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

TL;DR

This study uses DFT+U calculations to analyze how pressure-induced spin state changes of iron in MgGeO$_3$ differ from MgSiO$_3$, revealing that larger germanate phases require higher pressures for similar transitions, impacting phase stability.

Contribution

It provides new insights into the pressure dependence of iron spin states in germanate versus silicate phases, highlighting the effects of atomic size and iron substitution on phase transitions.

Findings

Iron spin transitions occur at higher pressures in MgGeO$_3$ than in MgSiO$_3$.

Fe substitution influences the pressure of the perovskite to post-perovskite transition.

Iron state changes are linked to specific Fe-O bond lengths regardless of mineral composition.

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.