TY - JOUR
T1 - Optical signatures of low spin Fe3+ in NAL at high pressure
AU - Lobanov, Sergey S.
AU - Hsu, Han
AU - Lin, Jung Fu
AU - Yoshino, Takashi
AU - Goncharov, Alexander F.
N1 - Publisher Copyright:
©2017. American Geophysical Union. All Rights Reserved.
PY - 2017/5/1
Y1 - 2017/5/1
N2 - 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.
AB - 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.
KW - crystal field splitting
KW - diamond anvil cell
KW - lower mantle
KW - new aluminous phase
KW - optical absorption
KW - spin transition
UR - http://www.scopus.com/inward/record.url?scp=85019883071&partnerID=8YFLogxK
U2 - 10.1002/2017JB014134
DO - 10.1002/2017JB014134
M3 - 期刊論文
AN - SCOPUS:85019883071
SN - 2169-9313
VL - 122
SP - 3565
EP - 3573
JO - Journal of Geophysical Research: Solid Earth
JF - Journal of Geophysical Research: Solid Earth
IS - 5
ER -