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

Karen Valencia,Neil L Allan,Guillaume Morard,Aldemar De Moya,Carlos Pinilla

doi:10.2138/am-2021-7268

Abstract

Abstract Using density functional theory electronic structure calculations, the equation of state, thermodynamic and elastic properties, and sound wave velocities of Fe3S at pressures up to 250 GPa 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 the c/a ratio leading to more isotropic compressibility, 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 2.7 wt% S, assuming the only components of the inner core are Fe and Fe3S, 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.

Highlights

The Earth’s core is composed of iron (Fe) alloyed with Ni (~5wt%) and lighter elements which account for a density deficit compared to pure Fe (~10 % for the liquid outer core and ~3% for the solid inner core) (Poirier, 1994)
In the Fe-S phase diagram, the intermediate compound Fe3S is stable from 21 GPa
Density functional theory (DFT) calculations were carried out using the Vienna ab-initio simulation package (VASP) (Kresse and Furthmüller, 1996), which expands the electronic wavefunction on a plane wave basis set and incorporates the projected augmented wave (PAW) method for pseudopotentials (Kresse and Joubert, 1999)

Summary

Introduction

The Earth’s core is composed of iron (Fe) alloyed with Ni (~5wt%) and lighter elements which account for a density deficit compared to pure Fe (~10 % for the liquid outer core and ~3% for the solid inner core) (Poirier, 1994). GPa. The variation with pressure for all three sites is very similar to those in calculations reported for the I4$ phase of Fe3P (Gu et al, 2014).

Results

Conclusion