New stable structures of OsN4 predicted using first-principles calculations

The crystal structure prediction algorithm has been performed using the swarm-intelligence-based CALYPSO method combined with density functional theory. The P4/mmm phase and R-3c phase are found for potential superhard OsN4, energetically much superior to the previously proposed Pbca-type structure. It is confirmed that the stable ground-state structure of OsN4 is the P4/mmm phase. More importantly, the phase transformation is firstly found from the P4/mmm phase to the R-3c phase at nearly 65.3 GPa. And the two phases of the OsN4 not only have the high bulk and shear modulus but also the low Poisson's ratio. This suggests that the OsN4 compound has the potential to be a low-compression material. According to a detailed analysis of the density of states and electronic local functions, it is revealed that the covalent bonding of Os-N and N-N are responsible for their structural stability and high hardness.