One major theme that has driven our work for years, and continues to be important, is the structural and dynamical nature of the glass to liquid transition. One aspect of this is to try to determine how liquid structure changes with temperature and pressure. The complement to this is to search for mechanisms controlling dynamics, such as diffusion and viscosity. For example, using in situ, high T static and MAS NMR we have been able to show that the fundamental step in controlling viscous flow in silicate melts is local-scale Si-O bond breaking, not longer-range, "polymeric" sorts of interactions. And we found the first good experimental evidence for a key transition complex that had often been hypothesized, which is the SiO₅ group. Also using high T NMR, we have recently learned a lot about the nature of sites for network modifiers such as Na⁺ and Mg²⁺ in melts, and the mechanism for diffusion of these species. We continue this sort of work on geological materials, as well as on technological glasses, such as borosilicates. In the latter, we again have found some of the first direct evidence for how melt structure changes with temperature, and for the microscopic mechanism for viscous flow and diffusion.

We have recently been using the great improvements in NMR spectral resolution obtained from multiple quantum MAS (MQMAS) NMR to quantify the different types of oxygen linkages in network-structured oxide glasses, for example Si-O-Si, Si-O-Al, and Al-O-Al in aluminosilicates or Si-O-B, B-O-B, etc., in borosilicates. We have used these previously unobtainable types of data to construct quantitative models of the configurational properties of the melts, and compared these to our recent ab-initio bond energy calculations. This work has made it clear that order/disorder among network cations is a key aspect of the overall melt energetics, including configurational heat capacity, total free energy, and the thermodynamic activities that control phase equilibria. We have also been able to use this experimental approach to directly determine the network cations (e.g. Si, Al, B) to which non-bridging oxygens are linked.

Fluoride and chloride ions dissolved in silicate melts and glasses can have major effects on thermodynamic and kinetic properties that are important in both geochemical and technological systems. NMR is uniquely capable at providing structural information about F and Cl sites, but ¹⁹F and ³⁵Cl pose major (but very different) technical challenges. We have recently reported some of the first highly informative studies of these nuclides in silicate and aluminosilicate glasses. We have begun to learn quite a bit about how they order their local chemical environments and what bonding configurations are most favorable, that are already providing new insights into their roles in solubilities and other bulk properties.

The mechanism of exchange of oxygen in H₂O with oxygen sites in silicates is of major importance in stable isotope geochemistry, in understanding mineral surface processes such as dissolution, and in catalysis and corrosion of ceramics and glasses. We have recently demonstrated that high-resolution ¹⁷O NMR can be used to determine exchange rates for specific kinds of structural sites in zeolites, and that these results make sense in terms of local site energetics. For example, Al-O-Si oxygens exchange faster than Si-O-Si oxygens. Further studies of site-specific exchange kinetics in zeolites and other silicates, such as sheet silicates, is continuing.

We have recently been making detailed studies of the extent of framework disorder in both natural and synthetic zeolites, with the aim of better understanding their thermodynamics and kinetics. We have used both "traditional" ²⁹Si MAS NMR and XRD, as well as very high-resolution ²⁷Al MAS NMR at fields as high as 21.1 Tesla and ¹⁷O triple and five-quantum MAS NMR. We’ve gained new insights into site occupancies and disorder in several groups of zeolites, that have led to improved models of their solid solution and temperature effects on their crystallization.

Pressure effects on silicate melt and crystal structure

We continue to be interested in the effects of pressure on the structure of molten and crystalline silicates. We’ve recently reported the first definitive evidence for SiO₅ groups in a high-P calcium silicate glass. Using the 18.8 T spectrometer and tiny (1 to 3 mg) simple-run samples from 26 GPa, we were able to clearly define the mechanism of solid solution of Al in MgSiO₃ perovskite grown from a stoichiometric glass composition.

We are using solid-state NMR, including in-situ, high T work to 1400 deg C, single crystal, and 2-D techniques to study the mechanisms of diffusion in silicates of geological and technological interest. High resolution techniques often allow different sites to be distinguished, and exchange rates among sites to be quantified. For the first time, we have been able to constrain specific diffusion pathways for cations such as Li⁺ and Mg²⁺ in materials such as lithium silicate ceramics and in olivine.

We are making several major efforts to extend the utility of solid-state NMR into new systems. The first of these is to experimentally calibrate NMR data such as chemical shifts and quadrupolar coupling constants with structural parameters such as bond length and coordination number in known crystal structures. This has opened up our ability to use nuclides such as ⁶Li, ¹¹B, ¹⁷O, ¹⁹F, ²³Na, ²⁵Mg, ³⁵Cl and ³⁹K to study materials with unknown structures, such as glasses and melts. We are also applying new 2-D techniques such as triple- and five-quantum MAS NMR to obtain new insights into glass and crystal structure. Finally, we are working to define the potential for "ultra-high" field NMR (currently meaning 18.8 and 21.1 Tesla and (?) above) to open new research areas. We have recently shown for example, that these fields provide unprecedented sensitivity and resolution for a wide range of nuclides, from ¹¹B to ¹⁷O and down to ³⁵Cl, ³⁹K, and even ⁷³Ge.