The structure of CaIrOb>3b> (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 Cmc2b>1b> is inconclusive. As CaIrOb>3b> (Cmcm) unit-cell volume changes, refinements indicate deformation of cation-centered coordination polyhedra, rather than tilting. Structure models demonstrate Ca2+-centered 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 MgSiOb>3b> 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.
