The iron-sulfur system is important for planetary interiors and is intensely studied, particularly for better understanding of the cores of Mars and Earth. Yet, there is a paradox about high-pressure stability of FeS: ab initio global optimization (at DFT level) predicts a Pmmn phase (with a distorted rocksalt structure) to be stable at pressures above ~120 GPa, which has not yet been observed in the experiments that instead revealed a CsCl-type phase which, according to density functional calculations, should not be stable. Using quasiharmonic free energy calculations and the dynamical mean field theory, we show that this apparent discrepancy is removed by proper account of electron correlations and entropic effects.
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