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Thermoelastic properties and crystal structure of CaPtO 3 post‐perovskite from 0 to 9 GPa and from 2 to 973 K
Author(s) -
LindsayScott Alex,
Wood Ian G.,
Dobson David P.,
Vočadlo Lidunka,
Brodholt John P.,
Knight Kevin S.,
Tucker Matthew G.,
Taniguchi Takashi
Publication year - 2011
Publication title -
journal of applied crystallography
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.429
H-Index - 162
ISSN - 1600-5767
DOI - 10.1107/s0021889811023582
Subject(s) - equation of state , orthorhombic crystal system , metastability , perovskite (structure) , ambient pressure , neutron diffraction , thermal expansion , debye model , thermodynamics , chemistry , debye , phase (matter) , crystallography , materials science , crystal structure , condensed matter physics , physics , organic chemistry
ABX3 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 θ 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 /γ′) = 3.16 (3) × 10 −17 J, where γ′ is a Grüneisen parameter. Combining the present measurements with heat‐capacity data gives a thermodynamic Grüneisen parameter γ = 1.16 (1) at 291 K. PPV‐CaPtO 3 , PPV‐MgSiO 3 and PPV‐CaIrO 3 have the same axial incompressibility sequence, κ c > κ a > κ b . However, when heated, CaPtO 3 shows axial expansion in the form α c > α b > α 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 α b > α a > α c is the same as 1/κ i ) and CaIrO 3 (where α b > α c > α 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.