Triple-quantum Magic-angle Spinning Research Articles

Unraveling Sodium-Ion Dynamics in Honeycomb-Layered Na2MgxZn2-xTeO6 Solid Electrolytes with Solid-State NMR.

All-solid-state sodium-ion batteries (SIBs) have the potential to offer large-scale, safe, cost-effective, and sustainable energy storage solutions by supplementing the industry-leading lithium-ion batteries. However, for the enhanced bulk properties of SIB components (e.g., solid electrolytes), a comprehensive understanding of their atomic-scale structure and the dynamic behavior of sodium (Na) ions is essential. Here, we utilize a robust multinuclear (23Na, 125Te, 25Mg, and 67Zn) magnetic resonance approach to explore a novel Mg/Zn homogeneously mixed-cation honeycomb-layered oxide Na2MgxZn2-xTeO6 solid solution series. These new intermediate compounds exhibit tailorable bulk Na-ion conductivity (σ) with the highest σ = 0.14 × 10-4 S cm-1 for Na2MgZnTeO6 at room temperature suitable for SIB solid electrolyte applications as observed by powder electrochemical impedance spectroscopy (EIS). A combination of powder X-ray diffraction (XRD), energy-dispersive X-ray (EDX) spectroscopy, and field emission scanning electron microscopy (FESEM) reveals highly crystalline phase-pure compounds in the P6322 space group. We show that the Mg/Zn disorder is random within the honeycomb layers using 125Te nuclear magnetic resonance (NMR) and resolve multiple Na sites using two-dimensional (triple-quantum magic-angle spinning (3QMAS)) 23Na NMR. The medium-range disorder in the honeycomb layer is revealed through the combination of 25Mg and 67Zn NMR, complemented by electronic structure calculations using density functional theory (DFT). Furthermore, we expose very fast local Na-ion hopping processes (hopping rate, 1/τNMR = 0.83 × 109 Hz) by using a laser to achieve variable high-temperature (∼860 K) 23Na NMR, which are sensitive to different Mg/Zn ratios. The Na2MgZnTeO6 with maximum Mg/Zn disorder displays the highest short-range Na-ion dynamics among all of the solid solution members.

Probing oxide-based glass structures by solid-state NMR: Opportunities and limitations

This Primer critically reviews the possibilities and limitations of probing the short-range structure of an oxide-based glass by routine magic-angle spinning (MAS) or triple-quantum MAS (3QMAS) nuclear magnetic resonance (NMR) experiments. We briefly outline the structural features of oxide-based glasses and the basics of solid-state NMR. Besides suggesting guidelines for selecting favorable experimental conditions and important considerations for recording high-quality MAS NMR spectra amenable for subsequent analysis, we review options for extracting NMR observables and their distributions from spins-1/2 as well as half-integer spin quadrupolar nuclei. Considering that the isotropic chemical shift remains the primary probe of local structure by MAS NMR, we thoroughly review its dependence on the short-range oxide-based glass parameters from the viewpoint of a very simple and intuitive but qualitative model. The utility of deconvolutions of notably 29Si and 31P MAS NMR spectra are discussed critically. We also suggest alternative yet qualitative analysis options that are available whenever MAS NMR spectral deconvolutions are not warranted without additional information, which incidentally applies to a majority of modern multicomponent glasses.

Measuring multiple 17O-13C J-couplings in naphthalaldehydic acid: a combined solid state NMR and density functional theory approach.

