x <_ 0.32) with a fluorine excess have been acquired by spinning faster than 20 kHz. These 19F spectra show 4 resonances whose intensities vary with x (Figure 9.9) and correspond to a normal lattice site, a slightly relaxed normal site and 2 distinct interstitial sites (Wang and Grey 1998). The spectra are comparable with that of the
A
B
l 25
-25
-75
rl' ;'' I'"''' I''' 25 -25
'1 . . . .
I'f'''l -75
'
19F shift ( p p m ) w.r.t. CFCI3 Figure 9.8. A. 19F MAS NMR spectrum of e~-PbF2acquired at an MAS speed of 22.5 kHz showing the well-resolved FPb4 and FPb5 sites at - 20.5 ppm and - 57.7 ppm. The broader resonance at - 39 ppm represents mobile fluorine jumping between these 2 sites. B. The same 19F MAS NMR spectrum with 2~ decoupling to remove the fine structure. Asterisks denote spinning side bands. From Wang and Grey (1995), by permission of the American Chemical Society.
554
Multinuclear Solid-State NMR of Inorganic Materials
CaF 2
J
-110
/
~
,
~
composition X
.
composition x $
0.03 -112 -95 / \ S -81 -70 ~_~ ~ -1O0 !
0
|
[
-200
20
0.0 0.10 |
9
#
o:.
~ ~ ~ 0 . 3 !
!
i
0
i
|
I
2 I
v
v
-200
19F shift (ppm) w.r.t. CFCI 3 Figure 9.9. 19FMAS NMR spectra of CaF2 and a selection of solid solutions Cal-xYxF2+xwith values of x as indicated. The resonance at - 112 ppm is from the normal fluorite site, that at - 95 ppm is from a slightly relaxed normal site while the 2 low-intensity features at - 7 0 and - 81 ppm arise from interstitial sites. The asterisks denote spinning side bands. From Wang and Grey (1998), by permission of the American Chemical Society.
mineral tveitite (x ~ 0.25-0.27) in which the fluoride ions are located in clusters. The anions in these clusters are mobile, with exchange occurring between the relaxed and normal sites as the doping level and temperature are increased. BaFBr is an important storage phosphor material which forms solid solutions and gives rise to nonstoichiometric compositions such as (BaF)l.lBro.9. The principal 19F MAS N M R resonance in BaFBr occurs at 150.9 ppm but the non-stoichiometric sample shows an additional line at 145.3 ppm with an intensity of 9%. The similarity of this intensity to the 10% excess fluorine in the sample led to the assignment of this resonance to fluorine on the antisites (the bromine sites) (Schweizer et al. 1998).
9.2.2.3 More complex fluorides. The sodium aluminium fluorides cryolite (Na3A1F6) and chiolite (NasA13F~4) are both industrially important materials, playing a fluxing role in the industrial electrochemical production of aluminium. The static 19F N M R spectra of these compounds are quite complex since significant homo- and heteronuclear dipolar coupling occurs in these systems. The MAS 19F spectrum shows 3 distinct fluorine resonances due to M - F - M , A1-F-Na4 and A1-F-Na3 (Zeng and Stebbins 2000). Significant averaging of the cryolite spectral lineshape occurs when the compound is heated from room temperature to 150~ but heating produces relatively little change in the
555
N M R o f Other Spin -1/2 N u c l e i
Table 9.3.
19F
chemical shifts of complex fluorides.
Compound
~iso(ppm)*
Reference
KA1F4 K3AIF6 K2SiF6
- 155 - 155 - 92,
Asseid et al. (1990) Asseid et al. (1990) Asseid et al. (1990), Schaller et al. (1992) Asseid et al. (1990) Schaller et al. (1992), Zeng & Stebbins (2000) Schaller et al. (1992) Schaller et al. (1992)
135
-
Cs2SiF6
- 92
Na3AIF6
- 189, - 191
K3A1F6
- 190
Alz(F,OH)2SiO4
- 140
(topaz) NazSiF6
NasA13F14 Sn3F3PO4 Sn2FPO4
-
- 166, -
-
152, 135, 151, 151 182, - 190 103 114.8
Hayashi & Hayamizu (1990), Schaller et al. (1992), Burum et al. (1978), Zeng & Stebbins (2000) Zeng & Stebbins (2000) White et al. (1995) White et al. (1995)
* chemical shifts quoted with respect to CFC13
spectrum of chiolite (Spearing et al. 1994). This difference in behaviour is attributed to the much stronger A1-F-A1 linkages in chiolite preventing large frequency oscillations on heating to 150~
9.2.2.4 A p p l i c a t i o n s to f l u o r o a p a t i t e studies. The particular importance of fluoroapatite in dental applications and also as a matrix material for fluorescent lamp phosphors has led to detailed 19F NMR studies. The dilute nature of the fluorine in this compound allows good quality 19F NMR spectra to be obtained under relatively slow MAS conditions, from which the composition of the fluorohydroxyapatite can be deduced. Such experiments have shown that exposing hydroxyapatite to 9.7 mM fluoride solutions forms surface layers of Cas(PO4)3(OH)I-xFx (x = 0.4-0.8) with the eventual formation of bulk fluoroapatite as the aging time increases. As the concentration of the fluoride ions in the aqueous solution increases, CaF2 tends to form as well (Kreinbrink et al. 1990). The 19F NMR shift of fluoroapatite has variously been reported to occur in the range - 9 8 . 9 to - 101 ppm. Doping with Sb 3+ has only a small effect (1.5 ppm) on the shift, and a similar difference (--~2 ppm) has also been reported between fluoroapatite and fluorohydroxyapatite (Kreinbrink et al. 1990). Braun et al. (1995) made a 19F MAS NMR study of fluorinated calcium apatites used as glass-ceramic bone implant materials, and conclude that fluoroapatite is a major constituent of these compounds. This work investigated a range of compositions of the solid solution Calo(PO4)6F2x(OH)2-2• where the fluorine content varied from x = 0.065 to 0.45. All
556
Multinuclear Solid-State NMR of lnorganic Materials
the CSA tensor elements showed a systematic variation (Braun and Jana 1995) with 8is o changing from 59.8 to 63.2 ppm, AS from 41 to 67 ppm and ~ from 0.7 to 0.2 (using the Haeberlen (1976) definition of the tensor elements). Glass ceramics containing fluorine are often used as components of medical implants, reacting with body fluids to form fluorine-containing apatite. 19F MAS NMR has been used to determine the fluorine content of the resulting apatite and other fluorine-containing phases including synthetic fluorphlogopite (8iso = - 9.5 _+ 0.2 ppm) (Jana and Braun 1996). SnF also occurs in dental materials but no SnF compounds were detected by 19F MAS NMR after hydroxyapatite was treated with SnF (White et al. 1995). Doping of calcium fluoroapatite by an Sb 3+ activator perturbs the 19F MAS NMR spectrum, producing, in addition to the major resonance at - 99 ppm, a shoulder at - 97.4 ppm and a sharp peak at - 94.4 ppm. A pulse sequence SPARTAN (Selective Population Anti-z and Rate Transfer to Adjacent Nuclei) has been developed to measure spin diffusion by selectively inverting one of the peaks. Inversion of the additional sharp peak led to cross-relaxation to the main fluoroapatite peak but not to the shoulder, indicating that antimony is perturbing the fluoride ions in 2 different chains. An additional 19F resonance at - 89.9 ppm found in some samples was assigned to a different substitutional site in which only 1 fluoride is perturbed by each antimony atom. The site distribution determined by 19F NMR is consistent with the replacement by SbO33- of a PO4 3 - group (Moran et al. 1992). The 2 substitutions differ depending on which oxygen in the nearest neighbour environment is replaced by the lone pair on the antimony. The fluorine atoms in these compounds occur in parallel linear chains, with calculated values of 5234 and 262 Hz for the maximum homonuclear (F-F) dipolar couplings of the intra- and inter-chain fluorines respectively. The largest P-F dipolar coupling was --- 2200 Hz, and the P-P coupling is ---640 Hz. More recently 31p-19F REDOR has been used to examine the connectivity of the different atoms via the dipolar coupling (Pan 1995). The method was used to compare the effect of fluoride treatment of hydroxyapatite with fluoroapatite itself. The results suggest that a layer of fluoroapatite less than 1 unit cell thick is formed on the hydroxyapatite and illustrate the potential of this approach for more detailed modelling of the fluorine distribution in these materials. 9.2.2.5 Fluorine in aluminosilicate minerals and related materials. Quite slow 19F MAS NMR has been used to examine the different fluorine sites in tremolite and fluoroscandium pargasite (Harris and Jackson 1991). Pargasite displays more sidebands and a broader MAS linewidth than tremolite, suggesting greater disorder in the former. Details of the sideband structure indicate 2 fluorine sites with different MgMgMg and MgMgSc nearest neighbours. Huve et al. (1992, 1992a) have studied natural and synthetic fluorinated 2:1 layer lattice minerals by 19F MAS NMR, showing that the 19F shifts are sensitive to both the chemical nature of the octahedral elements
NMR of Other Spin -1/2 Nuclei
557
and the type of octahedral sheet, allowing dioctahedral and trioctahedral structures to be distinguished. A number of peaks is observed in such minerals, typically at - 132 ppm (corresponding to dioctahedral sheets containing mainly A1 atoms), - 152 ppm (dioctahedral sheets containing A1 and Mg atoms), - 176 ppm (trioctahedral sheets containing mainly magnesium atoms) and - 182 ppm (trioctahedral sheets containing Mg and Li). There is, however, some dispute regarding these assignments since the shifts remain unchanged when the layer structure is broken down by heating. The major 19F NMR peak at - 100.9 ppm in the spectrum of a 2:1 dioctahedral layered gallium germanate has been attributed to fluorine in the germanate layer (Huve et al. 1992). Two significant 19F resonances have been reported at - 146.6 and - 169.1 ppm with a weaker line at - 186 ppm in the spectrum of a zinc/titanium substituted layer silicate. None of these peaks coincides with those previously observed in hectorites and other layered silicates. Comparison with previous work and the use of electronegativity arguments have led to the suggestion that these peaks could be due to fluorine in octahedral sheets with differing metals arising from the substitution of Zn for Mg (Luca et al. 1995). A relationship was derived between ~iso(ppm) and the group electronegativities (GEN) in such materials 6iso = 51.1(GEN)- 329
(9.4)
where GEN is the group electronegativity of the units directly attached to fluorine. Miller (1996) has given a detailed analysis of the implications of equation 9.4 for the 19F shifts observed in such materials. The 19F MAS NMR spectra reported for a wide range of clay minerals (Labouriau et al. 1995) seem to indicate that their 19F shifts are characteristic of the occupancy of the octahedral sites to which the fluorine is directly attached. A structural implication of this work is that fluorine substitution for hydroxyl groups does not occur randomly, but that the fluorine shows a preference for sites associated with magnesium rather than aluminium.
9.2.2.6 Surface interaction of fluorine with silica- and alumina-based materials. Interaction of fluorine with aluminium usually occurs where metal fluorides are supported on materials such as alumina, clays and zeolite-related materials. New fluorinecontaining species form on heating these systems. Adsorption of alkali metal fluorides on to alumina (Duke et al. 1990) produces relatively sharp 19F NMR peaks at - 115 ppm - 109 ppm and - 88 ppm attributed to KF, RbF and CsF respectively (although the latter has been suggested to arise from a mobile free F - ion) (Miller 1996). Broader peaks were also observed at - 156 ppm, - 139 ppm and - 111 ppm in systems containing KF, RbF and CsF respectively. On drying at 773 K these become the major 19F peaks and have been attributed to A1F63- species. Miller (1996) has reviewed in detail the literature up to 1996 concerning the interactions of other fluorides with silica and
558
Multinuclear Solid-State NMR of Inorganic Materials
alumina. The systems studied by 19F NMR include NH4F-A1203, NH4F-SiO2, NaF-SiO2, NaBF4-SiO2, and KF-SiO2. Dissolution of bayerite ([3-Al(OH)3) in a medium containing F - results in the appearance of 2 fluorine resonances in the 19F NMR spectrum. The signal at -131 ppm is due to fluoride bridges and that at - 1 4 2 ppm is from fluorine in terminal sites. An additional signal has been attributed to aqueous fluorinecontaining complexes in the pores (Nordin et al. 1999). When alumina is placed in contact with a fluoride-containing solution, the surface A1 interacts with F - by the displacement of hydroxyls by fluorine, leading eventually to the breakdown of the A1-O-A1 bridges. As fluorine enters the nearest neighbour coordination sphere, several types of octahedral unit can be identified by their 19F NMR peaks; A1OsF ( - 1 5 4 ppm), A104F2 ( - 1 4 3 ppm) and A103F3 ( - 1 3 0 ppm) (Fischer et al. 2000). At higher fluoride concentrations the remaining A1-O-A1 bonds anchoring the unit to the surface are sufficiently weakened that dispersed A1F3.nH20 forms. In studies of fluorinated zeolitic materials Caullet et al. (1995) reported 19F NMR signals in the region - 150 to - 175 ppm which they assigned to A1F species, and a signal at - 1 2 9 ppm attributed to an SiF species. An additional signal at - 3 8 ppm was assigned to fluoride ions trapped in the double 4-membered rings of octadecasil. Delmotte et al. (1990) assigned signals at - 1 5 1 to - 1 5 6 ppm in related systems to Si-F bonds. Frron et al. (1994) found a 19F NMR signal at - 1 7 9 ppm in a sodalite material and attributed it to a hydrated fluoride ion localised in the sodalite cavities. Similar 19F NMR results have been obtained for alkali metal fluorides adsorbed on to montmorillonite clay (Asseid et al. 1990). On heating to > 1000~ bonds can form between fluorine and aluminium or silicon, but the position of the resulting 19F NMR peak ( - 1 2 9 ppm) suggests they are Si-F rather than A1-F bonds. When montmorillonite particles are coated by CdF2, ZnF2 or CuF2 and heated, the 19F NMR spectra show peaks both in the - 151 to - 153 ppm region, characteristic of mixed fluoroaluminate units, and at - 197 ppm, corresponding to CdF2. Delmotte et al. (1990) used 19F NMR to study crystalline microporous alumino- and gallosilicates synthesised in a fluoride-containing medium. The spectra contained a resonance at - 129 ppm, assigned to Si-F species. The spectra of samples prior to calcination showed a peak at - 155 ppm, as was also found in a calcined galliumcontaining sample. A signal observed in the as-synthesised materials at - 64 to - 75 ppm was attributed to ion-paired free fluoride. Adsorption of gas molecules can be used to characterise the active sites in zeolites. Xenon may be used for this purpose and monitored by 129XeNMR (see Section 9.3.7), but adsorption of CF4, monitored by 19F NMR, has been suggested as an alternative (Yang et al. 2001). Such measurements place strong constraints on the modelling of the molecular level adsorption.
NMR of Other Spin -~/2 Nuclei
559
9.2.2.7 F l u o r i n e in alumino- and gallophosphates. The effect of HF on the incorporation of silicon into some aluminophosphate molecular sieves has been studied (Lohse et al. 1994). 19F signals observed in these materials at - 142 ppm arise from interaction of fluorine with aluminium, while a signal at - 123 ppm arises from fluoride which acts as a counter-ion to protonated cyclohexylamine. Schott-Darie et al. (1994) used 19F MAS NMR to examine cloverite, a structure containing sodalite-like cages with aluminium substituted by gallium. A total of 8 lines were observed in the range 60 to - 178.2 ppm, 5 of these, between - 6 7 . 7 and - 9 5 . 5 ppm being assigned to F(GaxA14-x). Two separate additional resonances were observed at - 145.6 and - 178.2 ppm, the former assigned to impurity species of the type HO-A1-F and the latter corresponding closely to the resonance of fluorine atoms in sodalite-like pseudo-cages. Schott-Darie et al. (1994a) have found in microporous alumino- and gallophosphates prepared in the presence of fluoride 3 19F resonances with shifts of -67.4, - 9 9 . 2 and - 1 1 3 ppm. The signal at - 6 7 . 4 ppm was assigned to fluorine occluded in a double-four ring, the other 2 resonances being assigned to F atoms involved in Ga bridges.
9.2.2.8 F l u o r i n e in oxygen-containing glasses. The addition of even small amounts of fluorine to silicate and aluminosilicate melts can have significant effects on their melting points and viscosity. Hence, fluorine additions are very important in the petrology and technical applications of such glasses. Speciation in these melts can be examined by comparison of the 19F NMR spectra with those of well-defined fluorine environments in crystalline compounds. One of the first 19F studies of this problem showed the formation of Si-F bonds as fluorine was added to silicate glasses (Duncan et al. 1986). A combination of 19F MAS NMR, 19F CRAMPS, 19F ---->29Si CP, and high power 19F-decoupling has been used to study dry and hydrous nepheline, dakeite and albite glasses doped with fluorine (Schaller et al. 1992). The observed shifts obtained from direct 19F NMR measurements of the glasses indicated that fluorine was probably associated with 1 aluminium and several sodiums. From 19F---->27A1 CP experiments on NaF- and cryolite-doped anhydrous jadeite glasses, Kohn et al. (1991) detected the previously unknown species A1Fs. Fast MAS (25 kHz) has been used to examine the fluorine sites in a range of aluminosilicate glasses (Zeng and Stebbins 2000). The complex spectra indicated that the fluorine in aluminosilicates is predominantly coordinated to 1 aluminium and several cations, in agreement with Schaller et al. (1992), although some intensity was detected from Si-F bonding. In aluminiumfree silicate glasses some Si-F bonding was again detected but the dominant fluorine coordination is cation-only or "free" fluoride. Fast (--~ 25 kHz) 19F MAS has been used to study a series of metal oxide-silica and mixed cation silicate glasses containing 2 wt % fluorine. In systems containing a single cation, the 19F shift is similar to that
560
Multinuclear Solid-State NMR of Inorganic Materials
of the simple metal fluoride, with additional indications of Si-F bonding in the Nacontaining system. Systems containing mixed cations show partially resolved peaks arising from the local coordination of the different cations. A tendency was also found for preferential bonding to the higher-field-strength modifier cation (Stebbins and Zeng 2000). 19F MAS NMR has been applied to a study of barium aluminofluorophosphate glasses. The 19F resonances in these glasses ( - 9 0 to - 9 9 ppm) are intermediate between those of BaF2 and A1F3, shifting towards the value for BaF2 as the Ba/A1 ratio increases; this result was taken to indicate a lack of preferential metal coordination to the fluoride ions. As nitrogen was added to the glass the 19F chemical shifts became generally more positive, due either to the presence of the nearest-neighbour nitrogen or to a change in the F:(Ba/A1) ratio resulting from fluoride volatilisation (Fletcher et al. 1990). The 19F NMR spectrum of sodium fluorophosphate glasses is dominated by a peak at - 73 ppm from fluorine in terminal positions on the phosphate chains. When aluminium is added to the system a second peak observed at - 175 ppm indicates the formation of fluorine-aluminium bonds. The data indicate the formation of further F-P bonds on remelting the glasses with NHaHF2 (Brow et al. 1992). Polymeric glasses of the type BO1.3Fo.4 have been investigated by both I~B and 19F MAS NMR. The structure is made up of trigonal units (e.g. BFO2, BF20, etc.) giving rise to at least 4 different fluorine sites with 19F resonances in the range 24.2 to 55 ppm. A model based on a cross-linked spaghetti structure was consistent with the NMR data (Boussard-P16del 1997). Heating these glasses above Tg causes their structure to change as BF3 is evolved and becomes trapped in the glass. The 19F spectra show a very sharp line from the gas which can be used for its quantification (Le Floch et al. 1998).
9.2.2.9 Fluoride glasses. Fluoride glasses have many potential applications arising from their interesting infra-red and optical properties, and their electrical properties based on fluoride ion motion. Early static NMR work on ZrFa-BaF2-LaF3 glasses (Bray and Mulkem 1986) showed non-Gaussian lineshapes, similar to the spectrum of crystalline ZrF4. This suggested that short-range structure in the glasses tended towards specific environments and was therefore more ordered than in oxide-based glasses. This concept was supported by a later study (Aujla et al. 1987) which also extended the work into the quaternary system with NaF. Two components were observed in the 19F NMR spectra, but a further study of the ZrF4-BaF2-LaF3 glassy system simulated the static 19F NMR spectrum in terms of 3 components (MacFarlane et al. 1989) without determining the exact nature of the structural species associated with each component. One of these species which is immobile over the temperature range examined is probably more strongly associated with the Zr 4+ ions. The other 2 peaks in the spectrum appear to be associated with more mobile species since on heating at 150-420 K the lines become more Lorentzian in shape and the shift difference decreases from 180 to 2 ppm.
561
NMR of Other Spin -~/2 Nuclei
Different fluoride sublattices have also been revealed by 19F NMR of compounds in the ZrF4-BaFz-ThF4-LiF system in which a comparison was made of the 19F static NMR spectra of the alkali-free systems ZrF4-AIF3-BaFz-(LaF3 or YF3) with the oL- and [3-forms of crystalline BaZrzFlo. A structural model of the glasses based on the units in crystalline BaZr2Flo appears valid. Three fluorine species have been suggested, 1 involving internal bridging between the zirconium atoms of the basic ZrzF13 bipolyhedron, 1 involving Zr-F-Zr bridges between different bipolyhedra and 1 non-bridging configuration in which 1 of the bonds is to another metal such as Ba (Senegas et al. 1994). The 19F MAS NMR spectra of glasses in the PbFz-ZnFz-GaF2 system have been compared with the spectra of the initial constituents, crystallisation products and model crystalline compounds containing interesting network connectivities of fluoride octahedra. The glass spectra reveal 3 fluorine-containing species, namely, fluorine which is not connected to a metal, and fluorine associated with (ZnF6) 4- and (GaF6) 3- species. Quantitative analysis of the spectra indicates that the glass structure can be accurately modelled by corner-sharing of such octahedral units (Bureau et al. 1997). Fast MAS was combined with the double resonance techniques CP and REDOR to investigate the proximity of aluminium and fluorine in the A1F3-BaFz-CaF2 system (Chan and Eckert 2001). The 19F shifts in this system range from - 138 to - 173 ppm (Figure 9.10). The 27A1{19F} REDOR curves of A1F3 and the glasses were similar. The 19F NMR spectra contained a broad-intensity component, some of which could be
AIF3-BaF2-CaF2
,1,
left shift
48 - 12 - 40
,esh, ft t ___j 45- 19 -36 ~
left shift
41- 19-40 ,,,
B
] CaF2 I
1]
, ft h,ft
AIF 3
,
200
.
~
0
,
I
-200
'
19F shift (ppm) w.r.t. CFCI3
~
500
. . . .
J
0
-500
0
~9F shift (ppm) w.r.t. CFCI3
Figure 9.10. 19F NMR spectra of A1F3and 3 A1-Ba-Cafluoride glasses showing the effect of leftshifting the FID to remove some of the broadening. From Chan and Eckert (2001), by permission of Elsevier Science.
562
Multinuclear Solid-State NMR of Inorganic Materials
removed by left-shifting the data prior to Fourier transformation to reveal at least 3 components, 2 narrow and 1 broad (Figure 9.10). The data show unequivocally that the glasses are composed of A1F6 units but their chemical shift, determined from 27A1{19F } CP HETCOR experiments, is different from the shift of A1F3.19F MAS NMR spectra of glasses in the system YF3-A1F3-BaFz-CaF2 show signals from bridging and nonbridging fluorines. The ratio of these sites and the glass-forming ability of the system changes with chlorine-doping (Yano et al. 1997). 9.2.2.10 Fluorine in other materials. 19F-31pJ-resolved spectroscopy has been used to study the mobile PF6- species in KPF6. Normally linebroadening effects obscure the heteronuclear indirect J couplings since homonuclear direct coupling of isotopically abundant nuclei usually dominate such spectra. However, the J-resolved spectrum of KPF6 could be obtained by using l qF multiple-pulse decoupling. Comparison of the spectra with and without decoupling shows that decoupling produces a marked improvement in resolution, revealing seven lines from the coupling of 31p to the 6 equivalent fluorine atoms. The same 747 Hz coupling to 31p can be seen in a doublet in the 19F MAS fluorine spectrum. 9.2.2.11 Fluorine as a source of cross-polarisation. Although CP experiments with 19F are much less common than with IH, cross-polarisation to 19F can readily be achieved, providing the opportunity both for signal enhancement and spectral editing. The high frequency (proton) channel of modem spectrometers can be readily switched, but it may be necessary to broaden the decoupling channel on the probe to tune to 19F. Recommended CP set-up compounds standards are Na2SiF6 and topaz [A12(F,OH)2 (SiO4)] for 29Si and Na2SnF6, for 119Sn. Contact times of 10 ms and 5 ms have been used for silicon and tin respectively. The much larger CSA of fluorine compared to protons and the effect of strong scalar coupling may significantly affect the CP dynamics. In many cases where CP could be used the fluorine is relatively dilute so that the assumption of an infinite spin bath (Chapter 2) may not hold. Zeolitic materials which have been synthesised in fluoride media have also been studied by 19F-29Si CP MAS (Hoffner et al. 1993) and show significant differences in the 29Si intensities by comparison with the proton CP experiment. Contact times as long as 100 ms are sometimes required for 19F CP experiments which should therefore be carried out cautiously since they place a significant strain on the amplifier. 9.2.2.12 Summary of 19F shift trends and other N M R properties. Fluorine shifts in inorganic solids are affected by both nearest and next-nearest neighbour effects. It is often difficult to make unequivocal assignments of lqF NMR resonances of samples which are either genuinely disordered or consist of low-coverage surface phases
NMR of Other Spin -1/2 Nuclei
563
containing a range of sites. Where both bulk and surface fluorine atoms are present they can have significantly different relaxation times, making it important to obtain quantitatively meaningful spectra which are not influenced by the differential T1. Two shift prediction schemes have been described, 1 based on next-nearest neighbour effects (Hayashi and Hayamizu 1990) and the other on group electronegativities (Luca et al. 1985). Both approaches require a knowledge of the structures and experimental shift values for a series of known compounds to be able to calculate the 19F shifts of other compounds, and, furthermore, group electronegativity is not a clear-cut concept. Recently, a large experimental dataset of ionic fluorides has been analysed in terms of a superposition model making the assumption that the chemical shift can be derived by superposition of effects from all neighbouring cations (Bureau et al. 1999). This approach, which was suggested to be generally applicable, allowed the identification of fluoride environments in complex materials.
9.3. DILUTE OR MEDIUM--yNUCLEI
9.3.1 ~3C N M R The dominance of 13C studies of the solid state NMR literature clearly reflects the central role played by carbon in organic polymers. The sensitivity of the 13C chemical shift allows detailed NMR characterisation of the different fragments present in organic polymers. The use of 13C NMR is widespread, despite its low sensitivity due to its natural abundance of only 1.1%, since the 13C signal is routinely enhanced by the use of 1H-13CCP. When CP cannot be used, 13Cmust be observed directly in a 1 pulse experiment; in such cases the low sensitivity and very long T~ values make ~3C a difficult nucleus. For these reasons 13C NMR studies of inorganic non-protonated solids are much more limited in number than polymer studies. TMS is taken as the primary shift standard, although secondary standards are usually used. Adamantane is a good secondary reference since it produces a narrow line under MAS and decoupling and does not require CP to produce a good signal. It is sensitive to shimming and has 2 narrow resonances, the more intense appearing at 38.4 ppm. Glycine is a useful secondary shift reference with a shift of 176.03 ppm, and is also good for setting up CP experiments. Since the carbonyl signal from glycine has a large CSA but an intrinsically narrow resonance, it can also be used to check the accuracy of the magic angle setting. The or-carbon of glycine has strong 1H dipolar coupling, making it ideal for optimising the decoupling performance since the S/N of this peak is sensitive to both the power and the frequency offset of the decoupler. 9.3.1.1 13CN M R of elemental carbon. 13C NMR has been reported for the crystalline forms of carbon and its amorphous derivatives. Carbon has many applications, including
564
Multinuclear Solid-State N M R of Inorganic Materials
the increasing use of carbon fibre and as a battery component. Because of their relatively high electrical conductivity, powdered graphite samples often have to be mixed with an insulator to allow the probe to be well tuned. The 13C NMR spectra of graphite show an axially symmetric CSA tensor with the extreme components at 0 _+ 5 and 180 ppm and an isotropic value of 119 ppm, as expected for an sp 2 hybridised system (Resing et al. 1985). A 13C MAS NMR study of powdered graphite yielded an isotropic chemical shift of 112 ppm (Freitas et al. 2001). The temperature dependence of the ~3C shifts of the resolved lines from highly oriented pristine pyrolytic graphite was determined on an ellipsoidal shaped block rather than a powder and was related to the large diamagnetic susceptibility of the material (Hiroyama and Kume 1988). The AB structural sequence of pyrolytic graphite gives rise to 2 distinct carbon sites when the magnetic field is parallel to the c axis. Careful analysis using the T~ values (616 +_ 116 and 275 + 171 s) together with the Korringa relation showed that the Knight shift contributes only --~7 ppm to the overall shift, indicating that the dominant contribution is from the chemical shift. The temperature-dependent part of the shift is thought to be originate from the 7r--rr* interaction (Hiroyama and Kume 1988). The ~3C NMR signal from diamond is difficult to observe because of the very long T1 (of the order of hours) due to the weak coupling to the rigid lattice. One resonance line is usually observed at 39 ppm in natural diamonds. Defects in diamonds are often centrally important to their most interesting physical properties. The most common defect centre is nitrogen which can be paramagnetic, allowing the use of investigative techniques using the electron signal either directly (e.g. electron paramagnetic resonance) or indirectly (e.g. dynamic nuclear polarisation or DNP). DNP provides such enhancement of the S/N ratio that even natural abundance 13C-13C coupling can be observed (Hill et al. 1999). Single crystal studies have been made of ~3C-enfiched samples (Lefmann et al. 1994), allowing more complex couplings to be observed. Magnetic resonance studies of diamond have recently been extensively reviewed by Reynhardt and High (2001). C6o or fullerene, a new form of carbon discovered in the mid-1980s, has been extensively studied and characterised by ~3C NMR. At room temperature the considerable reorientational motion of the C6omolecules within the structure give rise to a narrow ~3C NMR resonance with a reported isotropic shift of 148.6 ppm (Yannoni et al. 1991). T~ measurements as a function of temperature indicate a phase change at --~ 260 K, above which temperature the C6o molecules undergo continuous rotational diffusion. Below the transition temperature jumps occur between orientations that by symmetry are the same. As the temperature is reduced below 173 K unaveraged CSA effects become apparent, with a conventional CSA powder pattern being observed at 100 K (Figure 9.11) (Yannoni et al. 1991 a, Tycko et al. 1991). The tensor of this CSA has a span of --~ 180 ppm and a skew of 0.62. Since NMR depends strongly on the local bonding it can determine bondlengths in materials such as C6o which show orientational
565
NMR of Other Spin -1/2 Nuclei Temperature K
.
.
.
.
.
.
.
.
.
.
173 163
L
~
133
~
........ ~~tatie
!
300
!
200
K
.
)~
....
Temperature
I. . . . .
100
1 _
0
13C shift (ppm) w.r.t. TMS
I
300
i
,
CSA ::~'1
200
100
=
-
::
-
0
13C shift (ppm) w.r.t. TMS
Figure 9.11. 13CNMR spectra of polycrystalline C6o at various temperatures, showing motional narrowing of the NMR line due to rapid isotropic reorientation of the molecules above 140 K. The static CSA powder pattern lineshape was calculated using principal values of 213, 182 and 33 ppm. Adapted from Tycko et al. (1991). disorder. In such an experiment on a sample enriched in 13C to --~6 percent, the sample was cooled to remove dynamic disorder and an echo sequence was used to eliminate CSA effects. The resulting static spectra were used to determine the bondlengths from a knowledge of the dipolar coupling between 13C-13C pairs. The C6o sample showed two separate Pake doublets in the intensity ratio 2:1 corresponding to bondlengths of 1.45 _+ 0.15 and 1.40 _+ 0.15 A (Yannoni et al. 1991). These bond lengths provide strong experimental support for the existence of a truncated icosahedral form of C6o. The alkali fullerides possess a range of intriguing physical properties. NMR provides clear evidence of the lack of solid solutions between the metal ion and C6o but indicates the existence of stoichiometric compounds A• where x is an integer between 1 and 6 (Tycko 1994). The 13C NMR spectra show distinct resonances from the different phases rather than a continuous change in a single peak. In the potassium system stable phases with x = 1, 3, 4 and 6 which can be formed under different physical conditions, with the first two having ~3C shifts of 174 and 186 ppm. In A6C6o (where A - K, Rb or Cs) three closely spaced isotropic ~3C resonances are observed in the range 153.1-160 ppm, indicative of orientational ordering within the structure giving rise to three crystallographically inequivalent carbon sites in the ratio 2:2:1 (Hajji et al. 1996). C6o can be polymerised at elevated temperature and pressure to form new carbon phases such as the FCC and rhombohedral forms (Maniwa et al. 1996).
566
Multinuclear Solid-State N M R of lnorganic Materials
The spectra recorded at low temperature show a CSA lineshape similar to the sp 2 pattern from unpolymerised C6o, with the addition of a narrower component from sp 3 sites accounting for ---15-20% of the carbon-content (Figure 9.12). The sp 3 signal probably arises from bonding between C6o units as the system polymerises. So-called hard carbons can be formed in a similar manner but the spectra show no characteristic peaks from sp 3 sites (Figure 9.12). There is now a vast literature on the NMR of fullerene-based materials. C7o is a much less symmetric structure, the constitution of which is apparent from the presence in the ~3C NMR spectrum of 5 inequivalent carbon resonances at 150-130 ppm in the ratio 1:2:1:2:1. A two-dimensional NMR pulse sequence INADEQUATE was used to examine the connectivity of these units (Johnson et al. 1992 and references therein). Comparison of the ~3C MAS spectra before and after annealing at 1600~ revealed a large decrease in the anisotropy on annealing which was attributed to the removal of nickel impurities. After annealing, the sample had a ~iso value of 126 ppm with sidebands extending over --- 300 ppm (Goze Bac et al. 2001). This spectrum agrees with that of a single-walled nanotube reported by Tang et al. (2000). The CSA tensor of this material, determined from its static spectrum, had a span of 276 ppm and a skew of 0.50 (Goze Bac et al. 2001). All the ~3C NMR characteristics suggest an sp 2 graphene sheet type structure. The temperature dependence of the tensor indicated no motion leading to line narrowing in the range 10-350 K.
har
ca
/ har d ~ . _ . ~ / ~
rhombohedral
\
~
~f~
" .
i
~
L
I~__LLJ
400
~
L
~
I
0
~
,
t
I
,
a
~
I
j
-400
13C shift (ppm) w.r.t. TMS
. . . . .
I
....
1000
,
1
0
,
,
,_.L._l
,
,
,__L_I
-1000
13C shift (ppm) w.r.t. TMS
Figure 9.12. Low-temperature ~3C NMR spectra of various carbon phases, including hard carbons produced under pressure from C6o. The shaded resonances are attributed to sp3-1ike carbon atoms in the C6o cage. From Maniwa et al. (1996), by permission of Elsevier Science.
N M R of Other Spin -~/2 Nuclei
567
In disordered carbonaceous materials the ~3C NMR lines are often broad due to chemical shift dispersion effects. The optical properties of carbon black formed by resistive heating of carbon electrodes are significantly influenced by the local structure (--~1-100 nm). 13C NMR measurements of the sp2/sp 3 ratio have been used to characterise the number of bent and fiat planes, since the different carbons resonate in distinct ranges (8-80 ppm for sp 3 and 90-180 ppm for sp2). The spectral lineshapes were simulated using a Voight function. The major sp 2 peak occurs at 126 ppm with a minor peak at 167 ppm and the largest sp 3 peak is at --~ 20 ppm. Excellent agreement was obtained between the sp2/sp 3 ratio determined by NMR and electron energy loss spectroscopy. A dense amorphous sample produced from a carbon arc source was shown by 13C NMR to contain a large proportion of sp 3 hybridised carbon. The ~3C NMR spectrum is dominated by a peak at 66 ppm intermediate between graphite and diamond, with a small peak at 140 ppm arising from sp 2 carbon (Golzan et al. 1995). A small amount (--~ 1%) of hydrogen detected in this material was thought to compensate for the dangling bonds. Amorphous carbon-based materials are also important anode materials in lithium batteries. The formation mechanism and consequent structural changes on oxidation and carbonisation can be readily followed by 13C NMR techniques (Hayes et al. 1997). Amorphous carbon-hydrogen (a-C:H) is also technologically very important. The degree and nature of its heterogeneity, especially on the nanoscale, are important factors in determining the properties, and require accurate characterisation. The sp2:sp 3 ratio and fraction of hydrogen-beating species are 2 important constraints on the structural models for these materials. The first ~3C NMR studies clearly showed signals from both carbon hybridisations, with an sp 2 resonance at --~ 140 ppm and sp 3 at -~40 ppm (Kaplan et al. 1985, Jarman et al. 1986, Grill et al. 1987, Lukins et al. 1993). The precise shifts of the various peaks depend to some extent on the formation conditions (e.g. cathode voltage) of the a-C:H compound. This work suggested that the hardness of the films depends inversely on the hydrogen content and the sp3/sp 2 ratio (Kaplan et al. 1985). The protonation state of different carbon populations has been determined either by their response to decoupling (Carduner et al. 1991) or by the use of interrupted decoupling (Schwerk et al. 1993, J~iger et al. 1994). The differential relaxation time of the protons allows these regions to be discriminated. By applying a 7r pulse on the proton channel and varying the time for the ~H magnetisation to recover prior to the CP sequence J~iger et al. (1994) showed 2 distinct sp 3 signals (Figure 9.13). Combination of these ~3C data with ~H CRAMPS and ~H MQ led to the conclusion that on the nanometre scale the structure is highly heterogeneous. One suggested structural model contained short CH2 chains and regions of mixed sp2/sp 3 environments with randomly distributed protons. More recently 13C NMR has been used to investigate the structure of films grown at different temperatures containing varying degrees of protonation (Braddock-Wilking et al. 2001). The NMR data show how the 2 different sp 3 populations vary and are clearly consistent with a heterogeneous structure, although its exact nature is still debated.
568
Multinuclear Solid-State NMR of lnorganic Materials sp 2 B
/,\
sp5
A (c)
0
(b)
(a)
50
100
150
x (ms)
1000
(C)
sPY
I
-, 200
! 100
'
,
f 0
13C shift (ppm) w.r.t TMS
Figure 9.13. A. Biexponential relaxation behaviour of ]H spin-lattice relaxation in amorphous hydrogenated carbon. The 2 time constants of these curves are 14 ms and about 120 ms, showing no equilibrium between the 2 proton reservoirs up to about 100 ms. B. 12C CP NMR spectra acquired with different delays (-r) between the w-pulse and the start of the normal CP experiment. (a) normal CP spectrum obtained with "r > 200 ms, showing sp2 and sp 3 r e s o n a n c e s . (b) spectrum with "r = 35 ms using fast-relaxing protons for CP, still showing sp2 a n d sp3 resonances. (c) spectrum with 'r = 8 ms showing CP of only the sp3 carbons by the slow-relaxing protons. Note that the signal is now negative, as expected. From Jager et al. (1994), by permission of the copyright owner. 9.3.1.2 Silicon carbide. Silicon carbide is a very important material with good abrasive properties, high temperature strength, good thermal properties and a degree of semiconducting behaviour. Much work has been done to develop processes for its manufacture by low temperature chemical routes. SiC also has an intriguing diamondlike network structure showing extensive polytypism with --- 200 reported polytypes. All these structures are based on layers of tetrahedral units in different stacking sequences which give rise to the polytypism. The end-member structures are the cubic zincblende structured [3-SIC and the hexagonal wurtzite type oL-SiC. Samples differ in the periodicity of their stacking sequence and the displacement of the layers along the c-axis. The different polytypes are usually denoted by the Ramsdell notation (See Hartman et al. 1987 for an explanation). Despite some of the structures having extremely long repeat distances along the c-axis they contain only 4 structurally different carbon (and silicon) sites, although the proportions of these can effectively vary continuously. One of the difficulties of observing the ~3C NMR signal from these phases arises from the extremely long relaxation times that can be encountered. A detailed study of the relaxation has shown it to exhibit slightly stretched exponential behaviour with ~3C T~ values of up 14 hours (Hartman et al. 1993). In some samples T] varied by a factor of--~ 2.5 between different resonances in the same sample. The
569
NMR of Other Spin -1/2 Nuclei
addition of nitrogen reduced T1 to --~ 100 s. Some samples may contain disordered components and if the minor constituents have relatively short T1 values, these can give a misleading impression of the phase distribution if the NMR experiment is carried out using too short a recycle delay to observe the major component. A number of studies have reported the 13CMAS NMR spectra of the various polytypes (Finlay et al. 1987, Hartman et al. 1987, Apperley et al. 1991). The peak positions reported for the different polytypes agree within error, with values as follows: polytype 2H- 1 peak (15.0 ppm), polytype 4H- 2 peaks (13.5, 20.9 ppm), polytype 6H- 3 peaks (23.0-23.2, 20.2-20.5, 15.0-15.5 ppm), polytype 15R- 4 peaks (22.7, 20.7, 16.0 and 13.3 ppm). Typical spectra are shown in Figure 9.14A. The shift differences of the various sites are quite large since they all have the s a m e CSi4 nearest neighbour environment, suggesting that they reflect changes in the next-nearest neighbour environment (Hartman et al. 1987, Apperley et al. 1991, Richardson et al. 1992). Although several studies failed to observe the 13C signal from [3-SIC, presumably because of its very long T~ time, its resonance was eventually reported at 23.7 ppm (Wagner et al. 1989). Although there is good agreement between the reported ~3C NMR shifts for SiC, there is much less agreement regarding their crystallographic assignments. The 6H polytype is by far the most thoroughly studied by NMR, with authors agreeing that the 15 ppm peak corresponds to the C1 structural site (Richardson et al. 1992). To improve the A
20.2 23 2
15.2
6H
(c)
22.7/~0.7 16
E V -~
\
-
o
-
15R 0
60
0 .
.
120
180:
(degrees)
.
3
20
10
ppm
~3C shift (ppm) w.r.t. T M S Figure 9.14. A. 13CMAS NMR spectra of hexagonal SiC polytypes, from Hartman et al. (1987). The spectrum of the cubic (zincblende) structure was not detected by these authors under a wide range of conditions. B. Angular dependence of the 13C NMR lines of single-crystal 6H polytype of SiC. Curve (a) corresponds to the resonance at 21.9 ppm in this crystal, curve (b) corresponds to the resonance at 17.2 ppm and curve (c) to the resonance at 25.4 ppm. From Richardson et al. (1992). Both figures used by permission of the American Chemical Society.
570
Multinuclear Solid-State NMR of Inorganic Materials
peak assignments, elements of the CSA tensor have been determined from a single crystal rotation pattern. All the sites show relatively small CSA spans which, however, vary from 5.5 to 12 ppm and show different angular variations (Figure 9.14B). The resonances have been assigned on the basis of this additional information, but the improved shift calculations developed in the last few years could be usefully applied to these compounds to verify the peak assignments. Plasma-sprayed SiC can be thermally treated to form mixtures of the different polytypes in varying ratios. The very high resolution of these crystalline phases means that their 298i and 13C NMR spectra can be used to accurately quantify the phase distribution (Dando and Tadayyoni 1990). In many practical applications, SiC is used in the form of fibres and whiskers which have a more disordered structure. Commercial fibres such as Nicalon and Tyranno show broad ~3C NMR peaks centred at 26-30 ppm as expected from CSi4 environments, but with much greater variation in the sites (Hartman et al. 1987, Murthy et al. 1989). The signal from excess graphitic carbon in such fibres has remained elusive to NMR, presumably because of its broadness and long T1. The 13C MAS NMR signal detected from a [3-SIC whisker was broad compared to crystalline [3-SIC and showed a shoulder, possibly from a-SiC (Wagner et al. 1989). Interest in producing single-phase and tailored SiC-based ceramics at low temperatures has led to interest in advanced chemical processes involving polymeric precursors. NMR provides insights into the structural development and conversion chemistry of these amorphous precursor compounds. Numerous NMR studies of these systems include a 13C investigation of the changes occurring in vinylic polysilane when heated to 1000~ under N2 (Figure 9.15). The 13C NMR spectrum of the final product contains a major peak at 19.5 ppm arising from disordered SiC, the excess carbon showing up in the sp 2 region at 121 ppm (Schmidt et al. 1991). This chemical approach is also the starting point for the preparation of more complex systems such as Si-C-N and Si-C-O which have also been studied by ~3C NMR. Polymerisation and breakdown of Si(NHEt)4 at 800~ under argon forms a mixture of SiN4 units and free carbon which then reacts at 1500~ to form a mixture of SiC polytypes with some residual graphite (Narsavage et al. 1991). Silicon oxycarbides can be used as a matrix for SiC fibrereinforced glass composites and as precursors to the ceramic fibres themselves. A sol-gel synthesis route has been investigated by ~3C NMR, showing that as the structure breaks down, a peak assigned to sp 2 carbon forms, probably from a carbon-free phase which subsequently reacts at higher temperatures to form a Si-C-O network (Babonneau et al. 1994). These studies illustrate the use of 13C NMR to optimise sol-gel processing methods. 9.3.1.3 Other binary carbides. Boron and carbon form a series of boron carbide
phases of which B4C appears to be the most ordered. There are several competing structural models for boron carbide which have been investigated by liB and 13C
NMR of Other Spin -1/2 Nuclei
571
max. heating temperature (~
$ I000
650 400 250 196
~
~
151
unheated
200
0
13C shift (ppm) w.r.t. TMS Figure 9.15.
13C MAS NMR spectra of the polymeric SiC precursor vinylic polysilane heated to
various temperatures under a nitrogen atmosphere. The principal peak at 19.5 ppm in the sample heated at 1000~ is due to SiC. The asterisk marks an artifact from the teflon rotor insert. From Schmidt et al. (1991), by permission of the American Chemical Society.
