Abstract
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 Ni 14 S- C 2/ m, Ni 13 S- R 3̅, Ni 12 S- R 3̅, Ni 5 S- C 2/ m, Ni 4 S- P 1̅, and Ni 3 S- Cmcm is revealed. We also confirmed the absence of the stable Fe-bearing compounds between Fe and Fe 2 S 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 Ni 3 S, two phases with space groups I 4̅ and Cmcm were predicted. Ni 3 S- I 4̅ is stable at least from 100 GPa, whereas above 330 GPa, it transforms into Ni 3 S- Cmcm . The pressure of phase transition is almost independent of temperature. The Ni 2 S is stable in the entire pressure range and undergoes a single-phase transition from the Pnma - to P 6̅2 m -phase at 266 GPa and 0 K with a Clapeyron slope of 5 MPa/K. The S-rich sulfide NiS 3 is characterized by Im 3̅ 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.