A combined multinuclear solid state NMR and gauge included projected augmented wave, density functional theory (GIPAW DFT) computational approach is evaluated to determine the four heteronuclear 1J(13C,17O) couplings in solid 17O enriched naphthalaldehydic acid. Direct multi-field 17O magic angle spinning (MAS), triple quantum MAS (3QMAS) and double rotation (DOR) experiments are initially utilised to evaluate the accuracy of the DFT approximations used in the calculation of the isotropic chemical shifts (δiso), quadrupole coupling constants (CQ) and asymmetry (ηQ) parameters. These combined approaches give δiso values of 313, 200 and 66 ppm for the carbonyl (C[double bond, length as m-dash]O), ether (-O-) and hydroxyl (-OH) environments, respectively, with the corresponding measured quadrupole products (PQ) being 8.2, 9.0 and 10.6 MHz. The geometry optimised DFT structure derived using the CASTEP code gives firm agreement with the shifts observed for the ether (δiso = 223, PQ = 9.4 MHz) and hydroxyl (δiso = 62, PQ = 10.5 MHz) environments but the unoptimised experimental XRD structure has better agreement for the carbonyl group (δiso = 320, PQ = 8.3 MHz). The determined δiso and ηQ values are shown to be consistent with bond lengths closer to 1.222 Å (experimental length) rather than the geometry optimised length of 1.238 Å. The geometry optimised DFT 1J(13C,17O) coupling to the hydroxyl is calculated as 20 Hz and the couplings to the ether were calculated to be 37 (O-C[double bond, length as m-dash]O) and 32 (O-C-OH) Hz. The scalar coupling parameters for the unoptimised experimental carbonyl group predict a 1J(13C,17O) value of 28 Hz, whilst optimisation gives a value of 27 Hz. These calculated 1J(13C,17O) couplings, together with estimations of the probability of each O environment being isotopically labelled (determined by electrospray ionisation mass spectrometry) and the measured refocussable transverse dephasing (T2') behaviour, are combined to simulate the experimental decay behaviour. Good agreement between the measured and calculated decay behaviour is observed.

Nanostructure of CaO-(Na2O)-Al2O3-SiO2-H2O Gels Revealed by Multinuclear Solid-State Magic Angle Spinning and Multiple Quantum Magic Angle Spinning Nuclear Magnetic Resonance Spectroscopy

(Calcium,alkali)-aluminosilicate gel frameworks are the basis of modern cements and alkali-activated materials for sustainable infrastructure and radioactive waste immobilization and also find application in glass alteration, mineral weathering, and zeolite synthesis. Here, we resolve the nanostructure of these gels that dictates mass transport, solubility, and mechanical properties. The key structural motifs comprising hydrous (calcium,alkali)-aluminosilicate gels are identified via 17O, 23Na, and 27Al triple-quantum magic angle spinning and 29Si magic angle spinning nuclear magnetic resonance spectroscopy of a novel class of stoichiometrically controlled 17O-enriched multiphase gels. Increased Ca content promotes low-Al, high-Ca chain-structured “C-S-H-type” products exhibiting significant nanostructural ordering, low levels of chain cross-linking, predominant Ca coordination of nonbridging oxygen atoms, and an increase in proton association with CaO layers to form Ca–OH sites. Al substitution is identified in multiple sites in the silicate chains, including cross-linking, bridging, and pairing tetrahedra. Increased Al content increases the proportion of cross-linking sites and gel disorder. The large increase in SiIV–O–AlIV sites increases the relative amounts of Na+ and AlV species charge-balancing AlO4– tetrahedra and results in the formation of an additional disordered low-calcium, framework-structured alkali aluminosilicate (“N-A-S-H-type”) gel, with high Al and Na contents. Changes in bulk composition significantly alter the nanostructures of the C-S-H-type and N-A-S-H-type gels. Mean SiIV–O–AlIV bond angles for each type of AlIV site environment are highly consistent, with compositional changes dictating the relative proportions of individual AlIV species but not altering the local structure of each AlIV site. These findings reveal the molecular interactions governing the (calcium,alkali)-aluminosilicate gel nanostructure, which are crucial in controlling material properties and durability.

Inspecting the Structure and Formation of Molecular Sieve SAPO-34 via 17O Solid-State NMR Spectroscopy

Silicoaluminophosphates (SAPOs) are microporous frameworks with Brønsted acid sites that can be used as acidic catalysts. A firm understanding of SAPO structure, formation, and crystallinity is necessary for understanding and expanding SAPO applications in heterogeneous catalysis. Solid-state 17O NMR (SSNMR) spectroscopy is an ideal tool to probe structure and formation of SAPO-based materials; the 17O quadrupolar and chemical shift interactions are exquisitely sensitive to local electronic and magnetic environments. In this work, a pure trigonal SAPO-34 molecular sieve synthesized via the dry-gel conversion (DGC) method was investigated using a combination of 17O magic-angle spinning, 17O triple-quantum magic-angle spinning, 17O[27Al] transfer of population in double-resonance, and 17O[31P] rotational-echo double-resonance SSNMR spectroscopy, complemented by powder X-ray diffraction along with 27Al, 29Si, and 31P multinuclear SSNMR experiments. The four observed 17O resonances were simulated to extract NMR parameters, with each resonance assigned to individual oxygen local environments and connectivities in SAPO-34. The incorporation of oxygen from 17O-labeled water (i.e., H217O) during the DGC formation of pure trigonal SAPO-34 has also been investigated at various time intervals and stages of crystallization using 17O SSNMR. There is direct involvement of 17O-enriched water vapor during the DGC crystallization process of trigonal SAPO-34. The initial dry gel is amorphous, transforming to a crystalline layered AlPO4 phase during the first hour of heating and then progressing to a semicrystalline phase after 4 h of heating; formation of the crystalline trigonal SAPO-34 product was found to be complete after 2 days.

