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Publication Detail
Thermoelastic properties and crystal structure of CaPtO3 post-perovskite from 0 to 9 GPa and from 2 to 973 K
  • Publication Type:
    Journal article
  • Publication Sub Type:
    Article
  • Authors:
    Lindsay-Scott A, Wood IG, Dobson DP, Vocadlo L, Brodholt JP, Knight KS, Tucker MG, Taniguchi T
  • Publisher:
    WILEY-BLACKWELL
  • Publication date:
    10/2011
  • Pagination:
    999, 1016
  • Journal:
    J APPL CRYSTALLOGR
  • Volume:
    44
  • Print ISSN:
    0021-8898
  • Language:
    EN
  • Keywords:
    NEUTRON POWDER DIFFRACTION, THERMAL-EXPANSION, STRUCTURE MODEL, SINGLE-CRYSTAL, CAIRO3, MGSIO3, PHASE, COMPRESSION, MANTLE, STATE
  • Addresses:
    Wood, IG
    UCL
    Dept Earth Sci
    London
    WC1E 6BT
    England

    Nat Hist Museum
    London
    SW7 5BD
    England
Abstract
ABX3 post-perovskite (PPV) phases that are stable (or strongly metastable) at ambient pressure are important as analogues of PPV-MgSiO3, a deep-Earth phase stable only at very high pressure. The thermoelastic and structural properties of orthorhombic PPV-structured CaPtO3 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, Vocadlo, 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, alpha(T), of CaPtO3 can be represented by alpha(T) = a(0) + a(1)(T), with a(0) = 2.37 (3) x 10(-5) K-1 and a(1) = 5.1 (5) x 10(-9) K-2. Over the full range of temperature investigated, the unit-cell volume of CaPtO3 can be described by a second-order Gruneisen approximation to the zero-pressure equation of state, with the internal energy calculated via a Debye model and parameters theta(D) (Debye temperature) = 615 (8) K, V-0 (unit-cell colume at 0 K) = 227.186 (3) angstrom(3), K-0' (first derivative with respect to pressure of the isothermal incompressibility K-0) = 7.9 (8) and (V0K0/gamma') = 3.16 (3) x 10(-17) J, where gamma' is a Gruneisen parameter. Combining the present measurements with heat-capacity data gives a thermodynamic Gruneisen parameter gamma = 1.16 (1) at 291 K. PPV-CaPtO3, PPV-MgSiO3 and PPV-CaIrO3 have the same axial incompressibility sequence, kappa(c) > kappa(a) > kappa(b). However, when heated, CaPtO3 shows axial expansion in the form alpha(c) > alpha(b) > alpha(a), a sequence which is not simply the inverse of the axial incompressibilities. In this respect, CaPtO3 differs from both MgSiO3 (where the sequence alpha(b) > alpha(a) > alpha(c) is the same as 1/kappa(i)) and CaIrO3 (where alpha(b) > alpha(c) > alpha(a)). Thus, PPV-CaPtO3 and PPV-CaIrO3 are better analogues for PPV-MgSiO3 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 BX6 octahedra. Under pressure, all contract mainly by reduction in the B-X and A-X distances. On heating, MgSiO3 expands (at high pressure) mainly by lengthening of the Si-O and Mg-O bonds. In contrast, the expansion of CaPtO3 (and possibly also CaIrO3), at atmospheric pressure, arises more from changes in angles than from increased bond distances.
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Dept of Earth Sciences
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Dept of Earth Sciences
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Dept of Earth Sciences
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Dept of Earth Sciences
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