Computational study of the pressure behavior of post-perovskite phases

The recent discovery of the post-perovskite phase transition (CaIrO$_3$ structure) in MgSiO$_3$ has lead to theoretical and experimental investigations of silicates, germanates and oxides that could take this structure. We have employed density functional-theory to explore a series of new compounds with the post-perovskite structure under pressure. We analyze the effects of the Si substitution by tetravalent cations on the perovskite-to-post-perovskite transition and on the crystal structure of post-perovskite. Cations Ti$^{4+}$ and Zr$^{4+}$ prefer the post-perovskite structure. We also explore the sesquioxides Al$_2$O$_3$ and Rh$_2$O$_3$ and compare their structural evolution with the one of MgSiO$_3$. For Rh$_2$O$_3$ we observe an enhancement of the ionic character of the type II structure with pressure.