Ab Initio Phase Diagram of Gold in Extreme Conditions

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 finitetemperature ab initio 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 1 TPa. At higher temperature, we do not highlight any fcc-bcc transition line between 200 and 400 GPa, in agreement with ramp-compressed experiments. The present results only disclose a bcc domain around 140-235 GPa and 6000-8000 K, 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.