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–Grüneisen parameter, δ _T , 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 axial P–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 δ _T  = 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.