Solid Hydrogen at Ultra High Pressure

Abstract

In recent years the diamond anvil cell (DAC) technique has been developed to statically compress matter, with pressures reaching into the hundreds of gigapascals (100 GPa = 1 megabar). One of the most interesting substances to study under these conditions is solid hydrogen, which is predicted to have a number of phase transitions, including the insulator-metal (IM) transition. At very high pressures of order 300–400 GPa, hydrogen is predicted to become a metallic atomic solid (Wigner and Huntington, 1935; Ceperley and Alder, 1987). However, it is expected that hydrogen first becomes a metal at somewhat lower pressures, within the molecular solid phase, by an electronic band overlap mechanism (Ramaker et al., 1975; Friedli and Ashcroft, 1977). Phase transitions in hydrogen have been studied by a number of methods, including Raman scattering and optical reflection and absorption. An unexpected new phase has been shown to exist for pressures above 149 GPa by study of the pressure-temperature phase line (Lorenzana, Silvera, and Goettel, 1989). This phase is called the hydrogen-A (H-A) phase and it is suspected, on a number of grounds that the H-A phase is metallic. However, at this time there is no direct evidence to support this interpretation. On the theoretical side calculations have predicted band gap closure, or metallization, at a pressure of 150–180 GPa for a structure with the hcp lattice with molecules oriented along the c-axis (Garcia et al., 1990). Again, uncertainties arise, as recent calculations find a different structure to have a lower energy and a larger gap and metallization pressure. In this article I shall discuss the recent rapid developments in this challenging area of research.