NaMgF₃: A low-pressure analog of MgSiO₃

Using first principles computations we show that NaMgF₃ perovskite undergoes the same series of phase transitions as MgSiO₃ perovskite, that is, the postperovskite transition and the dissociation into CsCl-type NaF and cotunnite-type MgF₂. These fluorides also undergo the same series of phase transitions as MgO and SiO₂, the dissociation products of MgSiO₃. Since the phase transformations in NaMgF₃ are not accompanied by any soft mode, we compute quasi-harmonic free energies and the respective phase boundaries. They have positive and negative Clapeyron slopes, respectively, like MgSiO₃. However, the transition pressures in NaMgF₃ are much lower and could be easily achieved in diamond anvil experiments. Therefore NaMgF₃ should be a good low-pressure analog of MgSiO₃.