The mechanical properties of MoN under high pressure and effect of metallic bonding on its hardness

Using first-principles calculations, the structural, electronic and thermodynamic properties of MoN under high pressure are investigated, as well as the effect of metallic bonding on its hardness. Five structures are considered, i.e., δ-MoN, WC-MoN, NiAs-MoN, NaCl-MoN and CsCl-MoN. The obtained lattice constant, elastic constants are in good agreement with the available experimental data and other theoretical results. δ-MoN phase is found to be energetically the most stable phase closely followed by NiAs-MoN phase at ambient conditions. On the basis of the third-order Birch–Murnaghan equation of states, a pressure-induced structural phase transition from δ-MoN to NiAs-MoN is observed. Elastic properties, Poisson's ratio, Debye temperature, and thermal expansion coefficient of MoN under high pressure are derived. Furthermore, the bonding nature of MoN can be described as covalent-like due to hybridization of N and Mo states, together with ionic and metallic characters. Based on Mulliken population analysis, hardness of MoN is evaluated and the obtained results, namely, NiAs-MoN (26.6 GPa) and δ-MoN (24.7 GPa), are consistent with experimental data excellently. Moreover, the hardness of MoN increases with the pressure. This is a first-principles investigation on the structural and thermodynamic properties of MoN, and it still awaits experimental confirmation.