NMR. These phases have a rhombohedral structure with boron-rich ( B l l C ) icosahedra joined together either directly or via short chains. B4C has 3 13C MAS NMR peaks, at 1.0 ppm (66.9%), 81.9 ppm (31.4%) and 101.3 ppm (1.7%), the assignments of which have been the subject of some discussion (Duncan 1986, Alexander 1986). Higherfield studies using faster MAS produced similar spectra but with significantly better resolution than the earlier work (Kirkpatrick et al. 1991). The peak near 0 ppm results from carbon in chain-end positions. It is most likely that the peak at 81.9 ppm is from icosahedral carbon. The small peak observed at 101.3 ppm possibly arises from carbon substitution in the equatorial sites of the icosahedra. The phases B13C2 and BsC show more complex 13C NMR spectra with additional peaks resulting from differences in ordering within the short C-C-C joining chains resulting from boron substitution. The NMR work indicates that none of these phases is fully ordered. The 13C NMR spectrum of amorphous hydrogenated boron carbide shows a strong peak at 15 ppm, in the region of the resonance from the boron carbide icosahedra. Other peaks in this spectrum could arise from sp 2 and sp 3 carbon as in a-C:H, possibly indicating an amorphous matrix in which the icosahedra are embedded (Braddock-Wilking et al. 1999).
572
Multinuclear Solid-State NMR of lnorganic Materials
The 13C shift of 530 ppm in titanium carbide is considerably more positive than in many other carbides, reflecting the Knight shift contribution. As nitrogen is dissolved in TiC the ~3C line broadens and shifts down to 330 ppm for TiCo.28No.72 (MacKenzie et al. 1995), suggesting that the 13C shift could provide a sensitive alternative to XRD for determining the composition of titanium carbonitrides. The addition of nitrogen to an amorphous film of hydrogenated carbon nitride results in the appearance in the 13C NMR spectrum of sharp resonances in addition to the broad spectral features from the amorphous film. The sharp lines were assigned to nanocrystals of nitrogen-containing aromatic material (LaManna et al. 1999). The 13C NMR spectra of the ionic carbides CaC2 and BaC2 have been determined by Wrackmeyer et al. (1990) who report ~iso values of 206.2 ppm for CaC2 and 232.1 ppm for BaC2, both shifts with respect to TMS. The NMR results indicate that the environment of the carbon atoms in these compounds is not axially symmetric, and that the carbide unit in these "ionic" carbides is not comparable with the carbon-carbon triple bond in alkynes. Various routes have been proposed for synthesising materials in the B-C-N system. There is some dispute as to whether these structures consist of three-dimensional networks composed of the constituent elements or physical mixtures of the different phases. ~B and 13C MAS NMR have been used to study a number of compositions formed by pyrolysis of organic precursors (Andreev et al. 1995). Broad 13C peaks were observed at 100-200 ppm, but no evidence of a B4C phase. The NMR data together with XRD results were taken to indicate that the samples consisted of a mixture of graphite-like carbon, B N and amorphous boron. 13C NMR has been used to study the incorporation of carbon into S i-B-N-C ceramics (van Wtillen and Jansen 2001). New synthetic approaches use a single molecular precursor containing the atomic linkages required to be present in the product. The organic fragments are removed by pyrolysis to leave a dense but amorphous ceramic with a ~3C MAS NMR spectrum containing 2 sp 2 peaks, a narrow resonance at 115 ppm superimposed on a broader line at 130 ppm. ~3C-{liB} REAPDOR indicated that the 115 ppm resonance is due to carbon not associated with the network but the broader line experienced dipolar dephasing, suggesting that it was part of the network. It is likely that this carbon enters the network in cation positions replacing silicon and/or boron.
9.3.1.4 Ternary and quaternary carbides.
9.3.1.5 Carbonates. A
13CNMR study of 28 synthetic and natural carbonates includ-
ing samples of mineralogical and biogenic origin showed only a small shift change from 166.3 to 170.3 ppm as the cation is varied, reflecting the well defined nature of the CO32- group (Papenguth et al. 1989). This narrow range of shifts is slightly extended to 163.8 ppm in the scapolite series in which the carbonate group is more distorted than in the simple carbonates (Sherriff et al. 1987). Ionic ordering in (Mg,Ca)
NMR of Other Spin -1/2 Nuclei
573
carbonate minerals could not be determined from 13C NMR measurements because of the small chemical shift range. 13C NMR has been examined as a method for analysing mixtures of the calcium carbonate polymorphs calcite and aragonite. Long recycle delays in excess of 600 s were necessary to obtain quantitatively reliable data. The lowest level of aragonite detectable by NMR was 5 wt %. Dissolution of CO2 in magmas influences their physical properties by processes which can better be understood if the speciation can be determined. Such systems have been modelled by forcing CO2 into aluminosilicate melts under pressure then quenching from high temperature to form a glass. The 13C NMR spectra of albite glasses treated in this way show a signal at 125 ppm arising from the presence of molecular CO2 within the structure, and a carbonate signal in the range 155-170 ppm (Figure 9.16) (Kohn et al. 1991). The 13C NMR spectrum of molecular CO 2 shows an axial powder pattern with a span of 260-270 ppm whereas nepheline and sodamelilite glasses show only a carbonate signal (or 2 distinct carbonate signals in some of the samples). In albite glass the ratio CO32-'CO2 was found to increase in the presence of water. The ability of 13C NMR to distinguish these different species makes NMR a powerful technique for determining CO2 speciation. ~3C NMR has also been used to study the mechanism of CO32- substitution in fluorapatite (Regnier et al. 1993).
qr
300
200
100
~3C shift (ppm) w.r.t. TMS Figure 9.16. 13C MAS NMR spectra of aluminosilicate glasses containing CO2. Upper: anhydrous albite glass synthesised under reducing conditions. Lower: hydrous albite glass. The major resonance at 125 ppm in both glasses is from dissolved molecular CO2, and the resonances at 160-170 ppm arise from carbonate ions. The asterisk denotes a spinning sideband. From Kohn et al. (1991), by permission of the Mineralogical Society of America.
574
Multinuclear Solid-State NMR of lnorganic Materials
9.3.2 lSN N M R
~SN is potentially an attractive nucleus for solid-state NMR as it has a large chemical shift range that is sensitive to quite small structural changes. Its major drawback is the relatively low sensitivity resulting from its low natural abundance (--~ 0.4%). However, even modest isotopic enrichment can lead to significant improvements in sensitivity and the unique perspective offered by 15N NMR can often be crucial in understanding structure, particularly atomic ordering. The majority of ~SN NMR studies have used MAS and once the optimum acquisition conditions have been determined, obtaining 15N spectra from solid materials is straightforward. Apart from sensitivity problems, the major experimental difficulty with nitride systems is their very long T1 values. Since pure Si3N4 contains no strong coupling, its ~SN T~ is --- 3000 s (Harris et al. 1990). These long T~ values are a reflection of the lack of impurity relaxation and the highly rigid nature of such network materials, and make ~SN NMR studies of such materials less straightforward than for organic polymers, where CP can be used. However, typical ~SN spectra of some inorganic materials shown in Figure 9.17 illustrate the possibility of obtaining good dispersion from different structural units. A practical problem with the ~SN NMR literature arises from the wide range of secondary shift references which have been employed, making it necessary to correct shift values quoted directly against the secondary shift reference before the spectra from different studies can be compared. A common secondary standard is desirable, even if
15
15 NH4
Si31
NO 3
f""
'
I '
'
600
KIsNO
'
!
. . . . .
i
200
f
''r
i
,
,~
i
100
-200
,
,
,
i
,
,',
60
i
,"',
,
i
,
,
,
20
3
..... J 11.......... 1~
600
,
,
i
,
,
,
i
r',
200
~ i
,,
,
1.,
-200
,1
200
0
-200
~SN shift (ppm) w.r.t. 1SNH4NO3 Figure 9.17. A selection of 15N MAS NMR spectra of inorganic compounds. The sharp peak at 0 ppm in 15NH415NO3is from the ammonium group nitrogen, while that at 358 ppm is from the nitrate group, with its associated spinning sidebands. From Turner et al. (1987), by permission of the American Ceramic Society.
575
NMR of Other Spin -1/2 Nuclei
only amongst workers studying inorganic nitrides. The ammonium 15N resonance of NHaNO3 would be a good candidate since it is a narrow line with relatively few spinning sidebands (Figure 9.17). This group resonates at - 358 ppm relative to the primary shift standard of CH3NO2 (Turner et al. 1987). Other standards that have been used as secondary shift references are the NO3 resonance in NH4NO3 ( - 4.6 ppm) and NH4C1 ( - 338.2 ppm). The ammonium group in NH4NO3 is a good set-up compound for 1H --> 15N CP.
9.3.2.1 Nitrides. The nitrogen in Si3N4 occurs in NSi3 units. [3-Si3N4 contains 2 nitro-
gen sites in the ratio 3:1, differing in their orientation relative to the threefold axis. The ~SN MAS NMR spectrum reflects this site distribution. Replacement of the Si and N by A1 and O produces the structurally related phase [3-sialon in which most of the nitrogen remains coordinated to silicon, evidenced by the similarity in the 15N NMR spectrum. The presence of additional layers in the structure of oL-Si3N4 gives rise to 4 ]SN resonances (Figure 9.18) (Harris et al. 1990). The 15N NMR spectrum of A1N shows a major peak at - 294.3 ppm with a T] of 600 s. The cubic and hexagonal polymorphs of BN show an isotropic shift difference of 78.8 ppm (Jeschke et al. 1998). A natural abundance spectrum of the hexagonal phase was obtained by using ~]B--->~SN CP to enhance the signal. The shift difference between the 2 BN polymorphs reflects the change from NB4 to NB3 coordination, in agreement with detailed GIAO cluster calculations (Marian and Gastreich 2001). The lower site symmetry of the hexagonal form results in a measurable ~SN CSA of 160 + 20 ppm.
MgSiN2
~-Si3N4
. . . . . . . . .
9
-260
,
-310
-360
-160
LiSiON
-260
-360
:160
-260
-360
15N shift (ppm) w.r.t CH3NO2 Figure 9.18. A selection of ~SNMAS NMR spectra of nitrides and related compounds. From Harris et al. (1990, 1992), by permission of the American Chemical Society.
576
Multinuclear Solid-State N M R o f lnorganic Materials
Table 9.4.
15N
Phase ot-Si3N4 [3-Si3N4
LaSi3N5 Mg2SiN2 MgSiA1N3 c-BN h-BN TiN A1N A1ON
isotropic chemical shifts of pure nitrides and related phases. 15N ~iso ( p p m ) *
-
References
309.0, - 307.2, - 296.6, - 284.5 Harriset al. (1990) - 306.5, - 289.3 Harris et al. (1990) - 286.7 Harris et al. (1990) - 292.5, - 281.3 Harris et al. (1992) - 313.0, - 287.8 Harris et al. (1992) - 355.8 Jeschke et al. (1998) - 277 Jeschke et al. (1998) 24 MacKenzie et al. (1995) - 293.6, - 294.3 Kruppa, Dupree and Lewis (unpublished) - 303 Kruppa, Dupree and Lewis (unpublished)
* chemical shifts quoted with respect to CH3NO2
Extremely high-resolution ~SN NMR spectra of MgSiN2 have been obtained with linewidths of only 25 Hz for the 2 NSi2Mg2 environments (Figure 9.18) (Harris et al. 1992). These 2 sites have only minor crystallographic inequivalence, illustrating the sensitivity of the ~5N isotropic chemical shift. The 15N NMR chemical shifts of a series of titanium carbonitrides decrease from 24 to - 15 ppm as the nitrogen content decreases from 100 to 35.4% (MacKenzie et al. 1995). The metallic character of these samples resulted in relaxation times somewhat reduced by comparison with other nitride ceramics.
9.3.2.2 S i l i c o n a l u m i n i u m o x y n i t r i d e c e r a m i c s a n d glasses. As oxygen is dissolved
in A1N the structure transforms to a defective cubic spinel related to ~/-AleO3; however the presence of nitrogen stabilises this structure at temperatures where ~/-AleO3 would normally transform to ot-AleO3. Although some solid solution exists, the phase region is centred on 35.7 mol % A1N, corresponding to 5A1N:9AleO3. The eTA1 MAS NMR spectra (Dupree et al. 1988) indicate the presence of A1N4, A104 and A106 units, but the 15N isotropic chemical shift remains the same as for A1N albeit with considerable line broadening, indicating a range of next-nearest neighbours (Kruppa, Dupree and Lewis (unpublished)). 29Si NMR has been used to examine the atomic ordering in metal silicon oxynitride phases (see Chapter 4). In early studies of YeSi303N4 and YnSi207Ne (N-YAM), single e9si NMR signals were detected, leading to the postulation of structures with a single well-defined silicon site. Later work confirmed the 29Si MAS NMR spectra but augmented the structural information with 15N MAS NMR measurements. This is illustrated by the structure of N-YAM which contains "disilicate" SieO5Ne units for which 2 possible oxygen-nitrogen ordering schemes are possible (Figure 9.19A),
577
NMR of Other Spin -1/2 Nuclei
A
B
245 4
l
i
I
I
I
~
I
-60 ~-silicon
@-nitrogen Q- oxygen
-160 -260 -360 15N shift (ppm) w.r.t CH3NO2
Figure 9.19. A. The 2 possible oxygen-nitrogen atomic arrangements in the Si205N2 unit of Y4Si207N2. B. 15N MAS NMR spectrum of Y4Si207N2 showing 2 distinct nitrogen environments, consistent with N in the bridging site and 1 of the terminal sites (A, lower). From Hauck et al. (1993), by permission of the Royal Society of Chemistry. involving either SiO3N units only, or equal numbers of SiO2N2 and SiO3N units. Pauling's Second Crystal Rule suggests that nitrogen should occupy the bridging position. The 15N MAS NMR spectrum shows 2 resonances, at - 131.4 and - 245.4 ppm with respect to CH3NO2 (Figure 9.19B), with the more positive shift assigned to the more ionic terminal, non-bridging position (Hauck et al. 1993). The implication of this observation is that the single 29Si MAS NMR spectrum must be a composite of 2 unresolvable overlapping resonances. A similar 15N NMR study carried out on N-melilite again showed that the single 29Si MAS NMR peak should be interpreted cautiously, since the 15N MAS NMR spectrum could be decomposed into 3 overlapping peaks indicating a structure with 2 distinct silicon sites (Koroglu et al. 1996). These 15N NMR observations on N-melilite are again consistent with nitrogen favouring bridging positions. All the 15N NMR data for the crystalline silicon oxynitrides indicate that the shift range of NSi2 units is --~100 ppm higher than that of NSi3 (Figure 9.20). Similar studies have been carried out for oxynitride glasses in which 29Si MAS NMR indicates nitrogen enters the structure, forming a range of co-existing local SiOxN4-• units. A single broad 15N NMR peak has been reported at - 330 ppm in Mgsialon glass (Turner et al. 1987). The 15N NMR spectrum of an yttrium-sialon glass with predominantly tricoordinate nitrogen sites showed 2 peaks, 1 of which was a very broad and ill-defined resonance at - 270 ppm arising from NSi2 and NSi3
578
Multinuclear Solid-State N M R o f Inorganic Materials -- NH 4 -- NP 2 -
NP 3
9 NAI4 9 BN3 9 BN4
--
NSi2 NSi3
- -
NO2
- -
--
NO 3
t
I
200
I
0
I
I
I
I
-200
-400
lSN s h i f t ( p p m ) w.r.t. CH3NO2 Figure 9.20. Schematic diagram of the ~SN chemical shift ranges of the various N-containing units in nitrogen compounds. B
wt% N
wt% N
4.6
10.3
annealed 1100~
.~~.
7.0
10.6
unan
~ -60
-160
-260
-360
lSN s h i f t ( p p m ) w.r.t. CH3NO 2
. ~ 6 ,
i
,
-160
,
,
i
9
-360
~ 9 ~
~ ,
2 i
. ,
-160
,
0 ,
9
,
9
9
-360
15N s h i f t ( p p m ) w.r.t. CH3NO2
Figure 9.21. A. ~SNNMR spectrum of yttrium sialon glass (lower) and the same glass heated at 1100~ (upper). Note the additional peaks in the crystallised sample corresponding to [3-sialon ( - 305.3 ppm), Si2N20 ( - 314.8 ppm), Y2A1SiO5N ( - 209.5 ppm) and N-substituted Y3AIsOI2 ( - 73.1 ppm). From Kruppa et al. (1991), by permission of Elsevier Science. B. 15N MAS NMR spectra of nitrided NaPO3 glasses containing varying amounts of nitrogen. The NP3 units (at about - 270 ppm) can readily be distinguished from the NP2 units at about - 300 ppm. Note the ratio of NP3:NP2 units remains constant at about 1:3 irrespective of the nitrogen content over this composition range. Asterisks denote spinning side bands. From Bunker et al. (1987), by permission of the American Ceramic Society.
579
N M R o f O t h e r Spin -1/2 N u c l e i
Table 9.5. ~SNisotropic chemical shifts of crystalline metal sialon phases.
Phase
15N
Y4Si207N2 Y2Si303N4
-
SizN20
O', Sil.9Alo.lN1.9Oa.1 O', Sil.72Alo.z8N1.7201.28 ~', Sis.4Alo.6Oo.sN7.4
LaSiO2N La3SigO4Nll YzSiA1OsN LiSiON
-
~iso(ppm)*
- 131.4, - 245.4 214.6, - 231.6, - 252.9 - 317.7 - 317.2, - 306.4 316.9, - 306.2, - 289.2 - 305.6, - 289.2 - 203 - 238, - 298 - 209.5 - 303.6
References Hauck et al. (1993) Koroglu et al. (1996) Sjoberg et al. (1992) Sjoberg et al. (1992) Sjoberget al. (1992) Sjoberg et al. (1992) Harris et al. (1989) Harris et al. (1992) Kruppa et al. (1991) Leach et al. (1990)
*chemical shifts quoted with respect to CH3NO2
environments, and the other at - 317 ppm. On crystallisation, additional 15N NMR peaks were resolved, due to [3-sialon, Si2N20, metastable Y2SiA1OsN and nitrogen substituted into Y3A15012 (Figure 9.21A). The latter gave a 15N resonance at - 68.1 ppm (Kruppa et al. 1991). Nitrogen has been added to sodium metaphosphate glasses to improve their durability and increase their Tg (Bunker et al. 1987). Two distinct 15N NMR resonances were observed, 1 from NP3 units (at - 2 7 0 to - 2 6 5 ppm), the other from NP2 ( - 3 0 3 to - 2 9 5 ppm). The nitrogen was added as dry ammonia which displaces oxygen from the network and forms water. The ratio of NP3:NP2 units determined by ~SN MAS NMR was 1:3 and was independent of composition (Figure 9.21B). The effect of nitridation is to increase the cross-linking of the network, and the change in shielding on going from NP2 --+ NP3 is opposite to the effect with silicon, reflecting the different electronegativities of phosphorus and silicon relative to nitrogen.
9.3.2.3 Nitride ceramics from polymeric
p r e c u r s o r s . Formation of complex nitride based ceramics is facilitated when the component elements are intimately mixed in a polymeric precursor. The organic component of the polymer is removed by calcination, producing a monophase ceramic product at comparatively low temperatures. The intermediate steps, and sometimes the final products, are amorphous, making solid state NMR an ideal technique for studying their short-range order. A polymeric synthesis route starting from polyborazilene precursors has been suggested for BN. The ~SN NMR spectra of these compounds are often complex, being composed of several strongly overlapping lines. The different lines can be separated using inversion-recovery CP (IRCP) which exploits the fact that signals relax differently depending on the number of directly attached protons. A study of a series of model compounds including MexNH3-xBH3 showed that the IRCP sequence (Figure 9.22A) with a variable
580
Multinuclear Solid-State NMR of Inorganic Materials
B
1.0
A ~ "4
x
,ll
0.5
Y
Y
I
-Y
.~
s
~ o -0.5 "~---tc ~ t i
~
t~ - - - - - "
N
H 0.2
/
~
~
0.6
1.0
Inversion time (ms)
Figure 9.22. A. Schematic diagram of the Inversion Recovery Cross Polarisation (IRCP) pulse sequence exploiting the different relaxation behaviour of NH• groups in borane-ammonia complexes containing different numbers of protons. B. Evolution vs. inversion time of the ~SN IRCP MAS NMR signals from the different N-H groups in MexNH3-x.BH3 compounds. From Gervais et al. (1998), by permission of John Wiley and Sons Ltd. inversion time (ti) could readily distinguish the different NHx environments (Gervais et al. 1998). The magnetisation curves were analysed according to the degree of dipolar coupling. In cases where the coupling is small (either because there are no directly coupled protons or because the coupling is greatly reduced by molecular motion), then the magnetisation behaves as a single exponential function of ti
o,,c,[ expl
1]
(9.5)
where Tis is the standard time constant for the CP process (see Section 2.6). In the case of stronger dipolar interaction, analysed by Wu and Zilm (1993) and Sangill et al. (1994) the magnetisation shows multiple exponential behaviour
+
(9.6)
where Tc is a time constant for the coherent transfer of magnetisation related to the dipolar coupling (Hirschinger and Herv6 1994), n is the number of directly coupled
581
N M R of Other Spin -1/2 Nuclei
protons and TD is the decay due to isotropic spin diffusion. The different MexNH3-xBH3 units show very different recovery behaviour (Figure 9.22B). The spectra of the precursors show peaks from the tricoordinate units in the range - 325 to - 364 ppm and a separate peak at - 267 ppm from the tetrahedral units (Gervais et al. 1998). Heated polyborazilene shows complex 15NMAS NMR spectra containing up to 8 resonances, 7 of which are from the precursor matrix (Gervais et al. 2001). By using IRCP to identify the different species, 2 distinct local coordinations (NHB2, NB3) were assigned. Calculations of the shift allowed the different peaks to be assigned on the basis of second nearest neighbour effects (Figure 9.23B,C). The ceramic precursor contains only tricoordinated boron, and the ratio of the species NHBe:NB3, determined by NMR was 50:50, was important for modelling the ring statistics (Gervais et al. 2001). 15N MAS and spin-echo NMR methods have been used to identify the various nitrogen species formed in the reaction of [(tBuCH2)2TaN]5 with NH3 to form cubic TaNo.s (Holl et al. 1996). More complex (ternary and quaternary) nitride ceramics can also be formed from polymeric precursors. Thermal decomposition of [CH3Si(N = C = N)l.5]n has been shown by 15N NMR to form NSi3 units when the protons are lost from the system during heating at 600-800~ (Gabriel et al. 1999). The product is an amorphous mixture of Si3N4 and carbon which reacts to form an amorphous Si-C-N ceramic phase. Similar studies have been carried out on the formation of Si-B-N ceramics from single phase
A
B
-257 -268 -294 x a
C O)
(Iv) / ~ ,B
N taN/ \H
-253 ppm (II) .~N/R
(V) /t~-']['n
-274 ppm
-287 ppm
(III)
.,.,
-220
....
J,:
,i.,:.l
-280
,.J.,.i.
-267 ppm obse
simulate//
(VI)
. ) ~ ~ . ~ z- ~
_
v
.
-340
~SNshift (ppm) w.r.t. CH3NO 2
-285 ppm
-305 ppm
-220
-280
-340
lSN shift (ppm) w.r.t. CHaNO 2
Figure 9.23. A. 15N NMR spectra of polyborazilene precursor for the production of BN. Upper: solid state MAS spectrum, middle: solid-state CP MAS spectrum, lower: liquid-state spectrum in tetrahydrofuran. B. Schematic representations and calculated 15N chemical shifts of the various environments in hexagonal BN. C. Observed and simulated 15N MAS NMR spectrum of polyborazilene showing the fitted components with assignments according to the environments of Figure 9.23B. The fitted peaks from the BHN2 sites are shown by full lines, those from BN3 sites by broken lines. From Gervais et al. (2001), by permission of the American Chemical Society.
582
Multinuclear Solid-State NMR of lnorganic Materials
molecular precursors. Three different precursors which show a 15N NMR peak at --~ 42 ppm all thermally decompose to a material containing a single resonance at --~ 60 ppm (Mt~ller et al. 2000). The linewidth of the resonance in the product spectrum (1.5 kHz) spans the shift range of NSixB3-x units, suggesting that the sample is both amorphous and atomically disordered. 9.3.2.4 Nitrates a n d nitrites. The strengths of the bonds in nitrate and nitrite ions are
such that the charge balancing cation has very little effect on the value of the 15N isotropic shift. The shifts of NO3- in NH4NO3 and KNO3 differ by only --~ 6 ppm, although the span and the skew of the nitrogen CSA tensors of these 2 compounds are very different (248 ppm and 0.23 for the ammonium salt compared with 170 ppm and 0.1 for the potassium salt (Turner et al. 1987)). Nitrates and nitrites have only small but distinct shift ranges (Figure 9.20). The MAS and static 15N NMR spectra of NaNO3 and NaNO2 were used to determine the tensor elements which were then compared with values calculated using local origin methods (Barrie et al. 1994). The CSA tensor for the nitrite unit shows a span of 951 ppm and a skew of 0.37 whereas the nitrate unit CSA tensor has a span of 222 ppm and a skew of - 1. The much greater span of the nitrite reflects the more asymmetric electron density around the nitrogen in this unit. The 5 distinct crystalline phases of NH4NO3 have all been detected in a 15N NMR study although their differences in ~iso are only ,v 1.4 ppm (Anderson-Altmann and Grant 1993). More significant changes were found in the other CSA tensor parameters of these phases. Two resonances separated by only 10 Hz were found in the spectrum of the high temperature phase (V), confirming the presence of 2 slightly inequivalent nitrate sites as suggested by neutron diffraction data. The difference in the 2 types of nitrate is related to the degree of hydrogen-bonding to the ammonium ion. The effect of the previous thermal history on the phase transitions in KNO3 has been studied by variable temperature ~SN MAS NMR (Schonwandt and Jakobsen 1999). 15N MAS NMR was also used to study crystalline solid solutions Ba• where 0.08 < x < 0.93. Four resonances observed in a ~SN-enriched sample of composition x = 0.59 were explained in terms of the different combinations of the 4 metal ions surrounding the nitrate ion (Crundwell et al. 1999). Nitrate ions adsorbed on hydrotalcite can be located in both surface and interlayer sites. Variable temperature 15N static NMR showed 2 signals (Figure 9.24) that varied with temperature and relative humidity (Hou et al. 2000). The interlayer nitrate ion is quite strongly bound and showed a relatively broad CSA pattern over the temperature range of the study. The surface nitrate resonance is much narrower, indicating a higher degree of molecular motion, and has a lineshape which is much more sensitive to temperature and relative humidity. The NMR results indicate that the interaction of nitrate with hydrotalcite is intermediate between the behaviour of mobile chloride ions and tightly bound carbonate.
583
NMR of Other Spin -1/2 Nuclei
observed
I \
t
interlayer I \
Jc-
500
i
]
/ nitrate
/
'-
400
'
300
'
200
lSN shift (ppm) w.r.t. CH3NOz Figure 9.24. Observed and simulated 15N NMR spectra of nitrate ions in recrystallised hydrotalcite at room temperature and room humidity, showing the broad calculated CSA lineshape with a maximum at 448 ppm arising from interlayer nitrate ions and the narrower symmetric peak at 374 ppm from the more mobile surface nitrate ions. From Hou et al. (2000), by permission of the Mineralogical Society of America.
9.3.3 778e N M R 77Se has a natural abundance of 7.86%, is relatively easy to observe and is usefully sensitive to changes in its environment. Many of the published 77Se NMR studies are of organoselenium compounds, and thus outside the scope of this book. These compounds and other aspects of 77Se NMR have been extensively reviewed by Duddeck (1995). The primary reference compound for 77Se NMR is dimethyl selenate, (CH3)zSe, but (NH4)zSeO4 has been suggested as a useful secondary reference compound, as it gives a high quality one-line spectrum at 1040.2 ppm with respect to dimethyl selenate, and can also be used to set up CP experiments (Collins et al. 1986). The 77Se CP MAS NMR spectra of a number of selenium compounds have been reported by Collins et al. (1986). A selection of 77Se NMR parameters are given in Table 9.6. A 77Se and 39K NMR study of single-crystal KzSeO4 has been used to elucidate the nature of the structural transition occurring in this compound at low temperatures (Topic et al. 1988). Measurement of the angular dependence of the 77Se line shifts allowed the chemical-shift tensor to be determined in the paraelectric phase of KzSeO4.
584
M u l t i n u c l e a r Solid-State N M R o f l n o r g a n i c M a t e r i a l s
Table 9.6.
778e
NMR parameters for selenium compounds.
Compound
~iso (ppm)*
Se4(HS207)2 [(CH3)4N]SeO2C1
1939 1371 1288.1 1040.2 863.4 256.1 201.0,189.7,184.0,175.1,165.2
H2SeO3
(NHn)2SeO4 (NH4)zSeC16 (CH3)3SeI (NH4)2CSe
Reference Collins et al. Collins et al. Collins et al. Collins et al. Collins et al. Collins et al. Collinset al.
(1986)
(1986) (1986) (1986) (1986)
(1986) (1986)
* chemical shifts quoted with respect to (CH3)2Se
Unlike the 39K chemical-shift tensor in this compound, the 77Se tensor is largely unchanged when the material passes through the low-temperature phase changes (Topic et al. 1988). A 77Se CP NMR single-crystal study of KHSeO3 and RbHSeO3 has shown that the 77Se chemical shift tensors are as expected for the configuration of the HSeO3ion. Measurements of the spectra from room temperature to 108 K showed no evidence of phase transitions in this temperature range (Vinogradova et al. 1989). By contrast, phase transitions to superionic plastic phases with high conductivity were found at 417 and 446 K by 77Se NMR and conductivity studies in single crystals of NHaHSeO4, ND4DSeO4 and RbHSeO4 (Moskvich et al. 1985). A single-crystal 77Se NMR study of elemental selenium carried out between room temperature and the melting point indicates that the Zeeman spin-lattice relaxation time arises essentially from a 2-phonon (Raman) process over the entire temperature range (GUnther and Kanert 1985). The structure of the fast ionic conductor AgvPSe6 has been elucidated by 77Se MAS NMR which reveals 3 chemically distinct species, 2 of which give rise to spinning sideband patterns due to the large CSA of these sites. Acquiring the 778e NMR spectrum with 3~p decoupling leaves 2 of the resonances unchanged but simplifies the third resonance significantly, collapsing the multiplet into 2 distinct resonances (Figure 9.25) (Maxwell et al. 1990). Thus, the 778e{31p} double-resonance MAS NMR experiment not only differentiates between Se units associated and unassociated with P, but also distinguishes 2 types of P-Se atoms in the asymmetric unit. 31p-VVSe Spin Echo DOuble Resonance (SEDOR) has been used in a study of P-Se binary chalcogenide glasses to determine whether the P atoms are distributed in an isolated fashion or if there is a tendency for P clustering with the formation of P-Se-P bridges (Lathrop and Eckert 1990). SEDOR measurements of the fraction of P-bonded Se atoms were compared with theoretical predictions for the 2 possibilities and clearly indicated a preference for the P atoms in this system to exist in an isolated state. A number of other selenides have been studied by 778e NMR. The spectra of ZnSe, CdSe, HgSe and PbSe were reported to show 77Se chemical shifts of about - 1600 ppm,
NMR of Other Spin -112 Nuclei
without 3~p decoupling
A I~
585
II~
(c) decoupliug
~
. ~1
[
9 '/' 1500 ' ' i4'00 ' ' i300 "/'"i2i~0 / / / /
' '
/ /
/
/
,
/
,'
.," /
/ '1
(b){
I
I I
!L i
" I
]
1500
]
[
~'
i"
I
I
Z
|
I
i
1....... I'
i
r
i
500
|
i
~
i
~
|
i
-5 0
77Se shift (ppm) w.r.t CdSe
Figure 9.25. 77SeNMR spectrum of AgvPSe6 showing three distinct groups of resonances arising from three different Se environments. Inset (above) shows the signals from site (a) with and without 31p decoupling. Note the 2 distinct Se sites with an intensity 3:1 in the decoupled spectrum. From Maxwell et al. (1990), by permission of the copyright owner.
with linewidths which increased with the atomic number of the counterion (Koch et al. 1978). The polycrystalline semi-metallic layered compound TiSe2 has been investigated over the temperature range 1.5-342 K and shows a pronounced decrease in the isotropic Knight shift with decreasing temperature (Dupree et al. 1977). 7 7 8 e NMR measurements of the 2H polytypes of NbSe2 and TaSe2 as a function of temperature show a distribution of Knight shifts producing an asymmetric broadening of the NMR line below the critical temperatures of these compounds (Borsa et al. 1977). A 7 7 8 e NMR measurement of ZnSe has also been reported (Bastow and Stuart 1988). The effect of incorporating alloying elements such as Cu, Ag, T1, I and Ge into amorphous AszSe3 has been studied by 778e NMR, revealing a close connection between changes in the chemical shift and the electrical conductivity of the system (Ueda and Shimizu 1979). The results were taken to indicate that a fraction of the added elements is positively ionised, resulting in an increase in the number of negatively-charged dangling Se bonds and a corresponding shift in the Fermi level. A 7 7 8 e NMR study of molecular clusters in CdSe has revealed, in addition to a spectral component corresponding to the bulk material, other components of the spectrum which move upfield as a function of decreasing particle size (Figure 9.26). The line positions and distributions of the
586
Multinuclear Solid-State NMR of Inorganic Materials
. ~
g
9 ::'~
cluster:ize (~)
i. 9
9
9
-
d
t
,
. ~ ~ ,
-
15-18
30-35
............
_,._..~T _;,~,,~cr -
/~b~
.~.,, ,, bulk
ll,,llILIli,,llil]IZi[t'liJ]'ii[l[II]l'1]~
-200 77Se s h i f t ( p p m )
200
600
w.r.t CdSe
Figure 9.26. 77SeNMR spectra of CdSe molecular clusters isolated by encapsulation with covalently-bonded Se-phenyl or Se-butyl moieties. The 77Se spectra relate to the internal portion of the cluster rather than to the organoselenide ligand. Note the change in relative proportions of the three-line fitted components, shifting towards the position of the bulk material with increasing cluster size. From Thayer et al. (1988), by permission of the copyright owner.
chemical shifts could be related to the size dependence of the electronic structure and the local chemical environment (Thayer et al. 1988). The Se-containing species selenate and selenite are of signficant environmental concern, and 778e NMR has proved useful in studying their exchange and adsorption behaviours which are closely related to their mobility, bioavailability and recoverability. 77Se NMR has been used to investigate the uptake of SeO4 and SeO3 species in the layer lattice of hydrotalcite compounds (Hou and Kirkpatrick 2000). Although SeO4 is a tetrahedral anion, the 778e CSA was found to be uniaxial, reflecting the pattern of hydrogen bonding to octahedral layer hydroxyls and interlayer water. The 77Se spectrum of recrystallised samples is dominated by signals from the interlayer selenate, by contrast with the spectra of the uncrystallised material which show signals from selenate on the surface and in the disordered interlayers. The isotropic 778e chemical shifts in recrystallised hydrotalcite move regularly to less shielded (more positive) values with increasing temperature, probably reflecting the decreasing influence of hydrogen bonding at higher temperatures. Interlayer selenite shows a uniaxial CSA powder pattern independent of the relative humidity of the sample (Hou and Kirkpatrick 2000).
NMR of Other Spin -1/2 Nuclei
587
The similarity between the chemistry of selenium and sulphur has led to a
77Se NMR study of the formation of dimethylselenoxide (DMSeO) intercalated compounds with the clay mineral kaolinite. The DMSeO intercalates serve as model compounds for the technically more important dimethylsulphoxide (DMSO) intercalates which are less accessible to NMR investigation (Raupach et al. 1987). A 77SeCP MAS NMR experiment exploiting the protons present in this system indicated that the DMSeO molecules are in a single environment, supporting a cubic face-centred structure for the intercalate.
9.3.4 lllCd and 1~3Cd NMR The 2 NMR-active nuclei of cadmium are both spin I = 1/2, and have similar natural abundances (12.75% for 111Cd and 12.26% for 113Cd). However, the relative receptivity of ~13Cd is slightly better (see Chapter 1, Table 1.1), making it the nucleus of choice for many Cd NMR studies. Cadmium NMR has proved useful in studies of semiconducting oxide compounds and alloys, and of cadmium exchange in clay minerals and zeolites. The chemical shifts of Cd compounds are normally reported with respect to aqueous Cd(ClO4)2 solution. The 113Cd NMR spectra of CdO prepared by thermal decomposition of a number of cadmium compounds were found to be sensitive to the thermal history of the sample (Meinhold 1987). Samples prepared from cadmium hydroxide or carbonate showed broad resonances with large Knight shifts and rapid relaxation times similar to commercial CdO. Heating commercial CdO brought about changes in the 1~3CdNMR peak position, linewidth and relaxation rate (Figure 9.27A). The abrupt decrease in the CdO peak position in samples heated at 200-400~ (Figure 9.27B) has been ascribed to the decomposition of CdCO3 impurities, while the approximately linear increase in samples annealed at 400-1000~ reflects semiconductor behaviour arising from the formation of oxygen vacancies or interstitial Cd ~ (Meinhold 1987). The cadmium stannates CdzSnO4 and CdSnO3 show interesting photoelectronic properties with potential applications as UV-visible radiation filters or anodes for photoelectrochemical cells. When prepared under oxidising conditions, Cd2SnO4 is yellow but preparations under inert or reducing conditions result in a green form with lower electrical conductance. A 113Cd NMR study showed that the green form has a more positive chemical shift than the yellow, reflecting a larger concentration of electrons in the conduction band of the former resulting from a greater concentration of donors (Cardile et al. 1987). These donors, which could be oxygen vacancies, Cd or Sn 2+ give rise to electron scattering, decreasing the electron mobility and hence the electrical conductivity of the green form of CdzSnO4. The 113Cd MAS NMR spectra have been reported of the cadmium halides (Sakida and Kawamoto 2000) and model halide compounds CdF2 (containing CdF8 structural
588
Multinuclear Solid-State NMR of Inorganic Materials
A heating
temperature (~ 500 460
1 O
40O
1000
e~ U
300
|
~
|
|
,
,
l
570
,
,
,
,
,
,
,
470
l
J
~
,
,
,
..~.D-"
T
i
370
I
i
200
~13Cd shift (ppm) w.r.t. Cd(H~O)62§
7
I .....
600
io
1
1000
Temperature (~
Figure 9.27. A. l l3Cd NMR spectra of commercial CdO heat treated at different temperatures. B. l l3Cd peak shift of commercial CdO as a function of heat treatment temperature. From Meinhold (1987), by permission of Elsevier Science. Table 9.7.
l l3Cd
NMR parameters of cadmium compounds.
Compound
~iso(ppm)*
Reference
CdF2 KCdF3
226 105 183 314-419, 250-676
Sakida et al. (2001a) Sakida et al. (2001a) Sakida et al. (2001a) Mennitt et al. (1981), Meinhold (1987) Meinhold (1987) Meinhold (1987) Cardile et al. (1987) Cardile et al. (1987) Cardile et al. (1987)
CdC12 CdO CdCO3 Cd(OH)21.12H20 Cd2SnO4 (yellow) CdzSnO4 (green)
CdSnO3
-
- 32.4 139 185-196 230-262 232
* chemical shifts with respect to Cd(C104)2 solution
units), KCdF3 (containing CdF6 units) and CdCI2 (containing CdC16 units) (Sakida et al. 2001 a). The isotropic chemical shifts are shown in Table 9.7. 113Cd MAS NMR has been used to study the anion coordination environments of the Cd 2+ in halide glasses (Sakida and Kawamoto 2001) and a series of glasses in the system CdF2-NaC1-BaF2-BaC12 (Sakida et al. 2001a). Additions of 0.2 mol% NiF2 made to the glasses shortened the 113Cd relaxation time from 20 s to 1 s but had no
589
NMR of Other Spin -1/2 Nuclei CdCI6 CdF6 CdFs ~
~
i
composition x
,
-92,1 i -351 ~
' 611
I
. . . .
1000
I
'
I
0
i
~
,i,
,
I
. . . .
i
-1000
ll3Cd shift (ppm) w.r.t. Cd(H20)62+ Figure 9.28. 113CdMAS NMR spectra of glasses in the series 50CdF2.30NaC1. (20-x)BaF2.xBaC12 showing changes in the speciation with composition x. The broken lines show the positions of the expected structural units determined from model compounds. From Sakida et al. (2001 a), by permission of Elsevier Science.
effect on the resonance position and width. Changes in the l J3Cd resonance width and position with increasing BaC12 content are consistent with the presence in these glasses of CdFmCln polyhedra containing increasing concentrations of C1 (Figure 9.28) and a greater variety of coordination environments around the Cd. The adsorption of Cd onto the clay minerals montmorillonite, hectorite and kaolinite has been studied by 113Cd NMR from which different sites in the ion exchanged clay could be identified (Bank et al. 1989). The spectra were found to contain contributions from cadmium oxoanions giving differing linewidths depending on the preparation conditions. The spectral linewidths were found to depend more on dynamic processes and paramagnetic effects related to the iron impurities in the montmorillonite than chemical shift dispersion. By comparison with the 113Cd NMR spectra of Cd 2+ adsorbed on montmorillonite, which contain both narrow and paramagnetically broadened components, the spectra of Cd on hectorite and kaolinite (minerals containing no iron in the octahedral layers) show only the narrow resonance, arising in the case of kaolinite from a cadmium oxyspecies (Bank et al. 1989). The 113Cd NMR spectra of Cd-exchanged montmorillonite and hectorite have been acquired for both powder and gel samples (Tinet et al. 1991). From the spectra of the gel samples it was possible to
590
Multinuclear Solid-State N M R o f lnorganic Materials
differentiate between cadmium located in the outer and inner clay layers, while the ratio of the 2 sites thus determined provided evidence of the existence of a 3-moleculethick water layer between the clay sheets (Tinet et al. 1991). A single ~13Cd MAS NMRresonance observed in montmorillonite treated with aqueous CdC|2 solution located in the same position as the hydrated Cd 2+ ion has been taken as an indication that selective adsorption of cadmium is associated with the oxygen donors of the montmorillonite (Jun et al. 1996). ~l~Cd NMR has been used to study exchangeable Cd 2+ in the interlayer surface of swelling micas and vermiculites (Laperche et al. 1990). In this work ~ C d was used rather than ~3Cd since the MAS probe could not be tuned to the latter nuclide. The one-layer vermiculite hydrate showed a ~ ~Cd giso value of about - 15 ppm with a strong associated spinning sideband manifold. The dehydrated phase was characterised by a giso value of - 32 ppm. The effect of exchanging Cd 2+ into zeolite LTA has been studied by 1~3Cd NMR, enabling the identification of 2 different cadmium coordination states in the hydrated form of the zeolite. The effect of cadmium exchange on the silicon and aluminium environments of this zeolite was also investigated using 29Si and 27A1 NMR spectroscopy (Eldewick et al. 1999). Cadmium forms a number of telluride semiconductor alloys with manganese, iron and mercury, the behaviour and properties of which have been studied by ~13Cd NMR. Well-resolved ~3Cd MAS NMR spectra have been obtained for a series of compounds Cd~_xMn• where x (ranging from 0.01 to 0.055) is such that the compounds are diluted magnetic semiconductors (DMS). The temperature dependence of the resonances follows the Curie-Weiss law, indicating that the transfer of hyperfine interaction between the Mn 2+ and the Cd (or Te) nuclei is predominantly responsible for the NMR behaviour (Beshah et al. 1993). A full spectral assignment could be made on the basis of a random distribution of Mn in the lattice, also assuming that the transferred hyperfine interaction leading to the observed shifts in the spectral lines was dependent on the number of Cd-Mn bonds present. The resulting assignment indicated that the spin density transferred to the Cd lattice sites decreases tenfold from one Cd shell to the next (Beshah et al. 1993). The local electronic structure of these alloys at higher Mn concentrations (x = 0.01 to 0.6) has also been investigated by ~13Cd NMR, together with 55Mn and 125Te NMR (Bose 1991). The series of DMS alloys Cd~_ xFexTe where x = 0.01 to 0.05 has been studied by ~13Cd and 125Te NMR, showing shifts in the resonances arising from the transferred hyperfine interaction with the neighbouring paramagnetic ions (Gavish et al. 1993). The intensities of the resonance lines were correlated with the probabilities of finding iron atoms in distinguishable positions in proximity to the Cd or Te nuclei, and were consistent with a random distribution of magnetic ions in the zincblende crystal structure of the alloy. Measurements of the ~3Cd spin-relaxation times in these materials suggest that the relaxation mechanism is determined by the dipolar interaction (Gavish et al. 1993).