Probing the nanostructure, interfacial interaction, and dynamics of chitosan-based nanoparticles by multiscale solid-state NMR.

Chitosan-based nanoparticles (NPs) are widely used in drug and gene delivery, therapy, and medical imaging, but a molecular-level understanding of the internal morphology and nanostructure size, interface, and dynamics, which is critical for building fundamental knowledge for the precise design and efficient biological application of the NPs, remains a great challenge. Therefore, the availability of a multiscale (0.1-100 nm) and nondestructive analytical technique for examining such NPs is of great importance for nanotechnology. Herein, we present a new multiscale solid-state NMR approach to achieve this goal for the investigation of chitosan-poly(N-3-acrylamidophenylboronic acid) NPs. First, a recently developed (13)C multiple cross-polarization magic-angle spinning (MAS) method enabled fast quantitative determination of the NPs' composition and detection of conformational changes in chitosan. Then, using an improved (1)H spin-diffusion method with (13)C detection and theoretical simulations, the internal morphology and nanostructure size were quantitatively determined. The interfacial coordinated interaction between chitosan and phenylboronic acid was revealed by one-dimensional MAS and two-dimensional (2D) triple-quantum MAS (11)B NMR. Finally, dynamic-editing (13)C MAS and 2D (13)C-(1)H wide-line separation experiments provided details regarding the componential dynamics of the NPs in the solid and swollen states. On the basis of these NMR results, a model of the unique nanostructure, interfacial interaction, and componential dynamics of the NPs was proposed.

Changes in surface structure of sodium aluminoborosilicate glasses during aqueous corrosion analyzed by using NMR

Aqueous corrosion tests were performed for aluminoborosilicate glasses with different Na contents at 90°C for 20days. Multi-nuclear solid-state nuclear magnetic resonance (NMR) techniques were employed to characterize the structures of the pristine and altered glasses. 11Bmagic-angle spinning (MAS) NMR spectra showed the same line shape for the pristine and altered glasses, indicating that B species such as BO3 and BO4 were released into the solution congruently with the same fraction during the static dissolution experiments. The reconstruction of Si on the glass surface as hydrated phases was confirmed by the 29SiMAS and H1→Si29 cross-polarization magic-angle spinning NMR spectra of the altered glasses. 23NaMAS and triple-quantum magic-angle spinning NMR spectra showed that part of the Na released from pristine glasses was incorporated in the hydrated phases, which can be identified by a 23Nachemical shift at δiso=−3.3ppm. The availability of these structural data by employing solid-state NMR techniques can provide a better understanding of the formation of the hydrated phases and the effect of dissolution behavior on the glass composition.

Direct Detection of Al–O–Al Structure in Aluminosilicate Specimens: A Use of Homo-Nuclear DQMAS NMR

A general strategy of Al–O–Al structure in various aluminosilicate was evaluated by combining triple-quantum magic angle spinning (3QMAS) and double-quantum homo-nuclear correlation under magic angle spinning (DQMAS) solid-state nuclear magnetic resonance (NMR) measurements with the aid of high magnetic field NMR (800 MHz for 1H Larmor frequency). The results show that in many cases the direct detection of Al–O–Al sites in aluminosilicate crystals and glasses is possible; hence the extent of aluminum avoidance can be directly elucidated. Specifically, experimental evidence of Al–O–Al linkages in several aluminosilicate materials with Si/Al >1 was straightforwardly confirmed; and the existence of Al–O–Al is considered to have little correlation with the Si/Al ratio, but it may be strongly related to the cation and local structural arrangement. In addition, the presence of tri-clusters of (Si, Al)O4-tetrahedra in aluminosilicate framework was proposed, which was thought to act as nuclei for formation and incorporation of cations to achieve charge neutrality.

