High-pressure phase diagram of the exp-6 model: The case of Xenon

TL;DR

This study maps the high-pressure, high-temperature phase diagram of Xenon using the exp-6 model, revealing direct freezing into FCC and a narrow BCC stability region at extreme conditions.

Contribution

It provides a detailed numerical phase diagram of Xenon with the exp-6 potential, including the discovery of a BCC phase at high temperatures.

Findings

Fluid freezes directly into FCC solid under compression

A narrow BCC phase is stable above 4500 K

HCP phase has similar chemical potential to FCC

Abstract

We investigated numerically the high-temperature/high-pressure phase diagram of Xenon as modelled through the exp-6 interaction potential, which is thought to provide a reliable description of the thermal behaviour of rare gases under extreme conditions. We performed a series of extensive NVT Monte Carlo simulations which, in conjunction with exact computation of the solid free energy by the Frenkel-Ladd method, allowed us to precisely locate the freezing and the melting thresholds at each temperature. We find that, under isothermal compression, the exp-6 fluid freezes directly into a FCC solid; however, above 4500 K, an intermediate BCC phase becomes stable in a narrow range of pressures. The chemical potential of the HCP phase does never significantly differ from that of the FCC solid of equal T and P, though the former is found to slightly overcome the latter. We discuss our results in the light of previous numerical studies of the same model system and of the experimental data available for Xenon.