First-principles study of mechanical and magnetic properties of transition metal (M) nitrides in the cubic M4N structure

We report results from systematic calculations performed by density functional theory on mechanical properties of twenty-eight 3 d, 4 d and 5 d transition metal (M) nitrides (TMNs) in metal-rich cubic M4N structure as novel candidates for hard coatings materials. We have computed lattice constants, elastic constants, derived moduli and ratios which characterize mechanical properties, and other properties like magnetic moments, formation energies, Debye temperature and Bader charge transfer. Our calculations indicate that all M4N-type metal nitrides except V4N, Nb4N, and Pt4N are mechanically stable. All Group 7 TMNs in the M4N structure are found to have high Vickers hardness values with the highest being 24.3 GPa for Re4N. Our computed lattice constants and magnetic dipole moments for Mn4N and Fe4N, the two compounds for which experimental measurements exist, are consistent with their measured values. Spin-polarized computations reduce the hardness of some magnetic compounds like Mn4N and Fe4N. The total density of states calculation reveals that all 28 M4N phases are metallic. The hybridization of metal d and nitrogen 2p orbitals is found to be the key factor in determining mechanical stability and hardness in these compounds. In contrast, ionicity, as computed by Bader charge transfer, does not correlate with hardness. Our comprehensive database for binary transition metal nitrides in M4N structure offers wide possibilities for experimental synthesis of such materials with desirable physical properties for the hard-coatings application.