Theoretical study of the Ni–C system in the pressure range of 0–100 GPa

Abstract

This work is devoted to the search for stable compounds and structures in the Ni–C system in the pressure range of 0–100 GPa. Based on the density functional theory, a search for stable compounds and structures in the Ni–C system was carried out using modern algorithms for predicting crystal structures. As a result, one stable intermediate compound Ni3C with the structure of cementite, previously synthesized at 184 GPa, was revealed. Ni3C nickel carbide is dynamically stable, which is confirmed by the absence of imaginary modes in the phonon spectra. According to the results obtained, Ni3C is formed by the reaction of 3Ni + C ↔ Ni3C above 23 GPa and is stable up to at least 100 GPa. Spin-polarized calculations showed that the Ni3C has no magnetic moment in the entire pressure range. For carbon-rich compounds, performed calculations on the crystal structure prediction did not reveal any phase that would be energetically favorable relative to a mixture of pure nickel and carbon. Also, it was shown that the most energetically favorable modification of metastable carbide Ni7C3 is orthorhombic Ni7C3-Pbca.