Lattice dynamics and thermal equation of state of platinum

Abstract

Platinum is widely used as a pressure calibration standard. However, the established thermal equation of state (EOS) has uncertainties especially in the high P-T range. We use density-functional theory to calculate the thermal equation of state of platinum up to 550 GPa and 5000 K. The static lattice energy was computed by using the linearized augmented plane-wave method with local-density approximation (LDA), Perdew-Burke-Ernzerhof, and the recently proposed Wu-Cohen functional. The electronic thermal free energy was evaluated using the Mermin functional. The vibrational part was computed within the quasiharmonic approximation using density-functional perturbation theory and pseudopotentials. Special attention was paid to the influence of the electronic temperature on the phonon frequencies. We find that, in overall, LDA results agree best with the experiments. Based on the density-functional theory calculations and the established experimental data, we develop a consistent thermal EOS of platinum as a reference for pressure calibration.