Thermoelastic properties and crystal structure of CaPtO ₃ post-perovskite from 0 to 9 GPa and from 2 to 973 K

ABX ₃ post-perovskite (PPV) phases that are stable (or strongly metastable) at ambient pressure are important as analogues of PPV-MgSiO ₃, a deep-Earth phase stable only at very high pressure. The thermoelastic and structural properties of orthorhombic PPV-structured CaPtO ₃ have been determined to 9.27 GPa at ambient temperature and from 2 to 973 K at ambient pressure by time-of-flight neutron powder diffraction. The equation-of-state from this high-pressure study is consistent with that found by Lindsay-Scott, Wood, Dobson, Vočadlo, Brodholt, Crichton, Hanfland & Taniguchi [(2010). Phys. Earth Planet. Inter. 182, 113-118] using X-ray powder diffraction to 40 GPa. However, the neutron data have also enabled the determination of the crystal structure. The b axis is the most compressible and the c axis the least, with the a and c axes shortening under pressure by a similar amount. Above 300 K, the volumetric coefficient of thermal expansion, α(T), of CaPtO ₃ can be represented by α(T) = a ₀ + a ₁(T), with a ₀ = 2.37 (3)× 10 ^-5 K ^-1 and a ₁ = 5.1 (5)× 10 ^-9 K ^-2. Over the full range of temperature investigated, the unit-cell volume of CaPtO ₃ can be described by a second-order Grüneisen approximation to the zero-pressure equation of state, with the internal energy calculated via a Debye model and parameters D (Debye temperature) = 615 (8) K, V ₀ (unit-cell colume at 0 K) = 227.186 (3) Å ³, K′0 (first derivative with respect to pressure of the isothermal incompressibility K ₀) = 7.9 (8) and (V ₀ K ₀/′) = 3.16 (3) 10 ^-17 J, where ′ is a Grüneisen parameter. Combining the present measurements with heat-capacity data gives a thermodynamic Grüneisen parameter = 1.16 (1) at 291 K. PPV-CaPtO ₃, PPV-MgSiO ₃ and PPV-CaIrO ₃ have the same axial incompressibility sequence, c > a > b. However, when heated, CaPtO ₃ shows axial expansion in the form c > b > a , a sequence which is not simply the inverse of the axial incompressibilities. In this respect, CaPtO ₃ differs from both MgSiO ₃ (where the sequence b > a > c is the same as 1/i ) and CaIrO ₃ (where b > c > a ). Thus, PPV-CaPtO ₃ and PPV-CaIrO ₃ are better analogues for PPV-MgSiO ₃ in compression than on heating. The behaviour of the unit-cell axes of all three compounds was analysed using a model based on nearest-neighbour B-X and A-X distances and angles specifying the geometry and orientation of the BX 6 octahedra. Under pressure, all contract mainly by reduction in the B-X and A-X distances. On heating, MgSiO ₃ expands (at high pressure) mainly by lengthening of the Si-O and Mg-O bonds. In contrast, the expansion of CaPtO ₃ (and possibly also CaIrO ₃), at atmospheric pressure, arises more from changes in angles than from increased bond distances.