NMR of Other Spin -1/2 Nuclei
591
111Cd and 113Cd NMR has also been used to augment 125Te and 199Hg single-crystal studies of the semiconductor alloy Hgo.vsCdo.z2Te (Zax et al. 1993). By comparison with the 199Hg NMR spectra which are extremely sensitive to compositional effects in these alloys, the Cd NMR spectra were found to be relatively insensitive to composition and structural features. However, the spectra of the lattice Cd have been observed and assigned, and double-resonance Hg-Cd (and Hg-Te) experiments have enabled the shifts in the Te spectra to be assigned to distinct chemical environments (Zax et al. 1993). 9.3.5 ttSSn, tt7Sn and 119Sn NMR
All three NMR-active isotopes of tin are spin I = 1/2 with natural abundances of 0.35% (115Sn), 7.67% (117Sn) and 8.58% (119Sn). The relative gyromagnetic ratios of the 3 nuclei are 0.877:0.956:1. In practice, 119Snis usually observed although 117Sn is sometimes used as a viable alternative. J-coupling is sometimes manifest in the MAS NMR spectra of tin. The most frequent application of solid-state 119SnNMR, to the study of organotin materials, is outside the scope of this book, but is covered extensively by Sebald (1995) and Wrackmeyer (1999). In organotin compounds CP from 1H is routinely used to give both sensitivity enhancement and more particularly faster recycle delays. (C6Hll)4Sn, which resonates at - 9 7 . 3 5 ppm, is a useful compound for setting up CP experiments. When direct one-pulse observation is employed careful thought needs to be given to the recycle delay; T1 values of a few seconds are often reported but in our experience tin can often have relaxation times of tens of seconds, and, if possible, paramagnetic doping may usefully be employed to shorten the relaxation time. Well crystallised SnO2, resonating at - 604.3 ppm relative to the primary shift standard of (CH3)4Sn, is a useful secondary shift reference for MAS studies. Other interesting practical questions are whether to use MAS and what magnetic field to employ. For highly crystalline tin compounds MAS will certainly resolve different sites. Data collected for tin show that moderate to fast spinning speeds are best, although sufficient spinning sidebands should be present to define the shift tensor which can also be a sensitive probe of the structure. Often it is of fundamental interest to know the Sn(II) to Sn(IV) ratio. Although the ~iso values will reveal this difference, the CSA span is just as sensitive. The static NMR spectra of samples containing both Sn(II) and Sn(IV) have distinctive but overlapping lineshapes from these sites which can be deconvolved. Static probes tend to allow much larger samples to be used, improving the S/N ratio. The consequence of chemical shift dispersion in disordered materials is that fast MAS speeds are necessary to achieve narrowing, restricting the sample volume. In our laboratory we have therefore tended to use static spectroscopy at relatively low field (e.g. 5.6 T).
9.3.5.1 Crystalline oxygen-containing materials. The structural chemistry of tin is complex because it can take different coordination numbers and also different oxidation states. Both these effects influence the shift, and need to be disentangled before the
592
Multinuclear Solid-State N M R of lnorganic Materials
structural source of a resonance can be definitively stated. The 2 main oxidation states Sn(II) and Sn(IV) often occur in sites with very different symmetry. This point is illustrated by comparison of SnO2 (containing octahedral SnO6) and SnO (in which the tin is in a square pyramid configuration with 4 oxygens on the base and the lone pair opposite). The ~iso values corresponding to these 2 oxides differ by --~ 400 ppm. Differences in the site symmetries are reflected in the spans of the CSA tensors (--~130 ppm for SnOe and --~1013 ppm in SnO) (Cossement et al. 1992). The ll9Sn NMR spectrum of SnO also shows clear evidence of Sn-Sn J-coupling of 8300 Hz (Figure 9.29). l l9Sn MAS NMR has been used to study the effect of thermal manipulation of the particle size of SnOe. The spectra of 4-32 nm nanoparticles were compared with a bulk sample of 10 Ixm particle size (Tunstall et al. 1999). The CSA and T1 values were found to change most rapidly around 8 nm. The formation of SnO2 nanoparticles within the pores of zeolites has also been observed by 119Sn NMR (Wark et al. 1995). A ll9Sn MAS NMR study has reported the CSA tensor parameters of 14 MSnO3- and MeSnOa-type compounds (Clayden et al. 1989). Sn-O-Sn J-coupling observed in BaSnO3 had a value of 346.6 Hz, an order of magnitude smaller than for the direct J-coupling in SnO. The l l9Sn ~isovalues were shown to be very sensitive to the local coordination. In going from Li2SnO3 and Na2SnO3 to KzSnO3 the tin coordination changes from SnO6 to SnOs, reflected both by a change in ~iso of --~140 ppm and a large change in the span of the CSA. Although the change in the crystal structure of CaSnO3 from
-
.I .........
I .... 200
, ....
I .........
I ......... -200
I .........
! ......... -600
I .........
_ _ ~ -
-
__
! ..... -1000
119Sn shift (ppm) w.r.t (CH3)4Sn Figure 9.29. ll9Sn ]VIASNMR spectrum of SnO powder. From Cossement et al. (1992), by permission of John Wiley and Sons Ltd.
593
N M R o f O t h e r Spin -1/2 N u c l e i
orthorhombic to hexagonal does not change the gross nature of the local tin coordination, the ~iso value changes by 68.2 ppm (Dirken et al. 1996). In a series of stannates with strictly analogous structures, a correlation was found between parts of the CSA tensor and structural parameters (bondlengths and angles) (Clayden et al. 1989). Although l l9Sn MiSssbauer spectroscopy offers an alternative way of characterising the tin sites in a material, there are a number of cases where M r s s b a u e r and N M R results do not agree. For example, M r s s b a u e r studies of Sn2Nb207 (Cruz et al. 2001) indicate the presence of Sn(II)O8 and Sn(IV)O6 yet the 119Sn N M R shows only a single relatively broad resonance. The lack of a distinct second resonance may mean that the 2 sites have similar unresolved shifts, or that the Sn(II) site is not observed due to a large chemical shift dispersion effect, or that the M r s s b a u e r results are not being interpreted correctly. SnO2 can be i n c o r p o r a t e d b e t w e e n the layers of m o n t m o r i l l o n i t e by in s i t u h y d r o l y s i s of (NHn)2SnC16. The initially f o r m e d p a r t i a l l y - h y d r a t e d SnO2 has a l l9Sn N M R peak at - 590 ppm which subsequently hydrates to pure SnO2 on the outer surface of the clay (Hannus et al. 1995). A series of Mg-A1-Sn hydrotalcite double layer hydroxides gave a very broad l l9Sn signal with a m a x i m u m between 588 and - 599 ppm, indicating that the tin environment is SnO6. On heating, the structure breaks down to a mixture of crystalline phases, with tin incorporated in -
Mg2SnO4 (Velu et al. 1999).
Table 9.8.
l l9Sn
NMR parameters for oxide-based and oxygen-containing materials.
Compound SnO2
SnO MgSnO3 Ortho-CaSnO3 Hex-CaSnO3 SrSnO3
~iso (ppm)*
1~ (ppm)
Skew
Reference
604.3, - 603 - 603, - 604 - 208 - 586.3 - 611.7, - 610, - 611.7 - 543 - 640.8, - 640 640.8 - 679.2 - 512.6 - 483.5 - 484 - 471.4 - 546.3
125, ND 136, 121 1013 N N
--~1, ND 0.66, 0.84 0.87 N N
ND N
ND N
N N 142 ND ND 205
N N 0.35 ND ND 0.50
Clayden et al. (1989) Sebald et al. (1990) Cossement et al. (1992) Kulshreshtha et al. (2001) Cossement et al. (1992) Clayden et al. (1989) Clayden et al. (1989), Sebald et al. (1990), Dirken et al. (1996) Dirken et al. (1996) Clayden et al. (1989), Sebald et al. (1990) Dirken et al. (1996) Clayden et al. (1989) Clayden et al. (1989) Clayden et al. (1989) Sebald et al. (1990) Velu et al. (1999) Clayden et al. (1989)
-
-
BaSnO3 CdSnO3 Mg2SnO4 Ca2SnO4
594
M u l t i n u c l e a r Solid-State N M R o f l n o r g a n i c M a t e r i a l s
Table 9.8. (Continued) Compound
~iso (ppm)*
1~ (ppm)
Skew
Reference
Sr2SnO4
- 579.6 - 564.3 - 594.3 -471.4 - 606.7 - 444.3 - 444.9 - 480.3 - 564 - 312.8 - 570
171 ND 210 151 N 117 ND 112 ND 367 ND ND
0.55 ND 0.60 0.11 N - 0.22 ND - 0.29 ND 0.48 ND ND
Clayden et al. (1989), Dirken et al. (1996) Clayden et al. (1989) Clayden et al. (1989) Clayden et al. (1989) Clayden et al. (1989), Dirken et al. (1996) Clayden et al. (1989) Sebald et al. (1990) Clayden et al. (1989) Sebald et al. (1990) Clayden et al. (1989)
ND ND ND ND ND ND ND
ND ND ND ND ND ND ND
Cruz et al. (2001) Cruz et al. (2001) Grey et al. (1989) Grey et al. (1989) Grey et al. (1989) Sebald et al. (1990) Martinez et al. (1994)
ND ND
ND ND
Martinez-Juarez et al. (1995) Clayden & Pugh (1998)
Ba2SnO4 Zn2SnO4 Cd2SnO4 Li2SnO3 Na2SnO3 Na2SnO3.3H20 K2SnO3 K2SnO3.3H20 K3Sn307,
site 1 site 2 site 3
Sn2Nb207 SnNb206 Y2Sn207 La2Sn207 LuzSn207 Na4SnBe2Si309.2H20 Mono-LiSn2(PO4)3, site site Rhombo-LiSnz(PO4)3 NaSn2(PO4)3, site site
-
491.8
- 520.5 - 548.1 - 605 -
751
- 582 - 642 -
1 2 1 2
641
- 706 816 - 833 - 829 -
821
- 835
* chemical shifts quotedwithrespectto (CH3)4Sn N - negligible,ND- not determined.
9 . 3 . 5 . 2 O x i d e s o l i d s o l u t i o n s a n d g l a s s e s . Tin is one of the elements that can be
substituted into the f r a m e w o r k of zeolite structures. Small amounts of tin (Si/Sn > 40) have been found to enhance catalytic activity of a M E L - f o r m . This substitution has been studied by l l9Sn N M R , which indicated that almost all the tin was in octahedral coordination, in contradiction to an X R D finding that --~ 20% of the tin entered as SnO4 (Mal et al. 1995). This discrepancy m a y arise from difficulties in interpreting the N M R results because of complications from coordination and oxidation effects. The addition of tin to M C M - 4 1 resulted in the appearance of a ll9Sn N M R peak at - 6 7 7 to - 7 1 0 p p m which was attributed to tetrahedral tin, although the reasoning for this assignment is not clear (Chauhari et al. 1999). The entry of tin into the structure of m i c r o p o r o u s N a 2 0 - S n O 2 - S i O 2 occurs in an SnO6 site with 6 silicon n e x t - n e a r e s t neighbours, giving rise to reported 119Sn N M R shifts in the range - 688 to - 709 p p m (Lin et al. 1999, Lin et al. 2000, Ferreira et al. 2001). These shifts are close to the values reported for dense f r a m e w o r k sodium stannosilicates ( - 706 and - 708.3 ppm)
NMR of Other Spin -1/2 Nuclei
595
(Corcoran and Vaughan 1989). The Sn(II):Sn(IV) ratio in tin germanate glasses can best be determined by fitting the static 119Sn spectra to the components; these 2 species have very different CSA values and no significant narrowing could be achieved by spinning at 15 kHz at 5.6 T (Holland et al. 2001). The acid sites in catalytically-active AlzO3-SnO2 have been characterised by 31p NMR observations of trimethylphosphine probe molecules. The 119Sn NMR spectrum of these materials contains a narrow line corresponding to bulk SnO2 superimposed on a broad line thought to arise from interaction between the tin and the alumina (Sheng et al. 1994). Doping TiO2 with SnO2 promotes direct crystallisation to the mille form and helps to preserve the nanocrystalline nature of the TiO2. Two competing models have been proposed for the structure of this material; one involves the precipitation of TiO2 onto seed nuclei of SnO2 with the rutile structure, while the other involves the formation of a solid solution with the tin. The l l9Sn NMR spectrum of (TiOz)x(SnO2)l-x formed by a sol-gel process contained a range of peaks arising from numerous tin environments, clearly indicating that the solid solution model is correct (Bastow et al. 1995). The intensities of these NMR resonances were consistent with random site substitution. More recent work on co-precipitated samples over a wider composition range shows numerous isotropic peaks (Figure 9.30A) confirming the formation of solid solutions, although the intensity distribution could not be reconciled with purely random substitution (Kulshreshtha et al. 2001). An elegant 119SnNMR study has been made of lanthanide pyrochlores solid solutions which find important applications as phosphor materials. The paramagnetic contact shift from the rare-earth elements gives rise to a huge isotropic shift range (5400 to - 4200 ppm). The local coordination of the tin sites is SnO6Ln6, of which the 6 next-nearest neighbour Ln atoms can be different lanthanides. In the case of Yz-xSmxSn207 7 l l9Sn isotropic peaks are observed, ranging from - 592 ppm for 6 Sm next-nearest neighbours to - 92 ppm for 6 Y next-nearest neighbours (Figure 9.30B). The 119Sn NMR spectrum can thus be used as a sensitive probe of the structural disposition of the lanthanide dopants. The quantitative distribution can also be obtained if care is taken, recognising the wide range ofT1 of nearly 5 orders of magnitude between different peaks (Grey et al. 1989). 9.3.5.3 Non-oxide materials. The Sn-S system is a good glass former with a rich
structural chemistry arising from the ability to form a number of different coordination units and the possible variations in their manner of linking up by comer or edge-sharing. SnS contains a distorted octahedral Sn(II)S6 environment giving rise to a large CSA. SnS2 displays extensive polytypism; the centreband of its l l9Sn NMR spectrum shows 7 peaks over a range of 16.7 ppm (Peitrass and Taulelle 1997). The lack of a consistent separation between these rules out a J-coupled multiplet structure. Although XRD indicates the presence of SnS2, this technique is not sensitive to the several
596
Multinuclear Solid-State N M R o f lnorganic Materials
A
composition x
0
-200
-400
-600
tt9Sn shift (ppm) w.r.t. (CH3)4Sn
1.
.
.
.
.
.
.
.
.
|
.
-500
.
.
.
.
.
.
.
.
|
.
.
.
.
.
.
.
.
.
| . . .
.
.
.
.
.
.
.
-700
ll9Sn shift (ppm) w.r.t (Cit3)48n
Figure 9.30. A. l l9Sn MAS NMR spectra of Sn~-xTixO2 for samples of varying composition, heated at 800~ for 16 h. Spectra processed with side band suppression. From Kulshreshtha et al. (2001), by permission of the Royal Society of Chemistry. B. l l9Sn MAS NMR spectrum of YSmSn2OT. The 7 isotropic resonances are shaded; the remaining peaks are spinning sidebands. From Grey et al. (1989), by permission of the American Chemical Society. different possible layering sequences present which would nevertheless give rise to different values of ~iso. Sn2S3 forms a mixed compound in which 2 environments similar to SnS and SnS2 are detected by 119Sn NMR (Pietrass and Taulelle 1997). The presence of lithium, which can be intercalated into SnS2, changes the relative intensity but not the spacing of the 119Sn spectrum, suggesting its interference with the stacking sequence. Additional peaks appearing at - 7 4 1 and - 5 6 2 ppm are associated with local changes due to the lithium intercalation. A large loss of 119Sn NMR signal observed when the lithium is initially added is due to the delocalisation of electron spins from the lithium; the loss of this signal per lithium decreases as the lithium content increases and the electron spins begin to pair up. All the spectral changes can be rationalised by assuming that the lithium first occupies the interlayer octahedral sites, then the interlayer tetrahedral sites and finally the intralayer site (Pietrass et al. 1997). The various crystalline phases in the NaS-SnS2 system contain tin in a range of environments with a wide variety of symmetry. These can be distinguished in the l l9Sn NMR spectra, which show a correlation with the span of the CSA (Table 9.9, Figure 9.31), (Mundus et al. 1997).
N M R o f O t h e r Spin -1/2 N u c l e i
597
Table 9.9. l l9Sn NMR parameters from non-oxide materials. Compound
Structural unit
~iso(ppm)*
l~(ppm)
Skew
Reference
ND
Scotti et al. (1999)
SnS2
Sn(IV)S6
- 123, - 370 - 301.0, - 299 - 765.4, - 764 - 274, - 719 67.6 61.7 - 16.0 - 359.6 - 773.1
ND
SnS
Sn(IV)N4, Sn(IV)N6 Sn(II)S6
624, ND 105, ND ND
0.61, ND 1.00, ND ND
< 100 339 674 931 < 70
ND - 0.02 0.69 - 0.76 ND
Sn3N4,
SnaS3, Na4SnS4 Na6Sn2S7 Na4Sn3S8, Na2SnS3
site 1 site 2
site 1, site 2
site 1, site 2
Sn(II)S6, Sn(IV)S6 Sn(IV)S4 Sn(IV)S4 Sn(IV)S4, Sn(IV)S6 Sn(IV)S6
Mundus Pietrass & Mundus Pietrass & Pietrass &
et al. (1996), Taulelle (1997) et al. (1996), Taulelle (1997) Taulelle (1997)
Mundus et al. (1996) Mundus et al. (1996) Mundus et al. (1996) Mundus et al. (1996)
* chemical shifts quotedwith respect to (CH3)4Sn ND - not determined.
|
. . . .
,
. . . .
| , " . . . ,
150
. . . .
Ill |,,,,,,,,,|,,,,,.,,|,,
.......
300
, ....
i ....
i
. . . .
50
l, ........
Na6Sn2S7
i .........
100
i ....
| ....
-600
, ....
| ....
, ....
i
-100
i ....
, ....
-750
| ....
! ....
,
-900
II]i,
LJ,!_lJlJ.!U Na4Sn3Ss
,
!
,
i
,'
|
,
|
,
|
,
Jt
'
|
'
|
4O0 -2O0 -80O ll9Sn shift (ppln) w.r.t. (CH3)4Sn Figure 9.31. lWSn MAS NMR spectra corresponding to the main structural units of the stoichiometric compounds in the system Na2S-SnS2. The weak peaks in the spectrum of Na4SnS4 are ascribed to an Na6SneO7 impurity. From Mundus et al. (1996), by permission of the copyright owner.
598 9.3.6
Multinuclear Solid-State NMR of Inorganic Materials
123Tea n d 125TeN M R
The 2 NMR-active isotopes of tellurium are both spin-l/2 but all solid state NMR studies to date have used 125Te because its sensitivity is more than an order of magnitude greater than that of 123Te. Tellurium has a widely varying chemistry, its range of oxidation states from - 2 to + 6 resulting in a wide ~25Te chemical shift range (--~10000 ppm). Elemental tellurium contains a single axially symmetric site with an isotropic shift of 1003.1 ppm and a span of 1790 ppm (Orion et al. 1997). The large shift range enables crystallographic inequivalences to be detected readily by MAS NMR; the 6iso values of the 2 Te sites which can be resolved in both Te(OH)6 and TeCI4 differ by 6.7 and 25 ppm respectively (Orion et al. 1997). ~25Te can have long T~ values, a problem overcome in recent work (Sakida et al. 1999a) by doping the samples with 0.3 mol % Fe203 which speeded up the relaxation and allowed the use of 2.5 s recycle delays compared to delays of 20 s in the undoped samples. Telluric acid (Te(OH)6) can be used as a secondary solid shift reference for MAS NMR (Orion et al. 1997, Sakida et al. 1999a), the higher resonance frequency peak being taken as 692.2 ppm with respect to the primary shift reference compound (CH3)2Te. Te(OH)6 can also be used to set up 1H-125Te CP experiments (Collins et al. 1987). The shift ranges for ~25Te in different environments are shown schematically in Figure 9.32.
Tellurides can demonstrate interesting technological properties including semiconducting behaviour. Undoped semiconducting tellurides have shifts in the range - 1600 to - 1900 ppm (Willig et al. 1976, Balz et al. 1980, Bastow and Stuart 1988). Direct dipolar and J-coupling occurs in these compounds, the J-coupling being sufficiently large that the anisotropy in the J-coupling tensor could be determined (Balz et al. 1980). The extensive solid solutions which exist between 9.3.6.1 Crystalline teUurides.
9
TeCI
4
9 Te ~'~
TeO3 am TeO3+l e-TeO4
"--' 9~
!
2000
I
c-TeO4
Tellurides /
1000
I
I
0
I
I
-1000
, I
!
-2000
12STe shift (ppm) w.r.t. (CH3)2Te Figure 9.32. Schematic diagram of the range of ~25Teshifts occurring in tellurium compounds.
N M R of Other Spin -1/2 Nuclei
599
some of the binary tellurides can be used to tailor the physical properties. These solid solutions are examples of crystalline compounds with a high level of atomic disorder and are composed of tetrahedral networks in which the Te takes nearest neighbour arrangements of the form TeAxB4-x. The short-range nature of the interactions that influence solid-state NMR makes it a good technique for studying the atomic ordering in these compounds. 125Te NMR has been used to study semiconducting telluride alloy systems including Cd-Mn-Te (Bose 1991, Beshah et al. 1993), Hg-Cd-Te (Zax et al. 1987, Zax et al. 1993) and Cd-Fe-Te (Ganish et al. 1993). The formation of solid solutions is usually accompanied by significant broadening of the 125Te NMR spectrum but distinct resonances from different nearest neighbour configurations TeAxB4-x can be observed. Where there is a paramagnetic ion present, the hyperfine contact interaction causes these shifts to be larger than in systems containing only diamagnetic ions, and significant differential relaxation times can be encountered. The degree of randomness in the atomic ordering can vary; although in most cases the evidence suggests that atomic ordering is nearly random, mercury clustering has been suggested to occur in mercury-containing systems (Zax et al. 1987). 125Te NMR spectra reported for 11 transition metal tellurides all show large frequency shifts in the range 550 to - 7400 ppm (Orion et al. 1997) due to hyperfine effects where the paramagnetic shift increases with increasing overlap between the anionic sp bands and the cationic d levels. The shifts can therefore be used to provide an estimation of the electron transfer from Te --~ metal and hence the Te oxidation state, which varies from - 2 to - 1 in these tellurides (Orion et al. 1997). Similar shifts are observed in the ternary compounds NbSiTe4, Nb3SiTe6 and Nb3GeTe6 (Orion et al. 1997). The 125Te NMR spectrum reported for the sodalite-structured telluride Sr8(A102)12Te2 has a shift of - 1372 ppm (Dann and Weller 1996). 9.3.6.2 Crystalline tellurites a n d tellurates. In tellurites and tellurates there are 4 distinct
structural units, all with bonds to oxygen and with tellurium in the + 4 oxidation state. These units are TeO3 (containing both isolated and terminal Te types), TeO3+l (containing one long Te-O bond), TeO4 with 1 comer shared and TeO4 with 1 edge shared. The 125Te NMR shifts associated with these structural units have been determined in a study of 21 tellurium-containing crystalline materials (Figure 9.33) (Sakida et al. 1999a). General trends were identified in the ~i~ovalues (TeO3 I> TeO3+ ~ ~ edge sharing TeO4 i> comer sharing TeO4), but the strong overlap of the shifts corresponding to these units makes their unambiguous identification difficult. However, different TeOx coordinations present in a given compound display different resonances, the higher coordination state showing a smaller shift (Table 9.10). Additional information from the CSA tensor has been invoked to help identify the different structural units. A composite plot of-q vs. IASI identified broad regions associated with the different structural units although again there was overlap between the different coordinations (Sakida et al. 1999a).
600
Multinuclear Solid-State NMR of lnorganic Materials
MAS
1722 Li2TeO3 h * 1669
~ M
1669
~
3
t
.i_l,
~
,
+
I
units
I
corner-sharing TeO4 units
1569 e 0
~ ,
TeO~units
1463
ot-TeO2
~
isolated TeO3units
.
,
,
,
I
,
i
edge-sharing units
4 ,
,
l
,
,
,
,
I
,
2500 1500 500 12STe shift (ppm) w.r.t (CHj)2Te Figure 9.33. ~25TeMAS NMR spectra of model crystalline compounds containing the representative Te-O units occurring in tellurium materials. The asterisks mark the positions of the 125Te isotropic shift. From Sakida et al. (1999), by permission of Elsevier Science.
Table 9.10. J25TeNMR parameters of crystalline tellurites and tellurates. Compound Li2TeO3 Na2TeO3 K2TeO3 Ag2TeO3 PbTeO3 BaTeO3 ZnTeO3 Sr8(A102) I2(TeO3)2 Cs2Te205 Te2V209 Te3Nb2Oll, site 1 site 2 MgTe205 a-LizTe2Os, site 1 site 2 [3-Li2Te205, site 1 site 2
Unit i-TeO3 i-TeO3 i-TeO3 i-TeO3 i-TeO3 i-TeO3 i-TeO3 i-TeO3 t-TeO3 t-TeO3 t-TeO3 c-TeO4 TeO3+l TeO3+l TeO3 + 1 TeO3+l TeO3 + 1
~iso
~'~
(ppm)*
(ppm)
1722 1787 1732 1658 1690 1712 1728 1742 1887 1669 1756 1557 1669 1599 1613 1632 1618
580 681 ND 573 759 658 882 ND 910 1068 1053 ND 1238 1285 ND ND ND
Skew
Reference
0.86 0.81 ND 0.75 0.42 0.41 0.31 ND 0.74 0.75 0.63 ND 0.65 0.17 ND ND ND
Sakida et al. (1999) Sakida et al. (1999) Sakida et al. (1999) Sakida et al. (1999) Sakida et al. (1999) Sakida et al. (1999) Sakida et al. (1999) Dann & Weller (1996) Sakida et al. (1999) Sakida et al. (1999) Sakida et al. (1999) Sakida et al. (1999) Sakida et al. (1999) Sakida et al. (1999)
601
N M R o f Other S p i n J / e N u c l e i
Table 9.10. (Continued)
Compound Zn2Te308, Mg2Te308,
Unit site 1 site 2 site 1 site 2
ot-Te02 TiTe308 ZrTe308 HfTe308 SnTe308 [3-TeO2 NaVTeO5
KVTeO5
c-TeO4 TeO3+l
c-TeO4 TeO3+l c-TeO4 c-TeO4
c-TeO4 c-TeO4 c-TeO4
e-TeO4 e-TeO4 e-TeO4
~iso
~'~
(ppm)*
(ppm)
1545 1679 1517 1620 1463 1461 1439 1493 1489 1536 1569 1630 1725
1540 ND 1517 ND 1509 1295 1533 1486 1541 1579 1509 1687 1688
Skew
Reference
0.24 ND 0.31 ND 0.23 0.27 0.17 0.29 0.29 0.18 0.29 0.32 0.33
Sakidaet al. (1999) Sakidaet al. (1999) Sakidaet al. (1999) Orionet al. (1997) Sakidaet al. (1999) Sakidaet al. (1999) Sakidaet al. (1999) Sakidaet al. (1999) Sakidaet al. (1999) Sakidaet al. (1999) Sakidaet al. (1999)
* chemical shifts quoted with respect to (CH3)aTe * ND - not determined, i - isolated, t - terminal, c - comer-sharing, e - edge-sharing.
m a t e r i a l s . Sakida and colleagues have carried out a series of 125Te NMR studies of binary tellurite glasses, in which they identified the different units by comparison with their extensive data collected for crystalline materials (Table 9.10). The static 125Te NMR spectra were fitted to 2 CSA powder patterns corresponding to TeO3 and TeO4 units with a difference of --~ 300 ppm in their isotropic shifts. Typical simulations are shown in Figure 9.34A. 125Te NMR studies have been made of the systems M20-TeO2 where M = Li, Na, K, Rb, Cs (Sakida et al. 1999a), MO-TeO2 where M = Mg, Zn, Sr, Ba, Pb (Sakida et al. 1999c), MzO3-TeO2 where M = A1, Ga (Sakida et al. 2001) and g 2 0 5 - Y e O 2 (Sakida et al. 2000). Where possible the NMR spectra of the other nuclei (e.g. 27A1, 51V) were also obtained, giving new insight into the constitution of these glasses and allowing new structural models to be proposed. An increase in the content of the second metal oxide led to a general tendency for the number of TeO3 units to increase at the expense of TeO4 units (Figure 9.34B). The nature of the glass network and the rate at which the different tellurium coordinations change was found to depend on the glass-forming tendency of the second metal oxide. 9.3.6.3 G l a s s y t e l l u r i u m - c o n t a i n i n g
9.3.7
129XeN M R
1 2 9 X e is a nucleus with a natural abundance of 26.4% and good relative sensitivity (greater than that of 2 9 8 i and t3C). In samples containing pure Xe gas, ~29Xe NMR suffers from the drawback of long T1 relaxation times, but these are significantly shortened by the presence of a small amount of paramagnetic oxygen.
602
Multinuclear Solid-State N M R o f Inorganic Materials
A
composition x 100
-9, 1 , 1 . , 1
....
i,,,,l'r,,,|.J..|
....
| ....
i~-,!
~,,,
.Jl
~ 60 rJl
~
20 ,....i....|.u
0
20
40
100
.a,.~
~ 60 ~_
16 ~
'l~_l
tl
I I It
4000
Ilil,
t i ~ [ ~lllll,
2000
I,J,,J
~ I , ill
0
12STe shift (ppm) w.r.t. (CH3)~Te
20
VO4 VO4chain I ~ ~ V O s zigzag
oli
,.. ch.a!.,
0
20
,..
40
V20 s (mole %)
Figure 9.34. A. 125Testatic NMR spectra of xGa203(100 - x)TeO2 glasses with fitted peaks corresponding to simulated TeO4 and TeO3 lineshapes. From Sakida et al. (2001), by permission of the American Ceramic Society. B. Change in the content of tellurate and vanadate species in V2OsTeO2 glasses as a function of V203 content, determined from ~25Teand 5~V NMR data. From Sakida et al. (2000), by permission of the Institute of Physics.
In view of the chemistry of this inert element, the main application of Xe N M R is as a surface probe for studying meso and microporous solids and the free volume in polymers. The relaxation time for Xe adsorbed in solids is typically 10 ms to a few seconds. The use of 129Xe NMR as a probe for studying microporous solids has been extensively reviewed by Barrie and Klinowski (1992). A more recent example of the use of ]29Xe NMR to study surface interactions is provided by a study of borosilicalites with the ZSM-5 structure (Ngokoli-Kekele et al. 1998). The 129Xe shift of adsorbed xenon (referred to the shift of the pure gas extrapolated to zero pressure) was found to change regularly with boron content, with a discontinuity at a boron content of about one atom per unit cell ascribed to a change in the distribution of boron atoms in the lattice. A similar correlation between the 129Xe NMR shift and the aluminium content has been reported for the zeolite ZSM-5, in which the discontinuity occurred at about 2 A1 atoms per unit cell (Chen et al. 1992). One and two-dimensional 129XeNMR data have been obtained for the adsorption of Xe in NaY zeolite (Labouriau et al. 1999). Although the use of 129Xe N M R to gain
NMR of Other Spin -1/2 Nuclei
603
accurate information about the energetics or locations of the Xe atoms in the zeolite pores is hampered by a present systematic lack of understanding of the Xe shift behaviour, the two-dimensional data suggest that the Xe atoms spend most of their time in the oL-cages and, at lower temperatures, near the cage walls (Labouriau et al. 1999). A 129XeNMR study has also been made of 3 mesoporous silica materials synthesised by a sol-gel process (Pietrass et al. 1999). Measurements of the temperaturedependence of the chemical shift and the spin-spin lattice relaxation times suggest that the Xe does not penetrate the largely disordered silica structure. The relaxation data were confirmed by 2D NMR exchange experiments which indicate rapid exchange of Xe between the adsorbed and gas phase (Pietrass et al. 1999).
9.3.8 t9Spt NMR The considerable practical importance of elemental platinum in catalytic applications is reflected by the fact that of the few reported solid-state 195pt NMR studies, most are of catalyst systems. The principal importance of NMR to such studies arises from the observation that the ~95pt parameters are essentially independent of the nature of the support material but are sensitive to the surface conditions, allowing the local densities of states at the Fermi energy on "typical" metal surface sites (and even on sub-surface sites) to be deduced. Since these properties are considerably altered by the chemisorption of gas species such as hydrogen (Bucher et al. 1989), the NMR data can provide a sensitive probe of the surface interactions occurring under catalysis conditions. These principles have been used to interpret the results of a detailed 195pt NMR study of a platinum catalyst supported on anatase in which the. spectra were determined by a pointto-point spin-echo method and fitted with a number of Gaussian peaks, each describing the signal resulting from atoms at a particular depth from the surface. The integrated area of each fitted Gaussian peak is regarded as being proportional to the number of atoms in the layer. The results revealed in detail the effect of hydrogen chemisorption on the surface platinum atoms (Tong and van der K/ink 1994). 195pt NMR was also used to determine the effect on the platinum surface of electropositive additives such as alkalies which are commonly used as catalyst promoters (Tong and van der Klink 1994a). The results clearly indicate that alkali impregnation increases the local densities of states at the Fermi energy on the surface, in sharp contrast to the effect of chemisorbed hydrogen which diminishes the local densities of states at the surface. Since supported platinum catalysts are commonly prepared by deposition of the platinum species from aqueous solutions onto the supporting oxide, followed by thermal activation, a 195pt NMR study has been made of the initial stages of the deposition process on A1203 (Shelimov et al. 1999). Although sharp 195pt NMR signals were determined in the wet preparations and showed an increase in chemical shift with increasing pH of the solution, the signals were irreversibly lost after drying at 90~
604
Multinuclear Solid-State NMR of Inorganic Materials
overnight. This result was suggested to indicate the modification of the octahedral symmetry of the original Pt complexes to species with lower symmetry and greater CSA which are undetectable by static NMR experiments (Shelimov et al. 1999). 195pt NMR has been used to study platinum particles embedded in a zeolite and to compare their characteristics with the more common oxide-supported platinum catalysts (Tong et al. 1993). Spin-lattice relaxation measurements indicated that a measurable fraction of the platinum in a zeolite-Y sample is not in a metallic environment, but it was not clear whether the loss of metallic signal reflected very small particle sizes or was due to interactions with the framework and its counter-ions. The latter possibility was supported by the observation that part of the metallic signal was restored by chemisorption of hydrogen (Tong et al. 1993). A brief review of the 195ptCP NMR literature (coveting mostly organometallics and complexes) has been given by Sebald (1994).
9.3.9 199HgNMR 199Hg has a natural abundance of 16.84%. To date 199Hg NMR has been exploited mainly in studies of organomercury compounds which are outside the scope of this book, and only a few solid state studies of inorganic materials have been reported, mainly of semiconductor alloy compounds in the system Hg-Cd-Te. The earliest 199Hg NMR study of crystalline HgxCdl-xTe with varying values of x showed that the technique is capable of monitoring changes in the bulk composition and suggested a relationship with the bonding properties in the alloy (Willig et al. 1976). Subsequent single-crystal 199Hgwork on Hgo.78Cdo.22Te revealed only 1 Hg environment, and double-resonance Hg-Te and Hg-Cd experiments enabled assignments of the 125Te resonances to distinct chemical environments (Zax et al. 1993). Measurements of the temperature dependence of the 199Hg NMR spectra of these alloys in the composition range x = 0.204).28 have revealed the intrinsic Knight shifts in these compounds and provided a measure of the Hg orbital contributions to the conduction electron states (Shi et al. 1993). The results indicate a consistently strong average contribution from the Hg s-orbitals.
9.3.10 2~
and 2~
NMR
Of the 2 spin I = 1/2 thallium nuclei, 2~ is the most abundant (70.5 % by comparison with 29.5 % for 2~ The receptivity of 2~ is also high (it is the fourth most sensitive spin-l/2 nucleus), making this the preferred T1 isotope for NMR study. The chemical shift, coupling constants and relaxation times of the T1 NMR spectra are extremely sensitive to the chemical environment in which the nucleus is placed. The preferred oxidation states of T1 are + 1 and + 3, both of which show large chemical
605
N M R o f O t h e r S p i n -1/2 N u c l e i
shift ranges (about 7000 ppm for TI(III) and 3400 ppm for TI(I)). The origin of this large shift range lies in the paramagnetic term of the nuclear screening constant which is responsible for about 95% of the variation observed in thallium chemical shifts. Thallium in covalent systems can exhibit significantly large CSA effects, but in highly ionic T1 solids the ions do not experience strong directional electronic interactions and show only limited CSA. The T1 chemical shifts are normally referenced to an aqueous solution of T1NO3, but solid T1C1 is a potentially useful secondary reference since the cubic symmetry of its T1 site yields a readily detected narrow line at 383 ppm. The earliest 2~ N M R solid state studies were of the common thallium salts in both the solid phase and the melt, and of thallium silicate, borate and chalcogenide glasses. The 2~ N M R literature up to 1988 has been extensively reviewed by Hinton et al. (1988). The 2~ chemical shift data for a number of thallium compounds are presented in Table 9.11. In view of the large CSA values which can occur in T1 compounds, distortion of the lineshapes can occur by loss of signal during the probe deadtime. Some of the earlier 2~ N M R data may be corrupted in this way, and should be treated with caution. Table 9.11. 2~
shifts of thallium compounds.
Compound
~*(ppm)
T1C13 T1C13.4H20 KT1CI4 Cs2T1C15.H20 Na2T1C15.4H20 Na3T1C16.12H20 K3T1C16.2H20
2485 2051, 2960 2716 2022 1984 1972 2007 1926 1262 - 1271 - 1194 540 1965 1098 2960 2220 2220 1630, 2320, 1913, 1845 1100, 1070, 843, 820, 753, 325,600, 393, 383, 383 780, 1300, 680 1963, 1963 - 108, - 66, 1110 80, -- 45, -- 107, -- 107, -- 110 2 7 0 , - 135,- 135,- 147,- 147 - 525, - 544 (isotropic)
Cs3T12C19
KT1Bra.2H20 [Co(NH3)6]T1Br6
Cs3T12Br9 Tlo.3WO3 T1(C104)3
T1Br3.4H20 Zn(T1CI4)4 (NH4)3T1C16 K3T1C16 TII T1Br T1C1 T1F T13PO4
T12CO3 T12SO4
T1NO3 T1C104
Reference** Hinton et al. Hinton et al. Hinton et al. Hinton et al. Hinton et al. Hinton et al. Hinton et al. Hinton et al. Hinton et al. Hinton et al. Hinton et al. Hinton et al. Hinton et al. Hinton et al. Hinton et al. Hinton et al. Hinton et al. Hinton et al. Hinton et al. Hinton et al. Hinton et al. Hinton et al. Hinton et al. Hinton et al. Hinton et al. Hinton et al.
(1988) (1988) (1988) (1988) (1988) (1988) (1988)
(1988) (1988) (1988)
(1988) (1988) (1988) (1988) (1988) (1988) (1988)
(1988) (1988) (1988) (1988) (1988) (1988) (1988) (1988) (1988)
606
M u l t i n u c l e a r Solid-State N M R o f l n o r g a n i c M a t e r i a l s
Table 9.11. (Continued)
Compound
~*(ppm)
Reference**
T12Ba2CuO6
3397 3108 2917 3490 3151
Winzek et al. (1990) Winzek et al. (1990) Winzek et al. (1990) Panich et al. (2001) Panich et al. (2001)
T12Ba2CaCu208 T12Ba2Ca2Cu301o (Tlo.5Pbo.5)(Bao.1Sro.9)zCaCuzOy (Tlo.5Pbo.5)(Bao.2Sro.8)2Ca2Cu3Oy
* Chemical shifts quoted with respect to aqueous T1NO3 solution ** Shifts taken from Hinton et al. (1988) which should be consulted for the original references.
A 2~ NMR study has been reported of hydrated and dehydrated zeolites A, X and Y in which thallium was exchanged for sodium (Groombridge et al. 1993). The 2~ NMR lineshapes of these compounds were generally dominated by large CSA, necessitating the stepwise acquisition of the spectra by a frequency-stepped spin-echo method. MAS did not produce significant narrowing of the spectra. The 3-fold coordinated T1 sites in zeolite A adjacent to 6-membered tings were assigned to a resonance with a ~iiso value of 1183 ppm and a large CSA of - 1842 ppm, while the resonance corresponding to the remaining T1 site was isotropic, with ~iso = 300 ppm. The spectrum of dehydrated zeolite X shows an isotropic peak at 854 ppm associated with TI(I) in the double-ring site and a broader aniosotropic resonance possibly associated with TI(III) in the supercage. The NMR data suggest that the cation distribution in dehydrated zeolite Y is disordered (Groombridge et al. 1993). Semiconducting compounds such as T1Se have a chain structure and are of interest for their potential applications in optoacoustic and optoelectronic devices. A combination of 2~ and 2~ NMR has been used to study the indirect nuclear exchange coupling, electronic structure and wave-function overlap in single-crystal T1Se (Panich and Gasanly 2001). The NMR study indicates the existence of strong exchange coupling between the spins of the TI(I) and TI(III) ions in this compound arising from an overlap of the T1+ and T13+ electron wave-functions across the intermediate Se atom. This wave function overlap determines the electronic structure and properties of T1Se since it provides the dominant mechanism for the formation of the uppermost valence bands and the lower conduction bands (Panich and Gasanly 2001). The compound T12Te3 is a p-type semiconductor in which the thallium atoms are located in channels between puckered Te layers. A 2~ and 2~ NMR study of this material has revealed that significant indirect exchange coupling occurs between the nuclei by overlap of the thallium electron wave functions, mainly across the Te atoms. Good agreement was found between the NMR conclusions and the calculated electronic structure and density of states in this semiconductor (Panich and Doert 2000). Thallium occurs in a class of high-Tc superconducting compounds of general type Tl(Ba,Sr)2Can_lCunOy, of which the phase Tl(Ba,Sr)zCa2Cu3Oy has a Tc well above
NMR of Other Spin -1/2 Nuclei
607
110K, yields high critical currents and shows a relatively high irreversibility field. This phase can be more readily fabricated if the Ba is partially substituted by Sr, and doped with PbO and/or Bi203. Pb-doped T1 superconductors such as (Tlo.sPbo.5)(Bao.zSro.8)2 Ca2Cu3Oy and (Tlo.5Pbo.5)(Bao.lSro.9)zCaCu2Oy have proved to be excellent candidates for study by 2~ 2~ and 2~ NMR, yielding information about the valence states of these cations and the flux line dynamics in the vortex lattice (Panich et al. 2001). The 2~ spin-lattice relaxation times were determined at temperatures between 68 and 300 K, and the chemical shifts of the spectra indicated that the T1 is in the +3 valence state. A linear relationship was also observed between the 2~ shifts and the Tc values, with a similar relationship also operating for the 2~ shifts (Panich et al. 2001). Although the T1 atoms are located in the insulating T1-O layer of these superconductors, a positive Knight shift contribution has been reported, attributed to superexchange interactions resulting from the overlap of the T1 orbitals with the Cu 2+ dZz orbitals via the Pz orbital of the bridging apex oxygen (Winzek et al. 1990). Measurements of the temperature-dependence of the 2~ NMR linewidth have provided information about the vortex mobility, melting point and phase transitions of the vortex lattice, indicating anisotropic melting of the vortex state in the strontium and lead-substituted T1 superconductors (Panich et al. 2001).
9.3.11 207 P b N M R
2~ is a potentially useful nucleus for NMR spectroscopy, having a favourable gyromagnetic ratio (5.5968 • 10 -7 rad T-l), a reasonably high resonance frequency and a natural abundance of 22.6%. Its main drawback as a practical NMR nucleus is its large number of electrons whose polarisability can lead to a large CSA which in turn produces broad static NMR lineshapes and complicated MAS spinning sideband patterns even at spinning speeds of 15 kHz, resulting in phasing difficulties and ambiguities in identifying the isotropic bands. Where there are even small deviations from spherical symmetry of the Pb environment, the CSA can exceed several thousand ppm with isotropic chemical shifts coveting a range of 16,000 ppm. However, despite these drawbacks, the practical importance of lead compounds as environmental contaminants as well as ferroelectric and piezoelectric materials has maintained a high degree of interest in 2~ NMR and led to improved structural correlations with the chemical shift, allowing useful structural information to be extracted from 2~ NMR spectra. The chemical shifts of 2~ spectra are normally referenced to Pb(CH3)4, but crystalline Pb(NO3)2 with a shift of - 3473.6 ppm or aqueous Pb(NO3)2 solution at - 2941 ppm can be used as secondary reference compounds. 9.3.11.1 Correlations between Z~ chemical shifts and structure. On the basis of the 2~ NMR spectra of a number of lead oxides and silicates, Fayon et al. (1997) have deduced empirical relationships between ~iso and structural parameters. Provided
Multinuclear Solid-State NMR of lnorganic Materials
608
care is taken to correctly analyse the sometimes complex overlapping sideband patterns, the ~iso values for ionic compounds show good linear correlations with both the coordination number (CN) and the mean bond length in nm (Figure 9.35A). These relationships are described respectively by
~iso (ppm) = 622.8 - 349.7CN
(9.7)
6iso ( p p m ) = 2 0 8 5 4 - 86689.5(Pb-0)
(9.8)
For more covalent compounds, the best empirical correlation was obtained by taking into account the degree of oxygen s-p hybridization p, given by (9.9)
p = (cos 0)/(cos 0 - 1)
where 0 is the Pb-O-X bond angle. The correlation for covalent compounds also takes into account the next-nearest neighbour electronegativity (AN) in defining a parameter P P = p + AN
(9.10)
which shows a satisfactory linear correlation with the isotropic chemical shifts of a number of covalent lead compounds (Figure 9.35B) (Fayon et al. 1997) defined by the equation (9.11)
~iso (ppm) = 2076.2 - 2180.2P A
B o
t~
1ooo
o
o
~
15oo
-1000
.
t,o o
o
-3000
-500 t
0.22
500
I
0.24 0.26 0.28 Mean Pb-O length (nm)
-.
o:
0.4
0.8
1.2
P
Figure 9.35. A. Relationship between the Z~ isotropic chemical shift and the mean Pb-O bond length in lead compounds. Filled symbols denote sites with CN < 7, open symbols denote CN > 7. B. Relation between the 2~ isotropic chemical shift in lead compounds and P, a parameter defined in Equation 9.10 taking into account the degree of oxygen hybridisation and the next-nearest neighbour electronegativity. From Fayon et al. (1997), by permission of the American Chemical Society.
