Hydrogen solubility in FeSi alloy phases at high pressures and temperatures

1Suyu Fu,2Stella Chariton,2Vitali B. Prakapenka,3Andrew Chizmeshya,1Sang-Heon Shim
American Mineralogist 107, 2307-2314 Link to Article [http://www.minsocam.org/msa/ammin/toc/2022/Abstracts/AM107P2307.pdf]
1School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287, U.S.A.
2Center for Advanced Radiation Sources, University of Chicago, Chicago, Illinois 60637, U.S.
Copyright: The Mineralogical Society of America

Light elements alloying with metallic Fe can change the properties and therefore play a key role
in the structure and dynamics of planetary cores. Hydrogen and silicon are possible light elements in
the rocky planets’ cores. However, hydrogen storage in Fe-Si alloy systems remains unclear at high
pressures and high temperatures because of experimental difficulties. Taking advantage of pulsed laser
heating combined with high-energy synchrotron X‑ray diffraction, we studied reactions between FeSi
and H in laser-heated diamond-anvil cells (LHDACs) up to 61.9 GPa and 3500 K. We found that under
H-saturated conditions the amount of H alloying with FeSi (0.3 and <0.1 wt% for the B20 and B2 structures, respectively) is much smaller than that in pure Fe metal (>1.8 wt%). Our experiments also
suggest that H remains in the crystal structure of FeSi alloy when recovered to 1 bar. Further density
functional theory (DFT) calculations indicate that the low-H solubility likely results from the highly
distorted interstitial sites in the B20 and B2 structures, which are not favorable for H incorporation.
The recovery of H in the B20 FeSi crystal structure at ambient conditions could open up possibilities
to understand geochemical behaviors of H during core formation in future experiments. The low-H
content in FeSi alloys suggests that if a planetary core is Si-rich, Si can limit the ingassing of H into
the Fe-rich core.

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