The $5d^5$ iridate CaIrO${}_3$ is isostructural with the post-perovskite phase of MgSiO${}_3$, recently shown to occur under extreme pressure in the lower Earth's mantle. It therefore serves as an analog of post-perovskite MgSiO${}_3$ for a wide variety of measurements at ambient conditions or achievable with conventional multianvile pressure modules. By multireference configuration-interaction calculations we here provide essential information on the chemical bonding and magnetic interactions in CaIrO${}_3$. We predict a large antiferromagnetic superexchange of 120 meV along the $c$ axis, the same size with the interactions in the cuprate superconductors, and ferromagnetic couplings smaller by an order of magnitude along $a$. CaIrO${}_3$ can thus be regarded as a $j=1/2$ quasi-one-dimensional antiferromagnet. While this qualitatively agrees with the stripy magnetic structure proposed by resonant x-ray diffraction, the detailed microscopic picture emerging from our study, in particular, the highly uneven admixture of $t_{2g}$ components, provides a clear prediction for resonant inelastic x-ray scattering experiments.

• Received 18 May 2012

DOI:https://doi.org/10.1103/PhysRevB.85.235147