NMR of Other Spin -1/2 Nuclei
609
Attempts to establish relationships between the 2~ CSA values of lead compounds and the average deviation of the bond angles or bond lengths from those of an ideal polyhedron have proved less successful (Fayon et al. 1997). 9.3.11.2 Z~
N M R o f crystalline lead compounds. PbO occurs in 2 forms, the
tetragonal ~-form (red) having an oxygen-lead layer structure with interlayer Pb-Pb bonding, and the orthorhombic B-form (yellow) consisting of chain units with interchain Pb-Pb bonding. The 2~ NMR spectra of both oxides show significant CSA effects (Gabuda et al. 1999, Zhao et al. 1999) of - 2 2 1 2 ppm (red) and - 2 5 7 6 ppm (yellow) and isotropic chemical shifts which are paramagnetically shifted as a result of the electron density of the Pb sites being considerably distorted from spherical symmetry by the presence of a lone electron pair. The larger CSA of yellow PbO is consistent with the lower symmetry of the Pb site in this oxide (Zhao et al. 1999). The oxide Pb304 contains both Pb(IV) in a pseudo-octahedral environment and Pb(II) with 4 nearest neighbour oxygen atoms. The 2~ NMR spectrum contains 2 overlapping resonances corresponding to the 2 Pb environments (Zhao et al. 1999). The symmetric environment of the Pb(IV) site gives rise to a small CSA (98 ppm) whereas the Pb(II) site is significantly less symmetrical and has a CSA of - 1910 ppm. The 207 Pb NMR spectra suggest that scalar coupling occurs between the Pb(IV) and 2 of the 4 surrounding Pb(II) atoms, rather than between magnetically equivalent Pb(IV) sites as proposed by Fayon et al. (1997). The chemical shift of the Pb(II) site has been attributed to a Pb-Pb contribution to the shielding arising from a strong PbZ+-Pb 2+ exchange interaction (Gabuda et al. 1999a). Crystalline solid solutions formed between the nitrates of lead and strontium have been investigated by 2~ NMR which shows up to 13 lines arising from Pb sites with up to 12 nearest neighbour cations replaced by Sr (Figure 9.36) (Kye and Harbison 1998). The presence of the Sr produces an average shift in the 2~ resonance of 21.8 ppm per Sr2+, with similar but smaller effects being observed in lead-barium nitrate solid solutions. The observed shift in the Pb resonance, which is thought to be due to differences in polarisability of the 2 ions rather than size differences, provides a convenient method of investigating the microenergetics of solid solutions in these and similar systems (Kye and Harbison 1998). 2~ NMR has also been used to study a series of solid solutions in the system Ba• utilising the sensitivity of NMR towards nearest neighbour Pb 2+ and Ba 2§ distributions to re-evaluate the validity of Vegard's Law of solid solutions which was originally deduced from a study of this system (Crundwell et al. 1999). Vegard's Law is an empirical principle stating that for some miscible solids the lattice constant varies linearly with composition, but many exceptions to this simple rule are known. The intensities of the peaks in the 2~ NMR spectra and their deviation from a simple binomial dependence suggest that the Ba and Pb are incorporated in the bulk crystals in a non-statistical manner. A linear
610
Multinuclear Solid-State NMR of Inorganic Materials
Sr c o n t e n t (mole%) ,
*
9
**~,,
9
r
100 2~
0
4.6 _
_
~
',
1
-100
. . . .
,
-300
shift (ppm) w.r.t. Pb(NO3)2
Figure 9.36. 2~ MAS NMR spectra of (Pb,Sr)(NO3)2 solid solutions of various compositions. The asterisks denote spinning side bands. Note the shift in the 2~ resonance with increasing Sr-content, and the appearance of up to 13 lines arising from the Pb sites with up to 12 nearest neighbours replaced by Sr. From Kye and Harbison (1998), by permission of the American Chemical Society. relationship was found between the 2~ isotropic chemical shift and the number of Ba ions in the first coordination sphere, confirming Vegard's additivity relationship on a local scale but indicating that the inhomogeneities in bulk crystals are responsible for the documented violations of the rule (Crundwell et al. 1999). Lead compounds such as PbTiO3, PbZrO3 and the lead zirconium titanates form the basis of many important electronic materials. The 2~ NMR spectrum of PbTiO3 (Figure 9.37A) shows a single Pb(II) site in an environment of reduced symmetry giving rise to a CSA of - 772 ppm (Zhao et al. 1999). The isotropic chemical shift shows only a weak temperature dependence probably arising from changes in the average P b - O distance, but the CSA increases markedly at lower temperatures and shows a strong correspondence with the square of the tetragonal distortion parameter (Bussian and Harbison 2000), given by CSA (ppm) = 188,000(c/a - 1)2
(9.12)
where c and a are the lattice parameters. PbZrO3 contains 2 Pb(II) sites which are apparent from the static 2~ NMR spectrum (Figure 9.37B). Both sites have 12 nearest neighbour oxygen atoms, but the
611
N M R of Other Spin -lIe Nuclei
Pb environments are of reduced symmetry with 3 shorter Pb-O distances giving rise to CSA values of - 838 and - 546 ppm (Zhao et al. 1999). The larger CSA is associated with the Pb(II) site containing the shortest Pb-O bond length (2.26A). The 2~ MAS NMR spectrum of PbZrO3 contains complex overlapping spinning sideband manifolds which can be simplified by using a two-dimensional Phase-Adjusted Spinning Sideband (PASS) experiment (Vogt et al. 2000). This yields an isotropic/anisotropic correlation diagram which when sheared gives effectively an isotropic spectrum with no sidebands (Figure 9.37C). The normal 2D PASS experiment works best at lower MAS speeds (1-5 kHz), but since lead compounds generally have large CSA values requiting high MAS speeds, a variation of the PASS experiment using multiple MAS rotor cycles has been developed to avoid pulse overlap (Vogt et al. 2000) giving excellent results for PbZrO3. Other lead compounds of importance as electronic materials which have been investigated by 2~ NMR include PbNb206, lead zirconium titanate and lead magnesium niobate. All these compounds show very broad NMR resonances, necessitating their acquisition by the point-to-point incremental frequency method (Zhao et al. 1999). Measurements of the temperature dependence of the 2~ NMR spectra of PbMgo.33Nbo.6603 between 450 and 15 K reflect the existence of polar nanoclusters in this relaxor material and can be described by a spherical random-bond random-field model (Blinc et al. 2000). A
B
PbTiO3
C
PbZrO3
PbZrO3
,Static
MAS
-1000
j
tt. I MAS
-2000
-1000 2~
-2000
9~ ' , ' l , , , ' l , , i , I
''
-1000
PASS
'[
. . . .
I ' '
-2000
shift (ppm) w.r.t. (CH3)4Pb
Figure 9.37. A. Static and MAS 2~ NMR spectra of PbTiO3,B. Static and MAS 2~ NMR spectra of PbZrO3. From Zhao et al. (1999), by permission of the American Chemical Society. C. 2~ NMR spectra of PbZrO3 obtained under MAS conditions (upper) and from a two-rotor-cycle 2D PASS experiment (lower) showing the simplification obtained by this method giving essentially an isotropic spectrum with no sidebands. From Vogt et al. (2000), by permission of the copyright owner.
612
Multinuclear Solid-State NMR of lnorganic Materials
The lead silicates PbSiO3 and the high-temperature form of Pb2SiO4 contain multiple Pb sites giving rise to complex overlapping sets of spinning sidebands in their 2~ MAS NMR spectra (Figure 9.38). PbSiO3 contains 3 inequivalent lead sites, 2 of which are located in PbO4 square pyramids in the lead-oxygen spiral chains of the structure and the third Pb site is in PbO3 trigonal pyramids. The 2 isotropic peaks in the NMR spectrum at - 366 and - 166 ppm correspond to the PbO4 units and the remaining isotropic peak at 93 ppm arises from the PbO3 unit (Fayon et al. 1997). The high-temperature form of Pb2SiO4 contains 4 inequivalent Pb sites, giving rise to an even more complex sideband pattern (Figure 9.38). The isotropic peaks in this spectrum at 1382 and 1344 ppm have been assigned from a consideration of the Pb-O bond lengths to the 2 sites containing Pb-O-Pb linkages, the 2 other isotropic peaks at 329 and 634 ppm then being assigned to PbO3 units containing Pb-O-Si bonds. Two other low-intensity isotropic peaks at 744 and 710 ppm in this sample were attributed to the presence of a small amount of another Pb2SiO4 polymorph (Fayon et al. 1997). The location of the lead cations in a series of lead-containing A, X and Y-zeolites and their mobility on exposure to water have been studied by a combination of 2~ NMR and 27AI-2~ double resonance (SEDOR) experiments (Niessen et al. 2001). Systematic changes in the 2~ relaxation time and the average chemical shift with water content suggest that the chemical environment of the lead cations is strongly affected by the sorption of water and supports a 2-site exchange model for the formation of Pb 2+ O H - entities (Niessen et al. 2001). PbSiO 3
*
.
.
I IJ
N 1000
0
Pb2SiO4
**
-1000 .
-2000
qr
I
,~ [.[,~ | I t , I t . ' , i
2000
1000 2~
shift (ppm)
i
i
i
0
simulated i
-1000
i
i
-2000
w.r.t. (CH3)4Pb
F i g u r e 9.38. Observed and simulated 2~ MAS NMR spectra of PbSiO3 and the hightemperature form of Pb2SiO4. Note the complex sideband patterns arising from the 3 Pb sites in PbSiO3 and the 4 inequivalent Pb atoms in Pb2SiO4. The isotropic lines are denoted by asterisks. From Fayon et al. (1997), by permission of the American Chemical Society.
613
NMR of Other Spin -1/2 Nuclei
The 2~ MAS NMR spectra of a number of crystalline and glassy lead fluorides have been acquired by cross-polarising with the 19F nuclei (Bureau et al. 1999). The resulting spectra (Figure 9.39A) show a wide range of isotropic chemical shifts, from - 2 6 6 6 ppm in et-PbF2 to - 3 2 1 5 ppm in PbGaF5 (these shifts quoted with respect to Pb(CH3)4 rather than Pb(NO3)2 as in the original paper). The 2~ isotropic chemical shifts in these crystalline fluorides and related lead fluoride glasses show a linear correlation with the number of next-nearest-neighbour Pb 2+ atoms (n) (Bureau et al. 1999) (Figure 9.39B), described by (9.13)
t~iso (ppm) : -- 1077 + 116n The 2~ Table 9.12.
NMR chemical shifts for a number of lead compounds are collected in
9.3.11.3 2~ N M R of lead-containing glasses. Lead silicate, borate and phosphate glasses have a number of important optical and electronic applications by virtue of their high refractive indices and low melting temperatures. A 2~ MAS NMR study of lead silicate, borate and gallate glasses containing high lead-contents has indicated extremely broad resonances at about 3000-6000 ppm with respect to crystalline Pb(NO3)2, consistent with the presence of the lead in a wide variety of sites (Yoko et al. 1992). Comparison of the chemical shifts with those of crystalline lead compounds has led to the suggestion that the majority of the lead atoms occur as trigonal PbO3 or square pyramidal PbO4 environments. A
B glass iI
~ _ ~ . ~ _ _ J ~Pb3_Ga_z-"-_ ~ " ' ' t
....
i
FI~
f
Pb9Ga2F2 4 Pb~ZnF6
~u -2940 .~ ~
>,..o~.
~-PbF~ ~....
-2340 2~
-2940
13-PbF2
-3540
-4141
shift (ppm) w.r.t. (CH3)4Pb
J
-3540
/: 8
12
No. of nnn Pb 2+
Figure 9.39. A. 2~ CP-MAS NMR spectra of crystalline and glassy Pb-F compounds. Note the large range of the isotropic chemical shifts in these compounds. B. 2~ isotropic chemical shifts of crystalline and glassy Pb fluorides as a function of the mean number of next-nearest-neighbour Pb2+ in the compounds. Filled squares denote crystalline compounds, filled and open circles denote glasses. From Bureau et al. (1999), by permission of the copyright owner.
614
Multinuclear Solid-State NMR of Inorganic Materials
Table 9.12. 2~ Compound
NMR shifts of lead compounds. giso (ppm)*
Fayon et al. (1997), Gabuda et al. (1999), Zhao et al. (1999) 1515, 1525, 1536 Fayon et al. (1997), Gabuda et al. (1999), [3-PbO (yellow) Zhao et al. (1999) - 1105, - 1091, - 1101, - 1112 Fayon et al. (1997), Gabuda et al. (1999a), Pb304(Pb 4+) Zhao et al. (1999), Yoko et al. (1992) (Pb 2+) Fayon et al. (1997), Gabuda et al. (1999a), 795,804,808, Zhao et al. (1999) Fayon et al. (1997) PbSiO3 site 1 93 Fayon et al. (1997) site 2 166 Fayon et al. (1997) site 3 - 366 Fayon et al. (1997) 329 Pb2SiO4 site 1 Fayon et al. (1997) site 2 634, Fayon et al. (1997) 1344 site 3 Fayon et al. (1997) site 4 1382 Fayon et al. (1997), Yoko et al. (1992) - 1 7 1 7 , - 1728 PbC12 Fayon et al. (1997), Neue et al. (1994), - 2 6 6 7 , - 2 6 6 8 , - 2666 oL-PbF2 Bureau et al. (1999) Bureau et al. (1999) [3-PbF2 - 2793 Bureau et al. (1999) - 2786 Pb2ZnF6 Bureau et al. (1999) - 3661 PbGaF5 site 1 Bureau et al. (1999) site 2 - 3601 Bureau et al. (1999) site 3 - 3521 Bureau et al. (1999) site 4 - 3481 Bureau et al. (1999) - 3331 Pb3Ga2F12 site 1 Bureau et al. (1999) site 2 - 3221 Bureau et al. (1999) - 2941 Pb9Ga2F24 site 1 Bureau et al. (1999) site 2 - 2821 Bureau et al. (1999) site 3 - 2691 Fayon et al. (1997), Neue et al. (1996), - 3 4 9 4 , - 3 4 9 1 . 6 , - 3494 Pb(NO3)2 Zhao et al. (1999) Fayon et al. (1997), Neue et al. (1996), - 2630, - 2622.4, PbCO3 Nolle (1977), Yoko et al. (1992) - 2 6 4 1 , - 2638 Fayon et al. (1997), Neue et al. (1996), - 3 6 1 3 , - 3 5 0 5 , - 3611 PbSO4 Zhao et al. (1999) Fayon et al. (1997), Neue et al. (1996), - 2 0 0 9 , - 2 0 0 4 . 9 , - 1989 PbMoO4 Lauterbur & Burke (1965) Nolle (1977) PbCrO4 - 2292 Nolle (1977) - 2003 PbWO4 Zhao et al. (1999) 1419 PbTiO3 Zhao et al. (1999), Vogt et al. (2000) site 1 PbZrO3 - 1 3 6 3 , - 1349 Zhao et al. (1999), Vogt et al. (2000) site 2 1 0 1 7 , 1000 Zhao et al. (1999) site 1 134 PbNb206 Zhao et al. (1999) site 2 - 2829 Yoko et al. (1992) - 2581 PbSb206 Fayon et al. (1997) 2886 Pb3(PO4)2 site 1 Fayon et al. (1997) site 2 - 2016 ~-PbO (red)
1939, 1930, 1878
Reference
615
N M R o f Other Spin -~/2 Nuclei Table 9.12. (Continued)
Compound Pb(P03)2 Pb3P4013
Pb2P207
~iso (ppm)* site 1 site 2 site 1 site 2 site 3 site 1 site 2 site 3 site 4
PbC204
-
3023 2962 2926 2906 2533 2680 2635 2571 2412 1642
Reference Fayon et al. (1999) Fayon et al. (1999) Fayon et al. (1999) Fayon et al. (1999) Fayon et al. (1999) Fayon et al. (1999) Fayon et al. (1999) Fayon et al. (1999) Fayon et al. (1999) Zhao et al. (1999)
* chemical shifts quoted with respect to Pb(CH3)4.
The presence of covalent PbO3 and PbO4 units has been confirmed in binary lead silicate glasses by 2~ NMR which has shown the existence of a lead oxide-based glass network at PbO concentrations > 60 mol % (Fayon et al. 1998). The results suggest that at higher lead contents the silicate network is destroyed and polymeric interconnected PbOn chains predominate. The increased connectivity between the PbOn units at higher lead contents leads to a very wide distribution of 2~ chemical shifts and broad N M R lineshapes, which are further broadened by disorder in the second coordination sphere resulting from Pb for Si substitution, and bond length and bond angle distributions (Fayon et al. 1999a). Similar problems of broadening occur with the 2~ NMR spectra of lead phosphate glasses, in which the broad lineshapes remain unresolved by MAS and are identical to the corresponding static spectra. The isotropic and anisotropic components of the chemical shift tensors of these glasses have been resolved by using a shifted-echo version of the PASS experiment (Fayon et al. 1999) which allows a spectrum free of spinning sidebands (corresponding to an infinite spinning rate) to be deduced. The results show a continuous distribution of the lead environment in a phosphate glass with bonding ranging from more ionic (characterised by longer Pb-O bond lengths and high anisotropy) to more covalent (shorter Pb-O bond lengths and a more pronounced lone pair effect with high anisotropy) (Fayon et al. 1999).
9 . 3 . 1 1 . 4 2~
in s o l - g e l
prepared ceramics. Pyrolysis of sol-gel-derived precursors
of lead zirconate titanate ceramics and their subsequent crystallisation has been followed using 2~ MAS NMR (Brieger et al. 1998). Since the spinning rate was not fast enough to narrow the resonance, its width must be due to chemical shift dispersion, which appears however to be reduced on recrystallisation, since this process is
616
Multinuclear Solid-State NMR of Inorganic Materials
accompanied by significant narrowing of the spectra. The widths of the 2~ NMR lines of the crystalline product were also found to be strongly dependent on the preparation method, even though X-ray diffraction indicated that all the products were similarly crystalline. This is a further illustration of the ability of NMR to detect subtleties in the short-range atomic ordering that are not observed by X-ray diffraction.
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Multinuclear Solid-State NMR of lnorganic Materials
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Chapter 10
NMR of Other Quadrupolar Nuclei 10.1.
10.2. 10.3.
10.4.
6Li and 7Li NMR 10.1.1 General Considerations 10.1.2 6'7LiNMR of Crystalline Solids 10.1.3 Relation between 6Li Chemical Shifts and Structure 10.1.4 6'7LiNMR of Fast Lithium Ion Conductors 10.1.5 6'7LiNMR of Glasses 9Be NMR 51V NMR 10.3.1 General Considerations 10.3.2 5~V NMR of Vanadium Oxides and the Vanadates 10.3.3 51V NMR of Zeolites and Catalysts 63Cu and 65CuNMR 10.4.1 63CuNMR of Superconductors and Superfast Ionic Conductors
10.5. 69Ga and 71Ga NMR 10.5.1 General Considerations 10.5.2 69'71GaNMR of Crystalline Compounds 10.5.3 69'71GaNMR of Other Compounds 10.6. 87RbNMR 10.6.1 General Considerations 10.6.2 87RbNMR of Crystalline Compounds 10.6.3 87RbNMR of Rubidium Fullerides 10.7. 93Nb NMR 10.8. 133CsNMR 10.8.1 General Considerations 10.8.2 133CsNMR of Crystalline Caesium Compounds 10.8.3 133CsNMR of Minerals and Zeolites 10.8.4 ~33CsNMR of Fullerides, Superionic Conductors and Semiconductors 10.9. 139LaNMR References
629 629 630 634 636 638 639 642 642 642 646 649 650 653 653 655 657 658 658 658 661 662 665 665 666 669 673 674 678
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Chapter 10
NMR of Other Quadrupolar Nuclei The quadrupolar nuclei of greatest importance to materials science (27A1 and 170) have been dealt with in Chapters 5 and 6 respectively. Two other important quadrupolar nuclei (23Na and liB) have also been treated separately in Chapter 7. The present chapter deals with a number of the other quadrupolar nuclei encountered in solid state NMR studies of inorganic materials.
10.1. 6Li AND 7Li NMR
10.1.1 Generalconsiderations Both the lithium nuclides are suitable for NMR spectroscopy. The spin = 3/2 nucleus VLi is commonly used since it has a high natural abundance (92.5%) and favourable receptivity, but the quadrupole moment ( - 4 . 0 • 10 -3~ e m 2) can give rise to relatively broad lines from Li in non-symmetrical sites. The Larmor frequency of the spin = 1 nucleus 6Li is about 2.6 times smaller than 7Li and it has a much lower natural abundance (7.5%) and hence a less favourable receptivity, but its quadrupole moment is also significantly lower than 7Li and its homonuclear dipole-dipole interactions are much weaker, giving narrower resonance lines under MAS conditions. Thus, although 6Li has in the past been less widely used in solid-state NMR studies, its use can be preferable in circumstances requiring the resolution of two close resonances. Because of its smaller quadrupolar interaction, the 6Li shift may also more closely approximate the isotropic chemical shift and provide a better measure of the Li bonding environment, but these benefits may be offset by the relaxation times which are often very long, and require longer data acquisition times. Comparing 2 isotopes with very different interactions can also be used to understand the source of relaxation and hence the motion responsible. Figure 10.1A shows the 7Li MAS NMR spectrum of Li-containing beryl, one of the few materials in which the 7Li-VLi homonuclear dipole-dipole interactions are sufficiently small to be removed by magic angle spinning, revealing the second-order quadrupolar lineshape. In most other materials, dipolar-dipolar broadening renders the 7Li lineshape broad and featureless even under MAS conditions. By contrast, the 6Li MAS NMR spectrum of the same sample (Figure 10.1B) shows a sharp resonance very close to the isotropic chemical shift, since the second-order quadrupolar shift is negligible. 6'7Li NMR chemical shifts are commonly measured with respect to aqueous LiC1 solution.
629
630
Multinuclear Solid-State NMR of lnorganic Materials
A
B
7Li
6Li
ated ~observed '
2.4
'
'
'
0.8
~
-0.8
...... _ ~ ....
I ....
, .........
6
I .....
9........
3
I ..............
0
l ..........
-3
Li shift ( p p m ) w.r.t. L i C I Figure 10.1. A. 7Li MAS NMR observed and simulated spectra of Li-substituted beryl, spinning speed 10 kHz. Simulation parameters XQ = 0.66 MHz, r I = 0.2. B. 6Li MAS NMR spectrum of the same sample, spinning speed 6 kHz. From Xu and Stebbins (1995), by permission of the copyright owner.
10.1.2 6'7Li NMR of crystalline solids A s e l e c t i o n o f 6Li and 7Li N M R shifts for l i t h i u m c o m p o u n d s are s h o w n in T a b l e 10.1.
Table 10.1. 6'7Li NMR parameters for lithium compounds. Compound
~iso (ppm)*
LiI LiBr LiA1Si4Olo (petalite) LiA1Si206 (spodumene) LiA1SiO4 (eucriptite) Nao.3(Mg,Li)3SiaOlo(F,OH)2 (hectorite) K(Li,A1)3(Si,A1)40~o(F,OH)2 (lepidolite) BeAlzSi6018 (beryl) LiA1Si206.H20 (bikitaite) LiAla(Si3A1)Olo(OH)8 (cookeite) Laponite LizSiO3
- 2.312 - 2.152, - 0.359 0.1 - 1.0 0.2 - 0.8 - 0.8 to - 1.0 0 . 9 , - 1 . 0 , - 1.5 0.1 - 0.6 - 0.735 0.200, 0.44
Li2Si205 Li4SiO4 NaLiSiO4 NaLi3SiO4 LiSiON LiSieN3
0.2 1.5, 0.8, 0.2, - 0.7 - 0.69 0.76 0.27I" 1.315"
Reference
6Li Bond et al. (1991) Bond et al. (1991) Xu & Stebbins (1995) Xu & Stebbins (1995) Xu & Stebbins (1995) Xu & Stebbins (1995) Xu & Stebbins (1995) Xu & Stebbins (1995) Xu & Stebbins (1995) Xu & Stebbins (1995) Bond et al. (1991) Bond et al. ( 1991), George et al. (1998) Xu & Stebbins (1995) Xu & Stebbins (1995) Gee et al. (1997) Gee et al. (1997) Kempgens et al. (1999) Kempgens et al. (1999)
631
NMR of Other Quadrupolar Nuclei
Table 10.1. (Continued) Compound
~iso ( p p m ) *
NaLiSO4 NaLi3SiO4 LisB7S13 LIPS3 Li4P2S6 Li3PS4 LivPS6 LiSiON LiSizN3
- 0.69 0.47 -2 1.76"* 1.45"* 2.80** 2.08** 0.17t 1.27t
Reference
7Li
Gee et al. (1997) Gee et al. (1997) Griine et al. (1995) Eckert et al. (1990) Eckert et al. (1990) Eckert et al. (19900 Eckert et al. (1990) Kempgens et al. (1999) Kempgens et al. (1999)
* Chemical shifts quoted with respect to LiC1 solution. ** With respect to solid LiC1 ~ determined at 7.05 T
Lithium orthosilicate, Li4SiO4, has an interesting structure containing LiO4, LiO5 and LiO6 units. One of the LiO4 sites has one very long Li-O bond, making it effectively LiO3. These polyhedra are connected by edge-sharing to form the three-dimensional structure. The 6Li MAS NMR spectrum of an enriched sample (Figure 10.2A) shows 3 well-resolved peaks at 1.5 ppm (LiO3), 0.8 ppm (LiO4), - 0.7 ppm (LiO6) and a shoulder at about 0.2 ppm attributed to LiO5 units (Xu and Stebbins 1995). As the temperature of the sample is raised, these resolved spectral peaks merge into a single broad line due to hopping of the Li + between all the sites (Xu and Stebbins 1995a). Two-dimensional 6Li NMR exchange spectra (Figure 10.2 B) have been used to provide a detailed picture of the hopping rates of the Li + among the various sites. The two-dimensional spectra are determined at various mixing times. If the mixing time of the experiment is faster than the rate of exchange between the sites, the normal one-dimensional spectrum appears on the diagonal and there are no other peaks. As the mixing time becomes longer, crosspeaks appear at the coordinates of the 2 peaks involved in Li + exchange, allowing exchange rates and activation energies to be determined for the various sites (Xu and Stebbins 1995a). The static 7Li NMR spectra of LizSiO3 recorded as a function of temperature from ambient to 1150~ show a well-defined quadrupolar lineshape at all temperatures, indicating that the Li in this compound is not sufficiently mobile to display an averaged isotropic environment even at temperatures 50~ below the melting point (George et al. 1998). The room-temperature second-order quadrupolar lineshape can be simulated by assuming XQ = 0.15 MHz and xl = 0.65, but above 600~ an additional narrower quadrupolar lineshape appears, with XQ = 0.03 MHz and xl = 0. These results have been tentatively explained in terms of a partial dynamical averaging of the spectra caused by Li + exchange from one position to another. Complete averaging, evidenced
632
Multinuclear Solid-State NMR of Inorganic Materials B
4
6
LiO 4 3
uo~/i 4
~4-6 3-6 3
I
-1
6Li shift (ppm) w.r.t. LiC! Figure 10.2. A. 6Li MAS NMR spectra of 6Li-enriched Li4SiO4, from Xu and Stebbins (1995), with permission of the copyright owner. B. Two-dimensional pure-absorption exchange 6Li NMR spectra of Li4SiO4 at two different mixing times. Note at the slower mixing time of 47 ms (top spectrum) only the peaks corresponding to the primary units are seen. At the faster mixing time of 188 ms (lower spectrum) peaks corresponding to Li exchange between the various sites appear. The horizontal axis is to2. Adapted from Xu and Stebbins (1995a). by the collapse of the spectrum to a single narrow peak is not observed, suggesting that the Li motion does not sample a wide enough set of site geometries and orientations for their time average to achieve spherical symmetry (George et al. 1998). The 6Li MAS NMR spectrum of LizSiO3 consists of a single narrow peak with a typically tetrahedral shift (0.44 ppm) but the relaxation time (hundreds of seconds) is too long for most practical purposes (George et al. 1998). Lithium aluminate, LiA1508 with the inverse spinel structure, is a material with possible applications in ceramic blankets for thermal control of fusion reactors. 6Li and 7Li NMR has been used to measure the spin-lattice relaxation of lithium in this compound (Stewart et al. 1995). The results indicate that 6Li relaxes most significantly through interactions with paramagnetic impurities, whereas 7Li relaxes much more strongly through dipole-dipole interactions. Small unexpected differences in the crystal structure of LiKSO4 have been revealed by a single-crystal 7Li NMR study which has indicated XQ and xl values of 0.025 MHz and 0.15 respectively for this compound (Lim et al. 1996, Lim and Jeong 2001). These results, taken together with 39K parameters determined for the same sample, indicate a non-axially symmetric EFG tensor, suggesting that the LiO4 unit is slightly distorted from its expected symmetry, possibly resulting from an artifact of the crystal growing conditions. The temperature dependence of the 7Li XQ and xl values of single-crystal LiKSO4 indicates the occurrence at 190 K of a first-order transition to ferroelastic domains characterised by lowering of the Li site symmetry (Lim et al. 1997). The
NMR of Other Quadrupolar Nuclei
633
7Li and 39K NMR spectra of this material, measured at 180 K, have allowed the structure of the ferroelastic phase to be directly inferred from the domain pattern of the low-temperature NMR spectra (Lira and Jeong 2000). The temperature dependence of the 7Li NMR spectrum of single-crystal LiRbSO4 has been determined in the temperature range 140-400 K. The room-temperature value of XQ (20.4 kHz) decreases with increasing temperature and has been interpreted in terms of the torsional frequency of Li-O (Lim et al. 1997a). The 7Li NMR spectrum of single-crystal LiCsSO4 has also been determined, and shows 3 sets of signals explained in terms of 3 types of growth twin-domains rotated with respect to each other by 60 ~ around the c-axis (Lira and Jeong 1999). The temperature dependence of these spectra indicates a second-order phase transition to ferroelastic domains with lowered Li site symmetry below 200 K (Lira and Jeong 1998). Measurements of the temperature dependence of the 7Li NMR spectrum of singlecrystal LiNHaSO4 demonstrate the occurrence of a first-order phase transition at 285 K (Lim et al. 2000a). The room temperature values of • and Xl (25 kHz and 0.22 respectively) increase with decreasing temperature, explained in terms of a change in the Li-O torsional frequency. Differences in the temperature dependence of the 7Li XQ values for the series of single crystals LiXSO4 (where X = K, Rb, Cs and NH4) have been explained in terms of differences in the torsional motion of the LiO4 tetrahedra about the x-axis of the EFG tensor, which, in turn, can be related to differences in the atomic weight of the X ion (Lim et al. 2000). Laponite, [Mg,Li]6SisOzo(OH)4Nao.sv.nH20, a synthetic form of hectorite, is a trioctahedral layer silicate with the structure of talc in which some octahedral Mg is substituted by Li, the charge balance provided by interlayer Na + or Ca 2+. These substitutions give the material useful cation exchange properties and also influence its thermal decomposition behaviour. The thermal decomposition of laponite has been studied by a combination of VLi, 298i and 25Mg MAS NMR (MacKenzie and Meinhold 1994). Loss of interlayer water at about 200~ produces very little change in the 7Li spectra, but just prior to dehydroxylation at 650~ discontinuities in the downfield trend of the 7Li shift with increasing temperature suggest the movement of interlayer Na closer to the tetrahedral sheets, influencing the Li in the octahedral sites (MacKenzie and Meinhold 1994). A similar downfield shift with increasing temperature reported in the 6Li spectra of heated laponite has been ascribed to the movement of Li from trioctahedral sites to edge sites (Bond et al. 1991). By contrast with the spinel LiTi204, which contains only tetrahedral Li sites, the superconducting spinel phase Li~.33Ti1.6704 contains both tetrahedral and octahedral Li sites, but the chemical shift difference between them is too small to be resolved even with very high MAS spinning speeds. The static 7Li spectrum is considerably broader than that of LiTi204 due to dipolar broadening between 7Li sites. This observation has led to an interesting experiment in which the Li-Li separation was increased by
634
Multinuclear Solid-State N M R of lnorganic Materials
isotopically diluting the sample with enriched 6Li (Dalton et al. 1994). The resulting 7Li MAS NMR spectrum showed structure resolvable into 2 Gaussian lineshapes with shifts of 0.2 and - 0.09 ppm. These resonances were identified as the tetrahedral and octahedral resonances respectively, by comparison with the known Li shifts in other compounds (8 for octahedral Li = - 0.1 to - 0.6 ppm, 8 for tetrahedral Li = 2.4 ppm) (Dalton et al. 1994). The lithium silicon nitride phases LiSiON and LiSi2N3 have been studied by 6Li and 7Li NMR at 2 magnetic field strengths (Kempgens et al. 1999). The spectra of both compounds consist of an intense central resonance with associated spinning side band manifolds. Although the difference between the isotropic chemical shift is small (Table 10.1), the 2 phases can readily be distinguished by their 7Li XQ and xI values (130 kHz and 0.55 for LiSiON and 100 kHz and 0.9 for LiSizN3). The quadrupolar interaction in the 7Li NMR spectra is much larger than the chemical shift interaction, making it difficult to accurately determine the small CSA and the relative orientation of the 2 interactions by 7Li NMR. These parameters can, however, be determined much more accurately from the 6Li NMR spectra (Kempgens et al. 1999).
10.1.3 Relation between 6Li chemical shifts and structure The 6Li NMR spectra of silicates are dominated by chemical shift effects, and this, together with their superior resolution makes them potentially useful for providing structural information. The 6Li chemical shifts (peak positions) of a series of silicate and aluminosilicate minerals derived from MAS NMR measurements have been shown to correlate well with the Li coordination numbers derived from single-crystal X-ray measurements (Figure 10.3A) (Xu and Stebbins 1995). This systematic decrease in the 6Li chemical shift with increasing oxygen coordination number (CN) resulting from increased shielding can be described by the linear relationship: 6 (rLi) = -0.608(CN) + 2.91
(~o.1)
This trend, which is in the same direction as for 27A1, 29Si, 23Na and 25Mg, appears to be related to the increase in Li-O bond length and Li + ionicity with increasing coordination number. A similar trend in the 6Li isotropic shift with ionicity has also been reported in sulphide-based glasses (Eckert et al. 1990). This simple relationship between the 6Li shifts and Li coordination number has been found not to hold for crystalline lithium phosphates, indicating that other factors must be taken into account (Alam et al. 1999). By analogy with 23Na NMR (Chapter 7), Alam et al. (1999) sought a relationship between the 6Li shift and the average degree of polymerisation in the phosphate tetrahedra as reflected by the number of non-bridging
635
NMR of Other Quadrupolar Nuclei
oxygen atoms per phosphate tetrahedron. The scatter in this relationship indicates that it does not satisfactorily describe the situation in the lithium phosphates. However, the chemical shift parameter (A) derived for 23Na shifts by Koller et al. (1994) in terms of the bond valences of the neighbouring oxygens was found by Alam et al. (1999) to give a good linear relationship with 6Li shifts (Figure 10.3B), described by
6Li + (ppm) -- 4.30A - 5.85
(10.2)
Similarly, a linear relationship was found for the Li sites in crystalline Li4SiO4 (Figure 10.3B), described by
(10.3)
6Li ~ (ppm) -- 7.50A - 8.52
By contrast with the results for 23Na (Chapter 7), both these lines have a positive slope. This unexplained descrepancy suggests that the formalism for defining the parameter A requires refinement, although the treatment is potentially useful within limited groups of similar compounds. A
B 2
/ / p / / /
~, ]
/
~o
/
o 0
_~
9
I ....
i ....
-1
....
i ....
4
i ....
6
i ....
l .+++:+ _
8
Li coordination number
-2 I
I 1.1
v
P 1.3
:
4 1.5
Chemical shift parameter A
Figure 10.3. A. Relationship between the 7Li chemical shift of lithium silicates and aluminosilicates and the Li coordination number (CN). The open circles indicate the probable peak assignments of Li4SiO4, crosses indicate the hypothesized LiO6 and LiOs sites in Li-substituted beryl, with all other samples indicated by +. From Xu and Stebbins (1995), with permission of the copyright owners. B. Relationship between the 6Li chemical shift and the chemical shift parameter A (defined by equation 7.7, Chapter 7) for crystalline lithium phosphates (solid circles) and crystalline Li4SiO4 (open circles). From Alam et al. (1999), by permission of Elsevier Science.
636
Multinuclear Solid-State NMR of Inorganic Materials
10.1.4 6'7Li N M R o f f a s t lithium ion conductors
One of the most important practical applications of lithium compounds is as fast ion conductors with potential electronic applications such as solid electrolytes for lithium batteries. Li20 is a fast ion conductor in which the Li ions occupy a simple cubic sublattice with the antifluorite structure. Both MAS and static 7Li NMR spectra of Li20 have been reported, the former recorded as a function of temperature up to 1000 K (Xie et al. 1995). The effect of introducing vacancies on the Li sites by doping with LiF has been studied by high-temperature static 7Li NMR, which reveals the interaction of the Li defects > 600 K and the appearance of 2 distinct quadrupolar interactions at about 900 K. Measurements of the relative intensities of the satellite peaks as a function of temperature have provided evidence of thermal dissociation of an impurity-vacancy complex (Xie et al. 1995). The mechanism of Li motion in the novel thioborate LisBvS13 has been investigated by measuring the relaxation rates of 7Li as a function of temperature up to 650 K (Grtine et al. 1995). This compound displays pronounced Li + mobility but with rather complex relaxation behaviour indicating the operation of 3 different processes by which Li ions move within the crystal. Below room temperature, the lithium ions move in the extended channels in the structure, unhindered by the presence of S atoms such as those located in the channels of related thioborate compounds. A second process becoming significant at about 200 K involves jumping of the Li + between the holes of the porous anionic network, while the third process, above about 300 K, results from the movement of Li + between more isolated sites via pathways which become increasingly accessible because of thermal activation (Grtine et al. 1995). Lithium intercalation of compounds such as SnS2 are of technical interest for photochromic display materials and lithium electrodes which reversibly take up and release Li +. 6'7Li and ll9Sn NMR has been used to investigate the location of the Li insertion sites in this material (Pietrass et al. 1997). The 7Li spectra show a central transition which can be decomposed into 2 components with different XQ values corresponding to Li in octahedral and tetrahedral interlayer sites. As the Li concentration increases, the additional ions enter tetrahedral intralayer sites surrounded by 3 tin and 4 sulphur atoms and characterised by a broad VLi NMR spectral component. Further insertion of Li results in the material becoming amorphous by rupture of the layers (Pietrass et al. 1997). LiCoO2, an important electrode material for secondary lithium batteries, occurs in 2 polytypes, both of which have been investigated by 6'7Li and 59Co NMR at 3 magnetic fields (Siegel et al. 2001). Both polytypes show only 1 Li resonance corresponding to lithium in octahedral coordination with oxygen, with similar 7Li XQ values (25-36 kHz for the 02 polytype and 31-39 kHz for the 03 polytype). The superionic compound Li3Scz(PO4)3, studied by 7Li NMR up to 575~ (Vashman et al. 1992) has revealed the operation of three types of Li ion motion and allowed
N M R of Other Quadrupolar Nuclei
637
their activation energies to be determined. Both the 7Li quadrupole parameter and spinlattice relaxation rate change abruptly in the vicinity of a phase transformation at about 530 K. The 6Li NMR spectra of a 6Li-enriched sample were also measured as a function of temperature, and it was possible to derive a precise value of the quadrupole moment for 7Li of (2.56 + 0.05) • 10 -2 barn from observation of the 6Li and 7Li quadrupole spectra in the same compound (Vashman et al. 1992). The 7Li NMR spectra of the superionic conducting compound Li3Inz(PO4)3 have been obtained at temperatures up to 520 K, and, together with the corresponding 31p NMR spectra, provide evidence of phase transitions in this material at about 380 K and 420 K, at which the lithium ions are re-distributed between the different crystallographic sites (Pronin et al. 1990). Measurements of the 7Li relaxation rates indicate the presence of Li+-Li + contact pairs in the crystal lattice, leading to a suggested model for Li ion transport involving the movement of an interstitial configuration of metastable Li + pairs (Pronin et al. 1990). Lithium-doped BPO4, another candidate ceramic electrolyte material for lithium batteries has been studied by 7Li NMR relaxation and linewidth measurements of samples with Li doping levels up to 20 mol % (Dodd et al. 2000). Comparison of the NMR data with values of the second moment calculated for both random and homogeneous models of Li distribution indicate the existence of Li clusters with an internuclear distance of --~ 3A, possibly consisting of 1 Li ion fixed at a boron vacancy with additional 2 Li ions in the conduction channels surrounding the vacancy. The atomic jump time, determined from measurements of the 7Li motional narrowing behaviour, indicate a maximum in the Li ionic mobility at the 10 mol % doping level (Dodd et al. 2000). 7Li NMR has been used to study the processes by which Li ions move through the structures of the ionic conducting ceramic materials lithium lanthanum titanate and lithium aluminium titanium phosphate (Nairn et al. 1996). The 7Li static NMR spectra of Lio.33Lao.svTiO3show the quadrupolar powder pattern associated with significant Li ionic mobility, with a room-temperature XQ value of 900 Hz. A second Li site which becomes apparent in these spectra at higher temperatures has been attributed to the presence of less mobile defects. The 7Li NMR spectrum of Lil.3Alo.3Til.7(PO4)3 shows a powder pattern with a large room-temperature • (about 45 kHz) increasing smoothly to about 54 kHz at 400 K probably due to a temperature-induced lattice distortion (Nairn et al. 1996). Lithium vanadate bronzes are intercalated compounds with potential applications for lithium battery technology, since Li can be reversibly inserted into these structures by electrochemical reaction. 7Li NMR has been used to study the structure of "y-Lio.95V205 (Cocciantelli et al. 1992) and a series of related bronzes LixV205 (Cocciantelli et al. 1992a). The 7Li NMR spectrum of the ~/-phase indicates the presence of a single Li site, but as the Li content is increased beyond x = 1, new lines can be resolved, corresponding
638
Multinuclear Solid-State NMR of Inorganic Materials
to the Li sites in the 8-phase ( - 12 ppm) and g-phase ( - 10 ppm) which co-exist with the y-phase (5-15 ppm) in these compositions (Cocciantelli et al. 1992, 1992a).
10.1.5 6, 7Li N M R o f glasses
A series of binary lithium silicate glasses have been studied by 6Li MAS NMR, showing a linear relationship between the average isotropic chemical shift and the glass composition (Figure 10.4A) (Gee et al. 1997). This relationship, which is similar to that found for 23Na isotropic shifts in binary sodium silicate glasses, indicates that for both nuclei the isotropic chemical shift becomes more positive (i.e. the bonding becomes more covalent) as the concentration of non-bridging oxygen species in the glass increases (Gee et al. 1997). Both 6Li and 7Li NMR spectra have been reported for binary lithium silicate glasses and their crystallisation products (Dupree et al. 1990). On thermal recrystallisation of the glasses, the widths of the 6Li NMR spectra decrease, indicating a significant contribution by chemical shift dispersion to the linewidth of the glass. The static 7Li NMR spectra of the crystallised glasses exhibit splitting due to dipolar coupling of isolated pairs of 7Li nuclei with a separation of -~ 2.1/k (Dupree et al. 1990). The "mixed alkali" effect in silicate glasses refers to the observation that systems containing more than 1 alkali cation show ionic conductivity and dielectric behaviour which does not follow a simple linear combination of the properties of the pure components, but can often show a marked minimum in these properties at about the equiatomic composition. 7Li, 23Na and 29Si MAS NMR has been used to investigate this effect in (Li,Na) disilicate glasses (Ali et al. 1995). The 7Li and 23Na linewidths and shifts were found to change continuously as a function of composition, suggesting that the alkali ions are uniformly mixed rather than segregated into Li and Na-rich domains. This conclusion, which contradicts previous glass structure models, has been confirmed by 23Na-{ 7Li} Spin Echo Double resonance (SEDOR) studies (Gee and Eckert 1996), and by 29Si{VLi} and 29Si{23Na} Rotational Echo Double Resonance (REDOR) NMR results (Gee et al. 1997). The REDOR experiments were used to selectively enhance those silicon sites most strongly coupled to either Li or Na ions, allowing a comparison of their spectroscopic parameters. The isotropic chemical shifts of both the 7Li and 6Li MAS NMR spectra of (Na,Li) disilicate glasses have been found to become more positive with increasing Na content of the glasses (Figure 10.4B), following a similar trend found for the 23Na shifts (although the chemical shift range in 6'7Li is smaller, making the relationship with composition more subtle). These monotonic compositional dependencies of the alkali chemical shifts provide further evidence against glass structural models involving cation clustering (Gee et al. 1997). Both 6Li and 7Li MAS NMR has been used to investigate the local Li coordination environment in a series of binary lithium phosphate glasses (Alam et al. 1999). The 6Li chemical shifts, which approximate closely to the isotropic chemical shifts, increase
639
NMR of Other Quadrupolar Nuclei
A
B
0.4
~
0.3
w _t_
0.2 G<:) .p.m
0.6]
-~
~., ~
~-
0.4
"
..-q
o.1
~.~ 0.2
0 -
,
0
~
,
!
,
10
i
',
i
20
,--1
,
J
30
Mol % Li20
o
i
,-
i~
40
~
0.2
0.6
1.0
Na/(Na + Li)
Figure 10.4. A. Relationship between the 6Li isotropic chemical shift and the composition of lithium silicate glasses. B. Relationship between the VLi and 6Li isotropic chemical shift and the composition of a series of (Li,Na) disilicate glasses. The open circles denote the 7Li shifts, the filled squares denote the 6Li shifts. From Gee et al. (1997), by permission of Elsevier Science.
with increasing Li20 content, reflecting increased cross-linking of the tetrahedral phosphate network by Li-O-Li bridges. The 6Li chemical shifts were also found to vary monotonically through the anomalous glass transition minimum in this system, showing that this phenomenon is not related to any abrupt changes in the Li coordination environment. The NMR results for this series of phosphate glasses indicate that the average coordination number of the Li atoms is 4-5 (Alam et al. 1999).
10.2. 9Be NMR
9Be is a spin = 3/2 nucleus with a relatively small quadrupolar moment (5.3 • 10 -3o m2), good receptivity and 100% natural abundance. Despite its suitability for NMR studies, recent solid-state 9Be NMR studies are relatively rare, probably due to the extreme toxicity of the element and its compounds, but possibly also because the narrow chemical shift range of 9Be detracts from its utility as a characterisation tool. Most of the solid state 9Be NMR studies reported to date have been of minerals which are comparatively nontoxic. 9Be chemical shifts are commonly reported relative to solid BeO. The 9Be MAS NMR spectra of BeO, beryl (AlzBe3Si6018) and tugtupite (NasAlzBezSisO24C12) have been obtained by Skibsted et al. (1995). Because of the small quadrupole moment of 9Be, measurements of the satellite transitions (SATRAS) were used to determine the quadrupolar parameters of these materials. A considerable contribution from the central transition to the intensities of the first and second-order spinning side bands of BeO was ascribed to incomplete averaging of the 6 9Be-9Be dipolar couplings for each Be. The value of Xe for BeO obtained from the SATRAS measurements (0.039 MHz) is in good agreement with a previous single-crystal value of 0.0394 MHz (Thorland et al. 1972).
