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

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, KT0 = 168. 4(3); K′T0 = 4. 48(3) (both at 298 K); ∂KT0/∂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, δT , is 7. 6(7) at 298 K. ∂KT0/∂T for CaPtO3 is similar to that recently reported for CaIrO3, differing significantly from values found at high pressure for MgSiO 3 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 ∂K aT0 /∂T = -0. 038(4) GPa K -1 ; ∂K bT0 /∂T = -0. 021(2) GPa K -1 ; ∂K cT0 /∂T = -0. 026(5) GPa K -1 , with $ T = 8. 9(9), 7. 4(7) and 4. 6(9) for a, b and c, respectively. Although K T0 is lowest for the b-axis, its incompressibility is the least temperature dependent.