Spectroscopic evidence for the Fe3+ spin transition in iron-bearing δ-AlOOH at high pressure

Journal Article · Mon Nov 01 00:00:00 EDT 2021 · American Mineralogist

DOI:https://doi.org/10.2138/am-2021-7541· OSTI ID:1839917

Su, Xiaowan ^[1]; Zhao, Chaoshuai ^[2]; Xu, Liangxu ^[2]; Lv, Chaojia ^[2]; Song, Xitong ^[2]; Ishii, Takayuki ^[3]; Xiao, Yuming ^[4]; Chow, Paul ^[4]; Sun, Qiang ^[1]; Liu, Jin ^[2]

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1. Peking Univ., Beijing (China)
2. Center for High Pressure Science and Technology Advanced Research (HPSTAR), Beijing (China)
3. Center for High Pressure Science and Technology Advanced Research (HPSTAR), Beijing (China); Univ. of Bayreuth (Germany)
4. Argonne National Lab. (ANL), Argonne, IL (United States)

δ-AlOOH has emerged as a promising candidate for water storage in the lower mantle and could have delivered water into the bottom of the mantle. To date, it still remains unclear how the presence of iron affects its elastic, rheological, vibrational, and transport properties, especially across the spin crossover. In this study, we conducted high-pressure X-ray emission spectroscopy experiments on a δ-(Al_0.85Fe_0.15) OOH sample up to 53 GPa using silicone oil as the pressure transmitting medium in a diamond-anvil cell. We also carried out laser Raman measurements on δ-(Al_0.85Fe_0.15)OOH and δ-(Al_0.52Fe_0.48)OOH up to 57 and 62 GPa, respectively, using neon as the pressure-transmitting medium. Evolution of Raman spectra of δ-(Al_0.85Fe_0.15)OOH with pressure shows two new bands at 226 and 632 cm^–1 at 6.0 GPa, in agreement with the transition from an ordered (P2₁nm) to a disordered hydrogen bonding structure (Pnnm) for δ-AlOOH. Similarly, the two new Raman bands at 155 and 539 cm^–1 appear in δ-(Al_0.52Fe_0.48)OOH between 8.5 and 15.8 GPa, indicating that the incorporation of 48 mol% FeOOH could postpone the order-disorder transition upon compression. On the other hand, the satellite peak (Kβ') intensity of δ-(Al_0.85Fe_0.15)OOH starts to decrease at ~30 GPa and it disappears completely at 42 GPa. That is, δ-(Al_0.85Fe_0.15)OOH undergoes a gradual electronic spin-pairing transition at 30–42 GPa. Furthermore, the pressure dependence of Raman shifts of δ-(Al_0.85Fe_0.15)OOH discontinuously decreases at 32–37 GPa, suggesting that the improved hydrostaticity by the use of neon pressure medium could lead to a relatively narrow spin crossover. Notably, the pressure dependence of Raman shifts and optical color of δ-(Al_0.52Fe_0.48)OOH dramatically change at 41–45 GPa, suggesting that it probably undergoes a relatively sharp spin transition in the neon pressure medium. Together with literature data on the solid solutions between δ-AlOOH and ε-FeOOH, we found that the onset pressure of the spin transition in δ-(Al,Fe)OOH increases with increasing FeOOH content. These results shed new insights into the effects of iron on the structural evolution and vibrational properties of δ-AlOOH. The presence of FeOOH in δ-AlOOH can substantially influence its high-pressure behavior and stability at the deep mantle conditions and play an important role in the deep-water cycle

• Research Organization: Argonne National Lab. (ANL), Argonne, IL (United States)
• Sponsoring Organization: USDOE National Nuclear Security Administration (NNSA); USDOE Office of Science (SC); National Key Research and Development Program of China; National Natural Science Foundation of China (NSFC)
• Grant/Contract Number: AC02-06CH11357; 2019YFA0708502; U1930401
• OSTI ID: 1839917
• Journal Information: American Mineralogist, Vol. 106, Issue 11; ISSN 0003-004X
• Publisher: Mineralogical Society of America
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
Iron-bearing o-AlOOH
Raman spectroscopy
X-ray emission spectroscopy
high pressure
spin transition