High-pressure behavior of the Fe—S system and composition of the Earth's inner core
A.R. Oganovc,b,a aLomonosov Moscow State University, Vorobevy Gory, Moscow, 119991, Russian Federation bMoscow Institute of Physics and Technology (State University), Institutskii per. 9, Dolgoprudnyi, Moscow Region, 141700, Russian Federation cSkolkovo Institute of Science and Technology, Bolshoy Boulevard 30, bld. 1, Moscow, 121205, Russian Federation
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 $\bar M$ in the range 52.6—53.3 results for in the inner core, which is much higher than $\bar M=49.3$ determined for the inner core from Birch's law. In the case of oxygen (noting the equilibrium coexistence of suboxide Fe2O with iron under nuclear conditions), the inner core density can be explained by the hydrogen content of 13.2—17. 2 mol.% (4.2—5.6 mass%), which corresponds to $\bar M$ between 49.0 and 50.6. Combining our results and previous works, we arrive at four preferred compositional models of the inner core (in atomic %): (i) 86% (Fe+Ni)+14% C; (ii) 84% (Fe+Ni)+16% O; (iii) 84% (Fe+Ni)+7% S+9% H; (iv) 85% (Fe+Ni)+6% Si+9% H.
Keywords: crystal structure prediction, ab initio calculations, evolutionary algorithms, mineral physics PACS:61.50.Ah, 61.50.Ks, 61.50.Nw, 61.66.Fn, 64.30.−t, 91.60.Fe (all) DOI:10.3367/UFNe.2017.03.038079 URL: https://ufn.ru/en/articles/2017/10/d/ Citation: Bazhanova Z G, Roizen V V, Oganov A R "High-pressure behavior of the Fe—S system and composition of the Earth's inner core" Phys. Usp.60 1025–1032 (2017)