Iron spin crossover and its influence on post-perovskite transitions in MgSiO${}_3$ and MgGeO${}_3$

Abstract

MgGeO${}_3$-perovskite is known to be a low-pressure analog of MgSiO${}_3$-perovskite in many respects, but especially in regard to the post-perovskite transition. As such, investigation of spin state changes in Fe-bearing MgGeO${}_3$ might help to clarify some aspects of this type of state change in Fe-bearing MgSiO${}_3$. Using DFT+U calculations, we have investigated pressure induced spin state changes in Fe${}^{2+}$ and Fe${}^{3+}$ in MgGeO${}_3$ perovskite and post-perovskite. Owing to the relatively larger atomic size 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 cage. We showed that iron state transitions occur at particular average Fe-O bond-length irrespective of mineral composition (silicate or germanate) or functionals (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.

Under Review with Physics of the Earth and Planetary Interiors

References in corpus (2)

• Density functional theory in transition-metal chemistry: a self-consistent Hubbard U approach
• First-principles study of intermediate-spin ferrous iron in the Earth's lower mantle