High-pressure structure and bonding in CaIrO₃: The structure model of MgSiO₃ post-perovskite investigated with time-of-flight neutron powder diffraction

The structure of CaIrO₃, (Cmcm) has been refined at high pressure and at low temperature using time-of-flight neutron powder diffraction data. Evidence supporting deviation from space group Cmcm to Cmc2, is inconclusive. As CaIrO₃ (Cmcm) unit-cell volume changes, refinements indicate deformation of cation-centered coordination polyhedra, rather than tilting. Structure models demonstrate Ca²⁺-ccntered polyhedra are an order of magnitude more compressible than Ir⁴⁺-centered octahedra. Bond valence sums show significant chemical strain (over-bonding) of calcium and oxygen at ambient conditions. Implications for structure change in MgSiO₃ post-perovskite are discussed and a method for predicting the Clapeyron slope between perovskite and post-perovskite phases is proposed based on extrapolation of the volume-ratio between cation-centered polyhedra.