Structure and elasticity of Cmcm CaIrO3 and their pressure dependences:: Ab initio calculations

Cmcm CaIrO3 is isostructural to the newly discovered high-pressure postperovskite polymorph of MgSiO3. Due to the high transition pressure in MgSiO3 over 100 GPa, low-pressure analoglike CaIrO3 is substantial to investigate physical properties of the postperovskite phase experimentally. Here we perform ab initio density functional calculations of structural and elastic properties of CaIrO3 phases and compare them with the results reported for MgSiO3 phases in detail. Local spin density approximation produces a Pauli-paramagnetic metallic ground state for Pbnm CaIrO3, while a very weak but finite antiferromagnetic moment appears in Cmcm phase at 0 GPa. We also find that each component of the normalized elastic stiffness constants of CaIrO3 is largely discrepant from that of MgSiO3 particularly in shear components, though CaIrO3 shows the compression behavior roughly similar to MgSiO3. Those contrasts in shear constants cause the different shear wave anisotropy style. Results suggest that at least regarding shear wave polarization anisotropy, CaIrO3 is unlikely to be a good low-pressure analog of MgSiO3, and care must be taken when one extrapolates the elastic property of Cmcm CaIrO3 to the MgSiO3 postperovskite.