Effect of particle size on phase transitions in metastable alumina nanoparticles: A view from high-resolution solid-state 27Al NMR study

Nanoparticles are known to have physical properties different from their bulk analogs, a characteristic that bears wide technological potential. Detailed knowledge of the atomic structures of diverse metastable/stable polymorphs in alumina nanoparticles with varying particle size is essential to understand their macroscopic properties and the behavior of their phase transitions. In this study, we report high-resolution solid-state 27 Al 2D triple-quantum (3Q) magic-angle spinning (MAS) and 1D MAS nuclear magnetic resonance (NMR) spectra for alumina nanoparticles with varying temperature and particle size with an aim to explore the nature of phase transitions in alumina nanoparticles. Although the 27 Al MAS NMR spectra of alumina nanoparticles cannot fully resolve all the crystallographically distinct Al sites for metastable aluminas such as γ-, δ-, and θ-Al 2 O 3 , the simulation of 27 Al MAS NMR spectra collected at different magnetic fields following the Czjzek model allows us to obtain the quantitative fractions of alumina polymorphs in nanoparticles and the NMR characteristics of their Al sites. The 27 Al 3QMAS NMR spectra resolved crystallographically distinct [6] Al and [4] Al sites in (γ,δ)- and θ-Al 2 O 3 in the isotropic dimension for the first time. The fraction of θ-Al 2 O 3 gradually increases up to ~1473 K at the expense of a decrease in (γ,δ)-Al 2 O 3 . The onset of formation of α-Al 2 O 3 from metastable aluminas is observed above ~1493 K. Several phase transitions in alumina nanoparticles observed in the current study include, γ,δ → θ-Al 2 O 3 , γ,δ → α-Al 2 O 3 , and θ → α-Al 2 O 3 . Although the phase transition γ,δ → θ-Al 2 O 3 occurs gradually with increasing annealing temperature from 873 to 1473 K, the phase transitions γ,δ → α-Al 2 O 3 and θ → α-Al 2 O 3 occur dramatically within a narrow temperature range between 1473 and 1573 K. The observed difference in temperature range (gradual vs. dramatic) for phase transition γ,δ → θ-Al 2 O 3 and γ,δ,θ → α-Al 2 O 3 originates from the different structural disorder in the metastable aluminas (i.e., γ-, δ-, θ-) and α-Al 2 O 3 . The effect of particle size on the phase transition (γ,δ) → θ-Al 2 O 3 between 298 and ~1473 K is not observed significantly. On the other hand, the transition temperature for γ,δ,θ → α-Al 2 O 3 , where the 50% for alumina is α-Al 2 O 3 , apparently increases as the particle size increases (as evidenced by TEM observation), indicating a larger energy penalty for phase transitions into α-Al 2 O 3 in larger alumina nanoparticles. This could be due to higher surface energy of θ-Al 2 O 3 than that of α-Al 2 O 3 and/or the fact that the transition from θ-Al 2 O 3 to α-Al 2 O 3 is kinetically favored for smaller nanoparticles. The mechanistic details of phase transitions among alumina polymorphs provided in the current study yield insights into the nature of the phase transition mechanisms for other oxide nanoparticles ubiquitous in the Earth’s surface environment.

Resolving Multiple Non‐equivalent Metal Sites in Magnesium‐Containing Metal–Organic Frameworks by Natural Abundance 25Mg Solid‐State NMR Spectroscopy

In a spin: Directly differentiating multiple Mg sites in Mg-containing MOFs by (25)Mg solid-state NMR spectroscopy is very challenging at natural abundance. By performing (25)Mg two-dimensional triple-quantum magic-angle spinning solid-state NMR experiments at a magnetic field of 21.1 T at natural abundance, four non-equivalent Mg sites with very similar local environments in α-Mg(3)(HCOO)(6) were unambiguously resolved (see figure).

