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