UCL Discovery

Abstract

The thermal equation of state (EOS) for platinum has been calculated to 300 GPa and 3000 K using ab initio molecular dynamics employing the local density approximation (LDA) and the projector augmentedwave methods (PAW). Direct ab initio molecular dynamics avoids the simplifying assumptions inherent in empirical treatments of thermoelasticity. A third-order Birch-Murnaghan equation EOS fitted to the 300 K data yielded an isothermal bulk modulus of B-TO=290.8 GPa and a pressure derivative of B'(T) =5.11, which are in better agreement with the measured values than those obtained by previous calculations. The high-temperature data were fitted to a thermal pressure EOS and a Mie-Griineisen-Debye EOS. The resulting calculated thermal expansion coefficient, alpha(0), temperature derivative of the isothermal bulk modulus, (partial derivative B-T/partial derivative T)v, and second temperature derivative of the pressure, (partial derivative 2P/partial derivative T2)v, were 1.94 x 10(-5) K-1, -0.0038 GPa K-1, and 1.7 x 10(-7) GPa(2) K-2, respectively. A fit to the Mie-Griineisen-Debye EOS yielded values for the Gruneisen parameter, gamma(0,) and its volume dependence parameter, q, of 2.18 and 1.75, respectively. An analysis of our data revealed a strong volume dependence of the thermal pressure of platinum. We also present a qualitative analysis of the effects of intrinsic anharmonicity from the calculated Griineisen parameter at high temperatures. (C) 2010 Elsevier Ltd. All rights reserved.