Russell J. Hemley

Rus Hemley continues to explore the remarkable behavior of materials when they are subjected to extreme pressures, up to and beyond 300 GPa (3 Mbar), at temperatures ranging from millikelvins to above 5000 K. It is becoming increasingly clear that such studies have implications that span the physical sciences, from fundamental chemistry and physics, earth and planetary science, to materials science and high technology. Research within Hemley's group during the past year has continued to uncover new phenomena in molecular materials at very high pressures. For instance, the researchers pinpointed the transition pressure of H₂O-ice from its normal molecular form to its high-density nonmolecular state. They also found unusual quantum mechanical phenomena associated with this transition. These experiments, combined with related studies of hydrogen and methane, reveal the nature of materials that comprise the large planets of the solar system. Some of the group's other high-pressure studies of molecular systems include the study of superconductivity in organic conductors, and in situ monitoring of the organic reactions under hydrothermal conditions.

Hemley's group is also focusing on the oxides, silicates, and metals that form major constituents of our planet. Long thought to be a solved problem, the high-pressure behavior of silica continues to produce new surprises. Important advances have been made in both theory and experiment, including the evidence for new forms of silica that may exist deep within the mantle. Recent experimental data were used to develop a theory for silica's major high-pressure transition at 50 GPa; this provides a basis for evaluating the geophysical signature of free silica in the lower mantle. The pressure dependence of the stability of quartz -- the common form of silica found in the Earth's crust -- was tracked using light-scattering techniques. The experiment revealed the origin of the pressure-induced transformation of the material to a dense glass. The group had discovered this phenomenon in the 1980s. New x-ray diffraction and spectroscopic studies have uncovered novel phenomena in iron-bearing oxides and silicates, including electronic spin transitions and magnetic collapse. These latter studies also include the first direct measurements of the elasticity, texture, and flow properties of iron -- crucial information for understanding seismological observations of the Earth's core.

Most of the discoveries mentioned above were made possible by the group's ability to continually develop new techniques in high-pressure experimentation. Among the most important have been those involving high-intensity synchrotron radiation, such as new techniques for x-ray diffraction of materials at pressures and temperatures found at the Earth's core, x-ray fluorescence spectroscopy, and new high-pressure inelastic x-ray scattering. In fact, the group has launched a major program to build a high-pressure synchrotron x-ray facility at the Advanced Photon Source, Argonne National Laboratory to carry out these studies in a dedicated fashion. This new facility offers many opportunities and will complement the dedicated high-pressure synchrotron beamlines the group uses at the National Synchrotron Light Source at Brookhaven Lab, including the just-completed dedicated high-pressure synchrotron infrared beamline. This synchrotron work also complements technical advances in other areas. Some examples are high-pressure magnetic susceptibility and electrical conductivity techniques for investigating new superconductors and novel metals, and the development of new synthetic diamond anvils for compressing larger samples, reaching higher pressures, and performing an even wider range of measurements.