The phase diagrams of KCaF and NaMgF by ab initio simulations.

ABF compounds have been found to make valuable low-pressure analogues for high-pressure silicate phases that are present in the Earth's deep interior and that may also occur in the interiors of exoplanets. The phase diagrams of two of these materials, KCaF and NaMgF, have been investigated in detail by static ab initio computer simulations based on density functional theory. Six ABF polymorphs were considered, as follows: the orthorhombic perovskite structure (GdFeO-type; space group ); the orthorhombic CaIrO structure (; commonly referred to as the "post-perovskite" structure); the orthorhombic SbS and LaS structures (both ); the hexagonal structure previously suggested in computer simulations of NaMgF (6/); the monoclinic structure found to be intermediate between the perovskite and CaIrO structures in CaRhO (2/). Volumetric and axial equations of state of all phases considered are presented. For KCaF, as expected, the perovskite phase is shown to be the most thermodynamically stable at atmospheric pressure. With increasing pressure, the relative stability of the KCaF phases then follows the sequence: perovskite → LaS structure → SbS structure → 6/ structure; the CaIrO structure is never the most stable form. Above about 2.6 GPa, however, none of the KCaF polymorphs are stable with respect to dissociation into KF and CaF. The possibility that high-pressure KCaF polymorphs might exist metastably at 300 K, or might be stabilised by chemical substitution so as to occur within the standard operating range of a multi-anvil press, is briefly discussed. For NaMgF, the transitions to the high-pressure phases occur at pressures outside the normal range of a multi-anvil press. Two different sequences of transitions had previously been suggested from computer simulations. With increasing pressure, we find that the relative stability of the NaMgF phases follows the sequence: perovskite → CaIrO structure → SbS structure → 6/ structure. However, only the perovskite and CaIrO structures are stable with respect to dissociation into NaF and MgF.