Accurate understanding and prediction of geological and technological processes depend on knowledge of the atomic-scale structure of the materials involved. Our main interests are in quantifying that structure at the short- to intermediate-range, particularly in the disordered crystalline, glassy, and molten silicates and oxides that are so common among Earth and high-tech materials. For example, the free energy of a molten silicate (needed to predict phase equilibria, whether the melt is lurking in a magma chamber beneath a volcano or bubbling away in an industrial-scale glass melting tank), as well as its viscosity (needed to predict flow rates), depend strongly on how disordered its various cations and anionic links are, and how these change with temperature. Density increases of melts at high pressure, which determine whether magmas rise or sink in the mantle and whether they can erupt through the crust, are controlled by changes in cation coordination numbers, bond angles and bond lengths. The stability, reactivity, and transport properties of crystalline solid solutions, whether in natural minerals or in ceramics for fuel cells, depend on the extent of real, short-range order/disorder, which is often different from the average structure detected by diffraction methods. The local structure in oxide glasses, for technologies ranging from optical data transmission, to flat-panel computer screens and cell phone displays, to fiber-reinforced composites, to coatings on microelectronic and optical systems, needs to be known to efficiently tailor properties to applications. We are working on, or have recently worked on, all of these kinds of problems.
One of the most versatile tools for quantifying the structure of disordered solid and molten materials is Nuclear Magnetic Resonance (NMR) spectroscopy, which gives information about local structure around isotopes of many elements of key interest in silicates and oxides (as well as many other materials, of course!), such as 1H, 11B, 13C, 17O, 19F, 27Al, 29Si, 45Sc, 89Y and numerous others. NMR can also provide uniquely detailed information about structural dynamics (e.g. cation site hopping, network bond rearrangment) at the seconds to microseconds time scales. We specialize in the application of solid-state NMR to these sorts of materials, including challenging in-situ, high-temperature spectroscopy. We operate two of our own spectrometers, at field strengths of 9.4 and 14.1 Tesla (400 and 600 MHz 1H frequencies), and routinely use higher field instruments elsewhere at Stanford (800 MHz instrument in the Stanford Magnetic Resonance Laboratory). In collaborative studies, we also use other structural methods as needed, including Raman and X-ray spectroscopies. Recent advances in NMR technology, such as fast-spinning MAS probes, higher and higher field magnets, and two-dimensional and double resonance methods, have greatly improved the resolution and thus the information content of spectra, and widened the types of problems that can be addressed. Along the way, it has become feasible to answer structural questions about elements at lower and lower concentrations, in smaller and smaller samples. This has recently opened the doorway to studies of the structural environments of even minor substitutents such as Al and P in tiny samples (ca. 1 to 10 mg) of minerals and glasses synthesized at pressures to >25 GPa in multianvil high pressure apparatus in the labs of collaborators. Improved sensitivity for smaller samples has also allowed detailed studies of thin-film oxide coatings (separated from their substrates) for electronic and optical devices.