Potentially novel ultrahigh pressure form of ABX$_3$-type compounds*

By means of first-principles computations we have identified two new dynamically stable structures that are candidate ultra-high pressure forms of ABX$_3$-type compounds. To our knowledge, they have not been experimentally observed yet. They are produced by metastable pressure-induced transformations in $Cmcm$ NaMgF$_3$, a post-perovskite phase. The first transition to a $Pmcn$ structure is related to a soft phonon mode in post-perovskite. The second one is a regular enthalpically driven transition from $Pmcn$ to a $P6_3/mmc$ structure. In NaMgF$_3$ these phases are metastable with respect to the dissociation into CsCl-type NaF and cotunnite-type MgF$_2$. However, the $Pmcn$ phase might be observed at low temperatures. We have also identified a candidate post-perovskite material that prefers the $Pmcn$ phase over the dissociation into AX- and BX$_2$-type solids.