AbInitio Phase Diagram of Gold in Extreme Conditions.

Abstract

A phase diagram of gold is proposed in the [0; 1000]GPa and [0; 10 000]K ranges of pressure and temperature, respectively, topologically modified with respect to previous predictions. Using finite-temperature abinitio simulations and nonequilibirum thermodynamic integration, both accelerated by machine learning, we evaluate the Gibbs free energies of three solid phases previously proposed. At room temperature, the face-centered cubic (fcc) phase is stable up to ∼500 GPa whereas the body-centered cubic (bcc) phase only appears above 1TPa. At higher temperature, we do not highlight any fcc-bcc transition line between 200 and 400GPa, in agreement with ramp-compressed experiments. The present results only disclose a bcc domain around 140-235GPa and 6000-8000K, consistent with the triple point recently found in shock experiments. We demonstrate that this re-stabilization of the bcc phase at high temperature is due to anharmonic effects.