We have identified by first-principles computations two dynamically stable structures that are candidate ultrahigh pressure polymorphs of AB X3 -type compounds. To our knowledge, they have not been experimentally observed yet. They are produced by metastable pressure-induced transformations in Cmcm NaMg F3, a postperovskite phase. The first transition to a Pmcn structure is related to a soft phonon mode in postperovskite. The second one is a regular enthalpically driven transition from Pmcn to a P 63/mmc structure. In NaMg F3 these phases are metastable with respect to the dissociation into CsCl-type NaF and cotunnite-type Mg F2. However, other AB X3 -type compounds may favor these phases over the dissociation products. Even in NaMg F3, the Pmcn phase might be observed at low temperatures.
|Original language||English (US)|
|Journal||Physical Review B - Condensed Matter and Materials Physics|
|State||Published - 2006|