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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.
Bibliographical noteFunding Information:
This work was supported primarily by grants NSF/EAR 1319368 and NSF/CAREER 1151738 . Computations were performed at the Minnesota Supercomputing Institute (MSI) and at the Blue Waters System at NCSA. We thank Han Hsu for helpful discussions.
© 2015 Elsevier B.V.
Copyright 2015 Elsevier B.V., All rights reserved.
- Ferrous and ferric iron
- Iron-spin crossover
- Pv to PPv transition
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