Spin crossover in (Mg,Fe${}^{3+}$)(Si,Fe${}^{3+}$)O${}_3$ bridgmanite: effects of disorder, iron concentration, and temperature

Abstract

The spin crossover of iron in Fe${}^{3+}$-bearing bridgmanite, the most abundant mineral of the Earth's lower mantle, is by now a well-established phenomenon, though several aspects of this crossover remain unclear. Here we investigate effects of disorder, iron concentration, and temperature on this crossover using ab initio LDA + U${}_{sc}$ calculations. The effect of concentration and disorder are addressed using complete statistical samplings of coupled substituted configurations in super-cells containing up to 80 atoms. Vibrational/thermal effects on the crossover are addressed within the quasiharmonic approximation. The effect of disorder seems quite small, while increasing iron concentration results in considerable increase in crossover pressure. Our calculated compression curves for iron-free, Fe${}^{2+}$-, and Fe${}^{3+}$-bearing bridgmanite compare well with the latest experimental measurements. The comparison also suggests that in a close system, Fe${}^{2+}$ present in the sample may transform into Fe${}^{3+}$ by introduction of Mg and O vacancies with increasing pressure. As in the spin crossover in ferropericlase, this crossover in bridgmanite is accompanied by a clear volume reduction and an anomalous softening of the bulk modulus throughout the crossover pressure range. These effects reduce significantly with increasing temperature. Though the concentration of [Fe${}^{3+}$]${}_{Si}$ in bridgmanite may be small, related elastic anomalies may impact the interpretation of radial and lateral velocity structures of the Earth's lower mantle.

Under review with Earth and Planetary Science Letters

References in corpus (5)

• Density functional theory in transition-metal chemistry: a self-consistent Hubbard U approach
• Anomalous compressibility of ferropericlase throughout the iron spin crossover
• Thermoelasticity of Fe2+-bearing bridgmanite
• First-principles study of intermediate-spin ferrous iron in the Earth's lower mantle
• Origin of Fe3+ in Fe-containing, Al-free Mantle Silicate Perovskite

Cited by in corpus (6)

• Advanced capabilities for materials modelling with Quantum ESPRESSO
• Hubbard-corrected density functional perturbation theory with ultrasoft pseudopotentials
• Influence of the iron spin crossover in ferropericlase on the lower mantle geotherm
• Phonon dispersions throughout the iron spin crossover in ferropericlase
• Unconventional iron-magnesium compounds at terapascal pressures
• Bullen's parameter as a seismic observable for spin crossovers in the lower mantle