Pressure Induced Iron Spin Crossover in MgGeO3 Perovskite and Post-perovskite

Abstract:

MgGeO₃-perovskite is known to be a low-pressure analog of MgSiO₃-perovskite in many respects, but especially in regard to the post-perovskite transition. As such, investigation of spin state changes in Fe-bearing MgGeO₃ might help to clarify some aspects of this type of state change in Fe-bearing MgSiO₃. Using DFT+U calculations, we have investigated pressure induced state changes in Fe-bearing MgGeO₃ perovskite and post-perovskite. Owing to the relatively larger atomic size of germanium compared to silicon, germanate phases have larger unit cell volume and interatomic distances than equivalent silicate phases at same pressures. As a result, all pressure induced state changes in iron occur at higher pressures in germanate phases than in the silicate ones, be it a spin state change or position change of (ferrous) iron in the perovskite cage. The effect of iron in the post-perovskite transition is also investigated.