Spin-state crossover and hyperfine interactions of ferric iron in MgSiO${}_3$ perovskite

Abstract

Using density functional theory plus Hubbard $U$ calculations, we show that the ground state of (Mg,Fe)(Si,Fe)O${}_3$ perovskite, a major mineral phase in the Earth's lower mantle, has high-spin ferric iron ($S=5/2$) at both the dodecahedral (A) and octahedral (B) site. As the pressure increases, the B-site iron undergoes a spin-state crossover to the low-spin state ($S=1/2$), while the A-site iron remains in the high-spin state. Our calculation shows that the B-site spin-state crossover in the pressure range of 40-70 GPa is accompanied by a noticeable volume reduction and an increase in quadrupole splitting, consistent with recent X-ray diffraction and Mössbauer spectroscopy measurements. The volume reduction leads to a significant softening in the bulk modulus, which suggests a possible source of seismic velocity anomalies in the lower mantle.

11 pages, 4 figures, 1 table

References in corpus (3)

• Quantum ESPRESSO: a modular and open-source software project for quantum simulations of materials
• Density functional theory in transition-metal chemistry: a self-consistent Hubbard U approach
• Anomalous compressibility of ferropericlase throughout the iron spin crossover

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