High magnetic field solid‐state NMR analyses by combining MAS, MQ‐MAS, homo‐nuclear and hetero‐nuclear correlation experiments

A general strategy of structural analysis of alumina silicate by combining various solid-state NMR measurements such as single pulse, multi-quantum magic angle spinning, double-quantum homo-nuclear correlation under magic angle spinning (DQ-MAS), and cross-polarization hetero-nuclear correlation (CP-HETCOR) was evaluated with the aid of high magnetic field NMR (800 MHz for (1) H Larmor frequency) by using anorthite as a model material. The high magnetic field greatly enhanced resolution of (27) Al in single pulse, DQ-MAS, and even in triple-quantum magic angle spinning NMR spectra. The spatial proximities through dipolar couplings were probed by the DQ-MAS methods for homo-nuclear correlations between both (27) Al-(27) Al and (29) Si-(29) Si and by CP-HETCOR for hetero-nuclear correlations between (27) Al-(29) Si in the anorthite framework. By combining various NMR methodologies, we elucidated detailed spatial correlations among various aluminum and silicon species in anorthite that was hard to be determined using conventional analytical methods at low magnetic field. Moreover, the presented approach is applicable to analyze other alumina-silicate minerals.

Framework and extra-framework aluminium in wet ion exchanged Fe-ZSM5 and the effect of steam during the decomposition of N 2O

The role of extra-framework and framework aluminium in wet-ion exchanged Fe-ZSM5 has been studied using 29Si NMR and 27Al triple quantum magic angle spinning (3QMAS) NMR. A series of samples were studied, the parent material, the wet ion exchanged Fe-ZSM5 and Fe-ZSM5 that has been used in the decomposition of N 2O with varying reaction conditions. Various framework and extra-framework aluminium species have been identified. It was found that cationic Fe species prefer to replace the Brønsted acid protons in their charge balancing role at those aluminium sites associated with the largest quadrupolar product. The framework aluminium atoms that pertain to the smaller quadrupolar product, which are either charge balanced by extra-framework aluminium or a proton, are much less prone to exchange. In the catalytic decomposition of N 2O it seemed that water present in small amounts enhances the catalytic activity. However, water also decreases the long term stability and performance by dealuminating the zeolite framework. With a high amount of water present, Fe-ZSM5 was destabilised and catalytically inferior.

Concentration-dependent luminescent behaviour of Rhodamine 6G in AlPO 4 xerogel monoliths

Homogeneous monolith of AlPO 4 gel doped with Rhodamine 6G (Rh6G) in different dye concentration is prepared by one step process with sol–gel method using the precursors Al(lact) 3 and H 3PO 4. The optical properties of AlPO 4 gel doped with Rh6G have been characterized by UV–vis absorption spectra and fluorescence spectra. Rh6G molecular J-dimers and H-dimers even multimers are analyzed by excitation spectra based on Exciton theory. The structure of AlPO 4 gel doped with Rh6G is investigated by 27Al and 31P Magic Angle Solid Nuclear Magnetic Resonance (MAS NMR), 27Al { 31P} Rotational Echo Double Resonance (REDOR) NMR and 27Al Triple Quantum Magic Angle Spinning (TQ-MAS) NMR. Based on the results of optical spectra and the structural analysis by NMR techniques, The AlPO 4 gel doped with Rh6G dye with molar ratio of Rh6G/Al(lact) 3 of 1 × 10 −4 displays excellent optical properties.

ChemInform Abstract: The Structural Role of Lanthanum and Yttrium in Aluminosilicate Glasses: A 27Al and 17O MAS NMR Study.

Aluminosilicate glasses containing rare-earth and rare-earth analogue cations such as Y3+ and La3+ are interesting for a variety of technological applications, as well as for elucidating general principles of glass formation and structure. We present high-resolution 17O and 27Al NMR data on a series of lanthanum and yttrium aluminosilicates, which show the significant increase in concentrations of AlO5 and AlO6 groups caused by the smaller radii and higher field strengths of the modifier cation in the latter. Triple-quantum magic-angle spinning (3QMAS) spectra for both nuclides show much better resolution than standard MAS and demonstrate that the latter can give misleading results for compositional effects on speciation. Oxygen sites that resemble those in crystalline Y2O3 and La2O3 were not detected, suggesting that such extreme clustering is not significant in the compositions studied. The role of highly coordinated Al, and the configurational complexity generated by the formation of such species, is d...

