Gibbs Free Energy Calculation of Beryllium at Megabar Pressures

Abstract:

Phase transitions play a key role in our understanding of mineral physics at high pressure. Beryllium, a metal that is widely used in space science, plasma physics, and nuclear science because of its high stiffness and high thermal conductivity, has triggered many investigations of its equation of state over past decades. It is used in internal confinement fusion applications and in diamond anvil cell experiments where it has been exposed to megabar pressures. At high pressure, it transforms to a body centered cubic (bcc) structure, and the transition pressure decreases with temperature. Anharmonic effects are strong, due to its light weight, which makes Be a challenging material to study. It is therefore worthwhile to study its phase diagram under extreme conditions. Here we present results from ab initio computer simulations of solid and liquid beryllium in combination with a thermodynamic integration technique to derive Gibbs free energies. We study the h.c.p., b.c.c., and liquid phases, explore anharmonic effects and make predictions for the melting line. We investigate beryllium under pressure and temperature conditions that are comparable to those in the cores for terrestrial planets. We discuss the differences between our results and quasi-harmonic methods and the effect of anharmonicity at high temperature.