Identification of post-pyrite phase transitions in SiO₂ by a genetic algorithm

Using a first-principles genetic algorithm we predict an Fe₂P phase is the first post-pyrite phase of SiO₂ at low temperatures. This contrasts with a recently predicted cotunnite phase. Static enthalpy differences between these two phases are small near the transition pressure (0.69 TPa). While quasiharmonic free energy calculations predict an Fe ₂P-→cotunnite-type transition with increasing temperature, another phase, NbCoB type, is identified as being structurally and energetically intermediate between Fe₂P and cotunnite phases. This structure suggests a possible temperature-induced gradual transformation between Fe ₂P and cotunnite phases. This finding would change our understanding of how planet-forming silicates, for example, MgSiO₃ post-perovskite and its solid solutions, dissociate into elementary oxides at thermodynamic conditions expected in the interior of solar giants and exoplanets.