The stability, electronic structure, elastic and metallic properties of manganese nitrides

The phase stability, electronic structure, elastic and metallic properties of manganese nitrides (Mn4N, Mn2N0.86, Mn3N2, and MnN) were extensively studied by first principles calculations. The negative values of cohesive energy and formation enthalpy show that these compounds are thermodynamically stable. The bonding of Manganese nitrides is the combinations of covalent and metallic bonds. In addition, a strong hybridization exists nearby the Fermi level, being characteristic of N-p and Mn-d states. The elastic properties of these nitrides (Mn4N, Mn2N0.86, and MnN) were calculated, which included bulk modulus, shear modulus, Young's modulus, Poisson's ratio, and hardness. The calculated results reveal that Mn2N0.86 and MnN are ductile while Mn4N is brittle, and the hardness values of these nitrides are 24.35 GPa, 12.01 GPa, and 17.06 GPa, respectively. The MnN compound has the highest Debye temperature (632.1 K), while Mn2N0.86 has the lowest Debye temperature (390.5 K). The anisotropy of Young's modulus for Mn4N and MnN compounds is more prominent than Mn2N0.86 at the three crystalline planes. Moreover, the compound of Mn3N2 has an unstable structure in mechanical stability.