First-principles study of intermediate-spin ferrous iron in the Earth's lower mantle

Hsu, Han; Wentzcovitch, Renata M.

doi:10.1103/physrevb.90.195205

Cited by 40 publications

(69 citation statements)

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“…Theoretical investigations indicate that the B ‐site Fe 3+ undergoes a transition from the high spin (HS) to the low spin (LS) state, while A site iron remains in the HS state, regardless of its valence [ Hsu and Wentzcovitch , ; Hsu et al ., , , ; Mohn and Trønnes , ; Yu et al ., ]. This theory‐based view has found experimental support [ Catalli et al ., , ; Lin et al ., ] but is not yet widely accepted [ Kupenko et al, ; McCammon et al ., ].…”

Section: Main Textmentioning

confidence: 99%

Optical signatures of low spin Fe³⁺ in NAL at high pressure

et al. 2017

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The iron spin transition directly affects properties of lower mantle minerals and can thus alter geophysical and geochemical characteristics of the deep Earth. While the spin transition in ferropericlase has been documented at P ~60 GPa and 300 K, experimental evidence for spin transitions in other rock‐forming minerals, such as bridgmanite and post‐perovskite, remains controversial. Multiple valence, spin, and coordination states of iron in bridgmanite and post‐perovskite are difficult to resolve with conventional spin probing techniques. Optical spectroscopy, on the other hand, can discriminate between high and low spin and between ferrous and ferric iron at different sites. Here we establish the optical signature of low spin Fe3+O6, a plausible low spin unit in bridgmanite and post‐perovskite, by optical absorption experiments in diamond anvil cells. We show that the optical absorption of Fe3+O6 in new aluminous phase (NAL) is very sensitive to the iron spin state and may represent a model behavior of bridgmanite and post‐perovskite across the spin transition. Specifically, an absorption band centered at ~19,000 cm−1 is characteristic of the 2T2g → 2T1g (2A2g) transition in low spin Fe3+ in NAL at 40 GPa, constraining the crystal field splitting energy of low spin Fe3+ to ~22,200 cm−1, which we independently confirm by first‐principles calculations. Together with available information on the electronic structure of Fe3+O6 compounds, we show that the spin‐pairing energy of Fe3+ in an octahedral field is ~20,000–23,000 cm−1. This implies that octahedrally coordinated Fe3+ in bridgmanite is low spin at P > ~40 GPa.

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Section: Main Textmentioning

confidence: 99%

Optical signatures of low spin Fe³⁺ in NAL at high pressure

et al. 2017

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“…High-pressure studies have observed the occurrence of the extremely high QS values of Fe 2+ in the A site (as high as~4.4 mm/s) and the partial decrease of the Fe Kβ satellite peak, which have been interpreted as evidence for an electronic spin transition or transitions in bridgmanite in the lower mantle [e.g., Badro et al, 2004;Li et al, 2004Li et al, , 2006 , 2008;Grocholski et al, 2009;McCammon et al, 2008McCammon et al, , 2010Catalli et al, 2010Catalli et al, , 2011Hsu et al, 2010Hsu et al, , 2011Fujino et al, 2012Fujino et al, , 2014. However, it has been shown in recent firstprinciple calculations that the extremely high QS is a result of the pressure-induced local lattice distortion [Bengtson et al, 2008[Bengtson et al, , 2009Hsu et al, 2010Hsu et al, , 2011Hsu et al, , 2012Hsu and Wentzcovitch, 2014]. However, it has been shown in recent firstprinciple calculations that the extremely high QS is a result of the pressure-induced local lattice distortion [Bengtson et al, 2008[Bengtson et al, , 2009Hsu et al, 2010Hsu et al, , 2011Hsu et al, , 2012Hsu and Wentzcovitch, 2014].…”

Section: Introductionmentioning

confidence: 99%

“…However, it has been shown in recent firstprinciple calculations that the extremely high QS is a result of the pressure-induced local lattice distortion [Bengtson et al, 2008[Bengtson et al, , 2009Hsu et al, 2010Hsu et al, , 2011Hsu et al, , 2012Hsu and Wentzcovitch, 2014]. It has also been argued that Fe 3+ in the A site with much lower QS should also remain in the high-spin state especially in Al-bearing bridgmanite, in which Fe 3+ is expected to be predominantly in the A site [e.g., Vanpeteghem et al, 2006;Hsu and Wentzcovitch, 2014]. Based on these results, Fe 2+ in the distorted A site preserves its high-spin electronic configuration in the lower mantle, instead of assuming the intermediate-spin state or the low-spin configuration suggested in some experimental studies.…”

