Structural and thermodynamic properties of compressed palladium: Ab initio and molecular dynamics study

First-principles and classical molecular dynamics simulations have been performed to study the structural and thermodynamic properties of Pd under pressure. By comparing the Gibbs free energy, in the quasiharmonic approximation (QHA), of the face-centered cubic (fcc) phase with those of the hexagonal-close-packed (hcp) and body-centered-cubic (bcc) phases we found that the fcc phase is stable up to 500 GPa and 5000 K. The predicted high-temperature elastic constants of fcc Pd agree well with experiments. The phonon dispersion curves are obtained at various pressures. In contrast with experiments we did not observe any phonon anomalies in Pd. We reproduced the thermodynamic properties of Pd accurately by taking into account the electron and phonon contributions to the free energy of Pd. The obtained thermal expansion coefficient, Hugoniot curves, and specific heat capacity compare well with experiments. In particular, the excellent agreement of the thermal expansion coefficients with experiment supports the validity of the QHA for Pd at high temperatures. Our QHA-based Hugoniot curves also show good agreement with experiments and our dynamic shock simulations. Shocks along [100] produced a melting temperature with a superheating of 18.3% at 226 GPa, compared with our high-pressure melting curve of Pd from coexistence-phase simulations based on an embedded atom model.