Ab initiomelting curve of osmium

Abstract

The melting curve of osmium up to a pressure $P$ of 500 GPa is obtained from an extensive suite of ab initio quantum molecular dynamics (QMD) simulations using the Z method. The ab initio $P=0$ melting point of Os is $3370\ifmmode\pm\else\textpm\fi{}75$ K; this range encompasses all of the available data in the literature and corroborates the conclusion of J. W. Arblaster [Platinum Metals Rev. 49, 166 (2005)] that the melting temperature of pure Os is $3400\ifmmode\pm\else\textpm\fi{}50$ K and that the 3300 K typically quoted in the literature is the melting point of impure Os. The $T=0$ equation of state (EOS) of Os and the $P$ dependence of the optimized $c/a$ ratio for the hexagonal unit cell, both to pressures $\ensuremath{\sim}900$ GPa, are obtained in the ab initio approach as validation of its use. Although excellent agreement with the available experimental data ($P\ensuremath{\lesssim}80$ GPa) is found, it is the third-order Birch-Murnaghan EOS with ${B}_{0}^{\ensuremath{'}}=5$ rather than the more widely accepted ${B}_{0}^{\ensuremath{'}}=4$ that describes the QMD data to higher pressures, in agreement with the more recent experimental EOS by Godwal et al. The theoretical melting curve of Os obtained earlier by Joshi et al. is shown to be inconsistent with our QMD results, and the possible reason for this discrepancy is suggested. Regularities in the melting curves of Os and five other third-row transition metals (Ta, W, Re, Pt, Au) could be used to estimate the currently unknown melting curves of Hf and Ir.