V. V. Roizen | Searcharxiv

arXiv2017

The role of temperature and Coulomb correlation in stabilization of CsCl-phase in FeS under pressure

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.

cond-mat.str-el↗

arXiv2017

High-pressure behavior of the Fe-S system and composition of the Earth's inner core

Using evolutionary crystal structure prediction algorithm USPEX, we identify the compositions and crystal structures of stable compounds in the Fe-S system at pressures in the range 100-400 GPa. We find that at pressures of the Earth's solid inner core (330-364 GPa) two compounds are stable - Fe2S and FeS. In equilibrium with iron, only Fe2S can exist in the inner core. Using the equation of state of Fe2S, we find that in order to reproduce the density of the inner core by adding sulfur alone, 10.6-13.7 mol.% (6.4-8.4 wt.%) sulfur is needed. Analogous calculation for silicon (where the only stable compound at inner core pressures is FeSi) reproduces the density of the inner core with 9.0-11.8 mol.% (4.8-6.3 wt.%) silicon. In both cases, a virtually identical mean atomic mass M in the range 52.6-53.3 results for in the inner core, which is much higher than M = 49.3 determined for the inner core from Birch's law. For oxygen (where the relevant stable oxide at conditions of the inner core is Fe2O) we find the matching concentration in the range 13.2-17.2 mol.% (4.2-5.6 wt.%), which corresponds to M in the range 49.0-50.6. Combining our results and previous works, we find that inner core density and M can be explained by only four models (in atomic %): (a) 86%(Fe+Ni) + 14%C, (b) 84%(Fe+Ni) + 16%O, (c) 84%(Fe+Ni) + 7%S + 9%H, (d) 85%(Fe+Ni) + 6%Si + 9%H, and some of their linear combinations (primarly, models (c) and (d)).

physics.geo-ph↗