Spin states and hyperfine interactions of iron in (Mg,Fe)SiO3 perovskite under pressure

a r t i c l e i n f o Keywords: spin crossover intermediate spin quadrupole splitting iron-bearing perovskite lower mantle With the guidance of first-principles phonon calculations, we have searched and found several metastable equilibrium sites for substitutional ferrous iron in MgSiO 3 perovskite. In the relevant energy range, there are two distinct sites for high-spin, one for low-spin, and one for intermediate-spin iron. Because of variable d-orbital occupancy across these sites, the two competing high-spin sites have different iron quadrupole splittings (QS). At low pressure, the high-spin iron with QS of 2.3–2.5 mm/s is more stable, while the high-spin iron with QS of 3.3–3.6 mm/s is more favorable at higher pressure. The crossover occurs between 4 and 24 GPa, depending on the choice of exchange-correlation functional and the inclusion of on-site Coulomb interaction (Hubbard U). Our calculation supports the notion that the transition observed in recent Mössbauer spectra corresponds to an atomic-site change rather than a spin-state crossover. Our result also helps to explain the lack of anomaly in the compression curve of iron-bearing silicate perovskite in the presence of a large change of quadrupole splitting, and provides important guidance for future studies of thermodynamic properties of this phase. Lower mantle, located between the depth of about 600 km and 2890 km, with pressures and temperatures varying from ∼23 to 135 GPa and ∼1900 to 4000 K, is the largest region of the Earth's interior. Two major constituents of this region are ferropericlase, (Mg, Fe)O, and iron-and aluminum-bearing magnesium silicate perov-skite, (Mg,Fe)(Si,Al)O 3 , with ∼35 vol.% and ∼ 62 vol.%, respectively. The valence and spin state of iron directly affect the transport, elastic, and rheological properties of the host phase, as reviewed by Lin and Tsuchiya (2008) and Hsu et al. (in press). Ferrous iron (Fe 2+) has six 3d electrons. Its spin state can thus be high-spin (HS), S = 2, intermediate-spin (IS), S = 1, or low-spin (LS), S = 0, where S denotes for the quantum number of total spin. While the pressure-induced spin-state crossover in ferropericlase observed experimentally (see Lin and Tsuchiya (2008) for a comprehensive review) has been confirmed to be a HS-to-LS crossover by first-O 3 still remains controversial. This is mainly due to the complexity of the iron-and aluminum-bearing perovskite. For example, the fraction and the site occupancy of ferric iron is still not well characterized. Various experimental techniques …