The search for the new superconductors in the Ni-N system

Transition metal nitrides show a unique combination of superconducting and mechanical properties. In this study we perform a theoretical search of the intermediate compounds in Ni-N system, determine their superconducting transition temperatures ( T c ) and bulk moduli ( K 0 ) in the wide range of pressures, from 0 to 300 GPa. We have revealed several nickel-rich interstitial compounds, Ni 5 N 2 , Ni 3 N, and Ni 6 N, and one nitrogen-rich compound, NiN 2 , as stable structures. At pressures above 100 GPa, previously unknown NiN with anti-nickeline structure is also stabilized. All found compounds are non-magnetic. The most promising for superconductivity NiN 2 structure is stable against decomposition on the mixture of (Ni+N) from 3 to at least 300 GPa. The transition from marcasite-type NiN 2 - Pnnm to pyrite-type NiN 2 - P a 3 ¯ occurs at 70 GPa. NiN 2 - Pnnm is an insulator with the band gap of about 1 eV, while NiN 2 - P a 3 ¯ is a metallic phase. The calculated T c of NiN 2 - Pnnm equals to 10 −2 K, and the bulk modulus ( K 0 ) equals to 225 GPa at 10 GPa, which is in excellent agreement with available experimental data. The comparison of T c of compounds in the Ni-H and Ni-N systems with that of the Fe-H and Fe-N compounds, shows that hydrides and nitrides of nickel have lower T c than hydrides and nitrides of iron, which is explained by the significant differences in crystal structures.