Lindsay-Scott, A.; (2011) The thermoelastic properties of post-perovskite analogue phases. Doctoral thesis , UCL (University College London).

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Abstract

Post-perovskite MgSiO3 is a major component of the D'' zone at the base of the lower mantle, so knowledge of its physical properties is essential to understanding mantle dynamics. Unfortunately, MgSiO3 post-perovskite is stable only at Mbar pressures. This difficulty can be addressed by combining computer simulations with experiments on analogue post-perovskite ABX3 phases, stable at ambient pressure and temperature. In this project, the properties of MgSiO3 and other oxide and fluoride post-perovskites were simulated using both athermal and molecular dynamics ab initio methods. Correspondence with experiment was generally good, although better in compression than when heated. Because few ABO3 oxides with this structure are known, an empirical approach was developed to predict whether post-perovskite phases would occur for given A, B and X elements, and to estimate the stabilisation pressures required. Only CaIrO3 forms at ambient pressure; single crystals were prepared for structural measurements, but the extremely high absorption made X-ray diffraction unreliable. Powdered CaPtO3 was synthesised at high-pressure and its thermoelastic properties and structure determined at high-pressure and from low to high-temperature by neutron powder diffraction; its isothermal equation of state was determined by Xray powder diffraction. CaPtO3 (and CaIrO3 as measured previously) have the same axial compression sequence (k_^c > k_^a > k_^b) as MgSiO3, but all show different axial expansion at high-temperature. Across D'' significant changes in the physical properties of MgSiO3 post-perovskite are likely to arise from changes in temperature as well as from changes in pressure; the differences in axial expansion therefore suggest that CaPtO3 and CaIrO3 may not be suitable analogues. Computer simulations suggest that ABF3 compounds may provide better analogues for MgSiO3 postperovskite. A new post-perovskite NaNiF3 has been synthesised at 15 GPa and its unit cell parameters determined; further experiments on these fluorides are the subject of ongoing work.

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