Calculations of high-pressure properties of beryllium: Construction of a multiphase equation of state

We describe the construction of a three-phase equation of state for elemental beryllium. The phases considered are: the ambient hcp phase, the high-temperature bcc phase, and the liquid. The free energies of the solid phases are constructed from cold, ion-thermal, and electron-thermal components derived from ab initio electronic structure-based calculations. We find that the bcc phase is unstable near ambient conditions and that even at high pressures at which the bcc phase is stable, the bcc-hcp energy barrier can be as small as a few hundred kelvins. The liquid free energy is based on a model of Chisolm and Wallace and is constrained by using the melt curve (determined by ab initio two-phase simulations) as a reference. The high-temperature plasma limit is addressed with an average-atom-in-jellium model. Comparisons to experimental results, both for the ambient hcp phase and for the phase diagram as a whole, are discussed.