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 Pmn2(1) and Cmc2(1) for potential superhard OsN, energetically much superior to the previously proposed NaCl-type and WC-type structures. The Pmn2(1) structure which consisted of distorted OsN4 tetrahedra is stable up to 62 GPa, above which Cmc2(1) becomes energetically more favorable. The Cmc2(1) 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 (similar to 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 similar to 20 GPa. (C) 2010 Elsevier Ltd. All rights reserved.