Crystal structure and physical properties of OsN: First-principle calculations

Using ab initio   evolutionary methodology for crystal structure prediction, we have found two orthorhombic structures of Pmn21Pmn21 and Cmc21Cmc21 for potential superhard OsN, energetically much superior to the previously proposed NaCl-type and WC-type structures. The Pmn21Pmn21 structure which consisted of distorted OsN4 tetrahedra is stable up to 62 GPa, above which Cmc21Cmc21 becomes energetically more favorable. The Cmc21Cmc21 structure contains the Os–Os and Os–N–N chains and possesses the unique diatomic N–N bond. OsN within two orthorhombic phases is found to be an ultra-incompressible material due to the high bulk modulus (∼350 GPa), which originates from the strong and directional covalent bonds in two structures. Analysis of the calculated formation energy suggested that the two structures could be synthesized at moderate pressures of ∼20 GPa.