Ab initio study of structural, elastic and thermodynamic properties of Fe3S at highpressure: Implications for planetary cores

1Karen Valencia,1,2Aldemar De Moya,3,4Guillaume Morard,5Neil L. Allan,1,5Carlos Pinilla
American Mineralogist 107, 248–256 Link to Article [http://www.minsocam.org/msa/ammin/toc/2022/Abstracts/AM107P0248.pdf]
1Departamento de Física y Geociencias, Universidad del Norte, Km 5 Via Puerto Colombia, Barranquilla, Colombia
2Departamento de Ciencias Naturales y Exactas, Universidad de la Costa, Calle 58 No. 55-66, Barranquilla, Colombia
3Sorbonne Université, Institut de Minéralogie, de Physique des Matériaux, et de Cosmochimie (IMPMC), UMR CNRS 7590, IRD,Muséum National d’Histoire Naturelle, Paris, France
4Université Grenoble Alpes, CNRS, ISTerre, F-38000 Grenoble
5School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, U.K.
Copyright: The Mineralogical Society of America

Using density functional theory electronic structure calculations, the equation of state, thermody-
namic and elastic properties, and sound wave velocities of Fe3S at pressures up to 250GPa have been
determined. Fe3S is found to be ferromagnetic at ambient conditions but becomes non-magnetic at
pressures above 50 GPa. This magnetic transition changes thec/a ratio leading to more isotropic com-
pressibility, and discontinuities in elastic constants and isotropic sound velocities. Thermal expansion,
heat capacity, and Grüneisen parameters are calculated at high pressures and elevated temperatures
using the quasiharmonic approximation. We estimate Fe-Fe and Fe-S force constants, which vary with
Fe environment, as well as the 56Fe/54Fe equilibrium reduced partition function in Fe3S and compare
these results with recently reported experimental values. Finally, our calculations under the conditions
of the Earth’s inner core allow us to estimate a S content of 2.7wt% S, assuming the only components
of the inner core are Fe and Fe 3S, a linear variation of elastic properties between end-members Fe
and Fe3S, and that Fe3S is kinetically stable. Possible consequences for the core-mantle boundary of
Mars are also discussed.

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