Kelton is working to develop a better understanding of the principles governing phase formation and stability in condensed phases and the relations between phase transitions and atomic structures. Studies are focused on nucleation and growth processes in condensed phases, the relations between local atomic structure and the nucleation barrier, the coupling of phase transitions of different order, the glass transition, and the formation of metallic glasses and their crystallization to consolidated nanostructured materials. A new kinetic theory for nucleation, developed in this group, has led to an improved understanding of nucleation processes where long- range diffusion is important, such as for solid-state precipitation.

The structure and formation of quasicrystals, a unique form of condensed matter discovered in 1984, have long been of interest to this group. Icosahedral quasicrystals are ordered phases that produce sharp diffraction patterns, indicative of long- range order, but have a symmetry that is inconsistent with the translational periodicity that is characteristic of crystal phases. Most of the known titanium-, zirconium-, and hafnium- based quasicrystals have been discovered in Kelton’s laboratory. One of these, Ti₄₅Zr₃₈Ni₁₇, is the only known stable quasicrystal at low temperatures and might be the first case of a quasiperiodic ground state. These quasicrystals can also store significant quantities of hydrogen, making them of potential interest for hydrogen storage and battery applications.

Metallic glasses, like more common silicate glasses, are amorphous, containing no long-range translational order but significant short- and medium-range order. Kelton uses a wide range of experimental techniques to study the order in these glasses and in related transition metal alloy liquids. Working in collaboration with researchers at NASA MarshallSpace Flight Center and the Advanced Photon Source, he has recently developed a new technique for studying the structures of reactive metal alloys at temperatures up to 3000 K. This has led to the first proof of a 50-year-old hypothesis linking the liquid structure to the nucleation barrier and has improved understanding of the atomic structures of equilibrium and supercooled liquids. It has also provided new information on liquid-liquid phase transitions. The construction of a levitation facility that is optimized for structural and thermophysical property studies of equilibrium and nonequilibrium liquids has recently been constructed at Washington University. Through several national and international collaborations, new techniques are also used for structural investigations of the metallic glasses, including fluctuation electron microscopy and 3-D atom probe. These provide complementary information to structural transmission electron microscopy (TEM), energy- filtered TEM, and EXAFS studies, and kinetic calorimetry and resistivity measurements made at Washington University and in collaboration with researchers at other institutions.

Professor Patrick Gibbons’ research group uses the TEM with its electron- energy-loss and energy dispersive X-ray spectrometers to investigate a wide variety of materials.

In collaboration with members of the McDonnell Center, the group is studying diamonds of extraterrestrial origin found in meteorites, asking whether they differ from terrestrial diamonds in any measurable way other than their small (2 nm) crystal size. A collaborative group, including members of the Department of Chemistry in Arts & Sciences, is examining semiconductor nanoparticles, nanorods, and nanowires synthesized by simple processes with potential applications in optical computing.

Working with Kelton’s group, Gibbons studies amorphous metal alloys to detect and characterize medium-range (1–2 nm) order. Measurements are made using the new, high-resolution, scanning TEM. Data are analyzed by simulating the measurements using programs written by Gibbons and realistic structural models constructed by Kelton’s group.