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

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 ²T_2g → ²T_1g (²A_2g) 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.