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

• Pressure induced iron state changes have been investigated in MgGeO3.
• DFT + U has been used to capture correlation of d-orbitals in iron.
• Iron state changes occur at a particular value of average Fe–O bond-length.
• Fe2+ decreases Pv to PPv transition pressure, while Fe3+ increases it substantially.

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 + Usc or GGA + Usc). Ferrous iron substitution decreases the perovskite to post-perovskite (PPv) transition pressure while coupled ferric iron substitution increases it noticeably.