Title: Optical signatures of low spin Fe³⁺ 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 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, whichmore » 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.« less

Authors:

Lobanov, Sergey S.  ^[1];

• Search DOE PAGES for author "Lobanov, Sergey S."
• Search DOE PAGES for ORCID "0000-0001-9728-2363"

Hsu, Han  ^[2];

• Search DOE PAGES for author "Hsu, Han"
• Search DOE PAGES for ORCID "0000-0003-1205-4770"

Lin, Jung -Fu  ^[3];

• Search DOE PAGES for author "Lin, Jung -Fu"
• Search DOE PAGES for ORCID "0000-0002-0163-5329"

Yoshino, Takashi  ^[4];

• Search DOE PAGES for author "Yoshino, Takashi"
• Search DOE PAGES for ORCID "0000-0002-5422-7396"

Goncharov, Alexander F.  ^[5]

• Search DOE PAGES for author "Goncharov, Alexander F."
• Search DOE PAGES for ORCID "0000-0002-6422-8819"

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+ Show Author Affiliations

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:

Tue May 02 00:00:00 EDT 2017

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:

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