Perturbations to 27Al Electric Field Gradients in Nanocrystalline α-Al2O3 Studied by High-Resolution Solid-State NMR

(27)Al solid-state NMR has been employed to study the perturbations to (27)Al electric field gradients for the aluminum environments in nanocrystalline alpha-alumina. Triple quantum magic angle spinning experiments show that the octahedral aluminum coordination remains unchanged down to 12 nm, although severe perturbations to (27)Al electric field gradients are noticed at 28 nm and below. 3Q-MAS and SATRAS experimental data of nano alpha-alumina have been analyzed through extensive spectral simulations to probe (27)Al electric field gradients of aluminum in the grains and grain boundaries. While the aluminum in the grains has a unique field gradient tensor, the same octahedrally coordinated aluminum environments in the grain boundaries suffer a distribution of electric field gradients. This is evidenced by data analysis of both 3Q-MAS and SATRAS spectra. By invoking the Gaussian isotropic model, in which the (C(Q), eta(Q)) parameter space is discretely sampled by the Czjzek distribution, we have been able to analyze the (27)Al SATRAS spectra of nanocrystalline alpha-alumina samples having grain sizes of 52, 28, 20, and 12 nm. Good agreement between experimental and simulated spectra has led to the quantitative determination of grain and grain boundary components in these materials.

Nature of Chemical and Topological Disorder in Borogermanate Glasses: Insights from B-11 and O-17 Solid-State NMR and Quantum Chemical Calculations

Knowledge of the extent of chemical and topological disorder in topological disordered oxide glasses and melts is essential for understanding the atomistic origins of their macroscopic properties. Here, we report the high-resolution B-11 and O-17 triple quantum magic angle spinning (3QMAS) NMR spectra for binary borogermanate glasses. The NMR results, together with quantum chemical calculations of cluster energy difference, allow us to estimate the extent of chemical disorder and topology variation with composition. The B-11 NMR result shows that the boroxol ring fraction decreases nonlinearly with increasing mole fraction of Ge and is smaller than that in binary borosilicate glasses, suggesting that the Ge/Si content influences the topological changes. Whereas oxygen clusters are not well resolved in O-17 NMR spectra, the Ge-O-Ge fraction apparently increases with increasing GeO(2) content. The estimated degree of framework disorder (Q) in borogermanate glasses is approximately 0.4, according to quantum chemical calculations based on density functional theory. This is halfway between chemical order (Q = 1) and a random distribution (Q = 0). In contrast, Q is approximately -0.6 for borosilicate glasses, indicating a moderate tendency toward complete phase separation (Q = -1). This result confirms that the degree of framework disorder shows a strong dependence on the type of framework cations (Si or Ge). The predicted configurational enthalpy of borogermanate glasses, explicitly considering both chemical and topological disorder, shows a negative deviation as predicted from the positive Q value. The results demonstrate that the macroscopic properties of topologically disordered noncrystalline solids can be established from the detailed quantification of topological and chemical disorder.

27Al Solid-state NMR Structural Studies of Hydrotalcite Compounds Calcined at Different Temperatures

Hydrotalcites are anionic clays that are quite prevalent in nature and their importance is growing more and more because of their very wide range of potential applications and uses. Understanding the structural and compositional changes that occur on the molecular scale during the thermal decomposition of hydrotalcite compounds is essential for the basic prediction and comprehensive understanding of the behavior and technical application of these materials. In this study, several hydrotalcite compounds calcined at different temperatures for applications in a chlorine resistant textile were prepared and 27-Aluminm solid-state nuclear magnetic resonance (NMR) spectroscopy was used as a tool to study their local structure and behavior. The changes in the Al coordination of the hydrotalcite compounds were investigated with one dimensional (1D) solid-state magic angle spinning (MAS) NMR spectroscopy. The two broad resonances arising from the structurally different Al coordinations of these compounds were clearly resolved by two dimensional (2D) triple quantum magic angle spinning (3QMAS) NMR spectroscopy.

