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Title: Optical signatures of low spin Fe 3+ in NAL at high pressure

Abstract

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 Fe 3+O 6, 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 Fe 3+O 6 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 2g2T 1g ( 2A 2g) transition in low spin Fe 3+ in NAL at 40 GPa, constraining the crystal fieldmore » splitting energy of low spin Fe 3+ to ~22,200 cm –1, which we independently confirm by first–principles calculations. Together with available information on the electronic structure of Fe 3+O 6 compounds, we show that the spin–pairing energy of Fe 3+ in an octahedral field is ~20,000–23,000 cm –1. Furthermore, this implies that octahedrally coordinated Fe 3+ in bridgmanite is low spin at P > ~40 GPa.« less

Authors:
ORCiD logo [1]; ORCiD logo [2]; ORCiD logo [3]; ORCiD logo [4]; ORCiD logo [5]
  1. Carnegie Inst. of Washington, Washington, DC (United States); Russian Academy of Sciences (RAS), Novosibirsk (Russia)
  2. National Central Univ., Taoyuan City (Taiwan)
  3. The Univ. of Texas at Austin, Austin, TX (United States); Center for High Pressure Science and Technology Advanced Research (HPSTAR), Shanghai (China)
  4. Okayama Univ.,Tottori (Japan)
  5. Carnegie Inst. of Washington, Washington, DC (United States); Institute of Solid State Physics, Hefei (China)
Publication Date:
Research Org.:
Argonne National Lab. (ANL), Argonne, IL (United States). Advanced Photon Source (APS)
Sponsoring Org.:
USDOE Office of Science (SC); National Science Foundation (NSF); U.S. Army Research
OSTI Identifier:
1368278
Grant/Contract Number:  
FG02-94ER14466; AC02-06CH11357
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Journal of Geophysical Research. Solid Earth
Additional Journal Information:
Journal Volume: 122; Journal Issue: 5; Journal ID: ISSN 2169-9313
Publisher:
American Geophysical Union
Country of Publication:
United States
Language:
ENGLISH
Subject:
58 GEOSCIENCES; optical absorption; crystal field splitting; spin transition; lower mantle; diamond anvil cell; new aluminous phase

Citation Formats

Lobanov, Sergey S., Hsu, Han, Lin, Jung -Fu, Yoshino, Takashi, and Goncharov, Alexander F. Optical signatures of low spin Fe3+ in NAL at high pressure. United States: N. p., 2017. Web. doi:10.1002/2017JB014134.
Lobanov, Sergey S., Hsu, Han, Lin, Jung -Fu, Yoshino, Takashi, & Goncharov, Alexander F. Optical signatures of low spin Fe3+ in NAL at high pressure. United States. doi:10.1002/2017JB014134.
Lobanov, Sergey S., Hsu, Han, Lin, Jung -Fu, Yoshino, Takashi, and Goncharov, Alexander F. Tue . "Optical signatures of low spin Fe3+ in NAL at high pressure". United States. doi:10.1002/2017JB014134. https://www.osti.gov/servlets/purl/1368278.
@article{osti_1368278,
title = {Optical signatures of low spin Fe3+ in NAL at high pressure},
author = {Lobanov, Sergey S. and Hsu, Han and Lin, Jung -Fu and Yoshino, Takashi and Goncharov, Alexander F.},
abstractNote = {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. Furthermore, this implies that octahedrally coordinated Fe3+ in bridgmanite is low spin at P > ~40 GPa.},
doi = {10.1002/2017JB014134},
journal = {Journal of Geophysical Research. Solid Earth},
number = 5,
volume = 122,
place = {United States},
year = {Tue May 02 00:00:00 EDT 2017},
month = {Tue May 02 00:00:00 EDT 2017}
}

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