Section: Introductionmentioning

confidence: 99%

High‐spin Fe²⁺ and Fe³⁺ in single‐crystal aluminous bridgmanite in the lower mantle

Lin

Mao

Yang

et al. 2016

Geophysical Research Letters

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Spin and valence states of iron in single‐crystal bridgmanite (Mg0.89Fe0.12Al0.11Si0.89O3) are investigated using X‐ray emission and Mössbauer spectroscopies with laser annealing up to 115 GPa. The results show that Fe predominantly substitutes for Mg2+ in the pseudo‐dodecahedral A site, in which 80% of the iron is Fe3+ that enters the lattice via the charge‐coupled substitution with Al3+ in the octahedral B site. The total spin momentum and hyperfine parameters indicate that these ions remain in the high‐spin state with Fe2+ having extremely high quadrupole splitting due to lattice distortion. (Al,Fe)‐bearing bridgmanite is expected to contain mostly high‐spin, A‐site Fe3+, together with a smaller amount of A‐site Fe2+, that remains stable throughout the region. Even though the spin transition of B‐site Fe3+ in bridgmanite was reported to cause changes in its elasticity at high pressures, (Fe,Al)‐bearing bridgmanite with predominantly A‐site Fe will not exhibit elastic anomalies associated with the spin transition.

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“…In contrast, both Fe 3+ and Fe 2+ in the A site have been reported to remain in the HS state throughout the entire lower-mantle pressure range (24-130 GPa) (Dorfman et al, 2015;Hsu et al, 2010Hsu et al, , 2011Lin et al, 2016). The extremely high quadrupole splitting has alternatively been interpreted as the occurrence of the intermediate-spin state in bridgmanite (Hsu & Wentzcovitch, 2014;Lin et al, 2008;McCammon et al, 2008). The extremely high quadrupole splitting has alternatively been interpreted as the occurrence of the intermediate-spin state in bridgmanite (Hsu & Wentzcovitch, 2014;Lin et al, 2008;McCammon et al, 2008).…”

Section: Resultsmentioning

confidence: 99%

Abnormal Elasticity of Fe‐Bearing Bridgmanite in the Earth's Lower Mantle

Yang

Zhang

et al. 2018

Geophysical Research Letters

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We measured the effect of pressure on the compressional and shear wave velocity (VP, VS) as well as density of Fe‐bearing bridgmanite, Mg0.96(1)Fe2+0.036(5)Fe3+0.014(5)Si0.99(1)O3, using impulsive stimulated light scattering, Brillouin light scattering, and X‐ray diffraction, respectively, in diamond anvil cells up to 70 GPa at 300 K. A drastic softening of VP by ~6(±1)% is observed between 42.6 and 58 GPa, while VS increases continuously with increasing pressure. A significant reduction in Poisson's ratio from 0.24 to 0.16 occurs at ~42.6–58 GPa, while VS increases by ~3(±1)% above ~40 GPa compared to MgSiO3‐bridgmanite. Thermoelastic modeling of the experimental results shows that the observed elastic anomaly of Fe‐bearing bridgmanite is consistent with a spin transition of octahedrally coordinated Fe3+ in bridgmanite. These results challenge traditional views that Fe enrichment will reduce seismic velocities, suggesting that seismic heterogeneities in the mid‐lower mantle may be due to a spin transition of Fe in Fe‐bearing bridgmanite.

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First-principles study of intermediate-spin ferrous iron in the Earth's lower mantle

Cited by 40 publications

References 80 publications

Optical signatures of low spin Fe³⁺ in NAL at high pressure

Optical signatures of low spin Fe³⁺ in NAL at high pressure

High‐spin Fe²⁺ and Fe³⁺ in single‐crystal aluminous bridgmanite in the lower mantle

Abnormal Elasticity of Fe‐Bearing Bridgmanite in the Earth's Lower Mantle

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First-principles study of intermediate-spin ferrous iron in the Earth's lower mantle

Cited by 40 publications

References 80 publications

Optical signatures of low spin Fe3+ in NAL at high pressure

Optical signatures of low spin Fe3+ in NAL at high pressure

High‐spin Fe2+ and Fe3+ in single‐crystal aluminous bridgmanite in the lower mantle

Abnormal Elasticity of Fe‐Bearing Bridgmanite in the Earth's Lower Mantle

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Optical signatures of low spin Fe³⁺ in NAL at high pressure

Optical signatures of low spin Fe³⁺ in NAL at high pressure

High‐spin Fe²⁺ and Fe³⁺ in single‐crystal aluminous bridgmanite in the lower mantle