Multiphase equation of state for magnesium based on first-principles simulations

We perform first-principles simulations to construct a multiphase equation of state (EOS) for magnesium (Mg) up to a pressure-temperature (P-T) range of 600 GPa and 8000 K. The phases considered include the solid hexagonal close-packed (hcp) and body-centered cubic (bcc) phases and the liquid phase. The effects of lattice anharmonicity on the free energies of the solid phases are considered using the phonon quasiparticle method, which is a hybrid approach that combines first-principles molecular dynamics (FPMD) and harmonic phonons to address lattice anharmonicity. These effects are found to significantly increase the calculated hcp-bcc transition pressure at high temperatures by >10% at T >1000 K and >20% at T >1250 K, leading to an hcp-bcc phase line in good overall agreement with static measurements. The free energies of the liquid phase are fitted using the results of FPMD simulations of this phase and constrained by our previous melting curve calculated from FPMD using the solid-liquid coexistence method. The resulting melting curve from our EOS reproduces the anomalous behavior of reentrant melting above similar to 300 GPa from our previous calculations and shows better agreement with the results from shock experiments than with static measurements at high pressures. Our EOS places the hcp-bcc-liquid triple point at 25 GPa and 2030 K, higher in pressure and temperature than those from previous first-principles studies based on the quasiharmonic approximation (QHA). Comparisons are also made with other experimental data, including those from various static and shock experiments and generally indicate a high level of consistency.