Spin crossovers in iron-bearing MgSiO₃ and MgGeO₃: Their influence on the post-perovskite transition

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 spin state changes in Fe²⁺ and Fe³⁺ in MgGeO₃ perovskite and post-perovskite. Owing to the larger ionic radius of germanium compared to silicon, germanate phases have larger unit cell volume and inter-atomic 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 A site. We showed that iron state transitions occur at particular average Fe-O bond-length (i.e., ~2.22 (1)Å and ~1.86 (1)Å for Fe²⁺ and Fe³⁺ substitutions, respectively) irrespective of mineral composition (silicate or germanate) or exchange-correlation functionals used in the calculation (LDA+U_sc or GGA+U_sc). Ferrous iron substitution decreases the perovskite to post-perovskite (PPv) transition pressure while coupled ferric iron substitution increases it noticeably.