The effect of network-modifying cations on the structure and disorder in peralkaline Ca–Na aluminosilicate glasses: O-17 3QMAS NMR study

We report the detailed atomic configurations around the oxygens in peralkaline Ca–Na aluminosilicate glasses {[(CaO) 1 − x (Na 2O) x ] 1.5(Al 2O 3) 0.5 (SiO 2) 6 with varying Na/Ca ratios [X Na2O = Na 2O/(Na 2O + CaO)], CNAS} using 2D 17O 3QMAS (triple-quantum magic angle spinning) NMR at 9.4 and 14.1 T. The high-resolution O-17 NMR spectra reveals peaks due to several distinct bridging oxygens (BO, Si–O–Al and Si–O–Si) and three types of non-bridging oxygens (NBO, Ca-NBO, Na-NBO, and mixed {Ca,Na}–NBO). The local environment around the oxygens provides previously unknown details of the extent of chemical and topological disorders in quaternary silicate glasses. The chemical shielding of the NBO peak increases with the Ca contents (chemical shift decreases), and a significant fraction of the mixed-NBO peak is observed in intermediate compositions, suggesting a considerable extent of mixing between Ca and Na around NBO. While the framework disorder due to the mixing of [4]Si and [4]Al and degree of polymerization (i.e., fraction of NBO) is almost invariant with the Ca content, topological disorders (and thus, the topological entropy) in Si–O–Si and Si–O–Al due to bond angle and bond length distributions in the CNAS (CaO–Na 2O–Al 2O 3–SiO 2) glasses increases with the Ca content, indicating that the non-framework cations influence the framework topology of BOs (particularly, with Si–O–Al) and NBOs. The increase in the topological disorder in Si–O–Al with the Ca content is non-linear in that the degree of disorder abruptly increases near the Ca-end member. This trend, together with the non-linear variation of O-17 isotropic chemical shift of Si–O–Al, implies that Na + may have proximity to Si–O–Al at intermediate compositions; thus, Na + plays a preferential role as a charge-balancing cation, while Ca 2+ can act as a network-modifying cation in the glass composition studied here (non-randomness in the partitioning of non-framework cations between NBO and Si–O–Al), manifesting a deviation from the randomness in the complex silicate glasses and melts.

Local Structure of Magnesium in Silicate Glasses: A 25Mg 3QMAS NMR Study

We have reported the 25Mg triple-quantum magic-angle spinning (3QMAS) NMR spectra of silicate glasses. The two-dimensional spectra suggest that the magnesium ions in MgSiO3, CaMgSi2O6, Ca2MgSi2O7, Mg3Al2Si3O12, and Li2MgSi2O6 glasses are mainly in octahedral environments, although in Na2MgSi2O6, K2MgSi2O6, and K2MgSi5O12 glasses they form tetrahedral species. We discussed the coordination environments of Mg based on the field strength of competing Mg2+, Ca2+, Na+, K+, and Li+ cations, and convincingly demonstrated that there is a correlation between them. These results indicate that the two-dimensional NMR spectroscopy such as MQMAS technique is a very useful method to analyze the local environments of nonframework cations in noncrystalline solids.

17O and29Si NMR Parameters of MgSiO3Phases from High-Resolution Solid-State NMR Spectroscopy and First-Principles Calculations

The 29Si and 17O NMR parameters of six polymorphs of MgSiO3 were determined through a combination of high-resolution solid-state NMR and first-principles gauge including projector augmented wave (GIPAW) formalism calculations using periodic boundary conditions. MgSiO3 is an important component of the Earth's mantle that undergoes structural changes as a function of pressure and temperature. For the lower pressure polymorphs (ortho-, clino-, and protoenstatite), all oxygen species in the 17O high-resolution triple-quantum magic angle spinning (MAS) NMR spectra were resolved and assigned. These assignments differ from those tentatively suggested in previous work on the basis of empirical experimental correlations. The higher pressure polymorphs of MgSiO3 (majorite, akimotoite, and perovskite) are stabilized at pressures corresponding to the Earth's transition zone and lower mantle, with perovskite being the major constituent at depths >660 km. We present the first 17O NMR data for these materials and confirm previous 29Si work in the literature. The use of high-resolution multiple-quantum MAS (MQMAS) and satellite-transition MAS (STMAS) experiments allows us to resolve distinct oxygen species, and full assignments are suggested. The six polymorphs exhibit a wide variety of structure types, providing an ideal opportunity to consider the variation of NMR parameters (both shielding and quadrupolar) with local structure, including changes in coordination number, local geometry (bond distances and angles), and bonding. For example, we find that, although there is a general correlation of increasing 17O chemical shift with increasing Si-O bond length, the shift observed also depends upon the exact coordination environment.