High Pressure Phase Diagram of Beryllium from \emph{Ab Initio} Free Energy Calculations

TL;DR

This study uses first-principles simulations to map beryllium's phase diagram under high pressure, accurately predicting phase boundaries, melting behavior, and shock response, with implications for experimental ramp compression.

Contribution

It provides the first ab initio derived phase diagram of beryllium up to 1600 GPa, including melting line, phase boundaries, and shock Hugoniot, improving understanding of its high-pressure behavior.

Findings

Melting line fits Simon Glatzel equation, matching previous simulations below 6000 K.

Hcp-bcc phase boundary is underestimated by quasiharmonic approximation but aligns with phonon quasiparticle results.

Predicted triple point at 164.7 GPa and 4314 K, and shock Hugoniot agrees with experiments.

Abstract

We use first principles molecular dynamics simulations coupled to the thermodynamic integration method to study the hcp-bcc transition and melting of beryllium up to a pressure of 1600~GPa. We derive the melting line by equating solid and liquid Gibbs free energies, and represent it by a Simon Glatzel fit $T_m= 1564~\text{K} (1 + P/(15.6032 ~\text{GPa}))^{0.383}$, which is in good agreement with previous two-phase simulations below 6000~K. We also derive the hcp-bcc solid-solid phase boundary and show the quasiharmonic approximation underestimates the stability of the hcp structure, predicting lower transition pressures between hcp and bcc phases. However, our results are consistent with the stability regime predicted by the phonon quasiparticle method. We also predict that hcp-bcc-liquid triple point is located at 164.7~GPa and 4314~K. In addition, we compute the shock Hugoniot curve, and show that it is in good agreement with experiments, intersecting our derived melting curve at $\sim$235~GPa at 4900~K. Finally, we show that an isentropic compression path that intersects the melting curve at both low and high temperature in the liquid regime, can reappear in the solid after a gap as large as 7000~K. Therefore, we predict that a large section of the melting curve could be sampled, in principle, by a ramp compression experiment, where solid and liquid Be would coexist as the sample is compressed.