Russell J. Hemley

We present results of semi-empirical lattice dynamics calculations of the sound velocities in solid hydrogen under pressure based on the many-body intermolecular potential and first-principle density-functional theory (DFT). Both the sound velocities and elastic moduli are in excellent agreement with data from Brillouin scattering measurements while Silvera–Goldman and Hemley–Silvera–Goldman potentials tend to overestimate the sound velocity. It is shown that the stiffer is the potential the greater is overestimated the sound velocity. As was the case for equation of state and Raman-active lattice phonon calculations, the employed many-body potential works well for phases I and II (up to ∼140 GPa) while for higher pressures the use of the DFT is preferable.

Composite diamond anvils have been developed for high-pressure/high-temperature measurements of diamond anvil cells. The anvils are fabricated using single-crystal chemical vapor deposition (CVD) from previously used and/or slightly damaged anvils made of natural or synthetic diamond. These composite anvils can be fabricated to possess optical characteristics at least comparable to conventional diamond anvils, whereas the single-crystal CVD portion is more durable because of its enhanced toughness relative to natural diamond. The viability of such anvils is demonstrated in measurements on hydrogen at megabar pressures and high temperature.

The detailing of the intermolecular interactions in dense solid oxygen is essential for an understanding of the rich polymorphism and remarkable properties of this element at high pressure. Synchrotron inelastic x-ray scattering measurements of oxygen K-edge excitations to 38 GPa reveal changes in electronic structure and bonding on compression of the molecular solid. The measurements show that O ₂ molecules interact predominantly through the half-filled 1π _g * orbital _g * orbital in the low-pressure phases, leading to electron delocalization and ultimately intermolecular bonding between O ₂ molecules at the transition to the ε-phase. The ε-phase, which consists of (O ₂ ) ₄ clusters, displays the bonding characteristics of a closed-shell system. Increasing interactions between (O ₂ ) ₄ clusters develop upon compression of the ε-phase, and provide a potential mechanism for intercluster bonding in still higher-pressure phases.

There is a great interest in electronic transitions in hydrogen-rich materials under extreme conditions. It has been recently suggested that the group IVa hydrides such as methane (CH ₄ ), silane (SiH ₄ ), and germane (GeH ₄ ) become metallic at far lower pressures than pure hydrogen at equivalent densities because the hydrogen is chemically compressed in group IVa hydride compounds. Here we report measurements of Raman and infrared spectra of silane under pressure. We find that SiH ₄ undergoes three phase transitions before becoming opaque at 27–30 GPa. The vibrational spectra indicate the material transforms to a polymeric (framework) structure in this higher pressure range. Room-temperature infrared reflectivity data reveal that the material exhibits Drude-like metallic behavior above 60 GPa, indicating the onset of pressure-induced metallization.

It began some 400 years ago with a simple question: can water be compressed? At that time it was clear that gases were compressible, but liquids presented a greater experimental challenge. Francis Bacon and the Accademia del Cimento in Florence were the first to attempt an answer, by filling a metal sphere with water and sealing it with solder. Although able to indent the sphere by striking it with a hammer, the Florentines concluded that the volume lost was equal to that of the beads of water that forced their way through the insufficiently strong seal. Thus, they deduced that water is incompressible. A century later, however, John Canton overturned this finding with a more precise experiment, and the experimental science of high-pressure physics was set in motion.

National scientific user facilities are becoming increasingly available to many different scientific communities in a number of countries. There is a growing use of these facilities by Earth and environmental scientists to study a broad range of materials and processes under realistic P-T and environmental conditions at unprecedented levels of energy and spatial resolution and elemental and isotopic sensitivity. The results of these studies are providing new insights into biogeochemical processes operating at Earth's surface as well as petrological, geochemical, and geophysical processes in Earth's interior. The availability of national user facilities is changing scientific approaches and is leading to multidisciplinary studies that were not possible a decade ago.