Multinuclear Solid-State NMR of lnorganic Materials
640
The 9Be XQ value obtained from SATRAS measurements of beryl (0.495 MHz) agrees with the value of 0.504 MHz obtained from a previous single-crystal study (Brown and Williams 1956) and is more than an order of magnitude larger than in BeO, reflecting the more distorted BeO4 tetrahedra in beryl (Skibsted et al. 1995). The corresponding parameter for tugtupite (0.035 MHz) is very similar to that of BeO, consistent with the axial symmetry of the Be sites in both compounds. A single crystal 9Be NMR study of the gemstone alexandrite, BeAll.98Cro.o204, has shown the presence of 2 magnetically inequivalent Be sites which are, however, chemically equivalent (Yeom et al. 1995). The temperature dependence of • was also determined, and was found to increase with increasing temperature, by contrast with the more usual decrease found in other compounds, but the asymmetry parameter decreases with increasing temperature (Yeom et al. 1995). The 9Be MAS NMR chemical shifts have been measured for a number of beryllium sodalite framework structures of general formula M8[BeZO4]6X2 where M = Cd or Zn, Z = Si or Ge and X - S, Se or Te (Dann and Weller 1997). All the spectra show a single sharp 9Be resonance corresponding essentially to the giso value. The chemical shifts show linear correlations with the Be-O-Si and Be-O-Ge angles (Figure 10.5A) given by 9Be 6iso = - 0 . 0 6 2 6 B e - O - S i angle (~ + 7.91
(10.4)
9 B e ~iso -- - 0 . 0 6 1 7 B e - O - G e angle (~ + 8.25
(10.5)
A
B 4 0.8
0.8 ~u
o silicates
r~
a tes
o
"'~~ -0.8
silicates
-0.8 -1.6 120
130 140 Be-O-Z angle (o)
8.1
8.5 Lattice parameter (~)
8.9
Figure 10.5. A. Relationship between the 9Be chemical shift and the tetrahedral Be-O-Z angle for a series of sodalite framework structures, where Z -- Si or Ge. B. Relationship between the 9Be shift and the lattice parameter for the same series of sodalites. From Dann and Weller (1997), by permission of the copyright owner.
641
NMR of Other Quadrupolar Nuclei
Linear correlations were also found b e t w e e n (Figure 10.5B) given by 9 B e 6is o --
9 B e 6is o -
~iso
and the sodalite lattice parameter a
- 2 . 4 4 a (A) + 20.0 (beryllosilicates)
(10.6)
1.96a ( * ) + 17.1 (beryllogermanates)
(10.7)
-
The 9Be N M R interaction parameters in Be c o m p o u n d s are shown in Table 10.2. Small amounts of beryllium can be substituted into the tetrahedral f r a m e w o r k sites of Z S M - 5 zeolite by treatment with a m m o n i u m tetrafluoroberyllate. A l t h o u g h the amounts of Be substituted were too small for detection by thermal analysis and desorption measurements, the presence of Be 2+ in f r a m e w o r k sites was detected by 9Be M A S N M R , which showed a resonance at - 5.0 p p m with respect to aqueous BeSO4
Table 10.2. 9Be NMR interaction parameters for beryllium compounds. Compound
~iso
XQ (MHz)
xI
Reference Skibsted et al. (1995), Thorland et al. (1972) Skibsted et al. (1995), Brown & Williams (1956)
(ppm)* BeO
0
0.039, 0.0394
0.15
A12Be3Si6018 (beryl)
- 1.9, - 2.4
0.495, 0.504
0.14, 0.09
tugtupite
0.035
0.14
BezAsOaOH.4H20
- 3.0, 2.1 ND - 1.4"* - 0.77**
Zn8[BeSiO4]6S2
0.15 ~
0.3178 ND ND ND
Zn8[BeSiO4]6Se2 Zns[BeSiO4]6Te2 Cds[BeSiO4]6S2 Cdg[BeSiO4]6Se2 Cds[BeSiO4]6Te2
0.08 t 0.03' - 0.59 t - 0.73* - 0.80 t 0.83 t 0.77 * 0.64* 0.27* 0.07 t - 0.03 t - 1.6"*
ND ND ND ND ND ND ND ND ND ND
0.904 ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND
-
alexandrite Be3(AsOa)z.2H20
Zn8[BeGeO4]6S2
Zns[BeGeO4]6Sez Zns[BeGeO4]6Te2 Cds[BeGeO4]6S2 Cds[BeGeO4]6Se2 Cds[BeGeO4]6Tea Mg19Na58(BePO4)96
* chemical shifts quoted with respect to solid BeO t these shifts quoted with respect to aqueous BeC12 solution ** these shifts quoted with respect to aqueous Be(NO3)2 solution
Skibsted et al. (1995), Xu & Sherriff (1994) Yeom et al. (1995) Harrison et al. (1994) Harrison et al. (1993) Dann & Weller (1997) Dann & Weller (1997) Dann & Weller (1997) Dann & Weller (1997) Dann & Weller (1997) Dann & Weller (1997) Dann & Weller (1997) Dann & Weller (1997) Dann & Weller (1997) Dann & Weller (1997) Dann & Weller (1997) Dann & Weller (1997) Nenoff et al. (1992)
642
Multinuclear Solid-State NMR of Inorganic Materials
(Han et al. 1993). This shift value is in fair agreement with another reported value of - 5.8 ppm for Be inserted in ZSM-5 during synthesis (Romannikov et al. 1985). The 9Be MAS NMR spectrum of the dehydrated faujasite analogue Mg-exchanged sodium beryllophosphate has been reported to contain a single sharp resonance at - 1.6 ppm with respect to aqueous Be(NO3)2 solution (Nenoff et al. 1992). 9Be NMR has been used to detect slow atomic motion of beryllium in Zr-Ti-Cu-NiBe metallic glasses. The results, obtained by a spin alignment echo technique, are consistent with Be diffusion occuring by a mechanism involving thermal fluctuations of the spread-out free volume rather than by vacancy-assisted or interstitial diffusion mechanisms (Tang et al. 1998).
10.3. SlV NMR
10.3.1 G e n e r a l c o n s i d e r a t i o n s
5~V is a favourable nucleus for solid state NMR since it has a 99.76% natural abundance, a large magnetic moment and generally short relaxation times because of the nuclear quadrupole interaction of this spin = 7/2 system. Although the static spectra are often broad, MAS satisfactorily averages the first-order anisotropic shielding and quadrupolar interactions, yielding useful spectral information. In materials science, the principal use of 5~V NMR has been to study the industrially important families of V2Os-containing catalysts. Although the 5~V NMR spectra of vanadium compounds are very sensitive to the oxygen environment of the V atom, the 5~V ~iso values are less diagnostic than the chemical shift anisotropies (CSA). However, ~iso values for compounds with the same first coordination sphere are sensitive to the nature of the atoms in the second coordination sphere (Lapina et al. 1992).
10.3.2 st V NMR of vanadium oxides and the vanadates
The structure of V205 is built up from VO5 square pyramidal units sharing edges and comers. A 5~V MAS NMR study of V205 has shown a single resonance from the unique vanadium site, with a pattern of spinning sidebands which have been simulated to provide information about the relative orientation of the principal axes of the 2 anisotropic interactions (Fernandez et al. 1994). A partial structural phase transition in B-type VO2 between 300-180 K has been studied by 5lV NMR in which the identification of a spin-singlet state confirmed the operation of V 4+-V 4+ pairing in half the V sites of the low-temperature modification (Oka et al. 1993). Similar spin-pairing behaviour is also known in the structurally-related oxide phase V6013.
643
NMR of Other Quadrupolar Nuclei
Vanadium in the alkali metal orthovanadates M3VO4and pyrovanadates M4V207 is in nearly regular tetrahedral coordination, giving almost isotropic 51V NMR spectra illustrated by T13VO4 (Figure 10.6A). The distortion from tetrahedral symmetry progressively increases in the other T1 vanadates T14V207 and T1VO3 containing tetrahedral chains, finally assuming a distorted octahedral symmetry in T12V6016. The orthovanadates of the divalent metals show 51V isotropic chemical shifts of about - 5 2 0 to - 560 ppm (with respect to VOC13) where the cation radius is < 1A, and about - 6 0 0 ppm for cations of radius > 1A,. All divalent orthovanadates have CSAs of 100 ppm (Lapina et al. 1992). Divalent metal pyrovanadates have 8iso values > - 6 0 0 ppm where r < 1A, and - 6 0 0 ppm where r > 1A. The CSA of the divalent pyrovanadates varies from 80 to 300 ppm. The 51V NMR spectra of the divalent metavanadates M(VO3)2, in which the vanadium is in a distorted trigonal bipyramidal environment, have nearly axial CSAs varying from 500 to 700 ppm, considerably less than the CSA value for V205 (960 ppm) (Lapina et al. 1992). The 5~V NMR spectra of the trivalent metal vanadates show narrow, almost isotropic lines with 8iso values ranging from - 4 0 0 to - 7 8 0 ppm and CSA values < 140 ppm (Lapina et al. 1992). A number of vanadates have structures with V in both distorted tetrahedral and octahedral environments. The S~V NMR spectra (Figure 10.6B) show the superposition of the fully anisotropic line from V in the distorted tetrahedron with a line of axial symmetry from the V atoms in the distorted octahedra.
8iso<
A
B Tet Oct ,--'--,
Cs2V~ TI3
TI4V10~~ 1 | ! 0
Rb3V5Ol4y I~___
_!
-~obo
~ ~
| _
o
I
.
I
-looo
i_
SlY shift (ppm) w.r.t VOCI2 Figure 10.6. A. Selection of 5~V NMR spectra of thallium vanadates showing the effect of increasing distortion of the VO4 tetrahedra in progressing from T13VO4to T1VO3. The spectrum of TlzV6016 arises from V in a distorted octahedral environment with almost axial symmetry. B. A series of 5~V NMR spectra of vanadates with V in both distorted tetrahedral and distorted octahedral environments. From Lapina et al. (1992), by permission of Elsevier Science.
644
Multinuclear Solid-State NMR of Inorganic Materials
T h e 5~V N M R i n t e r a c t i o n p a r a m e t e r s f o r a n u m b e r o f v a n a d i u m c o m p o u n d s are c o l l e c t e d in T a b l e 10.3.
Table 10.3. 5~V NMR interaction parameters of vanadium compounds.
6i~o(ppm)*
Compound Li3VO4 Na3VO4 K3VO4 Cs3V04
T13VO4 KCaVO4 site site site site Mg3(VO4)2
1 2 3 4
Ca3(WO4)2 Sr3(VO4)2
Ba3(VO4)2 Zn3(go4)2
AIVO4
site site site BiVO4 YVO4 LaVO4 LuVO4 Na4V207 site site
1 2 3
1 2
K4V207
C s 4 V 2 0 7 site 1
site 2 T14V207 o~-Mg2V207 site 1 site 2 13-Mg2V207 site 1 site 2 Ca2V207 site 1 site 2 Sr2V207 site 1 site 2 site 3 site 4 Ba2V207 site 1 site 2 site 3 ZnV207
-
- 544 - 545 - 560 - 576 - 480 -580 - 589 - 616 - 623 - 557, - 554 - 615 - 610 - 605 - 522 - 668 - 747 - 780 - 420 - 664 - 609 - 663 - 560 - 575 - 578 - 543 - 567 - 504 5 5 5 , - 551 617, - 605 6 8 0 , - 642 650, - 497 - 574 - 578 - 557 - 582 - 588 - 592 - 579 - 588 - 600 - 625
XQ (MHz)
rl
Reference
1.52 ND ND ND ND ND ND ND ND ND, 0.49 2.05 0.53 0.75 ND ND ND ND ND 4.75 5.21 4.23 ND ND ND ND ND ND ND ND, 4.6 ND ND, 9.5 ND ND ND ND ND ND ND ND ND ND
ND ND ND ND ND ND ND ND ND ND, 0.63 ND ND ND ND ND ND ND ND 0 0.69 0 ND ND ND ND ND ND ND ND, 0.55 ND ND, 0.70 ND ND ND ND ND ND ND ND ND ND
Lapina et al. (1992) Eckert & Wachs (1989) Lapina et al. (1992) Lapma et al. (1992) Lapma et al. (1992) Lapma et al. (1992) Laplna et al. (1992) Lapma et al. (1992) Lapma et al. (1992) Laplna et al. (1992), Occelli et al. (1992) Lapina et al. (1992) Lapina et al. (1992) Lapina et al. (1992) Eckert & Wachs (1989) Lapina et al. (1992) Lapina et al. (1992) Lapina et al. (1992) Lapina et al. (1992) Gubanov et al. (1977) Laplna et al. (1992) Lapma et al. (1992) Lapma et al. (1992) Lapma et al. (1992) Lapma et al. (1992) Lapma et al. (1992) Lapma et al. (1992) Lapma et al. (1992) Lapina et al. (1992), Occelli et al. (1992) Lapina et al. (1992), Occelli et al. (1992) Lapina et al. (1992) Lapina et al. (1992) Lapina et al. (1992) Lapina et al. (1992) Lapina et al. (1992) Lapina et al. (1992) Lapina et al. (1992) Lapina et al. (1992) Lapina et al. (1992) Eckert & Wachs (1989)
645
NMR of Other Quadrupolar Nuclei
Table 10.3. (Continued). Compound
~iso (ppm)*
Xe (MHz)
qq
Cd2V207 Pb2V207 ZrV207 LiVO3
- 579 - 522 - 774 - 573.4, - 577.1 - 572, - 569.5, - 571.5
ND ND ND 3.18
ND ND ND 0.87
NH4VO3
ot-NaVO3
- 582, - 572.7, - 578.2
[3-NaVO3
510.4, - 516.4
-
T1VO3
528, - 529.1 -583 -
CsVO 3 RbVO3 KVO3
ND - 548, - 552.7, - 557.7
KV308 site 1 site 2
K2V6016 Rb2V6016 Cs2V6016 T12V6016 K2V8021 KVO3H20 NaVO32H20 o~-Mg(VO3)2 Ca(VO3)2
Ba(VO3)2 Zn(VO3)2 Pb(VO3)2 Cd(VO3)2 VOPO4 VOAsO4 VOC13 ( - 170~
548.1 - 510.0 - 503 - 503 508 - 700 - 570 606 - 530 -576 -575
-
-
-
660 -517 -533 - 500 - 734 -617 6 -
2.88, 2.95, 2.76, 2.95 3.7, 3.65, 3.15, 3.94, 3.80 4.20, ND ND, 3.67 3.92, 3.84 4.33 4.21, 4.35, 4.06, 4.34, 4.20 2.45 3.03 ND ND ND ND ND ND 3.94 6.79 3.30, 3.16, 2.81 ND ND ND ND ND ND 7.5, 5.7, 2.98, 5.4
Reference
Eckert & Wachs (1989) Eckert & Wachs (1989) Lapina et al. (1992) Skibsted et al. (1993), Hayashi & Hayamizu (1990) 0.3, 0.19, 0.37, Baugher et al. (1969), 0.30 Segel & Creel (1970), Skibsted et al. (1993), Hayashi & Hayamizu (1990) 0.52, 0.6, 0.64, Baugher et al. (1969), 0.64, 0.46 Spegel & Creel (1970), Skibsted et al. (1993), Hayashi & Hayamizu (1990) 0.55, Skibsted et al. (1993), ND Hayashi & Hayamizu (1990) Lapina et al. (1992), ND, Skibsted et al. (1993) 0.71 Segel & Creel (1970) 0.62, 0.63 Lapina et al. (1992) 0.72 Gomostansky & 0.65, 0.75, 0.76, Stager (1968) 0.77, 0.80 Baugher et al. (1969), Segel & Creel (1970), Skibsted et al. (1993), Hayashi & Hayamizu (1990) Skibsted et al. (1993) 0.44 Skibsted et al. (1993) 0.89 Lapina et al. (1992) ND Lapma et al. (1992) ND Lapma et al. (1992) ND Laplna et al. (1992) ND Laplna et al. (1992) ND Laplna et al. (1992) ND Laplna et al. (1992) 0.64 Laplna et al. (1992) 0.63 Segel & Creel (1970), 0.8, 0.6, 0.6 Gomostansky & Stager (1968) Lapina et al. (1992) ND Eckert & Wachs (1989) ND Eckert & Wachs (1989) ND Lapina et al. (1992) ND Lapina et al. (1992) ND Lapina et al. (1992) ND Paulsen & Rehder (1982), 0.08 Habayeb & Hileman (1980), Allerhand (1970)
646
M u l t i n u c l e a r Solid-State N M R o f l n o r g a n i c M a t e r i a l s
Table 10.3. (Continued). Compound
~iso
V205
-
K3VsO14 (oct)
-
(tet)
-
Rb3V5014
(oct)
-
(tet)
-
T13V5Oj4 (oct)
-
(tet) Cs2V4011
(oct)
-
(oct)
-
(tet) Hg4V209
(tet)
(ppm)*
XQ(MHz)
xl
610 500 620 496 619 500 594 510 575 518 516
0.8 ND ND ND ND ND ND ND ND ND ND
0.04 ND ND ND ND ND ND ND ND ND ND
Reference Gornostansky & Stager (1967) Lapina et al. (1992) Lapina et al. (1992) Lapina et al. (1992) Lapina et al. (1992) Lapina et al. (1992) Lapina et al. (1992) Lapina et al. (1992) Lapina et al. (1992) Lapina et al. (1992) Lapina et al. (1992)
* Chemical shifts referenced to VOCI 3
A 51V and 2~ NMR study of single-crystal ferroelastic BiVO4 as a function of temperature has shown that this material undergoes a second-order phase transition. The results have also identified the substitutional sites of Mn 2+ and Fe 3+ impurities in this compound as Bi 3+ and V 5+ respectively (Choh 1996). The magnetic and electronic properties of La•215 have also been investigated by 51V NMR (Mahajan et al. 1991). At the composition about x = 0.7, this material undergoes a transition from metallic behaviour to an antiferromagnetic insulator, reminiscent of the cuprate superconductors such as La2-xSr• No evidence was found in lanthanum strontium vanadates for antiferromagnetic spin fluctuations or superconducting properties, and the 51V NMR data in the metallic composition range are typical of a narrow d-band metal (Mahajan et al. 1991). 5~V NMR has been used to study a series of solid solutions in the system ZrVz-xPxO7, some members of which show negative isotropic thermal expansion properties over a broad temperature range up to at least 950~ This unique thermal expansion behaviour appears to be related to frustration in bending V-O-V (or P-O-P) angles away from 180 ~ in a cooperative manner (Korthuis et al. 1995).
10.3.3 st V NMR of zeolites and catalysts Vanadium plays an important role in many industrial catalysts used extensively in a variety of applications including the production of SO3 from SO2, selective oxidation of hydrocarbons, reduction of nitrogen oxides with ammonia, and in the manufacture of many chemicals and chemical intermediates. Such catalysts typically consist of vanadium compounds supported on oxides such as silica, alumina, titania, etc., and their activity depends on factors such as the chemical form and crystalline environment
647
N M R of Other Quadrupolar Nuclei
of the vanadium. 51V NMR has been extensively used to provide information about the surface species of oxide-supported vanadium catalysts, their interaction with the supporting material and with the reacting molecules during the catalytic processes. The earlier literature on 51V NMR studies of catalysts has been extensively reviewed by Lapina et al. (1992). Typical of such 51V NMR studies is an investigation of the surface state of vanadium impregnated by a wet process on a TiO2-ZrO2 support (Reddy et al. 1992). At lower vanadium loadings, the 51V NMR spectra reveal the presence of 2 types of tetrahedral vanadium units characterised by peaks at - 6 6 0 to - 6 8 0 ppm and - 4 5 0 ppm (Figure 10.7A). At higher vanadium loadings in excess of a monolayer coverage, a third species appears, characterised by 5~V NMR peaks at - 310 and - 1265 ppm. This arises from vanadium in a distorted octahedral environment and reflects the presence of crystalline V2Os (Reddy et al. 1992). 5~V static and MAS NMR have been used to determine the quadrupole parameters and CSA tensors of strongly bonded vanadium species in VOx/TiO2 catalysts, in which a very large value of 5~V XQ was found (14-16 MHz), but with CSA tensor components similar to those of bulk V205 (Shubin et al. 1999). A 51V NMR study of V205 supported on SnO2 and oL-Sb204 showed the presence at A
B Oct Tet
wt.%
- , , Tet 13
t,~
. ~.
t
V2U
-310 Ii/~ : /: k :
Oct -1265 '
5
t * 16
-350/"",-~
i
l, li
10
-350 Static ~
ed
j -350 MAS / / ~
I
!
0
!
-508 unheated
i
-5oo -1000
SlY shift (ppm) w.r.t. VOCI3
|
,,
0
-500
=.
-1000
SlV shift (ppm) w.r.t. VOCI3
Figure 10.7. A. 51V NMR spectra of a series of V20 5 catalysts supported on TiO2/ZrO2 after heating at 250~ for 2 h. showing the evolution of the distorted octahedral V-O species (V205) at the higher vanadium concentrations. From Reddy et al. (1992), by permission of the American Chemical Society. B. 51V NMR spectra of a mixed V205-WO3 catalyst supported on a TiO2/A1203 substrate. Note the lack of narrowing under MAS conditions and the loss of the V2W40~94resonance at - 508 ppm on heating at 200~ From Mastikhin et al. (1995), by permission of the copyright owner.
648
Multinuclear Solid-State N M R of lnorganic Materials
lower V concentrations of a dispersed oxide phase, with crystalline V205 appearing at higher V concentrations. The oxide phases in these catalysts were too dispersed to be detected by X-ray diffraction (Reddy and Mastikhin 1992). 51V spin echo and MAS NMR has been used to study the coordination and local order of V205 supported on SiO2 and indicates that the vanadia species on the silica surface is an (SiO)3V = O unit (Das et al. 1993). Mixed oxide catalysts of V205 and WO3 supported on TiOx/Al203 have also been investigated by 5Iv NMR (Mastikhin et al. 1995). A sharp 51V line at about - 508 ppm in the as-prepared catalyst (Figure 10.7B) was identified as resulting from the anionic species V2W40194-in solution in the pores of the catalyst. Two broader lines were also found, 1 at - 550 ppm arising from tetrahedral vanadium and the other at - 3 5 0 ppm being 1 branch of the octahedral V resonance typically found in these supported catalysts. Magic angle spinning has a minimal effect on the linewidth of the broad peaks (Figure 10.7B), indicating the presence of inhomogeneous broadening due either to a distribution of quadrupole and chemical shift parameters, or to dynamic processes in the second coordination sphere of the vanadium. Removal of the adsorbed water by evacuation at 200~ results in the loss of the peak from the vanadium-tungsten anion and the broadening of the entire spectrum, with further broadening occurring after heat treatment at 500~ (Figure 10.7B). By analogy with NMR results for V205 catalysts supported on ~/-A1203 and on TiO2, the spectra of the heated samples provided possible evidence for surface interactions between mixed WO3-V205 species and the TiOx support, although direct detection of V-O-W fragments was not possible due to the broadness of the 51V spectra (Mastikhin et al. 1995). The formation of A1VO4 at high vanadium concentrations on AlzO3-supported catalysts has been detected by 51V NMR which has also revealed the complex behaviour of this compound during subsequent calcination processes leading to fundamental changes in the structure of the catalyst surface (Sobalik et al. 1992). Changes in a series of vanadium-rhodium catalysts supported on silica substrates during calcination have also been monitored by 51V NMR which showed the presence of V205 at all temperatures, the appearance of a distorted VO4 species at 973 K and the formation of RhVO4 at 1173 K (Lapina et al. 1992a). Sepiolite, a layered magnesium silicate mineral, is used to stabilise cracking catalysts against metal contaminants in crude oils. 5~V NMR has been used to study the interaction of sepiolite impregnated with vanadium from vanadyl napthenate solution (Occelli et al. 1992). Hydrothermal treatment of the V-impregnated sepiolite samples results in the formation of disordered microcrystalline ot-MgzV207 (or ~-MgzV207 at higher temperatures), together with an amorphous surface phase identified by 51V nutation NMR as having a structural environment similar to [~-MgzV207 (Occelli et al. 1992). Zeolites are shape-selective catalysts with a silicate or aluminosilicate framework into which vanadium can be incorporated, producing materials especially useful in
NMR of Other Quadrupolar Nuclei
649
reactions involving the partial oxidation of hydrocarbons under mild conditions. Such catalysts can have new and improved properties; the catalytic activity of V-substituted silicalite is quite different from that of V203 supported on a silica substrate. Hydrothermal synthesis has been used to prepare a vanadium-substituted silicate with the structure of ZSM-12 (Moudrakovski et al. 1994). Although no 5~V NMR signal was detected in the as-prepared sample, calcination at 823 K followed by rehydration produced a spectrum characteristic of VO4 but with no indication of the presence of any V203. The 5~V NMR data support a structural model in which the V is connected to the lattice by 3 bonds, with the fourth oxygen as a double bond, possibly hydrogen-bonded to an adjacent SiOH group (Moudrakovski et al. 1994).
10.4. 63CuAND 6SCu NMR
63Cu and 65Cu are both spin - 3/2 nuclei with natural abundances of 69.09% and 30.91% respectively. The quadrupole moment eQ of 63CH is 1.07 times greater than for 65Cu, but the XQ values for both nuclei can be very large in copper compounds, militating against the possibility of significantly narrowing the 63Cu or 65Cu spectra by magic angle spinning. Although the ~/-value for 63Cu is 0.934 of the value for 65Cu making the latter a more sensitive nucleus, this gain in sensitivity is not sufficient to compensate for the lower natural abundance of 65Cu. Furthermore, since the first-order quadrupolar broadening is proportional to the quadrupole moment eQ, and since both nuclei are spin - 3/2, the second-order quadrupolar broadening is proportional solely to (eQ)2/~/resulting in the NMR lines of 63CH being broader than those of 65Cu by a factor of 1.25. Nevertheless, 63Cu tends to be the preferred nucleus for most NMR studies. The linewidths of the static spectra have often been observed to be due to a combination of quadrupole and CSA effects. The differences in eQ and the frequencies of each nucleus are such that the spectra acquired for the 2 nuclei at a single field can yield similar information to spectra acquired for a single nucleus at 2 fields, allowing the deconvolution of the quadrupolar and CSA broadenings. CuC1 is a useful secondary chemical shift standard for these nuclei. 63CH NMR has provided useful structural information in a series of mixed copper halide crystals in which the copper ion is located in a suitably symmetrical site. The 63Cu MAS NMR spectra of a series of solid solution within the system CuBrxll-x are broadened due to the overlap of resonances from the 5 species CuBr4, CuBr3I, CuBr212, CuBrI3 and CuI4 (Endo et al. 1993). The spectra could be resolved by fitting Gaussian peakshapes, providing information about the various atomic structures present in the solid solutions. Similar 63Cu NMR experiments have also been carried out in the system CuClxBrl_x (Endo et al. 1993a).
650
Multinuclear Solid-State NMR of Inorganic Materials
The temperature dependences of the 63Cu chemical shifts in a series of cuprous halides have been determined by Becker (1978). These results suggest that CuBr is the most ionic of these compounds and CuI is the most covalent. The 63Cu shifts of the low temperature (cubic) phases increase with temperature due to increasing vibrational overlap. At higher temperatures the 63Cu shifts become increasingly diamagnetic with temperature, reflecting the highly disordered state of the Cu + under these conditions (Becker 1978).
10.4.1 63CuNMR of superconductors and superfast ionic conductors
63Cu NMR and nuclear quadrupole resonance (NQR) have been extensively applied to studies of the electronic phenomena in high-temperature superconducting oxides. These studies have exploited the unique ability of NMR and NQR to distinguish the behaviour of different atoms at different crystallographic sites. A full discussion of the extensive 63Cu NMR literature on the superconducting compound YBa2Cu307_y and related materials is beyond the scope of this chapter, but a few examples are presented which give an idea of the range of NMR results for these compounds. The effect of oxygen stoichiometry on the temperature dependence of the Knight shift has been demonstrated by 63Cu NMR measurements on well-characterised YBa2Cu306.63 magnetically aligned to orient the powder grains and ensure that the lines from the powder are as narrow as possible (Takigawa et al. 1991). The 63CuNMR spectrum of such a sample (Figure 10.8A) shows a relatively sharp central resonance corresponding to the planar Cu(2) sites. A broad downfield feature corresponds to Cu(1) chain sites with 4 oxygen nearest neighbours, while a sharp upfield feature which shows a small magnetic shift and is easily saturated by fast pulse repetition is associated with Cu(1) chain sites with no O nearest neighbours (Takigawa et al. 1991). These 2 copper sites show sharply contrasting temperature (T) dependences of their 63Cu nuclear relaxation rates (Walstedt et al. 1988); in the normal state the chain sites exhibit a roughly linear temperature dependence as predicted by the simple Korringa model, while the planar sites relax with an approximately T ~/2 dependence. The anisotropy of the Cu relaxation in the planar sites has been examined in detail using the 63CI1NMR shift to partition the susceptibility and estimate the density of states (Walstedt et al. 1988). Comparison of the relaxation rates of 63Cuwith those of ~70 for the planar sites in the same YBa2Cu307 sample (Figure 10.8B) has revealed the existence of a characteristic temperature range (between 20 K and 110 K) in which both relaxation rates exhibit identical temperature dependences over almost 3 orders of magnitude (Hammel et al. 1989). These results have led to the conclusion that the copper relaxation is enhanced by antiferromagnetic spin fluctuations which are undiminished in the superconducting state. Comparison of the 63Cu NMR relaxation rate data for
651
N M R o f Other Quadrupolar Nuclei
A
YBa2CuaO6.63 planar
chain
YBa2Cu307
chain
I
I
I
100 54
f
56
planar
I
10
O
r-.
80
84 0.1
planar
1
10
100
i000
63Cu l/T1 (s -1) 82.0
82.8
H (kOe) Figure 10.8. A. 63CHNMR spectra of magnetically aligned YBa2Cu306.63. Upper: Central transition, T = 80 K, alignment parallel to c-axis. Centre: High-field quadrupole satellite (1/2, 3/2)transition, T = 50 K, alignment parallel to c-axis. Lower: Central transition, alignment perpendicular to c-axis, showing similar second-order effects in the distribution of the EFGs as in the satellite transition spectrum. From Takigawa et al. (1991). B. Relationship between the 170 relaxation rate and the 63Cu relaxation rate for the planar sites of YBazCu307 with temperature as an implicit parameter. The solid line of unity slope indicates the relationship 63Cu T1-1/170 T1-1 -- 19.3. The data deviate from this relationship above 110 K. From Hammel et al. (1989). Both figures used by permission of the copyright owners.
YBa2Cu307 in the normal state with inelastic neutron scattering results for the same material has revealed similar very short correlation lengths for the antiferromagnetic fluctuations (Gillet et al. 1994). Measurements of both the spin lattice relaxation and the Gaussian spin-spin relaxation times have been made on YBazCu408 by both 63Cu N M R and NQR methods (Corey et al. 1996). Excellent agreement was found between the N M R and NQR-derived Gaussian spin-spin relaxation times, which indicated a temperature regime just above Tc in which the Gaussian relaxation becomes independent of temperature, similar to behaviour found by T a k i g a w a et al. (1994) for YBazCu306.63. The data for the normal and superconducting states could be explained in terms of appropriate theory (Corey et al. 1996).
652
Multinuclear Solid-State NMR of Inorganic Materials
In addition to the much-studied Y-Ba cuprate series of high-Tc superconductors, several other types of cuprate superconductors have been investigated by 63CHNMR. The relaxation behaviour of one such compound, Bi2Sr2CaCu208, which has been found to be proportional to the spin paramagnetic shift at temperatures <100 K as in other cuprate superconductors, was interpreted in terms of spin-gap effects (Walstedt et al. 1991). 63Cu NMR has also been used to investigate the superconducting properties of HgBa2Ca2Cu308+~, a compound with a Tc of 133 K (Magishi et al. 1996). These studies suggest that the temperature dependence of the relaxation measured for both the planar (fourfold) and pyramidal (fivefold) CuO2 layers in this compound arises from combined relaxation channels to vortex cores, as well as from a residual density of states at the Fermi level associated with the gapless superconductivity (Magishi et al. 1996). The first anion-doped high-Tc cuprate superconductor to be discovered, Nd2CuO4-xF• with a Tc as high as 27 K, has been studied by 63Cu NMR (Dai et al. 1994). Measurements of the orientation dependence of magnetically aligned samples suggest that the electric quadrupole interaction at the copper site is surprisingly small for a structure in which the Cu is located in the centre of a square of oxygen atoms. A similar conclusion was reached for Nd2CuO3.sFo.2 by Sugiyama et al. (1993) on the basis of 63Cu and 19F NMR measurements. A 63Cu NMR study of the diffusion mechanism of Cu + in the superionic conductor CuA1Br4 as a function of temperature (Tomita et al. 1999) has shown a linear change in the 63Cu ~isovalues with temperature, attributed to thermal expansion of the crystal lattice. Changes in the central transition of the 63Cu powder NMR spectrum with temperature were simulated (Figure 10.9A), allowing the CSA to be determined. No change was found in the CSA with temperature, even at higher temperatures where the diffusion of Cu + is sufficiently rapid to average out the CSA, suggesting that the diffusion process proceeds via tetrahedral sites, since although larger octahedral interstitial sites are also available, their occupancy by Cu + would change the 63Cu CSA lineshape (Tomita et al. 1999). Analysis of the 63Cu satellite peak positions allowed the temperature dependence of the nuclear quadrupole coupling constant • to be established (Figure 10.9B). This behaviour suggests that in apparent contradiction to the conclusions from the CSA measurements, the Cu site approaches spherical symmetry at 420 K (Tomita et al. 1999). A similar effect was not observed in the 27A1XQvalues of the same samples (Figure 10.9B), indicating that the environments of these Cu and A1 sites are very different. The compound CuGeO3 is a one-dimensional Heisenberg antiferromagnetic material which undergoes a spin-Peierls (SP) transition at about 14 K, at which spin and lattice dimerisations may simultaneously occur. 63Cu and 65Cu NMR and NQR measurements have provided information about the Cu 2+ electronic state and the spin dynamics in this material (Itoh et al. 1995). An anisotropic Knight shift with axial symmetry was observed and analysis of its temperature dependence suggested a decrease in the spin
653
NMR of Other Quadrupolar Nuclei
A ~7AI
500
526 K 300
'
..... ,......s~mu.!ated_
294 K
100 !_
ved
................... ': i
'
71.87
i
" ---s_!mulated i
71 88
I
300 (ii)
400
500
Temperature K
-
71.89
Frequency (MHz) Figure 10.9. A. Observed and simulated 63CuNMR spectra of CuA1Br4 at two different temperatures. B. Temperature dependence of the 63CuXQvalues (solid circles) and e7A1 XQvalues (open circles) for CuA1Br4. From Tomita et al. (1999), by permission of Elsevier Science. susceptibility below the temperature of the SP transition. The data suggest that the Cu 2+ can be described by a single-ion model in an octahedral crystal field with tetragonal symmetry. The temperature dependence of the nuclear spin-lattice relaxation rate shows the appearance of a gap in the magnetic excitation spectrum below the SP transition temperature. These experiments showed that supertransferred hyperfine interaction, a characteristic of the planar Cu 2+ in cuprate superconductors, does not play a significant role in CuGeO3 (Itoh et al. 1995).
10.5. 69Ga AND 71Ga N M R
10.5.1 G e n e r a l c o n s i d e r a t i o n s The 2 NMR-active nuclei of gallium both have spin I - 3/2 but different quadrupole moments (Q = 1.71 • 10 -29 m 2 for 69Ga and Q = 1.07 • 10 -29 m 2 for 71Ga). Gallium
is chemically similar to aluminium, forming analogous compounds, but despite the similarity in site distortion between A1 and Ga compounds and the quadrupole moment of 27A1 being intermediate between those of 69Ga and 71Ga, well-resolved Ga NMR spectra are more difficult to obtain because the second-order quadrupole broadening of the central transitions of 69Ga and 71Ga for a given EFG is greater than for 2:A1 by a factor of about 11. However, both nuclei have good sensitivity, and the effect of going from 69Ga to 71Ga at 7 T produces similar benefits in terms of second-order quadrupolar effects equivalent to obtaining the same 69Ga spectrum at 17.5 T (Massiot et al. 1995).
654
Multinuclear Solid-State NMR of Inorganic Materials
The 71Ga chemical shifts of a number of gallium compounds are given in Table 10.4. From the known crystal structures of these compounds, 4-coordinated Ga which falls in the chemical shift range 107 to 222 ppm (with respect to Ga(H20)63+) can readily be distinguished from 6-coordinated Ga at - 80 to - 42 ppm. The 71aa shifts bear a linear relationship to the 27A1 shifts of the analogous compounds (Figure 10.10), allowing, in principle, the 71Ga shift of any unknown compound to be inferred from the 27A1 shift of the A1 analogue (Bradley et al. 1993) via the relationship 671Ga (ppm) = 2.83(627A1) (ppm) - 4.50
(10.8)
A more correct version of this relationship, in which all the points are true isotropic chemical shift positions and which includes a number of additional data points has been given by Massiot et al. (1999) as 671Ga (ppm) = 2.84(627A1) (ppm) - 1
(10.9)
Table 10.4. 71GaNMR parameters for gallium compounds.
Compound
~iso (ppm)*
• (MHz)
[3---Ga203 (tet)
(oct) oL-GaOOH.H20 NaGaO2 [3-LiGaO2 Y3Ga5Ol2 (tet) (oct) MgGa204 (tet) (oct) LaGaGe207 (Gav) Ga(PO3)3 GaPO4 (quartz struct.) GaPO4 (crist. struct.) NHn[Ga(SOn)2].12H20 Ga-ZSM-6
200-220, 198 40-50, 25 42 223 83 219 5.6 171 74 75.8 - 40 100.3 118 - 1.9 150-160
11.0, 1.1 8.3, 8.34 ND ND ND 13.1 4.1 7.6 7.6 15 ND 8.6 4.7 ND ND
Ga-erionite Ga-natrolite Ga-zeolite Y Ga-zeolite X Ga-sodalite Ga-gehlenite (tet) (oct)
157 169 172 174 183.5 250 233
ND ND ND ND ND ND > 13.5
* chemicalshiftsrelativeto Ga(H20)63+
Xl
Reference
0.85 Massiot et al. (1995), (1999) 0.08, 0.01 Massiot et al. (1995), (1999) ND Bradley et al. (1993) ND Bradley et al. (1993) ND Miyaji et al. (1992) 0.05 Massiot et al. (1999) 0.03 Massiot et al. (1999) ND Massiot et al. (1999) ND Massiot et al. (1999) 0.7 Massiot et al. (1999) ND Bradley et al. (1993) 0.51 Massiot et al. (1999) 0.45 Massiot et al. (1999) ND Timken & Oldfield (1987) ND Bayense et al. (1989), Chen et al. ( 1991), Kentgens et al. (1991) ND Bradley et al. (1993) ND Timken & Oldfield (1987) ND Timken & Oldfield (1987) ND Timken & Oldfield (1987) ND Thomas et al. (1983) ND Massiot et al. (1999) ND Massiot et al. (1999)
655
NMR of Other Quadrupolar Nuclei
9 r
3ooo
-~
150 "
~
-
~
CN=6
.p...~
~ -150 -50
|
0
l
50
,
100
27A| shift (ppm) w.r.t. AI(H20)63§ Figure 10.10. Plot of the 71Ga chemical shifts of gallium compounds with only oxygen in the first
coordination sphere vs. the 27A1 chemical shifts of the analogous aluminium compounds. The scatter is attributed to the fact that not all the shift values may be the isotropic shifts. From Bradley et al. (1993), by permission of John Wiley and Sons Ltd.
The 71Ga NMR parameters of the well-defined 5-coordinated Ga site in the compound LaGaGe207 have been determined from its static spectrum (Massiot et al. 1999). The giso value (75.8 ppm) falls between the typical shift range for Ga (~v~ and Ga (vI~, with a XQ value of 15 MHz and an estimated CSA of --~100 ppm (Massiot et al. 1999).
10.5.2 69,71GaN M R o f crystalline compounds Both the 69Ga and 71Ga MAS NMR spectra of ~-Ga203 have been reported (Massiot et al. 1995). One-half of the Ga atoms in this oxide are in tetrahedral sites and the other half are in octahedral sites. The spectral widths extend over at least 200 kHz and are thus too broad to be usefully narrowed by MAS. However, the static spectra from both 69Ga and 71Ga show similar features (Figure 10.11) and can be simulated with 2 second-order quadrupolar lineshapes corresponding to the tetrahedral and octahedral sites, although the site population ratio derived from the simulations show a systematic underestimation of the wider contribution. The resulting XQ values for the 2 sites (Table 10.4) indicate that the tetrahedral site is more distorted than the octahedral, consistent with the known crystal structure of this compound (Massiot et al. 1995). GaPO4 is similar to A1PO4 in being able to crystallise in structural forms related to the silica polymorphs quartz and cristobalite. The 71Ga NMR spectra of both GaPO4 structures have been recorded, the NMR parameters (Table 10.4) being consistent with the more symmetrical tetrahedral Ga environment in the cristobalite form (Massiot et al. 1999). Very fast spinning MAS speeds (up to 35 kHz) have been used to distinguish the tetrahedral and octahedral Ga sites in the 71Ga NMR spectra of a series of Ga-rich fluoro-amphiboles NaCazMg4Ga3Si6Oz2F2, since more conventional spinning
656
Multinuclear Solid-State NMR of Inorganic Materials
A
69Ga
B
~
10000
'
~-------ksimulated oct
oct
F--~] '
71Ga
~. o
'-iooo0'
tet
~ _ c
~~J
2000
tet 0
-2000
Ga shift (ppm) w.r.t. Ga(H20)63+ Figure 10.11. A. Observed and simulated 7 T static 69Ga NMR spectrum of [3-Ga203 showing the octahedral and tetrahedral contributions to the simulation. B. The corresponding 7 T static 71GaNMR spectrum and simulation. From Massiot et al. (1995), by permission of the
copyright owner. speeds (14 kHz) resulted in an overlap of the spinning sidebands of the tetrahedral and octahedral resonances (Sherriff et al. 1999). The resulting 7~Ga MAS NMR spectra (Figure 10.12A) showed a single tetrahedral peak at 230 ppm and 2 low-intensity peaks attributed to an octahedral quadrupolar doublet at about 40 ppm. The crystal structure of this mineral indicates that the occupation of the octahedral sites should be about onethird of the tetrahedral sites; the NMR measurement therefore significantly underestimates the intensity of the octahedral Ga site, possibly due to large quadrupolar effects associated with this distorted site (Sherriff et al. 1999). The formation of germanate phases with the mullite structure (Ga6Ge2013, (Ga,A1)6Ge20~3) from sol-gel precursors has been studied by multinuclear solid state NMR, including 71Ga MAS NMR (Meinhold and MacKenzie 2000). Many of the 71Ga spectra were dominated by the quadrupolar lineshape of 13-Ga203 from the structural units of the intermediate phase a-Ga4GeO8 or from unreacted starting material. The 71Ga NMR spectrum of crystalline Ga6GeeO13 was found to be broad and featureless, as were the 69Ga MAS NMR spectra of these samples. Static 69Ga NMR spectra obtained for quenched and annealed samples of the clinopyroxene LiGaSi206 have been interpreted as indicating the presence of 2 different electronic states of the octahedral Ga ions (Ohashi et al. 1995) A series of crystalline gallosilicate molecular sieves with the zeolite [3 structure synthesised by a rapid method from alkali-free hydrogels have been studied by NMR methods, including 7~Ga MAS NMR (Occelli et al. 1999). The 71Ga spectra (Figure 10.12B) show that most samples contain only tetrahedral Ga in framework
657
NMR of Other Quadrupolar Nuclei
A
BB
Oct
et ~ O c t
I
L
!
400
f
r
200
t
F
0
-
i
I
p
I
P
300
100
-100
71Ga shift (ppm) w.r.t. Ga(H20)63+ Figure 10.12. A. 71Ga MAS NMR spectra of 2 gallium-fluoro amphiboles spun at 28 kHz. The most Ga-rich sample (upper) shows a small octahedral Ga feature which may arise from a gallium sapphirine impurity. From Sherriff et al. (1999), by permission of the Mineralogical Society of America. B. 71GaMAS NMR spectra of 2 galliosilicate molecular sieves with the ~ zeolite structure. The tetrahedral Ga spectrum (upper) is typical of gallium in framework sites. The additional octahedral Ga resonance (lower spectrum) arises from extra framework Ga generated during thermal treatment of the sample. From Occelli et al. (1999), by permission of Elsevier Science. sites, but one sample clearly showed the presence of extra-framework octahedral Ga, due in part to the thermal treatment of the sample after synthesis. Gallium has also been incorporated into the structure of a microporous titanosilicate ETS-10 which unlike other zeolite-type materials contains framework atoms in octahedral coordination. 71Ga MAS NMR of the gallium-substituted sample ETGS-10 shows the presence of only tetrahedral Ga, indicating that its isomorphous substitution occurs only on the silicon sites to avoid the neighbouring titanium (Rocha et al. 1995). In this respect the behaviour of gallium is similar to that of aluminium in the analogous titanoaluminosilicate ETAS-10. A 69Ga and 71Ga MAS NMR study of a series of gallium analogues of the zeolite ZSM-5 at 2 applied magnetic fields has allowed the value of XQ (1.9-2.2 MHz) to be determined from the difference of the peak position of the 69Ga and 71Ga resonances (Kentgens et al. 1991). Changes in the linewidth as a function of the magnetic field revealed the presence of both second-order quadrupolar broadening, and broadening due to a chemical shift distribution.
