The isothermal equation of state of CaPtO3 post-perovskite to 40 GPa

ABX3 post-perovskite phases that are stable (or strongly metastable) at room-pressure are of importance as analogues of post-perovskite MgSiO3, a deep-Earth phase stable only at very high pressure. Commonly, CaIrO3 has been used for this purpose, but it has been suggested that CaPtO3 might provide a better analogue. We have measured the isothermal incompressibility, at ambient temperature, of orthorhombic post-perovskite-structured CaPtO3 to 40 GPa by X-ray powder diffraction using synchrotron radiation. A third-order Birch–Murnaghan equation of state fitted to the experimental data yields V0 = 228.10(2) Å3, K0 = 168.2(8) GPa and K′0=4.51(6)K′0=4.51(6). Similar fits to the cube of each axis of the unit cell shows that the b-axis is the most compressible (b0 = 9.9191(5) Å, K0 = 123.3(5) GPa, K′0=2.37(3)K′0=2.37(3)); the a-axis (a0 = 3.12777(8) Å, K0 = 195.7(8) GPa, K′0=6.63(8)K′0=6.63(8)) and c-axis (c0 = 7.3551(4) Å, K0 = 192(2) GPa, K′0=12.2(3)K′0=12.2(3)) are both much stiffer and have almost identical incompressibilities when the material is close to ambient pressure, but the c-axis shows greater stiffening on compression. Comparison of these axial incompressibilities with those of CaIrO3 shows that CaPtO3 is slightly less anisotropic under compression (possibly because of the absence of Jahn-Teller distortion), suggesting that CaPtO3 may be a somewhat better analogue of MgSiO3.Our sample also contained minor amounts of a cubic CaxPt3O4 phase, for which the third-order Birch–Murnaghan equation-of-state parameters were found to be: V0 = 186.00(3) Å3, K0 = 213(1) GPa and K′0=4.9(1)K′0=4.9(1).