Pressure-induced structural transition of OsN₂ and effect of metallic bonding on its hardness

Using first-principles calculations, the elastic constant, structural phase transition and effect of metallic bonding on the hardness of OsN₂ under high pressure are investigated by means of the pseudopotential plane-waves method. Five candidate structures are chosen to investigate for OsN₂, namely, the pyrite, CoSb₂-type, marcasite, simple hexagonal and tetragonal structures. A comparison among the formation energies of OsN₂ explains the synthesis of OsN₂ marcasite under high pressure. On the basis of the third-order Birch-Murnaghan equation of states, the transition pressure P_t (P_t=223 GPa) between the marcasite and simple tetragonal phase is determinated. Elastic constants, shear modulus, Young's modulus, Poisson's ratio and Debye temperature are derived. The calculated values are, generally speaking, in good agreement with experiments and other theoretical calculations. Our calculation indicates that the N-N bond length is one determinative factor for the ultrahigh bulk moduli of the heavy-transition-metal dinitrides. Moreover, based on Mulliken overlap population analysis in first-principles technique, a semiempirical method to evaluate the hardness of multicomponent crystals with partial metallic bonding is presented. The effect of metallic bonding on the hardness of OsN₂ is investigated and the hardness shows a gradual decrease rather than increase under compression, which is different from diamond. This is a quantitative investigation on the structural properties of OsN₂, and it still awaits experimental confirmation.