Phase Relations in the Ni-S System at High Pressures from ab Initio Computations

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.