$Cmcm$ $\mathrm{Ca}\mathrm{Ir}{\mathrm{O}}_3$ is isostructural to the newly discovered high-pressure postperovskite polymorph of $\mathrm{Mg}\mathrm{Si}{\mathrm{O}}_3$. Due to the high transition pressure in $\mathrm{Mg}\mathrm{Si}{\mathrm{O}}_3$ over $100\mathrm{GPa}$, low-pressure analoglike $\mathrm{Ca}\mathrm{Ir}{\mathrm{O}}_3$ 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 $\mathrm{Ca}\mathrm{Ir}{\mathrm{O}}_3$ phases and compare them with the results reported for $\mathrm{Mg}\mathrm{Si}{\mathrm{O}}_3$ phases in detail. Local spin density approximation produces a Pauli-paramagnetic metallic ground state for $Pbnm$ $\mathrm{Ca}\mathrm{Ir}{\mathrm{O}}_3$, while a very weak but finite antiferromagnetic moment appears in $Cmcm$ phase at $0\mathrm{GPa}$. We also find that each component of the normalized elastic stiffness constants of $\mathrm{Ca}\mathrm{Ir}{\mathrm{O}}_3$ is largely discrepant from that of $\mathrm{Mg}\mathrm{Si}{\mathrm{O}}_3$ particularly in shear components, though $\mathrm{Ca}\mathrm{Ir}{\mathrm{O}}_3$ shows the compression behavior roughly similar to $\mathrm{Mg}\mathrm{Si}{\mathrm{O}}_3$. Those contrasts in shear constants cause the different shear wave anisotropy style. Results suggest that at least regarding shear wave polarization anisotropy, $\mathrm{Ca}\mathrm{Ir}{\mathrm{O}}_3$ is unlikely to be a good low-pressure analog of $\mathrm{Mg}\mathrm{Si}{\mathrm{O}}_3$, and care must be taken when one extrapolates the elastic property of $Cmcm$ $\mathrm{Ca}\mathrm{Ir}{\mathrm{O}}_3$ to the $\mathrm{Mg}\mathrm{Si}{\mathrm{O}}_3$ postperovskite.

• Received 26 June 2007

DOI:https://doi.org/10.1103/PhysRevB.76.144119