Calculations of the Krypton Phase Diagram and Novel Plasticity

Abstract

The phase diagram for Kr, as represented by the Tadah! two-body potential is shown to have face-centred cubic (fcc), hexahonal close packed (hcp), and body centred cubic (bcc) regions. It has been assembled by combining several methods: direct liquid--solid coexistence for the melt lines, Gibbs--Helmholtz integration and Clapeyron slopes for the bcc--fcc line, slab coexistence for the liquid--gas line, static zero-temperature relaxations for the crystals, and the quasiharmonic approximation for the low-temperature fcc--hcp windows. The bcc phase contains highly mobile ``greedy snake" defects, which suggests a reinterpretation of the melt-curve data: the anomaly observed may be due to the speckle method detecting the bcc-fcc boundary, not the melt curve. While pair potentials have limitations, comparison with a foundation MACE model shows that a more flexible machine-learned model does not necessarily improve matters if inappropriately trained.