The P–V–T equation of state of CaPtO3 post-perovskite

Abstract

Orthorhombic post-perovskite CaPtO3 is isostructural with post-perovskite MgSiO3, a deep-Earth phase stable only above 100 GPa. Energy-dispersive X-ray diffraction data (to 9.4 GPa and 1,024 K) for CaPtO3 have been combined with published isothermal and isobaric measurements to determine its P–V–T equation of state (EoS). A third-order Birch–Murnaghan EoS was used, with the volumetric thermal expansion coefficient (at atmospheric pressure) represented by α(T) = α0 + α1(T). The fitted parameters had values: isothermal incompressibility, \( K_{{T_{0} }} \) = 168.4(3) GPa; \( K_{{T_{0} }}^{\prime } \) = 4.48(3) (both at 298 K); \( \partial K_{{T_{0} }} /\partial T \) = −0.032(3) GPa K−1; α0 = 2.32(2) × 10−5 K−1; α1 = 5.7(4) × 10−9 K−2. The volumetric isothermal Anderson–Gruneisen parameter, δT, is 7.6(7) at 298 K. \( \partial K_{{T_{0} }} /\partial T \) for CaPtO3 is similar to that recently reported for CaIrO3, differing significantly from values found at high pressure for MgSiO3 post-perovskite (−0.0085(11) to −0.024 GPa K−1). We also report axialP–V–T EoS of similar form, the first for any post-perovskite. Fitted to the cubes of the axes, these gave \( \partial K_{{aT_{0} }} /\partial T \) = −0.038(4) GPa K−1; \( \partial K_{{bT_{0} }} /\partial T \) = −0.021(2) GPa K−1; \( \partial K_{{cT_{0} }} /\partial T \) = −0.026(5) GPa K−1, with δT = 8.9(9), 7.4(7) and 4.6(9) for a, b and c, respectively. Although \( K_{{T_{0} }} \) is lowest for the b-axis, its incompressibility is the least temperature dependent.