10.5.3 69"71Ga N M R o f other compounds
A series of caesium gallate glasses x C s 2 0 . ( 1 - x)Ga203 where x = 0.3 to 0.7 have been studied by 69'71Ga NMR (Zhong and Bray 1987). Both static and MAS conditions were
658
Multinuclear Solid-State NMR of Inorganic Materials
used to establish the presence of tetrahedral and octahedral coordination in these glasses. The results indicate that in glasses with Ga:Cs ratios of less than 3:7, the Ga is in solely tetrahedral network-forming sites, but as the gallium content increases, the excess Ga enters octahedral sites. The 71Ga NMR spectra of the semiconductors GaAs and InGaAs show a single resonance in GaAs but the spectrum of InGaAs consists of a sharp intense peak as in GaAs, but with an underlying broad, weak resonance. Under MAS conditions, the InGaAs spectrum shows an extensive and complex sideband structure, by comparison with the simpler MAS spectrum of GaAs (Kushibiki and Tsukamoto 1986).
10.6. 87RbNMR
10.6.1 General considerations Rubidium compounds are important in a number of areas of materials science, as catalysts for ammonia synthesis and oxidation of methane, as a component of some glasses and as a dopant metal in buckminsterfullerene (C6o) causing it to become superconducting at 28 K. There are 2 NMR-active rubidium isotopes, 85Rb (I = 5/2, natural abundance 72.8%) and 87Rb (I = 3/2, natural abundance 27.2%). The sensitivity of 87Rb is greater than that of 85Rb, but its residual homonuclear dipolar broadening is larger and its relaxation time tends to be longer (100-300 ms for simple Rb salts) which is an advantage for acquiring DAS spectra. Most of the published solid state rubidium NMR uses 87Rb as the nucleus of choice, although the larger quadrupole moment of 85Rb can be useful in providing an indication of the number of chemically different Rb sites present in a salt (Cheng et al. 1990). The simple rubidium salts such as the halides and nitrate have small • values giving rise to narrow resonances, whereas the chromate, acetate, sulphate and hydroxide have larger XQ values giving wider central transition lineshapes (Cheng et al. 1990). The NMR interaction parameters of a number of rubidium compounds are collected in Table 10.5.
10.6.2 S7Rb NMR of crystalline compounds Dynamic Angle Spinning (DAS) has been used to obtain the 87Rb NMR spectra of several rubidium salts (Baltisberger et al. 1992), including RbNO3 which contains 3 inequivalent Rb sites and could not be resolved in the static NMR spectrum (Cheng et al. 1990). DAS NMR was found to narrow the 87Rb spectral lines significantly more than MAS or VAS (variable angle spinning), except in the case of RbC1 (Figure 10.13A), in which the nucleus is in a cubic environment with no second-order quadrupolar
659
NMR of Other Quadrupolar Nuclei
Table 10.5. 87RbNMR interaction parameters for rubidium compounds. Compound RbF RbCI RbBr RbI Rb2SO4 site 1 site 2 RbzCO3 site 1 site 2 RbzCrO4 site 1 site 2 RbC104
Reference
0 0, 0
0 0, 0
0 0 2.6 3.2 5.0 3.2 5.2 11.5
0 0 0.89 0.13 0.75 1.0 0.48 0.75
3.2, 3.2 4.3 1.83 2.07 1.85 2.8 6.9
0.16, 0.10 0.77 0.12 1.00 0.48 0.3 0.47
Cheng et al. (1990) Cheng et al. (1990), Baltisberger et al. (1992) Cheng et al. (1990) Cheng et al. (1990) Cheng et al. (1990), Baltisberger et al. (1992) Cheng et al. (1990) Cheng et al. (1990) Cheng et al. (1990), Baltisberger et al. (1992) Cheng et al. (1990) Cheng et al. (1990), Baltisberger et al. (1992) Cheng et al. (1990) Baltisberger et al. (1992) Baltisberger et al. (1992) Baltisberger et al. (1992) Cheng et al. (1990) Cheng et al. (1990)
XQ
(MHz)
50 128, 127 155 183 46.4, 42 3.0, 16 18.9 - 7.0 - 47.4, - 11 52.8 3.8, 16.2 30.5 - 26.2 - 26.8 - 30.9 0 7.6 -
RbOH.H20 RbNO3 site 1 site 2 site 3 RbOOCH.H20 Rb acetate.H20
"q
~iso
(ppm)*
* chemical shifts referred to RbNO3 solution
broadening. MAS is able to average the homonuclear dipolar interaction present in this compound but DAS will not, resulting in a broader DAS spectrum. DAS allowed the MAS powder patterns of RbNO3 to be separated (Figure 10.13B) and the quadrupolar parameters for each site to be determined by single-site simulation (Baltisberger et al. 1992). The isotropic shifts derived by DAS for a number of rubidium salts other than RbC1 are considerably different from those from the static 87Rb spectra (Table 10.5); the DAS parameters are claimed to be more reliable since they do not depend on simulations requiting a number of adjustable parameters (Baltisberger et al. 1992). A triplequantum MAS NMR approach has also been used to separate the 3 Rb sites in RbNO3, demonstrating the usefulness of this technique for such closely overlapping sites (Fernandez and Amoureux 1996). By simultaneously selecting the 2 mirror coherence transfer pathways (0)(_+ 3 ) ( - 1) the pure-phase 2D 87Rb spectrum of RbNO3 was obtained (Figure 10.14) with a significant gain in sensitivity. The resulting isotropic chemical shifts and quadrupolar products are in agreement with those determined by DAS measurements (Baltisberger et al. 1992). 87Rb NMR has been used to measure the temperature dependence of the secondorder shifts of the central transition of RbSCN, a compound which undergoes an
Multinuclear Solid-State NMR of Inorganic Materials
660 A RbCI
MAS -10 -
I
I
!
observed
A
DAS I
,,|
145 ~
-30
\
~
JAM __
L
-25
, ,,
_L_ _L_
t
105
125
RbNO3 /
t
simulated
11
-20 -50
' S 3 -45 ' -33 -45 -20
S7Rb shift (ppm) w.r.t. RbCI soln.
I
-30
-35 -40 S7Rb shift ( p p m ) w.r.t RbC! soln. Figure 10.13. A. 87Rb MAS and DAS NMR spectra of RbC1 (upper spectra) and RbNO3 (lower spectra) acquired at 11.7 T. B. 87Rb powder pattern cross-sections through the F2 dimension of a pure-phase MASdetected DAS spectrum of RbNO3 acquired at 11.7 T with simulations of the lineshapes from the 3 sites. From Baltisberger et al. (1992) by permission of the American Chemical Society.
~geetlon 3
s~ection 2 section 1
30 25
~,
20
-20 30
-40
v2 (ppm) Figure 10.14. 87Rbtwo-dimensional triple-quantum MAS NMR spectrum of RbNO3 showing the three Rb sites and their corresponding anisotropic sections. From Fernandez and Amoureux (1996), by permission of the copyright owner.
NMR of Other Quadrupolar Nuclei
661
antiferroelectric phase transition at 435 K from a high-temperature paraelectric tetragonal phase to a low-temperature antiferroelectric orthorhombic phase (Blinc et al. 1995). The 87Rb NMR results provide a physical picture of the structural transformation in terms of a disordering process connected with the formation of dynamic clusters or microdomains which are embedded in a long-range-ordered matrix below the transition temperature and which become random above the transition temperature (Blinc et al. 1995). 8VRb NMR has also been used to study the low-temperature phase of RbzZnC14, a one-dimensionally modulated incommensurate crystal which exhibits successive phase changes through 4 phases. Single-crystal measurements of the change in the 87RbNMR lineshape as a function of temperature in the vicinity of the low temperature commensurate phase change at 74.6 K have confirmed the crystal structure of this phase, while an anomaly in the temperature dependence of the spin-lattice relaxation time was interpreted in terms of the condensation of the soft mode inducing this transformation (Apih et al. 1992). Alkali metals such as rubidium are added to the surface of alumina catalysts to improve their efficiency in facilitating the partial oxidation of ethylene to ethylene oxide. 87Rb NMR has been used to gain an understanding of the interactions between Rb salts and the reactive sites of ~/-alumina (Cheng and Ellis 1989). When the alumina was impregnated with solutions of RbI, RbC1, RbNO3 and RbzSO4 at concentrations giving a submonolayer coverage, 4 rubidium species were identified after oven drying. Two of these species, described as surface salts, have 87Rbchemical shifts which depend on the nature of the impregnating anion, while the other species (described as surface species) are only weakly bonded to their oxo-anion and have chemical shifts which are essentially independent of the anion. The 87Rbrelaxation times suggest that both the surface salts and surface species can exist in a disordered form containing interstitial vacancies which provide a mechanism for migration of Rb + from site to site (Cheng and Ellis 1989). Alkalides and electrides are stoichiometric salts containing alkali metal cations complexed by crown ethers. Charge balance is provided by the alkali metal anions (alkalides) or trapped electrons (electrides). 87Rb and 85Rb NMR has been used to study a number of rubidium alkalides, electrides and related compounds (Kim et al. 1996). Spin-echo NMR measurements were used to obtain reliable values of giso, Xo and qq for these compounds.
10.6.3 87Rb NMR of rubidium fuUerides. Solid C6o (buckminsterfullerene) can be intercalated by alkali metal atoms to form MxC6ocompounds where x = 1--~6. The discovery that M3C6odisplays superconductivity, with transition temperatures as high as 33 K, has promoted considerable research interest, including a number of solid state NMR studies of both the structure and superconductivity mechanisms in these compounds.
662
Multinuclear Solid-State N M R o f Inorganic Materials
Above 450 K, the 87Rb NMR spectrum of Rb3C6o contains 2 sharp resonances arising from rubidium in non-equivalent octahedral and tetrahedral sites (Walstedt et al. 1993). The octahedral peak appears at about 52 ppm and the tetrahedral peak is at about 195 ppm with an octahedral:tetrahedral intensity ratio of 1:2, consistent with the known crystal structure. As the temperature is lowered these resonances broaden and shift slightly and a third tetrahedral resonance appears; at 200 K the 3 resonances occur at 40 ppm (octahedral), 165 ppm (tetrahedral) and 270 ppm (new tetrahedral). The formation of the second tetrahedral site has been explained in terms of alkali-metal vacancies which occur only in the tetrahedral positions (Apostol et al. 1996). Measurements of the 87Rb and 85Rb relaxation rates indicate a quadrupole relaxation mechanism involving phonons, and no change in either the NMR spectrum or the relaxation rates was found in the vicinity of Tc for this compound (Corti 1993). High-temperature superconductivity has also been demonstrated in mixed alkali metal fullerides, including the ternary alkali compound KRbCsC6o which has a Tc of 28-29 K (Maniwa et al. 1993). The 87Rb NMR spectra of this and the related binary alkali compound K2RbC6o (Figure 10.15A) show that whereas the rubidium ions occupy predominantly octahedral sites ( - 1 5 0 ppm) in the latter, they are located primarily in tetrahedral sites (about 0 ppm) in KRbCsC6o. These results, together with 133Cs NMR, provide evidence that the alkali metal atoms are site-selectively intercalated into the face-centred-cubic C6o lattice according to their size, with the largest ion (Cs) preferentially occupying the octahedral sites and the smaller K and Rb ions occupying mainly tetrahedral sites (Maniwa et al. 1993). 87Rb NMR has been used to study the electronic properties and phase transitions in another rubidium fulleride, RbC6o, in which the rubidium is located in octahedral sites of the NaC1 structure (Tycko et al. 1993). The 87Rb NMR spectra (Figure 10.15B) indicate a phase transition in this compound at about 300 K, the low-temperature phase containing a broad octahedral Rb resonance at - 120 ppm, being replaced above the transition temperature by a narrower tetrahedral line with a chemical shift decreasing strongly from 615 ppm at 353 K to 410 ppm at 473 K. The NMR data indicate that the high-temperature phase is a paramagnet in which the electronic dynamics are dominated by electron-electron effects. The electron-spin susceptibility is greatly reduced in the low-temperature phase in which most of the unpaired electron spins have become paired (Tycko et al. 1993).
10.7. 93Nb
NMR
93Nb is a nucleus with spin = 9/2, a relatively large quadrupole moment (Table 1.2, Chapter 1) and 100% natural abundance. 93Nb NMR is difficult since, in the solid state, electric field gradients arising from the electronic cloud at the nucleus can interact
663
NMR of Other Quadrupolar Nuclei
A
B
RbC6o
T(K)
Oct
/ ~
/
K2RbC6 o
Tet
313 KRbCsC6~
323
I
200
I
0
I
~.
| ....
-200
87Rb shift (ppm) w.r.t. RbCl soln.
800
400
0
-400
a7Rb shift (ppm) w.r.t. RbCI soln.
Figure 10.15. A. 87RbNMR spectra of the rubidium fullerides K2RbC6o(upper) and KRbCsC6o (lower), from Maniwa et al. (1993). B. 87RbNMR spectra of the rubidium fulleride RbC6oat various temperatures. Note the broad octahedral Rb resonance in the low-temperature phase progressively replaced by the narrower tetrahedral resonance above the phase transition temperature. From Tycko et al. (1993). Both diagrams used by permission of the copyright owners.
with the nuclear electric quadrupole, giving rise to considerable spectral broadening. However, the technical importance of a number of niobates as piezoelectric and optoelectric ceramic materials has provided the stimulus for several recent 93Nb NMR studies using techniques such as high-speed MAS, DAS and MQMAS to overcome broadening problems. Chemical shifts have been quoted in the literature with respect to solid Nb205 or a saturated solution of NbC15 in wet acetonitrile, the latter being the more commonly used reference substance. The 93Nb NMR spectra of a number of alkali and lead niobates have been acquired using MAS, DAS, MQMAS and two-dimensional nutation NMR to improve the resolution and determine values of the quadrupolar products PQ for these materials (Prasad et al. 2001). The 9.4 T 93Nb MAS NMR spectrum of LiNbO3 spun at 25 kHz (Figure 10.16A) shows a lineshape dominated by the second-order quadrupolar interaction, with only marginal improvement in resolution at a field of 14.1 T. For most of the metal niobates, the second-order interaction is not removed by MAS alone, even at high magnetic fields and fast spinning speeds. The expected improvement in 93Nb resolution provided by DAS spectroscopy is offset by significant homonuclear
664
Multinuclear Solid-State NMR of Inorganic Materials A
MAS
B
DAS
C
MQMAS
D
2D nutation
14.1 T
.1 00 ~ -750 -1000 -1250
lu
93Nb shift ( p p m )
-1000" 'r
,,I
.........
i .........
-1000
, .........
w .
.
.
.
~
-1400 ~
Frequency (ppm)
.
.
.
.
.
.
.
i,
11,ti, i! o
I
~
100
~
I ......... t ........ ~ ......... r ........
300
Frequency(ppm)
,
, ,
1
3
......... ......................... ,........ ,.....
5
Frequency (vm~)
w.r.t. NbCIs in acetonitrile Figure 10.16. 93NbNMR spectra of LiNbO3. A. MAS NMR spectra acquired at 14.1 T (spinning speed 18 kHz) and 9.4 T (spinning speed 25 kHz). B. DAS NMR spectrum also showing the 1D projection of the isotropic dimension. C. Triple-quantum MAS NMR spectrum also showing the 1D projection of the isotropic dimension. D. Pure-phase 2D nutation spectrum also showing the 1D projection of the nutation dimension. From Prasad et al. (2001) by permission of the copyrightowner.
Nb-Nb dipolar interactions which dominate the centreband in the isotropic dimension of the DAS NMR spectrum of LiNbO3 (Figure 10.16B). Although the use of MQMAS NMR to improve the resolution is hampered by the large quadrupole interactions of 93Nb, making multiple-quantum excitation and conversion less efficient, the triplequantum 93Nb spectrum of LiNbO3 (Figure 10.16C) shows an improvement in resolution of approximately an order of magnitude over the DAS spectrum (Prasad et al. 2001). The numerous spinning sidebands in the isotropic dimension of the MQMAS NMR spectrum arise typically from rotor modulation of the anisotropic chemical shift and quadrupolar interactions in the conversion period being different from that of the excitation period. Since the 93Nb quadrupolar products PQ of LiNbO3 and the related alkali niobates determined by MQMAS NMR are large (22.1-22.7 MHz), twodimensional nutation spectroscopy has been used to provide complementary information. The 2D nutation spectrum of LiNbO3 (Figure 10.16D) shows a single Nb site with its centre of gravity at 4VRF corresponding to a XQ value of 20 MHz (Prasad et al. 2001). Lead magnesium niobate, Pb(Mgo.33Nbo.66)O3, a relaxor ferroelectric material with a high dielectric constant and useful electrorestrictive properties, occurs in both perovskite and pyrochlore structures. The perovskite contains Nb(V) in the multiple B-sites of the structure, with a 93Nb MAS NMR spectrum showing 2 distinct Nb environments for which the quadrupolar parameters were determined from the triplequantum MAS NMR spectrum (Cruz et al. 1999). The broader of these 2 resonances, centred at - 980 to - 1000 ppm with respect to NbC15 in acetonitrile, has been assigned to a range of axial or rhombic Nb(ONb)6_•215 B-site configurations occurring in the Nb-rich regions (Fitzgerald et al. 2000). The shift of the other sharper
665
NMR of Other Quadrupolar Nuclei
Table 10.6. 93Nb NMR interaction parameters for niobium compounds. Compound
giso(ppm)*
XQ (MHz)
xI
Reference
LiNbO3
0.82
KNbO3
- 1050
22.1t, 22.1 22.7 t, 19.7 23.1
0
NaNbO3
- 1004, - 1009 - 1073
PbNb206
- 1113, - 1090
16.8 t, 19
0.5
1003 978 995 1013 975 999 951 995
13.C 17.0 t 13.7* 16.6 t 17.9 t 18.9 t 20.6 ~ 13.7 t
-
Prasad et al. (2001), Kind et al. (1968) Prasad et al. (2001), Kind et al. (1968) Fitzgerald et al. (2000), Kind et al. (1968) Prasad et al. (2001), Prasad et al. (1999) Prasad et al. (2001) Prasad et al. (2001) Prasad et al. (2001) Prasad et al. (2001) Prasad et al. (2001) Prasad et al. (2001) Prasad et al. (2001) Prasad et al. (2001)
- 1014
26.8 t
-
Prasad et al. (2001)
Pb2Nb207 " Pb3Nb4013 PbsNb4015 " Pb3Nb208 " Pbl.g3Nb1.vlMgo.2906.39 (perovskite) Pb1.83Nbl.71Mg0.2906.39 (pyrochlore)
-
0.80
* chemical shiftsrelativeto NbC15in acetonitrile t quadrupolarproductvaluesPQ
r e s o n a n c e at - 902 p p m is u n u s u a l for B-site N b ( O N b ) 6 configurations, and has b e e n tentatively e x p l a i n e d in terms of 1" 1 M g / N b ordering in M g - r i c h regions w h e r e the local s y m m e t r y is nearly cubic (Fitzgerald et al. 2000). T h e 93Nb M A S N M R and M Q M A S N M R spectrum of the pyrochlore form contains a single resonance suggesting a d i s t r i b u t i o n of isotropic c h e m i c a l shifts a n d q u a d r u p o l e c o u p l i n g s a t t r i b u t e d to disorder in the N b e n v i r o n m e n t (Cruz et al. 1999). Structural details of b o t h the perovskite f o r m of lead titanium niobate and a series of lead niobate p y r o c h l o r e s h a v e also b e e n studied by 93Nb D A S N M R (Prasad et al. 2001) and by 2D nutation s p e c t r o s c o p y (Prasad et al. 1999, Fitzgerald et al. 2000, Prasad et al. 2001).
10.8.
133CsN M R
10.8.1
Generalconsiderations
133Cs is a spin I - 7/2 nucleus of 100% natural a b u n d a n c e and a very small quadrupole m o m e n t of - 3.4 x 10 -31 m 2. 133Cs in inorganic c a e s i u m c o m p o u n d s (Table 10.7) shows a m o d e r a t e c h e m i c a l shift range (about 300 ppm). T h e c h e m i c a l shifts are normally referenced to aqueous CsC1 solution.
666
Multinuclear Solid-State NMR of Inorganic Materials
Table 10.7.
133CsNMR interaction parameters of caesium compounds.
Compound
~iso (ppm)*
XQ (kHz)
xl
Reference
CsOH.H20 CsBr
ND 258.2, 260.3 223.2, 228.1 275 0
98.2 0
"~0 0
0
0
ND 135, 126 115 168 95 140
ND 0.09, 0.08 0 0.45 0 ND
ND, 38.0 ND ND 370 270 272 134 134 ND ND ND ND 148 181 153 212 207 105 ND
ND
Amm & Segel (1986) Mooibroek et al. (1986), Haase et al. (1977) Mooibroek et al. (1986), Haase et al. (1977) Haase et al. (1977) Mooibroek et al. (1986), Tarasov et al. ( 1991) Tarasov et al. (1990a) Tarasov et al. (1990) Mooibroek et al. (1986) Mooibroek et al. (1986), Haase et al. (1977) Mooibroek et al. (1986), Haase et al. (1977) Haase et al. (1977) Haase et al. (1977) Tarasov et al. (1992) Tarasov et al. (1992) Mooibroek et al. (1986) Mooibroek et al. (1986) Mooibroek et al. (1986) Hartman et al. (1998) Kohn et al. (1994) Kohn et al. (1994) Kohn et al. (1994) Kroeker et al. (1997) Lim et al. (1999) Lim et al. (1997b) Lim et al. (1997b) Lim & Jeong (1999a) Dupree et al. (1982) Dupree et al. (1980)
CsC1 CsI CsC104 CsBrO4 CsIO4 CsCN CsNO3 CszSO 4
Cs2CO3 Cs2CrO4 CsTcO4 site 1 site 2 CsAuC13 CsSD ( - 81~ CsSeD ( - 61 ~ CsA1TiO4 Cs2CdSi5012 Cs2ZnSi5012 CszMgSisOl2 CsCd(SCN)3 LiCsSO4 CsMnC13 site 1 site 2 CsPbC13 Cs3Sb CsAu
ND ND 135.3 - 9.2, - 14.9 100, 91.7 152.7 - 61.3 - 83** - 115"* 128 257 276 66.0 77.1, 25.7 64.4, 8.0 62.9, - 4.3 94.4 620 375
ND ND --~0 --~0 0 0 0 ND ND ND ND 0.98 0.92 0 0 0.38 ND ND
* chemical shiftsquotedwithrespectto aqueousCsC1solution ** chemical shifts quotedwithrespect to aqueousCsBrsolution
10.8.2
133C$N M R
of crystalline
caesium compounds
A comprehensive single-crystal 133Cs N M R study of Cs2CrO4 has yielded the magnitude and orientations of the
133Cs chemical
shielding and quadrupolar tensors for the 2
crystallographically distinct Cs sites in this compound, indicating that the chemical shielding and quadrupolar interactions are not coincident for the 2 distinct caesium positions (Power et al. 1994). The temperature d e p e n d e n c e of the 133Cs N M R interaction parameters of solid CsC104, CsBrO4 and CsIO4 have been determined as a function of temperature. In
667
NMR of Other Quadrupolar Nuclei
CsC104 the temperature dependence of XQfor both 133Cs and 35C1 is linear, with a negative temperature coefficient, but the temperature dependence of the 133Cs -q-value shows an anomaly at 342 K (Figure 10.17A) which is not related to any abrupt structural change but to a change in the relative values of the tensor components of the electric field gradient (Tarasov et al. 1991). 133Cs and 79'81Br NMR of CsBrO4 show lineshapes dominated by quadrupolar effects. An increase in 133Cs • and a decrease in ~q with increasing temperature (Figure 10.17B) is thought to be associated with anisotropic behaviour of the lattice a-parameter (Tarasov et al. 1990a). The behaviour of CsiO4 is more complex, being shown by 133Cs and 127INMR to involve 2 phase transitions, one at 243-300 K and the other at 420M40 K. The first phase transition shows temperature hysteresis effects, and the 2 phases can coexist over a finite temperature range. The 127I NMR spectra also suggest the samples are showing reverse piezomagnetism which is unusual in non-magnetic crystals (Tarasov et al. 1990). 133Cs NMR measurements have been used to study the nature of phase transitions occurring in several caesium compounds. CsSCN undergoes a first-order structural phase transition at 470 K from a low-temperature orthorhombic antiferroelectric phase to a high-temperature cubic paraelectric phase. Measurements of the ~33Cs spin-lattice relaxation time of this compound suggest that the high-temperature cubic phase is a rotationally-disordered plastic phase (Furukawa et al. 1991). A single crystal study of the relaxation rates of this compound in the vicinity of the phase transition indicates the onset of large amplitude reorientations of the thiocyanate groups, with no evidence of orthorhombic microdomains above the transition temperature (Blinc et al. 1995a), by
A
CsCIO 4 I
t
B
I
'
I
CsBrO4
t20
200
ZQ " ~ " " "'--"-'~"
N
11o N
100 rJ~
- 0.4
0
0.4
~, 100
q
e~
-. , 100
I 200
.
.
. . 300
Temperature
.
.
0 400
K
~.2 q
"~
0.2
90 80
|
100
,
,
I
,
,
I
160 220 Temperature
,
,
J
280
K
Figure 10.17. A. Temperature dependence of the 133CsNMR parameters XQand ~1for polycrystalline CsC104. From Tarasov et al. (1991). B. Temperature dependence of the 133CsNMR parameters Re and "q for polycrystalline CsBrO4. Adapted from Tarasov et al. (1990a).
668
Multinuclear Solid-State NMR of Inorganic Materials
contrast with KSCN and RbSCN. 133Cs NMR has also been used to study the phase transitions occurring in CsSnC13 and CsPbBr3 (Sharma et al. 1991). Single-crystal 133Cs NMR studies have been reported of CsCd(SCN)3 allowing the relative orientations of the EFG tensors to be determined (Kroeker et al. 1997), and of ferroelastic CsPbC13 in which the NMR measurements revealed the presence of a twinned crystal structure (Lim and Jeong 1999a). The temperature dependences of the 133Cs quadrupolar parameters have also been determined for single crystal LiCsSO4 (Lim et al. 1999) and single crystal CsMnC13 (Lim et al. 1997b). A 133Cs NMR study of CsTcO4 has revealed the presence of 2 crystallographically inequivalent caesium sites of which the relative populations vary with temperature. The results provide information about changes in the crystal field potential in the vicinity of the cations accompanying a first-order phase transition from an orthorhombic to a tetragonal form at 389 K (Tarasov et al. 1992). An incommensurate phase existing in single-crystal Cs2HgBr4 over a narrow temperature range has been studied by 133Cs NMR. Below the onset temperature of the incommensurate phase the compound occurs as a paraelectric phase, changing to an antiferroelectric phase at higher temperatures. Changes in the 133Cs resonance intensity and lineshape at 243 K provide evidence of the existence of the incommensurate phase, and unequivocally indicate a soliton lattice in this phase (Boguslavskii et al. 1990). 133Cs NMR has been used to investigate an unusual phase transition which occurs in the compound CsCuC13 at 4.2 K under the influence of a magnetic field applied parallel to the c-axis. At a critical value of the magnetic field (11.19 T) the 133Cs resonance abruptly disappears, with no hysteresis in the sweep direction of the magnetic field. Analysis of the spectral lineshapes above and below this critical field value suggests that the anomalous change in the magnetisation occurs as a result of the quantum spin fluctuations of a one-dimensional ferromagnet with s - 1/2 (Chiba et al. 1993). The compound CsOH.H20 exists as a hexagonal [3-phase at room temperature, undergoing a slight modification to a hexagonal a-phase above 340 K. Below 232 K the Cs and O atoms of the hexagonal [3-phase undergo a further small change resulting in the structure becoming monoclinic. A ~33Cs NMR study of "pseudosingle crystal" CsOH.H20 shows at room temperature all 7 expected energy level transitions, from which the XQ value are derived (Amm and Segel 1986). The 133Cs XQvalues show an almost linear decrease with increasing temperature throughout the hexagonal [3 to hexagonal c~ transition (Figure 10.18), but a discontinuity of about 13 kHz ocurs at the hexagonal [3 to monoclinic transition at 236 K, indicating that the monoclinic phase has a longer longitudinal relaxation time than the hexagonal [3-phase. The 133Cs NMR results, complemented by ~H NMR, indicate that the hexagonal [3-phase is affected by an additional relaxation mechanism such as atomic diffusion, but the
669
NMR of Other Quadrupolar Nuclei
160 ~o N
120
rj~
80
~176 1
monoclinic
hexagonal
hexagonal
(a)
40
!
I
I
I
200
300
400
500
Temperature K
Figure 10.18. Change in the 133Csquadrupole coupling constant XQwith temperature for pseudosingle crystal CsOH.H20. Note the 13 kHz discontinuity at the monoclinic to hexagonal 13transition at 236 K. Adapted from Amm and Segel (1986). change in ~33Cs XQ, which continues down to 180~ must involve a different mechanism (Amm and Segel 1986).
10.8.3 133CsNMR of minerals and zeolites 133Cs and 295i MAS NMR has been used to study 3 caesium compounds with the structure of leucite (Kohn et al. 1994). The 133Cs NMR spectra of Cs2CdSisO12, Cs2ZnSisO12 and Cs2MgSisO12 all show 2 narrow resonances of approximately equal area, consistent with the expected occurrence of 2 alkali sites in leucite structures with 6 tetrahedral T-sites. This result for Cs2ZnSisO12 is not consistent with the proposed structure which predicts 3 Cs sites with relative occupancies of 2:1:1, suggesting a need to reassess the structural space group in the light of the NMR data. The ~33Cs shifts are influenced by the framework cation, becoming more negative from Cd to Zn to Mg (Table 10.7) (Kohn et al. 1994). Barium hollandite, Bal.4(A1,Ti)2.28Ti6016, is an important component of Synroc, a synthetic material developed for the immobilisation of high-level waste from nuclear reactor fuel. The hollandite component of Synroc takes up alkali metal ions such as radioactive Cs + by substitution for Ba 2+ in the structural channels. This uptake has been studied by 133Cs MAS NMR which shows a single resonance at 211 ppm from Cs in the channel sites in the absence of paramagnetic ions (Hartman et al. 1998). Replacement of A13+ by Ti 3+ in the channel walls causes the 133Cs NMR peak to broaden and shift to 640 ppm, and also provides a sensitive means of monitoring the formation of water-soluble CsA1TiO4 which, if present, would compromise the aqueous durability of Synroc.
670
Multinuclear Solid-State N M R of lnorganic Materials
Cation adsorption onto phyllosilicate minerals is an important process with practical consequences for soil/water systems, sediments, natural hydrothermal processes, metamorphic environments and waste disposal sites. The adsorption of Cs + by a number of clay minerals has been studied by 133Cs NMR which provides information about the number and nature of the adsorption sites, and the hydration state of the cation. 133Cs MAS NMR at various temperatures shows that adsorption of Cs on the clay mineral hectorite, (Mg,Li,A1)3Si4Olo(OH)2.Cs+o.33, occurs in several distinctly different chemical sites between which motional averaging occurs at about - 40~ if interlayer water is present (Weiss et al. 1990). Below about - 60~ motional averaging of the adsorbed Cs is sufficiently slow for 2 Cs resonances to be resolved, 1, at about - 30 ppm, arising from Cs relatively tightly bound to the basal oxygens, the other, at about - 8 to + 30 ppm, arising from Cs in a region of compositional gradient (called the Gouy diffuse layer). After dehydration of the hectorite at 500~ the NMR spectra indicate that the adsorbed Cs remains in the interlayer in 2 new sites giving rise to 133Csresonances at about 30 and - 120 ppm. The latter peak corresponds to more highly coordinated Cs (CN --~ 12) located in the hexagonal holes formed by oxygen atoms on both sides of the interlayer, and the former peak corresponds to less highly coordinated Cs (CN ---9) associated with the hexagonal hole on only one side of the interlayer and interacting with fewer oxygen atoms on the opposite side (Weiss et al. 1990). ~33Cs NMR has been used to study the hydration state of the cation in Cs-exchanged vermiculite, a swelling mica mineral with a typical formula (Mg,Ti,Fe,A1)3 (Si,A1)4Olo(OH)2X2+o.45 (Laperche et al. 1990). The results indicate that the 133Cs isotropic chemical shift is directly related to the hydration state of the mineral and depends on the configuration of the oxygens from the lattice and water ligands and the back-donation from the ligand to the cation. The large • value found for 133Cs in the vermiculite interlayer (about 6.7 MHz) results from an appreciable degree of stacking disorder due to the large radius of Cs + which prevents its engagement with the pseudohexagonal lattice oxygen network. The corresponding value of ~1 is close to unity (Laperche et al. 1990). 133Cs MAS NMR has been used to examine the structural sites occupied by Cs adsorbed on a variety of phyllosilicate minerals with a view to determining possible relationships between the 133Cschemical shift and the chemical and structural parameters of the clays (Weiss et al. 1990a). Significant differences are found between the 133Cs NMR spectra of samples in the form of an aqueous slurry and those fully dehydrated by heating at 450~ consistent with an increase in the direct bonding of the exchanged Cs + to the basal oxygen atoms with increasing dehydration. For the hydrated slurry samples, a reasonable correlation was found between the ~33Cs chemical shift and the ratio of tetrahedral A1 to total tetrahedral atoms of the clay mineral, with the data for dioctahedral and trioctahedral minerals falling on different lines (Figure 10.19A). The 133Cspeak becomes less shielded as the content of tetrahedral A1
671
NMR of Other Quadrupolar Nuclei
increases, since although motional averaging occurs in these fully hydrated samples, the position of the Cs resonance is the weighted average of the peaks from the various hydrated sites. Only very poor correlations were found with the degree of tetrahedral distortion and with the total layer charge of the hydrated clay minerals. The 133Cs NMR spectra of the fully dehydrated samples contain 2 resonances, each of which shows a reasonable correlation with the degree of tetrahedral A1 substitution (Figure 10.19B), but with no separate trends apparent for dioctahedral and trioctahedral minerals. Somewhat similar correlations were also found with the total layer charge of the fully dehydrated minerals (Weiss et al. 1990a). Calcium silicate hydrates are nanocrystalline porous materials of variable composition and poor crystallinity analogous to the compounds occurring in hydrated cements. The behaviour of these materials is of practical interest in determining the possible performance and long-term durability of storage facilities for nuclear waste and other hazardous substances. In a study aimed at improving the understanding of the surface chemistry of calcium silicate hydrate compounds, their interaction with CsC1 and NaC1 has been studied by 133Cs and 23Na NMR (Viallis et al. 1999). The NMR results indicate that both Cs and Na have an affinity for the calcium silicate hydrate surface, on which they are located in a diffuse ion swarm. Freeze-drying changes the environment of the adsorbed cations, reflected in the 133Cs chemical shift (200-250 ppm) A
B
20 4
[]
10
dioctahedral
~
~~ t
o
~
dioctahedral + trioctahedraltrioctahedral
0 ~-~ .40
r~ r~
-10
"~
trioctahedral
-20 0
0.06
0.12
AI(IV)/(AI(W)+ Si)
{ 0.18
J ~ -120~ -
- -- ~
dioctahedral + trioctahedral
0.12
0.24
- 1 i
0
Al~
[
,
- t
0v) + Si)
Figure 11).19. A. Relationship between the 8.45 T 133CsMAS NMR room temperature chemical shifts of fully hydrated Cs-exchanged clay minerals and their degree of tetrahedral A1 substitution. Open squares denote the dioctahedral minerals, open circles denote the trioctahedral minerals. Note that due to motional averaging in these samples, only one caesium resonance is observed. B. The same relationship for samples fully dehydrated at 450~ The 2 lines correspond to the 2 133Cs resonances observed in these samples. Note the similar behaviour of the dioctahedral and trioctahedral minerals when dehydrated. From Weiss et al. (1990a) by permission of the Mineralogical Society of America.
672
Multinuclear Solid-State N M R of Inorganic Materials
arising from inner-sphere surface complexes formed by interaction of the dehydrated Cs cations with the oxygen atoms of the bridging Si units. The Cs involved in these inner-sphere complexes occurs in 2 distinct environments, with and without chloride in the coordination sphere (Viallis et al. 1999). ~33Cs NMR has been used to monitor the dehydration of the Cs-exchanged zeolite mordenite (Chu et al. 1987). Both the static and MAS 133Cs NMR spectra of the fully hydrated material show a single resonance at - 64 ppm arising from motional averaging of the fully hydrated Cs + (Figure 10.20). Progressive dehydration results in the migration of the Cs into zeolite lattice sites characterised by 133Cs resonances resolved by MAS as peaks at - 157 and - 2 4 ppm (Figure 10.20B). The more intense and broader resonance can be fitted by 2 peaks, at - 157 and - 186 ppm with XQ = 3.1 MHz and ~1 = 0.6. The quadrupolar fitting parameters of the - 24 ppm resonance are very similar. The 2 Cs sites with similar chemical shifts have been identified as being near the centre of an 8-membered oxygen ring in the mordenite structure, with the other site located off-centre of a 6-membered oxygen ring (Chu et al. 1987). Binary caesium-lanthanum oxides supported on the mesoporous molecular sieve MCM-41 have potential catalytic applications for base-catalysed reactions. A 133Cs MAS NMR study of this system revealed shorter Cs-O bond lengths in the A
Static
B <___
MAS
fully anhydrous
1' increasing dehydration
<._..
200
-200
-600
fully hydrated 200
-200
-600
133Cs shift (ppm) w.r.t. CsCI soln. Figure 10.20. A.
133Cs static and B. MAS NMR spectra of caesium mordenite in various stages of dehydration. Asterisks denote spinning side bands in the MAS NMR spectra. From Chu et al. (1987), by permission of the American Chemical Society.
NMR of Other Quadrupolar Nuclei
673
MCM--41-supported system by comparison with the unsupported mixed oxides, while a small difference observed in the 133Cschemical shifts of the hydrated and dehydrated materials has been taken to indicate a weak interaction between water and the Cs + (Kloetstra et al. 1997). Changes with temperature have been recorded in the ~33Cs NMR spectrum of fully hydrated zeolite A containing a mixture of Cs, Li and Na as the exchange cations (Ahn and Iton 1991). Below 268 K the single Lorentzian 133Cspeak is resolved into 3 resonances corresponding to zeolite sites associated with the single 8-membered ring, the single 6-membered ring and an a-cage site near the 4-membered ring. Simulation of the experimental lineshapes has provided information about the chemical exchange kinetics between the 3 sites, indicating that the rate-defining step is the exchange of Cs + between the 6 and 8-membered ring sites (Ahn and Iton 1991).
10.8.4 t33Cs NMR of fullerides, superionic conductors and semiconductors The sites occupied by Cs in the mixed alkali fullerides CsRb2C6o and Cs2RbC6o have been identified by 133Cs NMR (Maniwa et al. 1992). The 133Cs NMR spectrum of CsRb2C6o (Figure 10.21A) shows a single resonance at - 370 ppm with respect to aqueous CsC1, indicating that the caesium in this compound is located solely in octahedral sites. By contrast, the 133Cs NMR spectrum of Cs2RbC6o (Figure 10.21A) contains both this octahedral resonance and a tetrahedral resonance at - 120 ppm. The chemical shifts of both the octahedral and tetrahedral sites show a very small but similar temperature dependence (Figure 10.21B). The room-temperature spin-lattice relaxation time of the octahedral site was found to be longer than that of the tetrahedral site, suggesting that the latter experiences a larger electric field gradient consistent with its smaller cavity size and lower symmetry (Maniwa et al. 1992). CsHSO4is a proton superionic conductor displaying a liquid-like proton self-diffusion constant above 417 K. The room-temperature monoclinic structure undergoes reconstructive first-order phase transitions at 318 K and 417 K. The nature of the translational disorder in the superionic plastic phase formed above 417 K has been investigated by a 133Cs NMR single crystal study of the deuterated compound CsDSO4 (Dolin~ek et al. 1986). The 133CsNMR spectrum of the room-temperature phase suggests the presence of 2 physically non-equivalent (but chemically equivalent) Cs sites. The formation of the superionic phase is accompanied by a decrease in the ~33Cs quadrupole coupling which, however, remains non-zero for both ~33Cs and 2H, indicating that the translational diffusion between these 2 nuclei involves well-defined lattice sites and is not completely random (Dolin~ek et al. 1986). The alkali antimonide compounds such as Cs3Sb are of technical importance as semiconductors with high photoelectric quantum efficiency in the visible region. The room-temperature ~33Cs chemical shift of Cs3Sb has been determined to be 620 ppm
674
Multinuclear Solid-State NMR of Inorganic Materials
A
400
. . . . . . . . . . . . . . . . . . . . .
0
, . . . . . . . . .
tetrahedral site 1
~~,,,lq
C
~
~
2"r") ~'~ -4oo
Cs2RbC6o ",a --~ o
|
f
oetahedral site CsRb2C6o
300
.
,
i
,
,
i
,
-300
i
I
,
L
-900
133Cs shift (ppm) w.r.t. CsCl soln.
-800 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 100 200 300 Temperature K
Figure 10.21. A. 1 3 3 C s NMR spectra of the fullerides Cs2RbC60and CsRb2C60.The resonance at - 370 ppm arises from Cs in the octahedral site and that at about - 120 ppm is from Cs in the tetrahedral site. B. Temperature dependence of the octahedral 133Csshifts in CsRb2C60 and Cs2RbC6oand of the tetrahedral shift in Cs2RbC6o,from Maniwa et al. (1992), by permission of Elsevier Science. (Dupree et al. 1982), indicating its much higher p-character than CsI (275 ppm) and CsAu (375 ppm). Furthermore, this shift decreases markedly with temperature up to 500~ leading to the conclusion that a previously proposed ionic structural model is incorrect (Dupree et al. 1986).
10.9. 139LaNMR 139La is a spin I - 7/2 nucleus with a 99.9% natural abundance and good NMR sensitivity, but its utility for solid-state NMR studies is limited by its quadrupole broadening which is greater by a factor of about 30 than for 27A1 in sites with the same structural distortion and in the same magnetic field. The chemical shifts of 139La are normally quoted with respect to aqueous LaC13 solution. The broad 139La NMR spectrum of the single La site in polycrystalline La203 (Figure 10.22A) has been determined by the frequency-swept spin-echo method (Bastow 1994), yielding an isotropic chemical shift of 424.8 ppm and XQ = 58.52 MHz. The lanthanum site in this compound has axial symmetry, making xI = 0. The frequency-swept technique has also been used to determine the 139La NMR spectra of the lanthanum perovskite compounds LaCrO3, LaMnO3 and LaCoO3 (Bastow 1994). LaCrO3 has an orthorhombic structure with a 139La NMR spectrum showing a well-defined second-order quadrupolar lineshape (Figure 10.22B). By contrast, the 139La NMR spectrum of orthorhombic LaMnO3 (Figure 10.22C) shows an
675
NMR of Other Quadrupolar Nuclei A
LazO3
LaMnO3
C
oO~176 o
o
o o
o
o
o
o
o
o o
o
o
o
o oo
o o
o o o oo%r ~ o :
oo
o
oo
15
5
o o
MHz
o %
0.4
0
LaCo03
-0.4
MHz
D
o o
LaCrO 3 g
g
o
00 o o
o
o
oOoo
%
o
~176 o
o o
o
o
o o
o
o o o
o
O~o
oOoO
Oo
o
o
0% 0 o
o o o
ooo
o
c~ o o O o~
o ooo
o o
0%0
oO .
0.5
0
MHz
-0.5
.
.
.
.
~
1.0
,
0
MHz
Figure 10.22. A selection of 139LaNMR spectra of La203 and perovskite-type La oxide compounds, determined by the frequency-swept spin-echo technique. From Bastow (1994) by permission of the copyright owner.
approximately symmetric lineshape with no visible quadrupolar structure. The shift of this compound (7 MHz) indicates a transferred hyperfine field from the Mn to the La which apparently swamps the quadrupolar contribution to the linewidth (estimated to be no more than 0.75 MHz) (Bastow 1994). The 139La NMR spectrum of LaCoO3 (Figure 10.22D) has sharply-defined quadrupolar structure arising from its smaller value of XQ compared with the other compounds of this type. The spectrum is sufficiently narrow for the singularities of the (1/2, 3/2) and ( - 3/2, - ~/2) satellite transitions to be clearly visible. 139La NMR has also been used to study the low-spin to high-spin transition in LaCoO3 (Itoh and Natori 1995). The lanthanum environment in LaA103 is a symmetric LaO12 unit, giving rise to a static 139La NMR resonance sufficiently narrow to produce a recognisable quadrupolar lineshape without the use of an echo pulse sequence which is narrowed further by MAS (Dupree et al. 1989). The highly symmetric La site in LaB6 has also allowed the 139La NMR spectrum of this compound to be obtained readily (Lutz and Oehler 1980). The 139La NMR interaction parameters of some lanthanum-containing compounds are collected in Table 10.8. Doping the lanthanum perovskites with a divalent metal cation such as Sr2+ produces
676
Multinuclear Solid-State NMR of lnorganic Materials
Table 10.8.
139La
NMR interaction parameters of lanthanum compounds.
