Based on the ab initio calculations within the density functional theory and crystal structure prediction algorithms, the structure and stability of compounds in the Ni-S system at pressures of 100-400 GPa were determined. As a result, a homologous series of discrete compounds (Ni and S) consisting of Ni14S-C2/m, Ni13S-R3¯, Ni12S-R3¯, Ni5S-C2/m, Ni4S-P1¯, and Ni3S-Cmcm is revealed. We also confirmed the absence of the stable Fe-bearing compounds between Fe and Fe2S in the studied pressure range. At the Earth's core pressures, 4 wt % of sulfur can be dissolved in solid fcc-Ni without deformation of the structure. Significant deformations in the Ni structure occur at sulfur contents from 4 to 15 wt %. In contrast, up to 0.45 wt % of sulfur could be dissolved in hcp-Fe at 350 GPa and 0 K. For Ni3S, two phases with space groups I4¯ and Cmcm were predicted. Ni3S-I4¯ is stable at least from 100 GPa, whereas above 330 GPa, it transforms into Ni3S-Cmcm. The pressure of phase transition is almost independent of temperature. The Ni2S is stable in the entire pressure range and undergoes a single-phase transition from the Pnma- to P6¯ 2m-phase at 266 GPa and 0 K with a Clapeyron slope of 5 MPa/K. The S-rich sulfide NiS3 is characterized by Im3¯ m symmetry and is thermodynamically stable from 100 to 318 GPa. Our new data on Ni sulfides might be important to constrain detailed thermodynamic models for Fe-Ni-bearing Earth and planetary cores.
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