Phase stability and thermodynamic properties of FeS polymorphs

Umberto Terranova,Nora H De Leeuw

doi:10.1016/j.jpcs.2017.07.033

Umberto Terranova, Nora H De Leeuw

Open Access

https://doi.org/10.1016/j.jpcs.2017.07.033

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Abstract

We present a Hubbard-corrected density functional theory investigation of FeS polymorphs based on the quasi-harmonic theory of lattice vibrations. We show that the first temperature transition of troilite FeS cannot involve the MnP-type FeS phase, as sometimes reported in the literature, while the sequence of polymorphs in the pressure range 0–100 GPa at room temperature supports the experimental observations. Although with some differences in the critical pressures, our ab initio phase diagram is in line with those derived from X-ray diffraction studies. The thermodynamic properties of troilite FeS are in good agreement with those measured, which lends support to the accuracy of our predictions for the other FeS phases that are less accessible experimentally.

Highlights

FeS polymorphs are of significant relevance to condensed matter physics and planetary science
We note that the calculated bulk modulus of FeS VI (158.4 GPa) is remarkably close to the experimental value at room temperature (156 GPa), reflecting the fact that volume–pressure data could be collected for a window of several tens of GPa in the work of reference [21]
We have presented an investigation of the FeS polymorphs based on Hubbard-corrected density functional theory within the quasi-harmonic approximation

Summary

Introduction

FeS polymorphs are of significant relevance to condensed matter physics and planetary science. FeS phases are thought to form the cores of Earth and Mars, which is suggested by the presence of FeS I in many meteorites [6] This possibility has sparked interest in investigating the numerous polymorphs forming at different thermodynamic conditions, especially with the goal of modelling the cores of the terrestrial planets [7,8,9,10,11,12,13,14,15]. A further increase in pressure results in a second transition at 6.4 GPa to FeS III, which has a non-magnetic monoclinic structure (P21=a) [17,18]. At pressures above 36 GPa and room temperature, a non-magnetic MnP-type phase (FeS VI) becomes stable over FeS III [14,21].

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