Compound
8iso(ppm)*
XQ(MHz)
~1
Reference
La203 LaCrO3 LaCoO3 LaAIO3 LaAll ~Ols Lao.ssSro.15CrO3 La(NO3)3.6H20 La acetate LaB6
424.8 442.5 4230 375 46** 442 - 100 - 30 - 128
58.52 48 23.8 6 ND 33 21.5 11 ND
0 0.15 0 0 ND 0 0.85 0.65 ND
Bastow (1994) Bastow (1994) Bastow (1994) Dupree et al. 1989 MacKenzie et al. 1999 Bastow (1994) Thompson& Oldfield (1987) Thompson& Oldfield (1987) Lutz & Oehler (1980)
* chemical shift with respect to aqueous LaCI3 solution ** central position of a broad featureless resonance
compounds with interesting magnetic and electronic properties, some of which may find useful applications as high-temperature fuel cell electrodes. The frequency-swept 139La NMR spectrum of Lao.85Sr0.15CrO3 consists of a featureless peak in which the secondorder quadrupolar lineshape is blurred by disorder arising from Sr substitution. The shift of the peak is similar to that of LaCrO3 but the mean quadrupole interaction is lower than in LaCrO3, having been driven by the presence of the Sr to assume a structure closer to a cubic perovskite with La in a more regular octahedral site (Bastow 1994). The compound Lao.8Sro.2MnO3 is ferromagnetic at room temperature and also exhibits metallic conduction properties. As in LaMnO3, the 139La NMR lineshape is broad and featureless but skewed towards the high frequency side. The shift of this peak indicates a transferred hyperfine field from the Mn to the La, with the broad distribution of magnetic hyperfine fields swamping the quadrupolar contribution to the linewidth (Bastow 1994). 139La NMR has been used in combination with 27A1 MAS NMR to study the formation of crystalline LaAl11018 by heat-treatment of a precursor gel (MacKenzie et al. 1999). The spectra, acquired using a Hahn spin-echo pulse sequence, are very broad and featureless (Figure 10.23A), but as the solvent and by-products are progressively removed from the gel samples by heating at increasingly high temperatures, the centre-of-gravity (cog) of the 139La resonance progressively moves to less-shielded values (Figure 10.23B). On crystallisation of LaAl11018 at about 1000~ the peak cog abruptly becomes narrower and adopts an even more deshielded value, gradually settling down to its final value as the La moves into its lattice position in the mirror plane of the hexaluminate. Although La occurs in a 12-coordinated oxygen polyhedron in both LaA103 and LaAl110~8 the 139La NMR spectrum of the latter is much broader due to the lower symmetry of its La site in which 6 of the 12 coordinating oxygens are located significantly further away (MacKenzie et al. 1999). Lanthanum-exchanged zeolite-Y is an important catalyst used for fluid cracking in
677
NMR of Other Quadrupolar Nuclei
A
nab
250-
o ~
crystallisation
0 100
~ o
-250
1200oc
6
~., -500 /
~.
-750
~
,
!
500 i
4000
i
i
0
!
i
,
-4000
.
!
1000
1500
Temperature (~
139La shift (ppm) w.r.t. LaCI 3 soln. Figure 10.23. A. 139LaNMR spectra of LaAlllOlo gel precursor during the thermal evolution of the crystalline hexaluminate phase. B. Change in the 139La NMR peak position of LaAlllOls gel during heating. Note the discontinuity in the peak position at the point of crystallisation. From MacKenzie et al. (1999).
the petroleum industry. The presence of lanthanum in zeolite-Y catalysts is thought to significantly improve their thermal stability due to the presence of oxygen-bridged La polynuclear cations in the sodalite cages. The static 139La NMR spectra of calcined La-exchanged zeolite-Y contain 2 signals (Herreros et al. 1992); a broad underlying resonance is ascribed to La ions which have migrated into the small cages in the structure during calcination while a superimposed sharp symmetric signal at - 34 ppm with X Q - - 8.2 MHz is attributed to La cations located in the supercages. 139La NMR has also been used to monitor the migration of lanthanum cations from the large cavities to the SI' position in the sodalite cages in a study of La-exchanged zeolite sodium-Y. The lanthanum migration was found to cause the Si-O-T and A1-O-T angles to become strained, a result confirmed by 27A1 and 29Si MAS NMR (Hunger et al. 1995). Lanthanum compounds, particularly the cuprates and related phases, show interesting and potentially useful electronic properties ranging from superconductivity (as in the lanthanum strontium and lanthanum barium cuprates) to metallic behaviour (as in lanthanum cuprate). 139La NMR has been used to elucidate structural details of the metallic conductor LazCuO4+~ (Hammel et al. 1993). The electric field gradient (EFG) at the lanthanum site in a single crystal sample has provided information about the strongly temperature-dependent distribution of lateral displacements of the oxygens forming the apices of the oxygen octahedra in this structure. These effects
678
Multinuclear Solid-State NMR of lnorganic Materials
are considered to be an intrinsic response of the structure to hole doping, and are remarkable for the marked influence of a rather low concentration of holes (Hammel et al. 1993). 139La NMR has also shed light on the structural changes occurring in this compound below 220 K in which the oxygen octahedra develop a significant tilt upon the appearance of the highly disordered low-temperature structure (Hammel et al. 1991). ~39La NMR has been used to examine the possibility that substitution of Sr for La in LaNiO4 can bring about a change from an antiferromagnetic state to a metallic state in La2-xSrxNiO4+~ where x --~1 (Furukawa and Wada 1992). Strontium substitution of the antiferromagnetic phase produces a monotonic decrease in the internal magnetic field at the La sites up to x --~1, whereupon a transition to a non-magnetic phase occurs. The 139La Knight shift and spin-lattice relaxation behaviour of this phase is typical of a normal metallic state, confirming the nature of the phase transition (Furukawa and Wada 1992).
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Chapter 11
Solid State NMR of Metals and Alloys ll.1. Introduction 11.2. Experimental Approaches 11.3. Metallic Elements 11.4. Intermetallic Alloys 11.5. Phase Transformations, Ordering and Defect Sites 11.6. Phase Composition and Precipitation 11.7. Atomic Motion References
687 689 691 693 696 698 700 701
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Chapter 11
Solid State NMR of Metals and Alloys 11.1. INTRODUCTION The applications of solid state NMR described in this book are organised on the basis of the various nuclei. However, this chapter deals with the use of NMR as a technique for investigating metals, an application which is significantly under-represented in the literature. On first inspection, metals have very good NMR characteristics, displaying a range of shifts much larger than the normal chemical shift effect because of the contribution of the Knight shift from conduction electrons. The shifts in metals are very sensitive to the electron density and hence potentially useful as indicators of alloy composition. The presence of conduction electrons usually ensures that relaxation is rapid. A combination of shift and T~ measurements constrains the density of states, and linewidth measurements or pulsed field gradient experiments can, in principle, provide information about self-diffusion of the metal atoms which is directly related to the high temperature behaviour such as creep resistance. The application of NMR to metals followed rapidly from the initial development of the technique, with an extensive early study reported of a range of metals and alloys (Bloembergen and Rowlands 1953). To obtain representative and quantitative spectra it is important to use carefully prepared powder samples in which the particle size is less than the skin depth. The spectral intensity for quadrupolar nuclei is very dependent on sample preparation and also on the degree of atomic disorder within the alloy. This is well illustrated by the effect of zinc and silver additions to copper. Both copper isotopes (63'65Cu) are spin-3/2 so the intensity distribution between the central and satellite transitions is 2:3 (Chapter 2, Table 2.3). The observed intensity in pure copper was found to vary between 0.4-0.5 and 1, the latter value pertaining to very well annealed copper and the former being found for cold-worked copper filings (Figure 11.1A). The 63Cu NMR signal intensity from a cold worked sample could be recovered by annealing. The changing intensity in these samples is related to the introduction by coldworking of dislocations with associated strain fields which set up electric field gradients (efgs) such that the satellite transitions are lost. The loss of signal intensity can be even greater in alloys (Bloembergen and Rowlands 1953). The process was modelled as an "all or nothing" effect, i.e. the signal from a copper atom located near one of the alloying atoms is completely wiped out whereas the central transition remains at full intensity for copper atoms located further away. The static 25Mg NMR spectra shown in Figure 11.1B provide another example of the way in which the signal intensity is affected by work hardening (Bastow and Smith 1995). In these spectra 687
Multinuclear Solid-State NMR of Inorganic Materials
688 A
Mg + 6 % A l
10~
8
v..
annealed
~
partially annealed
oj,U
filed and etched
6 oH
9-
Mg, work-
filed
4 e~ ,,o
2
0
10
20
Solute concentration C (%)
3000
1000
-1000
25Mg shift (ppm) w.r.t MgSO 4 soln.
Figure 11.1. A. Changes in the intensity of the 63Cu NMR signal in annealed and cold-worked copper and in alloys with zinc and silver. The maximum NMR absorption is corrected for the number of copper nuclei in the sample and plotted as a function of the solute concentration C. Data points for zinc alloys denoted by crosses and by open circles for silver. From Bloembergen and Rowlands (1953) by permission of Elsevier Science. B. 25Mg NMR spectrum of pure annealed magnesium (lower), magnesium work-hardened by filing (middle) and a Mg + 6 % A1 alloy (upper). The insets in the upper spectrum shows the satellite powder pattern features enlarged vertically. From Bastow and Smith (1995), by permission of the copyright owner. the vertical scale has been expanded to show singularities of the quadrupole satellite transitions. Filing the powder has the effect of work hardening the sample, broadening the central transition from 450 Hz in the annealed state to 550 Hz and smoothing out the singularities by introducing a range of quadrupole interactions. The width of the central line in a magnesium sample containing 6% aluminium broadens to 2.3 kHz with a blurring of the quadrupole singularities, but there is a small decrease in the mean Xo compared to pure magnesium. This implies that the broadening observed in the main peak is due to an increased range of shifts resulting from the atomic disorder now present and not an increase in the quadrupole interaction. These potential complications of the quadrupole interaction and broadening due to atomic disorder discouraged the widespread use of solid state NMR as a structural probe for metals. Much of the continuing NMR work directed towards probing densities of states in metals was carried out at low temperatures, a further disincentive to the widespread application of NMR to these materials. Examples of the early use of NMR
Solid State NMR of Metals and Alloys
689
in the characterisation of metals are provided by the work of West (1964) and Drain and West (1965), with the field as it was at that time summarised by Drain (1967). For a discussion of NMR work on metals and alloys prior to 1977 the reader is referred to a comprehensive three-volume review by Carter et al. (1977). Although examples have continued to appear in the literature they still represent only a minority NMR activity, even though metallurgy remains one of the most important areas of materials research. Hence there is much scope for the application of NMR in this field, particularly since NMR techniques have evolved so much over the last 20 years, with significant improvements in the quantification of spectra from quadrupole nuclei. The availability of much higher applied magnetic fields is one of the major advances making NMR of metals worth reconsidering. The examples given below are of work over the last decade or so using high field magnets in room temperature NMR studies of problems related to metals. No discussion is given of metals containing large internal electronic magnetic fields. For an introduction to NMR in magnetic systems see Guimarges (1998). We hope that the following illustrations will provide impetus for the use of solid state NMR to provide atomic scale information on metallurgical systems. The evidence suggests that NMR can offer significant complementary information to standard characterisation techniques used in metallurgy (XRD, electron microscopy and calorimetry).
11.2. EXPERIMENTAL APPROACHES
The most obvious difference between the preparation of metal samples and insulating, diamagnetic materials for NMR investigation is the need to take into account skin depth effects in metals. The skin depth d is given by equation 3.4 in Chapter 3. Equation 3.4 becomes significantly less favourable at higher frequencies, so the particles should be smaller than the skin depth to ensure that the signal is coming from the total sample in the coil. Many intermetallic alloys have conductivities significantly poorer than for pure metals, offsetting the skin depth effect by decreasing ~. As an example consider Ti-A1 alloys, for which the electrical resistivities in the single phase samples (as opposed to multiphase compositions) are in the range 5.0 • 10 -7 to 2.2 X 10 -6 ~m. This corresponds to a skin depth of 33-73 txm for 27A1 at a resonance frequency of 104 MHz. Hence one of the most significant problems in sample preparation is production of sufficiently small particles. Filing is the usual method of producing powders from metallic specimens, but in the case of tough alloys a significant amount of the file can end up in the sample. The presence of ferrous debris in the sample will significantly distort the lineshape, making it advisable to remove the contaminating particles by running a small magnet over the sample. If time is spent on this laborious process it will remove almost all of the iron-containing particles. Further particle size reduction can be accomplished by grinding in an agate mortar. If the signal-to-noise ratio is sufficiently
690
Multinuclear Solid-State NMR of lnorganic Materials
good that loss of signal can be tolerated, experiments can be carried out on bulk samples and wires, but this assumes that the surface is representative of the whole specimen. The maximum particle size examined can be controlled by sieving. The key parameter is the spacing of the sieve, quoted in older papers in terms of the mesh size. Mesh size is the number of equally spaced wires per inch and can be converted to the maximum particle dimension in txm. Our recent work has used particles with a maximum dimension of --~50 txm. Reducing a metal to such relatively small particles by filing results in significant cold working, introducing plastic deformation and internal stresses. The early NMR work on metals demonstrated that this affects the spectrum, necessitating the use of a stress-release annealing cycle. This should provide sufficient thermal energy to remove stresses and dislocations without changing the phase distribution since the latter is of central interest in studies of materials. Single-phase samples can be annealed at elevated temperatures for significant periods (> 1 hour) but shorter periods of heat treatment are often advisable for multiphase samples. An example is provided by the Ti~_• alloys. The stress induced by filing the alloy Tio.szAlo.48 broadens the 27A1NMR spectrum (Figure 11.2A) but annealing at 1473 K for 20 minutes narrows the resonance significantly (Figure 11.2B) and removes the tail at the left hand side of the peak arising from a range of A1 environments. Annealing at different temperatures, including lower temperatures, will remove the stress but the phase distribution can change significantly, as illustrated by the spectra of the alloy Tio.6oA10.4o annealed at 1473 K (Figure 11.2D) and 1173 K (Figure 11.2E). After their initial manufacture, filing and stress relief, metal samples are ready for observation. However, to obtain the maximum benefits of particle size reduction, good electrical insulation must exist between the particles. In many metals this is provided by a natural thin oxide coating, but if this is not present or is ineffective, the particles can be dispersed in oil or set in wax. Use of a separate phase to disperse the metal particles degrades the available signal-to-noise ratio. A frequently-used approach is to mix the metal particles with MgO; this technique can be used even with magnesium metal samples as the shifts of the metallic components are usually very different from those of the oxide. For quantitative work the samples need to be carefully weighed and the probe tuned precisely, preferably using the impedance-frequency curve to determine whether changing between samples significantly changes the Q of the circuit. The NMR data must be of sufficient quality to allow the extraction of the interaction parameters since it is from these that the main characteristics of the different sites, and hence the material are deduced. A combination of static and MAS is often helpful in providing the necessary information. As discussed in Chapter 2, the different interactions exert differing effects on each transition. The lineshapes of the central transition of quadrupole nuclei in metals are likely to contain a more significant contribution from the shift anisotropy arising from conduction electron effects than from a chemical shift contribution arising from bonding electrons. The large Knight shift
Solid State NMR of Metals and Alloys
A
~
B
~
filed
filedand annealed
C
~
D E
/
f~
0
filedand
~,,, annealed1473K
/~
. . . . . . . . . . . . . . . . . . . . . . . . .
2000
1473K
filed
J .
691
filedand
~_annealed1173K . . . . .
~ . . . . . . . . .
i
-2000
27A1shift (ppm)w.r.t. AI(H~O)63+
Figure11.2. 27A1NMR spectra of Ti-A1 alloys showing the effects of particle reduction by filing and stress release by annealing at different temperatures. Spectra A-B, Tio.52Alo.48.Spectra C-E, Ti0.6oA10.4o.From Smith et al. (1996) by permission of the copyright owner. anisotropy (KSA) means that although the lineshape of the central transition may resemble a second-order quadrupole lineshape, misleading parameters may be obtained if the peak is fitted as such. However, the various interactions can be estimated by recording the singularity positions of both the central and satellite transitions. Although the satellite transitions may be very broad, allowing only the singularities to be observed with significant intensity in pulsed experiments, this is still sufficient to deduce the interactions. By field sweeping or frequency stepping (Section 3.4.2) the whole of the spectrum can be accurately recorded. Examples of all of these measurements are shown below. Experiments at more than one magnetic field strength can also help to unambiguously deconvolve the interactions.
11.3. METALLICELEMENTS The Knight shifts of most of the metallic elements were measured in the early NMR studies (summarised by Carter et al. 1977) but more accurate recent high field measurements have been made, for example the 67ZnNMR measurement by Bastow (1996) in which both the central and satellite transitions were recorded at room temperature (Figure 11.3A). The isotropic shift and XQ value were precisely determined to be 1776 ppm and 11.983 MHz respectively, and the measurements also provided the first
692
Multinuclear Solid-State NMR of Inorganic Materials
determination of the axial component of the Knight shift (Kax) ( - 124 ppm). A very small value of Ka• of "~ 3 ppm has been deduced in magnesium (Bastow and Smith 1995), agreeing with the value of 2 _+ 1 ppm determined from single crystal work (Dougan et al. 1969). The temperature dependencies of the shift and the quadrupole coupling constants in these metallic elements have also been studied. The quadrupole coupling in metals tends to increase as a metal is cooled, usually following a function of the type XQ = A -
BT"
(11.1)
where A and B are positive constants depending on the metal and n is around 1.5 (Kaufman and Vianden 1979). A value of n of 1.5 was found to be satisfactory for magnesium (Bastow 1991) but n = 2.2 was more appropriate for zinc (Bastow 1996). Higher-precision measurements also have allowed the temperature dependence of Kiso to be determined more accurately. A comparison has been made between the temperature dependencies of the hexagonal metals magnesium (Bastow and Celotto 1999), zinc (Bastow 1996) and cadmium (Seymour and Styles 1964). All three metals show a
A
8000
1800
sii-
re'J 300
0
Zn
-300
kHz (-3/2,-1/2)
9
.-
,~
1400
.a~--+ sooo
....-~ Cd
(1/2,3/2)
...--
:~
. ...~"
41,
Mg
1000
2000 400
800
Temperature K
1.0
0 MHz
-1.0
Figure 11.3. A. 67Zn NMR room-temperature powder lineshape of zinc metal showing the central transition (upper) and the (_+ 3/2,_+ 1/2) satellite transitions (lower). From Bastow (1996) by permission of the Institute of Physics. B. Variation of the isotropic Knight shifts of Mg, Zn and Cd metal with temperature. Data for Zn taken from Bastow (1991), for Cd from Seymour and Styles (1964) and for Mg from Bastow and Celotto (1999).
Solid State NMR of Metals and Alloys
693
positive shift with increasing temperature (Figure. 11.3B). The change in the value for zinc over the temperature range 149 to 432 K is 190 ppm, while the shift in magnesium varied by only --~ 40 ppm in going from 293 K to almost its melting point (923 K). The positive temperature coefficient was explained in terms of lattice vibrations which weaken the bonds and make them more isotropic, thus contributing more s-like character to the Fermi surface (Bastow and Celotto 1999).
11.4. INTERMETALLIC ALLOYS Many of the properties of metallic alloys originate from the solid solutions and phase mixtures formed in these systems, but well-defined stoichiometric intermetallic compounds can show interesting properties. 63Cu NMR studies have been reported of the intermetallic phases Cu3Au, CuAu and CuAu3 (Chepin and Ross 1994). The trialuminides A13M show interesting structural variations, the majority of these phases adopting the tetragonal D022 structure with two inequivalent aluminium sites in the ratio 1:2. Site 1 has 4 nearest neighbours in the (001) plane while site 2 has 2 nearest neighbours in the (100) and (010) planes. The quadrupole and shift interaction parameters have been deduced for these sites from their 27A1 NMR spectra (Lue et al. 1998, Bastow et al. 1998), (Table 11.1). In the DO22 structure the 2 sites have very different quadrupole interactions. The second element M plays little part in determining the quadrupole interaction, as can be seen by comparing • for site 1 in A13Ti and A13V (Table 11.1). The second element does however contribute electrons to the band structure, reflected in the shift variation (Table 11.1). The relatively small Kiso and long T1 values are consistent with a low density of states at the Fermi surface. The larger Kax and XQ values of site 1 indicate strong directionality of the bonds and have been used to explain the brittleness of these materials. A 51V NMR study of A13V provided evidence of local magnetic moments in this intermetallic compound in which the change in the static linewidth of the central transition with the applied magnetic field indicates that KSA is the predominant broadening mechanism (Lue et al. 1998). The 51V line is narrowed by a factor of approximately 4 by MAS. In the cubic phase A13Sc only 1 site is observed by NMR, as expected from the structure. A13Zr is also tetragonal but has a DO23 structure in which the 3 aluminium sites are inequivalent. The full intensity satellite spectrum obtained by sweeping the magnetic field shows these 3 sites (Figure 11.4A) and indicates that 1 of the sites (assigned to AI(1)) has a small but non-zero ~1 (Bastow et al. 1998). In AlzTi the 2 aluminium sites can be separated by MAS (Bastow et al. 1998) (Figure 11.4B) and the 49Ti NMR spectrum shows a distinctive quadrupole lineshape (Figure 11.4C, Bastow et al. 1998a). The formation of intermetallic compounds by high-energy milling is a potentially
694 Table 11.1.
Multinuclear Solid-State NMR of Inorganic Materials
NMR interaction parameters for some intermetallic alloys.
Phase
Nucleus, site
Kiso (ppm)
A13Ti
Al,1 A1,2 Ti AI,1 A1,2 V
410, 335 270, 253 275 - 170, - 130 - 100, 97 185 350 260 580 540 756 1100 455 384 687 40 330 610 3800 - 170 260 200 3600 3155 393 2100 2700 990 1445 1705 1300 - 200 ---60
A13V
Kax (ppm)
•
(MHz)
200, ND 11.09, 11.37 - 40, ND 3.30, 3.30 800 14.30 280, ND 11.47, 11.39 80, ND 2.50, 2.50 --~100 6.13
Reference
"q ND, ND, 0 ND, ND, 0
0 0 0 0
Lue et al. (1998), Bastow et al. (1998) Bastow et al. (1998a) Lue et al. (1998), Bastow et al. (1998) Bastow et al. (1998a)
-
A13Nb AI3Ta A13Sc A13Zr
AlleTi
A1Ti A1Ti3 TiAg ZrNiAI Fe2VA1 MglvAll2
Ni3AI MgB2
AI,1 A1,2 AI,1 A1,2 AI, Sc AI,1 A1,2 A1,3 Zr AI,1 A1,2 Ti AI, Ti A1, Ti Ti A1 A1 V Mg,1 M g,2 Mg,3 A1 A1 B
580 - 50 800 150 --~40 ND ND ND ND --~0 ND ND ND 130 14.0 270 ND ND 150 ND ND ND ND ND ND 800 ND
14.13 3.77 15.33 3.17 6.80 0 9.05 3.84 11.33 7.33 6.0 9.2 8.5 8.15 0 "-~0 13.9 --~0 3.3 ND ND 2.20 0 < 1.2 0.72 ND 1.67
ND Lue et al. (1998) 0 ND Lue et al. (1998) ND 0 Bastow et al. (1998) 0 0.03 Bastow et al. (1998) 0 0 0 0.9 Bastow et al. (1998), (1998a) 0.47 0.70 0 Bastow etal. (1998), (1998a) ND Bastow et al. (1998), (1998a) 0.1 0 Bastow et al. (1998a) ND Nowak & Hayashi (2000) ND Lue & Ross (1998) ND 0.95 Bastow & Smith (1995) 0 --~0 ND ND Scherrer et al. (1998) 0 Jung et al. (2001)
ND- not determined.
useful technique for producing a m o r p h o u s alloys. Solid state N M R provides a m e t h o d for detecting subtle atomic-scale changes in the structure of metals during alloying, w o r k i n g or annealing. 63Cu N M R has b e e n used to study the c h a n g e s i n d u c e d in Cu59Zr41 p o w d e r s during ball milling (Li and Xiao 1992) (Figure. 11.5A). The 63Cu N M R spectrum of pure copper metal shows a single resonance with a Knight shift of 2396 ppm. On milling, the copper metal peak is progressively replaced by a broader
695
Solid State N M R of Metals and Alloys
MAS
|A1(2)1 I ! IAi(2)2 [ l ] I ]AI(1)1 ~ ~l~ JSAI(3), /. ~ .
-1500
i
0
27A1 (kHz)
simulated
J
A1(3)2
1500
simulated
t 600
27A1 shift
(ppm)
400
200
w.r.t. AI(H20)63+
200
0
-200
49Ti (kHz)
Figure 11.4. A. 27A1 full intensity satellite NMR spectrum of A13Zrobtained by sweeping the magnetic field, allowing the 3 A1 sites in this phase to be distinguished. From Bastow et al. (1998). B. Observed and simulated 27A1MAS NMR spectrum of AlzTi showing the resolution of the two A1 sites. Asterisks denote spinning sidebands. From Bastow et al. (1998). C. Observed and simulated 49Ti NMR spectrum of AlzTi showing the quadrupolar lineshape. From Bastow et al. (1998a). All spectra used by permission of the copyright owners.
resonance feature with a Knight shift of about 1490 ppm arising from amorphous alloyed clusters of Cu59Zr41. Milling copper powder alone broadens the 63Cu resonance due to the introduction of lattice defects but produces no change in the Knight shift. The NMR results suggest that mechanochemical alloying of the Cu-Zr system is accompanied by a reduction in the s-electron density of states and polarisation effects (Li and Xiao 1992). A similar 63Cu NMR study of mechanochemically alloyed Cu-Zn compositions shows a progressive change in the copper Knight shift with the zinc content up to 40 at % zinc (Figure 11.5B). At this composition most of the copper atoms are surrounded by a shell of approximately 8 zinc atoms and the addition of further zinc produces little further change in the Knight shift, which is close to the value for meltalloyed [3-brass (Li and Wang 1993). The quadrupolar and shift tensor parameters of ZrNiA1 have been deduced, together with their relative orientation (Nowak and Hayashi 2000). A 63Cu and 119Sn NMR study of the ternary intermetallic compound U3Cu3Sn4 has shown that the 63CuKnight shift is anisotropic, while the small negative value of the hyperfine coupling constant suggests d-like conduction electron states at the Cu sites. The temperature dependence of the 63Cu nuclear relaxation rate indicates that U3Cu3Sn4 is an exchange-coupled system (Kojima et al. 1992). 51V NMR nuclear spin-lattice relaxation rates have also been measured as a function of temperature in the superconductor material V3Si (Ohno et al. 1993). A combination of 9Be, l~ and ~IB NMR was used to probe the substitution of low concentrations of boron into the heavy-fermion superconductor UBe~3 at 4 K. Two beryllium signals were observed, corresponding to the 2
696
Multinuclear Solid-State NMR of Inorganic Materials
A milling time (h)
/ \\\
,oo
0.25
~
0.20
r~
~
0.15
~
0.10
r,.)
0.O5 I
6000
i
I
.
i
2000
,
i
,
I
.
0
.
.
.
,
,
,
25
.
.
|
.
50
75
Zn content (at. %)
-2000
63Cu Knight shift (ppm) w.r.t. CuCl Figure 11.5. A. 63CuNMR spectra of Cu59Zr41 powders balled milled for varying times. Note the progressive replacement of the Cu Knight-shifted resonance at about 2400 ppm by the broad resonance at about 1490 ppm due to clusters of amorphous Cu-Zr alloy. From Li and Xiao (1992). B. Change in the 63Cu Knight shift of Cu-Zn powders milled for 12 h, as a function of Zn content. The plateau in the Knight shift corresponds to the formation of [3-brass, the 63Cu resonance frequency of which is independent of composition. From Li and Wang, (1993). Both diagrams used by permission of the copyright owners. inequivalent sites in this compound; the assignment of the broader of the 2 signals to the site of lower symmetry was confirmed by the agreement between the observed and expected intensity ratio of--~1:12. Only 1 boron signal was observed in the B-substituted phase, with a relative shift and linewidth consistent with B substitution only into the cubic site (Ahrens et al. 1993). Intense interest has been aroused in the intermetallic phase MgB2 by the observation of a superconducting transition at 39.2 K at zero applied magnetic field. I~B NMR has shown only a small Knight shift in the normal state with negligible s character at the Fermi surface (Jung et al. 2001).
11.5. P H A S E T R A N S F O R M A T I O N S , O R D E R I N G AND D E F E C T S I T E S
An important class of phase transformations occurring in solids, particularly metallic alloys, are termed martensitic transformations. These solid-solid transformations involve no change of composition or atomic diffusion, but merely a local displacive change which can be usefully studied by NMR since a local atomic-scale rearrangement is involved. A series of papers from Rubini et al. has shown that the NMR spectra of metals can be used to observe and quantify the degree of martensitic transformations, since there
Solid State N M R of Metals and Alloys
697
is a distinct shift between the precursor austenite and martensite phases. The phase transformation in Cu-Zn-A1 alloys was accompanied by the appearance of 2 27A1 signals as extensive martensitic regions rapidly formed (Rubini et al. 1991). The shift generally found to accompany the austenite ~ martensite transformation is consistent with an increase in the s--electron density at the Fermi surface. NMR has the potential to provide quantitative information, allowing the amount of martensite to be deduced and the kinetics of the transformation determined. 113CdNMR has been used to monitor such changes in Ag-Cd alloys, in which the isotropic shift changes by 250 ppm upon transformation, in Ni-A1 alloys, where the 27A1 shift changes by --~ 80 ppm and in the 63CuNMR of Cu-Zn alloys (Rubini et al. 1992, Rubini et al. 1994a, Scherrer et al. 1995). The particle size of Ni-A1 alloys was found to influence the phase transformation in these materials; where the particles were <- 150 Ixm in size, the transformation was incomplete (Rubini et al. 1994b). Small additions of copper to oL-NiA1 produced 2 signals in the NMR spectrum, attributed to copper substitutions on the aluminium and nickel sublattices (Bastow and Rossouw 1998). A recent study of the same transformation employed 6~Ni, a low-~/quadrupole nucleus with a natural abundance of only 1.19 % (Crevoiserat et al. 1999). Despite the problems inherent with such a nucleus, 61Ni NMR spectra were readily obtained from a near equiatomic Ni-Ti alloy and showed resonances from both phases, with a shift difference of ~ 1000 ppm. Spectra were obtained directly from both wires and thin films; an additional peak seen in the spectra of the thin films was attributed to the formation of Ti2Ni precipitates. The effect of ball milling and subsequent annealing on the atomic ordering of Ni3A1 has been followed by 27A1NMR (Scherrer et al. 1998). The large increase in the disordered state was reflected in the range of A1 shifts arising from varying numbers of neighbouring defects. The axial powder pattern observed in the ordered material (Figure 11.6) was recovered when the milled sample was annealed, and measurements of the linewidth provided an estimate of the amount of transformed phase, allowing the ordering kinetics to be determined. The ordering process could be explained by a simple model of nickel diffusion via vacancy sites with an activation energy 1.8 ___0.2 eV. Thus, NMR can probe the diffusional processes which determine many of the properties of such alloys. 59Co NMR spectra have been reported from dilute cobalt alloys with additions of chromium, iron, ruthenium and copper. These spectra show satellite peaks corresponding to the shift from the hyperfine fields produced by nearest neighbour impurity atoms (Meny et al. 1993). Site disorder in ~/-TiA1 gives rise to a minor peak at 610 ppm in the 27A1NMR spectrum from aluminium atoms on titanium sites (termed antisite aluminium) (Smith et al. 1996).
698
Multinuclear Solid-State NMR of lnorganic Materials
owm~
e'
e
ordered --O-- disordered
~ O
.
'Q o
o~ m
110 00"
04
|. . . .
86
Ir
"O
o -~
"o
II, 0 "O'Oo 0 0
t.
.
|
1
,.-
I
88
,
.I
90
9
,
I 92
Frequency (MHz)
Figure 11.6. 27A1NMR spectra of ordered Ni3A1(sharp resonance) and the same material disordered by ball milling. The broad spectrum of the disordered material was determined by a point-by-point spin-echo determination. From Scherrer et al. (1998) by permission of the copyright owner. 11.6. PHASE COMPOSITON AND PRECIPITATION
The interesting and useful properties of alloys are often conferred by the atomic and phase composition of the sample. NMR is a highly useful technique capable of probing the details of metallic solid solutions and determining the phase composition. 27A1NMR has been used to monitor the phase composition of Til-xAl,, alloys where 0.25 -< x -< 0.55 (Smith et al. 1996). This composition range spans a two-phase region in which the NMR spectrum shows signals from both phases, identified by comparison with the signals from the constituent phases x = 0.25 and 0.50 (Table 11.1). However, the intensities of the 2 NMR signals from the two-phase region did not agree with the expected quantitative phase distribution. If quantitative information is required, the sample must be weighed and the NMR spectrometer carefully retuned between samples. The signal can then be normalised to the "NMR signal per nucleus x" which should remain constant, providing quadrupole effects do not complicate the spectra. A significant drop in this number indicates loss of signal. This value was found to remain constant for the single-phase samples in the Ti-A1 system, but decreased by 10--40 % in the 2 phase region, reaching a minimum for a sample with x = 0.48. Physical mixtures of the two phases gave the correct intensity distribution, indicating that the loss of signal must be related to the microstructure of the two-phase samples. Interfacial strains between the phases of the two-phase regions are relieved by dislocations which are likely to be preferentially distributed in the oL2component in this system; this is the phase from which the greatest
699
Solid State NMR of Metals and Alloys
signal loss always occurs. The dislocations have an associated strain field which results in the displacement of atoms with the setting up of large electric field gradients. Thus, although NMR can have a strong quantitative advantage, care should be exercised in interpreting the NMR spectra of alloys, particularly multicomponent systems, to ensure that the samples are fully representative of all the sample components. The increased sensitivity available with high-field magnets enables the detection of the dilute component of dilute alloys. 45Sc NMR of dilute alloys of scandium in aluminium shows 2 4SSc signals, 1 at 1700 ppm from scandium in the aluminium matrix and the other at 1100 ppm from the A13Sc precipitate phase (Celotto and Bastow 2000). The intensities of these 2 peaks were used to determine the amount of precipitation and the solubility of scandium in the aluminium. NMR provides a direct measure of the number of atoms associated with each signal, and hence a direct measure of the amount of each phase. This approach is highly sensitive, allowing scandium levels in the aluminium matrix as low as 0.02 at % to be detected. NMR has also been used to determine the solubility of aluminium in magnesium. Two 27A1 NMR signals could readily be detected in a sample of magnesium containing 6 wt % aluminium, 1 of which (at 1890 ppm) originates from aluminium in solid solution in the magnesium matrix and the other, at about 1320 ppm, is from the MglvAla2 precipitate phase (Bastow and Smith 1995). A more detailed study included alloys containing 9 wt % aluminium with and without zinc doping (Celotto and Bastow 2001). The 27A1 NMR signals from the matrix and precipitate were again detected (Figure 11.7A). Aging causes the amount of precipitate to increase, and NMR measurements of the temperature dependence
B
1960
j
/
aged.
\
1260 ",,, "~. 1230
""~}.. -.
[ / agedln"
_
-
5 2000
I '~,
1600
27A1shift (ppm)
.. ~ltate
0
5 W t % AI
10
0
1.5
3.0
Wt % Zn
1200 w.r.t. AI(H20)63§
Figure 11.7. A. 27A1NMR spectra of an A1-Mg alloy containing zinc, showing the signal from the matrix (upper) and from the precipitate, represented by stoichiometric MglvA112(lower). The 2 middle spectra show the effect of aging time at 200~ on the relative amounts of matrix and precipitate phases. B. Knight shift of the matrix of binary Mg-A1 alloys as a function of aluminium content. C. Knight shift of the precipitate phase in Mg-A1 alloys doped with zinc as a function of the Zn content. From Celotto and Bastow (2001) by permission of Elsevier Science.
700
Multinuclear Solid-State NMR of Inorganic Materials
revealed details of the kinetics and the homogeneous nature of the precipitation process. The NMR data provided even more subtle details about the processes in these alloys, since changes in the shift indicated the occurrence of small changes in the atomic composition of the matrix and the precipitate phase (Figure 11.7B, C). An increase in the aluminium-content of the matrix increases the electron density and the shift decreases. The shift decrease observed when zinc is present suggests that the dopant migrates from the matrix and preferentially enters the precipitate. 11.7. ATOMIC MOTION Many of the properties of metals and alloys such as ductility and high temperature creep resistance ultimately depend on atomic motion. Direct information about the local motion in metals is provided by NMR measurements of the relaxation times and by pulsed field-gradient measurements of the diffusion constant. Furthermore, the information about atomic motion in metals provided by NMR is element-specific (Ailion 1971). Typically the NMR linewidth in metals begins to narrow at about 60% of the melting point of the system. Much smaller changes at lower temperatures correspond to changes in the atomic dimensions as the lattice parameter changes. In a recent NMR study of various solid solutions of NixAl~-x, Bastow et al. (i997) found that in the composition range 0.48 -< x -< 0.54, only the x = 0.54 sample showed significant line-narrowing below 900~ (Figure 11.8A), indicating that the presence of vacancies can greatly assist diffusion. Such a curve can, in principle, provide an estimate of the diffusion constant and the activation energy of the motion but the analysis requires a knowledge of the motionally-narrowed limit of the linewidth. In an earlier study, this analysis was applied to metallic aluminium (Seymour 1957). The room-temperature 27A1NMR spectrum of Ni2A13 shows 2 overlapping quadrupole lineshapes originating from the atoms in 2 A1 sites. As the temperature is raised, the 2 A1 lines merge into a single line with a • value close to the weighted average of the room temperature values (Figure 11.8B). The change in the spectrum with temperature can be explained in terms of hopping of the aluminium atoms between the two aluminium sites via the plane of condensed vacancies known to be present in Ni2A13 (Bastow et al. 1997). It should be noted that such A1 motion does not produce an isotropic average. 9Be NMR at 9 . 4 T has been used to detect ultraslow atomic motion in Zr-Ti-Cu-Ni-Be metallic glasses (Tang et al. 1998). Quadrupole order was created in the system by the use of a Jeener-Broekart sequence, requiting careful selection of the interpulse spacing in order to remove shift distributions and create a state of pure quadrupole order. Slow atomic motions were found to contribute to the decay of the echo magnetisation near the glass transition temperature, and the activation energy for this motion was determined to 1.2 + 0.15 eV, in close agreement with the value determined from elastic backscattering measurements (Tang and Wu 1998).
701
Solid State NMR of Metals and Alloys Temperature
A
(~
810 16
~~~'~.~...~
Nio.s~Alo.48 A1o.54
8
Nio.496Alo.5o4
680
~'X 530
~ i
|
0
i
300
i
i
i
,
600
Temperature (~
,.
i
20
!
9 0
120
40
-40
27A1 (knz)
Figure 11.8. A. Width of the 27A1NMR resonance of various cubic Nil-xAlx alloys as a function
of temperature. Note that only the highest-Ni alloy Nio.46Alo.54shows a significant change in linewidth with temperature. B. Temperature dependence of the 27A1powder lineshape of NizA13. Note the evolution of the 2 room-temperature overlapping quadrupole lineshapes into a single line as the aluminium atoms jump between the 2 sites with increasing temperature. From Bastow et al. (1997) by permission of the Institute of Physics.
REFERENCES Ahrens, E.T., Hammel, P.C., Heffner, R.H., Reyes, A.P., Smith, J.L. & Clark, W.G. (1993) Phys. Rev. B, 48, 6691. Ailion, D.C. (1971) Adv. Mag. Reson., 5, 177. Bastow, T.J. (1991) J. Phys.: Condensed Matter, 3, 753. Bastow, T.J. (1996) J. Phys.: Condensed Matter, 8, 11309. Bastow, T.J. & Celotto, S. (1999) Solid State Commun., 110, 271. Bastow, T.J., Forwood, C.T., Gibson M.A. & Smith, M.E. (1998) Phys. Rev. B, 58, 2988. Bastow, T.J., Gibson, M.A. & Forwood C.T. (1998a) Solid State Nucl. Mag. Reson., 12, 201. Bastow, T.J. & Rossouw, C.J. (1998) Philos. Mag. Lett., 78, 461. Bastow, T.J. & Smith, M.E. (1995) J. Phys.: Condensed Matter, 7, 4929. Bastow, T.J., Smith, M.E. & West, G.W. (1997) J. Phys.: Condensed Matter, 9, 6085. Bloembergen, N. & Rowlands, T.J. (1953) Acta Metallurgica, 1,731. Carter, G.C., Bennett, L.H. & Kahan, D.J. (1977) Metallic Shifts in NMR Parts 1-3, Oxford, Pergamon. Celotto, S. & Bastow, T.J. (2000) Philos. Mag. A, 80, 1111. Celotto, S. & Bastow, T.J. (2001) Acta Mater., 49, 41. Chepin, J. & Ross, J.H. (1994) Mater. Res. Soc. Symp. Proc., 332, 267. Crevoiserat, S., Scherrer, P., Dimitropoulos, C. & Gotthardt, R. (1999) Mater. Sci. Eng. A, 273-275, 357.
702
Multinuclear Solid-State NMR of lnorganic Materials
Dougan, P.D., Sharma, S.N. & Williams, D.L. (1969) Can. J. Phys., 47, 1047. Drain, L.E. (1967) Metallurgy Rev., 119, 195. Drain, L.E. & West, G.W. (1965) Philos. Mag., 11, 1061. Guimar~es, A.P. (1998) Magnetism and Magnetic Resonance in Solids, John Wiley and Sons, New York. Jung, J.K., Baek, S.H., Borsa, F., Bud'ko, S.L., Lapertot, G. & Canfield, P.C. (2001) Phys. Rev. B, 64, 012414. Kaufman, E.N. & Vianden, R.J. (1979) Rev. Modern Phys., 51, 161. Kojima, K., Hukuda, Y., Miyata, S., Takabatake, T., Fujii, H. & Hihara, T. (1992) J. Mag. Mag. Mater., 104-107, 49. Li, B. & Xiao, K. (1992) J. Appl. Phys., 71, 3917. Li, B. & Wang, Y. (1993) Phys. Rev. B, 47, 16582. Lue, C.-S., Chepin, S., Chepin, J. & Ross, J.H. (1998) Phys. Rev. B, 57, 7010. Lue, C.-S. & Ross, J.H. (1998) Phys. Rev. B, 58, 9763. Meny, C., Jedryka, E. & Panissod, P. (1993) J. Phys.: Condensed Matter, 5, 1547. Nowak, B. & Hayashi, S. (2000) Solid State Nucl. Mag. Reson., 18, 59. Ohno, T., Kishimoto, Y., Kotaki, H., Yamanishi, T., Kanashiro, T., Michihiro, Y. & Yamada, Y. (1993) Physica B, 186--188, 1034. Rubini, S., Dimitropoulos, C., Gotthardt, R. & Borsa, F. (1991) Phys. Rev. B, 44, 2019. Rubini, S., Dimitropoulos, C., Aldrovandi, S., Borsa, F., Torgeson, D.R. & Ziolo, J. (1992) Phys. Rev. B, 46, 10563. Rubini, S., Dimitropoulos, C. & Borsa, F. (1994a) Phys. Rev. B, 49, 12590. Rubini, S., Dimitropoulos, C. & Borsa, F. (1994b) Phys. Rev. B, 49, 9331. Scherrer, P., Rubini, S., Dimitropoulos, C. & Borsa, F. (1995) J. Physique IV, 5, 449. Scherrer, P., Dimitropoulos, C., Borsa, F. & Rubini, S. (1998) Phys. Rev. B, 57, 10462. Seymour, E.F.W. (1957) Proc. Phys. Soc. A, 66, 85. Seymour, E.F.W. & Styles, G.A. (1964) Phys. Lett., 10, 269. Smith, M.E., Gibson, M.A., Forwood, C.T. & Bastow, T.J. (1996) Philos. Mag. A, 74, 791. Tang, X.-P., Busch, R., Johnson, W.L. & Wu, Y. (1998) Phys. Rev. Lett., 24, 5358. Tang, X.-P. & Wu, Y. (1998)J. Mag. Resort., 133, 155. West, G.W. (1964) Phil. Mag., 9, 979.
Subject Index A c~-sialon 27A1 322 298i 253 Ab initio calculations 224, 346, 348, 381 Acid sites 27A1 290 Acquisition sequential 126 simultaneous 126 time 126 Activation energy 104 Addition of paramagnetic ions 202 Adiabatic demagnetisation 185 Adiabaticity parameter 84 Adsorbed selenate and selenite 778e 586 nitrates 15N 583 lo7,1O9Ag catalysts 472 fast ion conduction 471 glasses 470 halides 469 thiolates 469 Air bearings 141 Alkalides 87Rb 661 Alloys 25Mg 487,489 61Ni 525 47'49Ti 506, 509, 690,698 91Zr 516 A1PO4 31p 448 silicon incorporation 449 A1POs 305 Alumina general 291 170 372 films 293 interaction with fluorine, 19F 557 sodium adsorption 120 surfaces 293 Aluminate glasses 301 Aluminates cements 313 6'7Li 632
23Na 409 170 372 Aluminium borate 307 crystal 274 CSA 272 five-coordinated crystal 277,281,282 five-coordinated gels 284, 296 five-coordinated glass 284, 286, 299, 301 five-coordinated zeolites 287 fluoride 308 molybdate 307 nitride 316 oxynitrides 316 shift correlations 279 shift range 273 sialons 317 titanate 313 Aluminium production 27A1 299 Aluminium-based gels 284 Aluminofluorophosphates 19F 560 Aluminophosphate glasses 303 Aluminophosphates 27A1 304 19F 559 170 384 Aluminosilicate gels 27A1 294 Aluminosilicate glasses 27A1 299 23Na 413 170 379 fluorine interaction 19F 559 Aluminosilicate shift ranges 298i 206 Aluminosilicates 27A1 275,301 19F 556 23Na 410 170 375 Amorphous components in minerals 303 compounds 298i 230 hydrogen-carbon 13C 567 Angle setting 141 Angular distortion 283 Anisotropy 703
704
Multinuclear Solid-State NMR of lnorganic Materials
chemical shift 43,201 scalar coupling 42 Antiecho pathway MQ 98 Antiferromagnet 139La 678 Asymmetric splitting in J 72 Asymmetry chemical shift 46 Knight shift 50 quadrupole 51 Attenuation, 117 Autocorrelation function 102 Average Hamiltonian 62
B 11B
borates crystalline 421 borate glasses 424 boroaluminate glasses 428 boroaluminosilicate glasses 428 borosilicate glasses 425 borosilicophosphate glasses 429 carbides 422 carbonitrides 430 nitrides 422 NMR interactions 420 oxycarbide glasses 431 oxyfluoride glasses 430 sol-gels 427 zeolite 431 13-sialon 27A1 318 29Si 247 ~35,~37Ba fluorohalides 525 sol-gels 523 superconductors 524 titanates 522 Background signals 123 Baseline correction 130 Bayer process 27A1 299 by-products 23Na 413 9Be correlations 640 metallic glasses 642 zeolite 642 Bioglass 23Na 417 31p 450 29Si 257 Biomaterials
1H 550 31p 451
BLEW 12, 29 Bloch equations 99 Bloch Siegert shift 183 Boltzmann distribution 27 Bond angle 170 346 Bond angle distribution 170 342 in glasses 231 Bone implant materials 19F
555
Bore magnet 115 Borate glasses 23Na 415 Borates 170 381 Boroaluminate glasses liB 428 170 383 Boroaluminosilicate glasses liB 428 Boron, see liB Boron carbide ~3C 570 Boron nitride 15N 575 Borophosphates 31p 445 Borosilicate glasses 233 23Na 414 170 382 Borosilicophosphate 31p 445 Borosilicophosphate glasses 11B 429 BPP approximation 103 BR24, 79 Bridging oxygen 364 BrOnsted acid sites in zeolites 291,431,538 in Na a alumina 420 Brownian motion 78 C
13C
amorphous carbon-hydrogen 567 boron carbide 570 C7o, 566 carbides 570 carbonates 572 CSA 564 CO2 dissolution 573 CP 563 diamond 564 disordered carbonaceous materials 567 fullerene 564 graphite 564 SiC polymeric precursors 570 silicon carbide 568
Subject Index C7o 13C 566 43Ca carbonates 503 CSA 503 CP 502 interactions 502 sol-gel silicate 504 superconductors 505 Cadmium, see 113Cd Cadmium exchange 113Cd 590 Caesium, see 133Cs Carbides liB 422, 570 of Ba 572 13C 422 of Ca 572 29Si 255,570 of Ti 510,572 Carbon adsorption 133Cs 670 Carbonates 13C 572 43Ca 503 170 385 Carbonitrides 11B 424, 430 29Si 257 of Ti 511,576 Carbothermal synthesis 319 o~-sialon 25 ~-sialon 248, 319 volatile products 249, 319 Carr Purcell technique 201 Catalytic activity l~ 472 vs Si/A1 ratio (27A1) 300 Catalysts A1203-SnO2, 595 Mo compounds 519 oxide supported Ag 472 Pt catalysts 603 87Rb in alumina 661 super-five materials 290 zeolites 290, 538,677 113Cd clay minerals 590 halide glasses 588 semiconductors 590 Cements 27A1 313,315 43Ca 504 133Cs nuclear waste storage cements hydration 259,313, 369, 411,544
298i 257 Centre of gravity of shift DOR 77 MAS 68,146 Chalcogenide glasses 31p 447 29Si 238
Chalcogenides 95'97M0 521
671
Chemical shielding definition 46 introduction 44 Chemical shift Ag halides 469 27A1 273,300 aluminosilicates 207 liB 426, 432 definition 46 effects 295i 204 71Ga vs 27A1 31p borophosphate glasses 445 reference compounds 13, 14, 16 skew 48 span 47 tetrahedra1295i 205,208, 212 Zn compounds 513 Chemical shift anisotropy 46, 203 l~ 469, 471 27A1 272 13C 564, 570 43Ca 503 63'65Cu 652 1H 538, 545,550 39K 496 95'97Mo 517 15N 575 170 340, 358 31p 433,440 2~ 607 Q types in silicate 201, 231,236 335 488 775e 586 119Sn 592, 595 125'127Te 599 2~176 606 51V 643,647 89y 464 67Zn 512 91Zr 515 Chemical shift distribution 338 Chemical shift range 2~ 607, 631 Chemical shift relation to structure 27A1 279
705
706
Multinuclear Solid-State NMR of lnorganic Materials
9Be 640 19F 557, 563 1H 541 6Li 634 23Na 403 170 342, 348, 352 31p 439 2~ 607,613 29Si 204, 217 49Ti 509 Chlorides 35'37C1 491 35,37C1 clusters in zeolites 495 chlorides 491 perchlorate 492 Class of amplifier 118 Clay minerals 27A1 310 ll3Cd 590 1H 542 7Li 633 39K 500 23Na 412 295i 210 tin incorporation 593 CO2 dissolution 13C 573 Coherence level 32 order 32 pathway 33 Cold working 687 Contact interaction 43, 49 Cooley-Tukey algorithm (FFT) 128 Cooling rate borate glasses 427 Coordination effects 295i 204 Copper, see 63'65Cu Core polarisation 49 Correlation function 102 Correlation method 168 Correlation time 103 Correlations CSA vs c/a (2~ 613 CSA vs P-O, DI (31p) 440 gQ vs strain (27A1) 280 ~Q VS bond angle (170) 346, 360, 364 ~Q VS DI (67Zn) 513 ~Q vs DI, ~ (25Mg) 483 ZQ vs ionicity (170) 352, 377 ~Q vs r (23Na) 406 ~Q VS ~ (47'49Ti) 508 8(71Ga) vs ~5(27A1) 654 8 vs bond angle (170) 348
8 vs bond angle, bondlength (TBe) 640 8 vs CN,A (6Li) 634 8 vs M,X (liB) 426 8 vs r (170) 352 8iso vs angular distortion 283 8iso vs bond angle (27A1) 280 8iso vs bond angle (298i) 219 8iso vs bondlength (295i) 218,226 8iso vs CN, Pb-O, PN (2~ 608,623 8iso vs d(O-H-O) (1H) 541 8iso vs GEN (19F) 557 8iso vs Na-O 403 8iso vs nn, EN, z/r, S (31p) 439 8iso vs shift parameter (23Na) 403 8iso vs T-T distance (27A1) 8iso VS Ti-O (47'49Ti) 509 1"1vs bond angle (170) 346 COSY 158 of glasses 237 CP, see cross-polarisation CPMG sequence 135, 186 53Cr 525 CRAMPS amorphous C-H 567 biomineral materials 550 glasses 19F 559 hydrous glass 1H 547, implementation 152 31p 535 theory 82 Cross-polarisation 12, 85 27A1 290 27A1-170 345 27A1-31P 305 27A1-29Si 229 l~ 469 13C 563 43Ca 502 double 177 dynamics 88, 580 equilibrium 88,90 19F 562 19F-2~ 613 19F- 295i 229 19F-119Sn 562 1H - 29Si 227 23Na-31p 416 23Na- 29Si 229 95'97Mo 520 15N 575 23Na 399 170 345
Subject Index 31p 432 quadrupole nuclei 177 775e 583 set up compounds 19F 177, 229 set up compound 1H 174, 228 119Sn 591 125Te 183W 474 89y 462 Cryoshims 115 Cryostat 114 Crystalline solids 23Na NMR 406
133Cs
carbon adsorption 670 fast ion conductors 673 fullerene 673 interactions 667 minerals 669 phase transitions 667 semiconductors 674 zeolite exchange 672
63,65Cu
CSA 652 fast ionic conductors 562 solid solutions 649 superconductors 650 temperature dependent shifts Curie-like behaviour 28 Curve fitting glass spectra 235 29Si spectra 209 zinc phosphate spectra 438 CYCLOPS phase cycle 125
650
D
DAS, see dynamic angle spinning Deadtime electronic 133 probe 131 Dealumination zeolites 287 Decibel 117 Deconvolution 235 Decoupling 12 heteronuclear (CW) 78 homonuclear (multipulse) 79 homonuclear (Lee-Goldburg) 81 improved efficiency 175 Dehydroxylation 215 Diamond 13C 564 Diffusion effect of T2, 101
707
measurement 187 metals 700 Digitisation 126 Dipolar interactions 37,201 14N 475 alphabet expression 37 classical 37 powder pattern 39 secular terms 38 Dipolar oscillations 89 Disordered carbonaceous materials 13C 567 Distribution bond angle 342, 349 chemical shift 338 interactions 69, 71 Double angle rotation (DOR) 27A1 271,306, 308,449 BN 423 170 158,340, 382 23Na 409, 418 probe 151 theory 75, 92 DOR, see double angle rotation Double CP 177 Double frequency sweep 98 Double quantum filtering 161 Double resonance probe circuit 120 Drift of magnetic field 115 Droop, pulses 119 Dump resistors 114 Dwell time 125 Dynamic angle spinning (DAS) laB 425 23Na tellurite glasses 407,415 23Na zeolites 419 93Nb niobates 170 340, 342 probe 156 87Rb 658,660 silicate glasses 29Si 236 theory 75, 92
E
Echo formation DAS 92 Hahn 101,134 MAS 63 MQMAS 93 Solomon 134 Echo spectra 133 Eigenfunctions 24
708
Multinuclear Solid-State NMR of Inorganic Materials
Electrides 87Rb 661 Electric field gradient 51 Electron magnetic moment 43 Energy level diagram spin- 1/2, 26 spin-5/2, 54 Energy level splitting 24 Energy levels 23 Energy store in magnetic field 113 Enrichment procedure 170 334 EXAFS 3 Exchange spectroscopy 159 Experimental methodology 170 337 general 3 metals 689 quadrupole nuclei 172 Exponential multiplication 128
F
19F aluminofluorophosphate glass 560 aluminophosphate 559 aluminosilicate glasses 559 aluminosilicate minerals 556 boron oxyfluoride glass 560 complex fluorides 554 CP 562 effect of MAS 550 fluoride glasses 559 fluorides 551 fluoroapatite 555 gallophosphate 559 interaction with alumina 557 interaction with silica 557 ionic motion in PbF2 553 J coupling 553,562 phosphors 554 shift trend 562 solid solutions 553 Fast ion conductors 471 l~ 471 133Cs 673 63Cu 652 6'7Li 636 95Mo 521 170,357 778e 584 89y 465 Fermi contact interaction 3 Fictitious spin- 1/2, 53
Field dependence linewidth 340 peak position 68, 146 Filters 125 First-order phase correction 130 Five coordinated 27A1 crystalline 281 27A1 glassy 283 27A1 zeolite 287 29Si 225 Flipback sequence 175 Fluctuations 102 Fluoride 19F 551 aluminium 308 Fluoride glasses 19F 560 2~ 613 Fluorine, see 19F Fluorine in aluminosilicate glasses 27A1 302 Fluoroaluminate glasses 303 Fluoroapatite 19F 555 Fluorohalides 135'137Ba 583 Four coordinated 29Si 205 Fourier transform principle 111 Frame transformation 45, 52, 59, 76 Free induction decay 9 Frequency amplitude modulation (FAM) 98 Fullerenes 13C 564 133Cs 661 39K 501 87Rb 661
G 69,71Ga correlations 654 gallate 656 gallosilicate 656 glasses 657 oxides 685 semiconductors 658 zeolites 656 Gas ceramic foams 430, 446 Gallophosphate 19F 559 Gallosilicate 170 375 Gas exchange 170 336 Gas flow heating 188 Gaussian fitting silicate glasses Gaussian multiplication 129 23Ge 523
235
Subject Index Gels 27A1 284, 294, 297 calcium silicate 504 hydrous 170 387 mullite 241,294 oxide precursors 354, 370 PZT precursors 242 silica 29Si 240 SiO2-SnO2 595 TiO2 508, 354 ZrO2 352 Geopolymers 27A1 304 39K 500 23Na 413 29Si 259 Germania 170 359 Glasses 27A1 299 aluminate 301 aluminoborate 170 383 aluminofluorophosphate 303 aluminofluorophosphate 560 aluminophosphate 303 aluminosilicate 170 379 aluminosilicate 19F 560 aluminosilicate 23Na 413 aluminosilicate 299 borate liB 424 borate 170 382 boroaluminate liB 428 boroaluminosilicate liB 428 boron oxyfluoride liB 430 boron oxyfluoride 19F 560 borophosphate 31p 445 borosilicate 11B 425 borosilicate 23Na 414 borosilicophosphate 11B 429 connectivities 237 chalcogenide 31p 447 fluoride 560 fluoroaluminate 303 gallium-containing 69'71Ga 657 germanate 170 368 germania 170 367 1H 546 halide ll3Cd 588 hydrous 170 387 ionically conducting95'97Mo 521 lead-containing 613,632 2SMg in molten silicates 483 oxynitride 15N 577
phosphate 15N 579 phosphate 23Na 415 phosphate 31p 441 phosphoaluminoborosilicate 31p 447 phosphoaluminosilicate 31p 446 phosphosilicate Si(VI) units 233 phosphosilicate 31p 443 29Si silicate 231 sialon 27A1 323 sialon 29Si, AAA silica 170 366 silicate 170 367 silicate 23Na 413 silicate units 232 silver-containing l~ 470 tellurium-containing 304, 601 tin-containing 595 Graphite 13C 564 Gyromagnetic ratio 23
H
1H biominerals 550 chemical shift anisotropy 538 clay minerals 542 hydration kinetics 538 hydrogenated carbon 545 hydrogenated silicon 544 hydrous glasses 546 hydroxyls 541 MAS linenarrowing 537 nominally hydrous minerals 543 pressure determination 549 shift correlation 541 silicates 543 sol-gels 549 water 541 Hafnates 170 256 Halide glass 113Cd 588 Halides 39K 497 Hamiltonian 24 Hartmann-Hahn condition theory 12, 87, 89 1H_23Na 1H_170 1H_29Si 1H_89Y 1H_183w Heteronuclear coupling 40 HETCOR biominerals 1H-alp 550
709
710
Multinuclear Solid-State NMR of lnorganic Materials
glasses 27Al-19F 562 glasses 1H-298i 549 implementation 168 High pressure experiments 189 High temperature approximation 27 High temperature NMR 6Li in silicates 631 25Mg 483 29Si 211 albite 405 experimental 188 nepheline 412 silicates 234 High Tc superconductors, see superconductors Historical development 6 Homogeneous interaction 57 Homogeneity magnetic field 115 Homonuclear coupling 40 Homonuclear spins (non-equivalent) 74 Hydrides 89y 468 Hydrogenated carbon 1H 541 Hydrogenated silicon 1H 544 Hydrous gels 170 387 Hydrous glasses IH 543,546 23Na 417 Hydrogen, see 1H Hydrogen-containing materials 170 386 Hydroxide 25Mg 483 170 386 Hydroxyls 1H 541 Hyperfine interaction 43
I
In situ phase transformation 189 Inhomogenous interaction 57 Interaction chemical shielding 44 dipolar 37 homogeneous 57 inhomogeneous, 57 internal 35 Knight shift 48 paramagnetic 43 quadrupole 50 rf 32 scalar coupling 40 tensor 36 Zeeman 24 Intermediate range order, silicate glasses
287
Intermetallic alloys 693 Invisible NMR signal 146, 290 Ionic conductors, see fast ion conduction Ionic motion 19F 552 170 357 Ionicity 377 Iron, paramagnetic effects 203 Irradiation width 111 Isotopic enrichment schemes ~70 384 J
J-coupling 13C-95'97Mo 520 introduction 40 31p-19F 562 2~ 19F 553 perturbation 72 119Sn 592 ~25Te 598 Jeener Broekart sequence 185 Joule-Thompson effect 141 J-resolved spectra 158
K
39K clay minerals 500 fullerene 501 geopolymers 500 halides 497,499 magnetic materials 499 NMR interactions 496 order-disorder 497 phase changes 496 Knight shift Ag metal 472 cadmium oxide 587 contributions 49 core polarisation 49 definition 48 experimental values 692 graphite 564 Mg metal 487,489 Mo metal 521 phosphides 438 Ru metal 525 selenides 585 TiB2, 510 Ti metal 509 Zn metal 512 Zr metal 516
Subject Index L
139La antiferromagnet 678 interaction parameters 674 sol gel 676 solid solutions 676 superconductors 677 zeolite 676 Larmor Frequency 8, 23 Layer aluminosilicates 27A1 275 298i 210 Lead, see 2~ Lee-Goldburg decoupling 81 Left shifting 129 Legendre functions 65 Lewis sites zeolites 290, 539 6,7Li aluminates 632 clay minerals 633 correlations 634 fast ion conduction 636 glasses 638 high temperature NMR 631 mixed alkali effect 638 oxynitrides 634 phase changes 632 silicates 631 Linear back prediction 129 Lineshape chemical shielding 48 dipolar 39 quadrupole MAS 67 quadrupole static 56 simulation 143 Linewidth Vr dependence 145 Longitudinal magnetisation 33 relaxation (T1) 99 strain 280 Lowe-Tarr circuit 123 Lowenstein's rule 209, 210, 378 Lowering operator 25 Low-7 nuclei associated problem 461 definition 461
M
Magic angle spinning angle setting 141 first-order effects 59
higher-order effects 63 introduction 10 linenarrowing 1H 537 partial averaging 64 residual broadening 143 temperature effects 141 Magic angle turning 156 Magnesium, see 25Mg Magnet 113 Magnetic field 26 field drift 115 field homogeneity 115 field intensity (H) 26 induction field (B) 26 materials 39K 499 moment 23 quantum number 25, 33 torque 23 Magnetisation definition 26 equilibrium 28 Match condition CP 87 modulating 90 Mechanochemical processing alumina 294 aluminosilicates 243,296 kaolinite 303 magnesia-silica 244 silicates 242 Meiboom-Gill modification 186 Melts 27A1 301 23Na 415 Metals alloys 693 alloys 25Mg 487 alloys 47'49Ti 509 alloys, 91Zr 516 cold working 687 diffusion 700 experimental approaches 689 glass 9Be 642 milling 695 phase distribution 697 precipitation 699 temperature dependence of ~Q 692 work hardening 687 25Mg correlations 483 minerals 480
711
712
Multinuclear Solid-State NMR of Inorganic Materials
high temperature NMR 483 hydroxides 480 metal 487 oxides 480 shift ranges 479 sintering aids 486 thermal decomposition 484 Milling metals 695 Mixed alkali effect 6'7Li 638 23Na 414 Mixed silica gels 241 95,97Mo carbonyl 520 chalcogenides 521 CP 520 CSA 517 interactions 517 ionically conducting glasses 521 Moment analysis 40 Motional narrowing 700 MREV-8, 79 Mullite gels 241 Multiple quantum dipolar coupling 160 efficiency 97 frequency offset 167 generation 96 MAS 93, 161 proton counting 544 referencing 167 transitions 77 Multipulse sequences 79
N
14N dipolar coupling 475 integer spin effects 475 nitrates 478 nitrides 476 overtone spectroscopy 475 relaxation times 476 shift references 476 15N CSA 575 nitrates 582 nitrites 582 phosphate glasses 579 polymeric precursors 579 shift ranges 578 shift references 574
sialons 576 23Na alumina surfaces 410 aluminates 409 background 399 correlations 403 CP 399 crystalline solids 406 hydrous glasses 417 interactions 400 phosphates 408 phosphate glasses 415 silicates 408 silicate glasses 413 thermal reactions 412 zeolites 412 93Nb relaxor phases 664 atomic ordering 665 Next nearest neighbour effects 29Si 207 61Ni 525 Nitrates 14N 582 15N 582 phase changes 582 adsorbed 583 Nitrides 27A1 316, 476, 575 11B 422, 476, 575,579 metal silicon e9si 253 laN 476 29Si 244 47'49Ti 477, 510, 520 89y 464 Nitrites 15N 582 Nitrogen, see 14'15N NMR experiment introduction 7 NMR methodology 5 Nominally anhydrous minerals 543 Non-bridging oxygens 364 Noise figure 124 Non-quaternary suppression 153 Nuclei 7, 13 Nutation NMR ~B in zeolites 432 background 153 23Na in zeolites off-resonance 154 Nyquist criterion 125
Subject Index 0 170 alumina 372 aluminoborate glasses 382 aluminophosphates 384 aluminosilicates 375,379 bond angle distribution 342 borates 381 carbonates 385 chemical shift distribution 340 correlations 346 CP 345 CSA 341,358 crystalline silica 359 gallosilicates 375 hydroxides 386 multiple quantum 342 oxynitrides 385 silicates 361 silicate glasses 367 silicate gels 369 superconductors 388 Off-resonance nutation 154 Optical heating 188 Order-disorder effects aluminosilicates 378, 379 cordierite 210 leucite 210 niobates 665 phosphates 437 spinel 274, 373 thiocyanides 497 ultramarine 209 Order resolved sidebands 155 O-sialon 27A1 320 298i 250 Overtone spectroscopy 475 Oxidation ~-sialon 248, 320 O-sialon 250, 320 X-sialon 253,321 Oxides alumina 291 binary crystalline 352 glasses 2~ 25Mg 480 sol-gel 352 ternary crystalline 355 S9y 464 67Zn 511 91Zr 515
713
Oxycarbides 298i 256 Oxyfluoride glasses 11B 430 Oxygen, see 170 defects 358 gas exchange 336, 378 isotopic enrichment 334 Oxynitrides 27A1 316 glasses 15N 6'7Li 634 metal 298i 253 rare-earths 323 relaxation times 202 298i 244
P
31p A1PO4s 448 bioglass 450 borophosphate glasses 445 borosilicophosphate glasses 445 chalcogenide glasses 447 CSA 440 phosphates 433 phosphate additions to silicates 444 phosphate glasses 441 phosphoaluminoborosilicate glasses 447 phosphoaluminosilicate glasses 446 phosphosilicate glasses 443 shift correlations 438 shift ranges 436 Pake patterns 39, 545,565 Paramagnetic interaction 43 relaxation effects 202 PASS 156 Pauli susceptibility 48 2o7pb correlations 608,610 CSA 607 fluoride glasses 613 oxide glasses 613 phosphate glasses 615 relaxor phases 611 shift ranges 607, 609 silicates 612 sol gels 615 solid solutions 609 zeolites 612
714
Multinuclear Solid-State NMR of Inorganic Materials
Perchlorates 35'37C1 492 Permeability 27 Perturbation quadrupolar 52 Phase correction 130 cycles 135 generation 117 pulses 117 Phase sensitive detection, 1D 124 2D 163 Phase distribution in metal alloys Phase transformation ~33Cs detected 667 39K detected 495 7Li detected 632 metals 697 nitrates 582 SYRb detected 659 selenates 583 silica 360 titania 351 tridymite 211 51V detected 646 Phosphates 23Na 408 order-disorder 438 31p 433 Phosphate glasses
Polarisation transfer rate Polymeric precursors boron nitride 422 15N 579 silicon carbide 570 Polytypoid sialons 27A1 317
298i 253
698
6,7Li
15N 578 23Na 415 31p 441 2~ 615 Phosphoaluminoborosilicate glasses 31p 447 Phosphoaluminosilicate glasses 31p 446 Phosphors 137Ba 525 35C1 495 19F 556
170 358 ll9Sn 595 89y 464 Phosphorus, see 31p Phosphosilicate glasses 29Si 233
31p 443 Platinum (see 195pt)
89
Potassium, see 39K Powder pattern CSA 48 first-order quadrupolar 56 Pake 39 second-order quadrupolar 67 Preamplifier 124 Precipitation in metals 699 Pressure variation 189 determination via 1H 549 Principle axis system 45, 50 Probes 120 Processing FID 128 Protons, see iH Proton species 536 Pseudocontact interaction 44 Pseudo dipolar coupling 42 195pt surface sites 603 embedded particles 604 Pulse imperfection 119 irradiation 111 phases 117 types 9, 13 Pyrochlore solid solutions 119Sn 595
S9y 465
Q QPASS 156 Q-spoiling 131 Quadrature detection 125 Quadrupole anisotropy in MQMAS 94 broadening under MAS 68 effects on coupled spins 72 effects on pulselengths 53 experimental methodology 343,376 interaction 50 isotropic shifts 67
715
Subject Index linewidths 57 moment 51 product 145,287 Quantification quadrupole perturbed spectra 145 Quantum description 24 Quality factor 120 Quench 114 Q-units definitions 206 disproportionation 232 distribution 233 silicates 212
R
Radial distribution functions 3 Ramp CP 90, 175 Raising operator 24 87Rb alkalides and electrides 661 fullerenes 661 NMR interactions 659 phase changes 659 surface layers 661 Receptivity 13, 34 REAPDOR background 182 13C-11B of Si-B-N-C precursors 572 29Si-11B of gels 242 29Si-11B of B-C-N gel precursors 430 REDOR 27Al-19F of glasses 561 11B-29Si of gels 242 background 172, 179 1Hin glasses 549 31p-zvA1 A1PO4s 305,448 31p-zvA1 ]t-A1203 293,450 31p-19F biomaterials 556 295i-llB B-C-Nprecursors 430 295i-11B BN 424 Relaxation 13C SiC 568 CP 176 definition of times 98 determination 183 introduction 9, 98 mechanisms 101 170 337 paramagnetic effects 202, 203 29Si 202, 255 125Te 598
129Xe 601 Relaxor phases 93Nb 664 2~ 611 Residual broadening in MAS Residual coupling 72 RF field 30, 121 Hamiltonian 32 RIACT 98, 163 Rigid lattice limit 105 Ringing 132 Ripple 119 Rotating frame introduction 29 relaxation time 89 Rotational echoes 63 ROTISSERIE description 137 14N 475 Rotors 139 99'l~ 525
143
S 33S sulphates 491 sulphides 488 Satellite transitions (SATRAS) aluminoborates 307 BN 423 calcium aluminate 313 introduction 70, 142, 150 mullite 276 e3Na 406 170 340 Scalar coupling 40 Hamiltonian 41 Scale factors chemical shift MQ 95, 165 multipulse 80 isotropic quadrupole MQ 95 Lee-Goldburg 82 Schrodinger equation 24 77Se' adsorbed selenates and selenites CP 581 CSA 586 selenates 583 selenides 584 Second moment
586
716
Multinuclear Solid-State NMR of Inorganic Materials
definition 40 under MAS 67 Second-order quadrupole broadening 67 width 70 Second rank tensor 45 SEDOR 27Al_2OVpb zeolites 612 introduction 178 23Na-VLi glasses 638 31p-77Se, chalcogenide glasses Selenates 583 Selenides 584 Selenium (see 77Se) Semiconductors l l3Cd 587, 590 133Cs 674 71Ga 658 199Hg 604 tellurides 598 2~176 606 Sensitivity 13 Separated local fields 170 Shear strain 280 Shielding introduction 44 tensor 45 Shift reference compounds, 16 scales 47 skew 48 span 47 Shift range 27A1 273 25Mg 479 15N 578 31p 436
29Si 205 125,127Te 2~176 605 47'49Ti 508 51V 643 89y 462
67Zn 512 Shift trend 19F 562 Shimming 115
295i
biocompatible glasses 257 carbides 255 chalcogenide glasses 238 densification of glasses 231
584
nitride 245 oxynitride glasses 235 shift range 205 shift structure relations 217, 223 sialons 247 silica polymorphs 210 silicate glasses 231 silicate minerals 212 six coordinated SiO 225 Sialons ( also see ~-, I]-, O-, polytypoid, X-) 27A1 317 fibres 299 15N 576 29Si 247 sintering aids 247, 320 89y 464 Sideband suppression MAS 143 DOR 152 Signal 34 Silica gel 170 387 interaction with fluorine 519 170 359 29Si 210 Silicates 1H 543 6'7Li 631
23Na 408 170 2~
361 612 29Si 212 Silicate gels 170 369 Silicates glasses 1H 546 6'7Li 638 23Na 413 170 367 2~ 615 29Si 231 Siliceous ZMS-5,205 Silicon incorporation in A1PO4s 449 Silicon carbide 13C 568
29Si 255 Silicon monoxide 231 Silicon nitride, 14N 476 15N 575 295i 245 sintering aids 247
Subject Index Silicothermal synthesis O-sialon 250 X-sialon 251,321,464, 486 Solid electrolytes 6'7Li. 636 170 357 Sintering aids 27A1 323 25Mg 486 298i 247 89y 464 Skew 48 Skin depth 121,689 Small angle scattering 3 1198n clay minerals 593 comparison of isotopes 591 CP 591 CSA 592 glasses 595 J-coupling 592 pyrochlore solid solutions 595 Sn (II) vs Sn (IV) 592 sol gels 595 stannates 591 sulphides 595 zeolites 594 Sodium, see 23Na Sol gel 135'137Ba 524 borates 427 43Ca 1H 522 139La 676 170 enrichment,335 oxides 352 2~ 615 phosphosilicate glasses 443 silicates 370 119Sn 595 47'49Ti 508 Solid state reaction 374 Solid solution 63'65Cu 649 19F 553 139La 676 2~ 609 silver iodide 469 SlV 646 Solomon echo 134 Span 47 Spectral
deconvolution 235 density 102, 104 Spectrometer components 112 Spikelet spectroscopy 136 Spin concept 23 diffusion 159 echo 12 echo formation 100 -lattice relaxation (T1) 99 locking 83 operator 25 -spin relaxation (T2) 99 Spinning choice of speed 61 Spinning sidebands 27A1 271 1H 541,546 origin 61, 63 2~ 611 298i 202 suppression 143 Split-t1 sequence 162 Squared sine bell 129 Stannates 119Sn 591 STEAMER experiment 138 14N 476 Stepped experiments field 138 frequency 137 Sternheimer antishielding factor 15,51 Strain parameter 280 Structural disorder 3 heterogeneity 4 minerals disorder 209, 210 Structure determination methods Sulphates 33S 491 Sulphides 33S 488 119Sn 595 67Zn 511 Superconductors, 135'137Ba 524 43Ca 505 63'65Cu 650 139La 677 7Li spinel phase 633 170 388 87Rb 662
717
718
Multinuclear Solid-State NMR of Inorganic Materials
2~176 607 89y 467 Surface layer 87Rb 661 layer 51V 646 oxide 95'97Mo 520 probe 129Xe 602 sites ~95pt 603 Susceptibilty 27, 28 Symmetric transitions
78
T Tl183 Tli~ 183 Tip 183 T299, 185 TAPF 175 TEDOR 27A1-29Si, SAPOs introduction 172, 178, 181 31p-27A1AlPOns 305,448 125,127Te' CSA 599 glasses 601 J-coupling 598 semiconductors 598 shift range 598 tellurates 599 tellurides 598 tellurites 599 Tellurates glasses 601 125'127Te 599 Tellurides, 125'127Te 599 Tellurites glasses 601 23Na 407 125'127Te 601 Temporal arguments CRAMPS 82 MAS 61 Temperature calibration 189 dependent shifts 63'65Cu 650 dependent ZQ 692 effects of MAS. 141 variations 187 Tensor interactions 36 Ternary oxides 355 Thermal decomposition
27A1 310 kaolinite 216, 312 25Mg minerals 484 23Na 412 298i minerals 214 Thallium, see 2~176 Thermodynamics of spins 87 Theta REDOR 180 Thiolates ~~ 469 Time-dependent Hamiltonian 59 Time modulation MAS 61 Tin see ll9Sn Titanates 135'137Ba 522 170 350 47'49Ti 508 Titania in silica 241 47'49Ti 507 47,49Ti carbides 510 correlations 508 experimental considerations 506 metals 509 nitrides 510 sol-gel oxides 508 shift range 508 titanates 508 titania polymorphs 507 2o3,2O5T1 CSA 606 semiconductors 606 shift ranges 605 superconductors 607 TI(I) vs T1 (III) 604 zeolites 606 Toggling frame 80 TOSS, 27A1 272 pulse sequence 144 TPPM 175 Transformations PAS---~Lab 52 PAS--->MAS---~Lab 59 PAS---)Rotor 1--~ Rotor 2---~Lab 76 Transitions central 55 satellite 58 symmetric 78 Transmitters 116 Transverse relaxation (T2) 99 TRAPDOR
Subject Index
1H-27A1,),-alumina 1H-23Na hydrous glasses 548 introduction 172, 182 31p-zvA1 T-alumina 293,450 31p-zvA1 glasses 446, 447 31p-Z3Na glasses 446, 447 Trapezoidal multiplication 129 Triclusters 27A1 276, 285,312 170 375 Tungsten, see 183W Two-dimensional experiments introduction 12, 90 from solution NMR 157
U Ultrasonic narrowing
78
V 51V CSA 643 oxides 642 phase transitions 646 shift ranges 643 solid solutions 646 surface layers 647 zeolites 649 Variable angle spinning 75 Vegard's law 2~ 609
W 183W
CP 474 tungstates 473 Water aluminosilicate glasses, 23Na 414, 417 aluminosilicate glasses 170 387 IH 536 WHH-4 79 Window functions 128 Windowed observation 79 Work hardening 687
X
XANES 3 129Xe gas 601 surface probe
602
719
X-rays 3 X-sialons 27A1 321 29Si 250 89y 464 XY CP 17 7
Y 89y atomic distributions 465 CSA 464 hydrides 468 nitrides 464 oxides 462 phosphors 467 rare earth doping 464, 465 relaxation times 462 shift ranges 463 sialons 464 sintering aids 464 superconductors 467
Z Zeeman interactions classical 24 quantum 25 Z-filter 162 Zeolite, see Mineral Index Zero-order coherence 33 Zero-order phase corrections Zirconates 23Na 407 170 355 91Zr 516 Zirconia in silica 242 67Zn correlations 513 CSA 512 oxides 511 shift range 512 sulphides 511 91Zr metal alloys 516 oxides 515 zircon 515 zirconates 516
130
This Page Intentionally Left Blank
Mineral Index akermanite 298i of 212 25Mg of 479, 481 albite 298i ordering of 210 23Na of 145,402, 410 23Na temperature dependence of 405 298i of 213 27A1-298i CP of 229 23Na-298i CP of 230 1H-29Si CP and HETCOR of 549 albite glass 23Na of 147 1H-29Si CP of 228 170 of 337, 380 170 MQ MAS of 379 1H in 546 19F-298i CP of fluorine-doped 559 13C of CO2-treated 573 alexandrite 9Be of 640, 641 alite 27A1 of guest ions in 315 allophane thermal decomposition of 216, 313 amphibole 298i ordering in 210 analcime 298i of 213 23Na of thermal dehydration 412 analcite 1Hof 539, 540 anatase 170 of 349, 351 conversion to rutile 351 49Ti of 506, 507 195pt of supported catalyst 603 andalusite 27A1 of 144, 277,282 298i of 212 anorthite 29Si of 213 antigorite 25Mg of 481 apatite
43Ca of 503 IH of biomineralisation 550 apophyllite 298i of 213 aragonite 43Ca of 503,504 13C of 573 armenite 298i of 213 augelite 27A1 of 282 bauxite dissolution in NaOH 299 bayerite thermal decomposition of 291 27A1 of 292 17Oof 373 beidellite 298i of 213 belite 27A1 of guest ions in 315 benitoite 298i of 213 beryl 298i of 213 7Li of lithium-containing 629, 630 9Be of 639, 640,641 bikitaite 6Li of 630 boehmite set-up compound for 1H CP 177 ground 284, 294 27A1 of 292 thermal decomposition of 291 in anodised films 293 1H-170 CP of 345 170 SATRAS of 340 170 of 273 boracite 11B of 421 boralite 11B of 431,432 borax 23Na of 401 11B of 421 721
722
Multinuclear Solid-State NMR of Inorganic Materials
brookite lVO of 349 49Ti of 506, 507 brucite 25Mg of 480 thermal decomposition of 484 brushite IH of 540 buddingtonite ~H of 541 calcite 43Ca of 503,504 13Cof 573 carnegieite 29Si of 213 cancrinite 298i of 213 celsian from gels 242, 298 137Ba of 523,524 chabazite 29Si of 213 chiolite 27A1 of 309 19F of 554, 555 chlorite 298i of 213 chondrite 298i of 212 170 of 362, 386 chrysotile thermal decomposition of 217,485 25Mg of heated 482 clinoenstatite 29Si of 212 170 DAS andDOR of 341 170 of 362 clinohumite ~70 of 362, 386 clinopyroxene 1Hin 543 cloverite 19F of 559 coesite 29Si of 213 ab initio calculations 224 170 DAS of 347 lVO of 359 relation between ~70 CQ and structure 346 relation between 170 h and structure 360 colemanite 11B of 421
cookeite 6Liof 630 cordierite 29Si ordering in 210 298i of 213 from gels 242, 298 corundum 27A1 of 291 ground 294 cristobalite 298i of 213 ab initio calculations 224 17Oof 359 relation between 170 h and structure structural relation to glass 367 cryolite 27A1of 309 19F of 554 dakeite 19F-29Si CP of fluorine-doped 559 danburite 29Si of 213 11B of 421 datolite 298i of 212 JiB of 421 1H of 540 dawsonite 23Na of 400, 413 diamond JH in 545 13C of 564, 566 diaspore ~H of 537 diopside 29Si of 212 170 DAS and DOR of 341 ~70 of 361,363 25Mg of 479, 481 dolomite 25Mgof 480, 481 thermal decomposition of 484 elbaite ~H of 540 emerald 29Si of 213 endellite 29Si of 213 enstatite from chrysotile 216 25Mg of 481 1Hin 543
360
M i n e r a l Index
~-eucryptite 298i ordering in 210 298i of 213 170 of glass 380 eucryptite 6Li of 630 faujasite 27A1 of 287, 289 SAPO with similar structure 306 170 MQ of 343,344 170 ab initio calculations 348 17Oof 363 23Na of 418 feldspars relation between 298i ~ and structure 221 in carbothermal sialon formation 248 23Na of 414 1H of 542 ferrierite ab initio calculations 224, 348 170MQof 343 17Oof 363 fluoroapatite 1H of biomineralisation 550 19F of 555 31p-19F REDOR of 556 13C of carbonate-substituted 573 fluorohydroxyapatite 19F of 550, 551,555 fluorophlogopite 298i of 213 fluoroscandium paragsite 19F of 556 forsterite from chrysotile 216, 485 170 DAS and DOR of 341 170 ab initio calculations 348 170 of 361,362 25Mg of 481 25Mg exchange in 483 1H in 543 fresnoite 298i of 213 17Oof 365 gallium fluoroamphibole 71Ga of 657 garnet 1H in 542 gehlinite 298i of 212 gibbsite ground 284, 294
723
27A1 of 292 thermal decomposition of 291 gmelilite 298i of 213 grandidierite 298i of 212 27A1 of 282 liB of 421 25Mg of 479,480, 481 graphite 13C of 564, 566 gypsum 43Ca of 503 1H of 540 halloysite thermal decomposition of 216 harkerite 27A1 of 277 hectorite 298i of 213 thermal decomposition of 217,485,633 25Mg of 481 113Cd of adsorbed cadmium 589 6Li of 630 133Cs of adsorbed caesium 670 heulandite 298i of 213 hollandite sol-gel preparation 313 133Cs of barium-containing Synroc 669 holtedahlite 32p of 438 1H of 540 hyalite 1H of 540 hydrocalumite 35C1 of interlayer ions 495 hydromagnesite 25Mg of 480, 481 hydrotalcite thermal decomposition of 313,484, 486 25Mg of 480, 481 35C1 of interlayer ions 495 15N of adorbed nitrate 582, 583 778e of interlayer Se species 586 119Sn of tin-containing 593 hydroxyapatite formation from Bioglass 257 31p of in bioglasses 451 1H of 537, 540,541 ilerite 1H of 540
Multinuclear Solid-State NMR of Inorganic Materials
724 illite
29Si ordering in 29Si of 213
298i ordering in
210 thermal decomposition of ilmenite 49Ti of 506 imogolite thermal decomposition of inderite liB of 421 inyoite liB of 421 jadeite
298i of
313
212
lepidolite 29Si of 213 6Li of 630 leucite
relation between 29Si ~ and structure 220 relation between 27A1 8 and structure 279 39K of 501
133Cs of caesium-substituted 216, 313
27A1 of 301 19F-27A1CP ofF-doped glass 559 kalsilite 298i of 213 39K of 501 kanemite 23Na of 411 kaolinite 27A1 of 146 set-up compound for ~H CP 174, 227 29Si of 213 conversion to sialon 248,252, 319 thermal decomposition of 215,222, 310 23Na of NaOH-leached 413 1H-29Si CP of 228 778e of intercalated dimethylselenoxide 587 113Cd of adsorbed cadmium 589 kenyaite 23Na of 411 kernite set-up compound for ~H CP 177 11B of 421 kyanite 27A1 of 177, 277 29Si of 212 laponite 6Li of 630 thermal decomposition of 633 larnite in portland cement hydration 258 170 DAS and DOR of 341 17Oof 362 lawsonite
29Si of 212 43K of 503
210
lorenzenite 29Si of 213 magadiite 23Na of 411 magnesite 25Mg of 480, 481 thermal decomposition of margarite 29Si ordering in 210
298i of
669
484
213
makatite 23Na of 408, 411 1H of 540 metakaolinite 298i of 215 27A1 of 284, 310 conversion to geopolymer 304 mica IH in 542 microcline 298i of 213 vapour-phase formation 319 23Na of 410 milarite 298i of 213 monetite 1H of 540 montmorillonite 298i of 213 thermal decomposition of 216, 312 1H-29Si CPof 228 25Mg of 481 39K of exchanged ions 500 19F of fluoride-exchanged 558 l l3Cd of adsorbed cadmium 589, 590 l l9Sn of interlayer tin 593 monticellite 29Si of 212 mordenite 27A1 of 289 ~33Cs of exchanged caesium 672 mullite 298i of 212 27A1 of 276 from gels 240, 294 ground and reheated 285
Mineral Index
as a sialon oxidation product 248,253,320, 321 mechanochemical preparation 243 relation to X-sialon 251,321 muscovite 29Si of 202, 213 29Si ordering in 210 thermal decomposition of 216, 313 39K of exchanged ions 500 N-melilite 27A1 and 29Si of Sm compound 323 89y of 465 15N of 577 narsarsukite 29Si of 213 natrolite 295i of 204, 213 nepheline 29Si of 213 170 of glass 380 23Na of 412 1Hin 542 19F-29Si CP of F-doped 559 13C of CO2-treated glass 573 octadecasil 19F-29Si CP of 229 octosilicate 23Na of 411 1H of 540 oligoclase 295i ordering in 210 29Si of 213 olivine 29Si of 212 omphacite 29Si of 212 orthoclase 39K of 500 orthoenstatite 29Si of 212 25Mg of, from talc decomposition 485 palygorskite 29Si of 213 25Mg of 481 paragonite 295i of 213 pargasite 1H of 540 pectolite 295i of 212 1Hof 540 43K of 503
725
penkvilksite 29Si of 213 periclase 25Mg of 480 petalite 6Li of 630 phenacite 29Si of 212 phlogopite 29Si ordering in 210 29Si of 213 25Mg of 481 phonolite 1H of 549 phosphoellenbergerite 31p of 438 1H of 540, 541 piemontite 295i of 212 pseudoboehmite thermal decomposition of 221 27Alof 292 in anodised films 293 pyrope 29Si ordering in 210 25Mg of 481 pyrophyllite 295i of 213 thermal decomposition of 216, 312 ground 312 1H of 540, 541 quartz 29Si of 213 ab initio calculations 224 170 of 359 relation between 170 h and structure 360 structural relation to glass 367 1Hin 542 rankinite 29Si of 212 reedmergnite 11B of 421 ruby 170 of 372 rutile 170 of 349, 351 49Ti of 506, 507 formation from gel 508 sanidine 170 of 387 170 of glass 388 vapour phase formation of 319
726
Multinuclear Solid-State NMR of lnorganic Materials
sapphire 27A1 CSA of 272 saponite 298i of 213 scolecite 298i of 204, 213 senegalite 27A1 of 282, 284 sepiolite 298i of 213 25Mg of 481 1H of 542 51V of vanadium-impregnated catalysts 648 serpentine 298i of 213 sillimanite 298i of 212 29Si ordering in 210 27Alof 277 sodalite 29Si of 213 relation between 298i ~ and structure 220 35C1 of encapsulated AgC1 495,496 9Be of beryllium-containing 640 relation between 9Be ~ and structure 641 sorensenite ~H of 540 sphene 298i of 212 spinel degree of disorder 273 spodumene 298i of 213 6Li of 630 stilbite 298i of 213 170 of 376 170 MQ MAS of 378 stishovite 298i of 225,226 170 of 359 talc 298i of 213 thermal decomposition of 217,485 1H-170 CP of 345 25Mg of 481 1H of 540 thaumasite 29Si of 226 thompsonite 298i of 204, 213 thorveitite
298i of 212 tielite sol-gel preparation 313 titanite 298i of 213 topaz 298i of 212 thermal decomposition of 216, 313 1H of 540 19F of 555 tourmaline 298i of 213 11B of 421 tremolite 298i of 213 1H of 539, 540,541 19F of 556 tridymite 29Siof 210, 211,213 tugtupite 9Be of 639, 641 tveitite 19F of 554 ulexite I~B of 421 ultramarine 29Si ordering in 209 vermiculite 298i orderingin 210 298i of 213 39K of exchanged ions 500 113Cd of exchanged cadmium 590 133Cs of exchanged caesium 670 vesuvianite 27A1 of 283 vinogradovite 298i of 213 wadeite 298i of 226 170 of 362 whitlockite 3~p of in bioglasses 451 wollastonite 170 of 76, 362 170 DAS and DOR of 341 29Si of 213 xonotlite in portland cement hydration 259 zeolite 27A1 of 149 170 DAS of 157, 158 170 DOR and MQ of 342, 376, 377
Mineral Index
170 enrichment of
336
298i of 206, 209 relations between 298i ~ and structure 219, 220, 221 relation between 170 8 and structure 342, 348 ab initio calculations 224, 348 27A1 of 287 1H-ZVA1TRAPDOR of 290 surface catalytic sites of 290 23Na of 418,419 liB of 431 l~ of catalyst supported by 472 1H of 538 19F of fluorinated 558 19F of adsorbed CF4 558 19F-29Cp of F-containing 562 113Cd of exchanged cadmium 590 119Sn of tin-substituted 594
129Xe of adsorbed xenon 602 195pt of supported catalyst 604 2~ of thallium-containing 606 9Be of beryllium-substituted 641 51V of vanadium-containing catalysts 71Ga of gallium-containing 656 139La of lanthanum-substituted 677 zircon 298i of 212 91Zr of 514, 515 zirconia 91Zr of 515 zoisite 27A1 of 144 zorite 298i of 213 zunyite 27A1 of 278
727
649
