Exploring the Real Ground-State Structures of Molybdenum Dinitride

Molybdenum dinitride (MoN 2 ) was recently synthesized at a moderate pressure of 3.5 GPa, and a layered MoS 2 -type structure has been proposed. However, our first-principles calculations of thermodynamic, mechanical and dynamical properties suggest that this layered R 3 m structure is unstable. Therefore, stable structures of MoN 2 at pressures from atmospheric pressure to 100 GPa have been further examined by utilizing a widely adopted evolutionary methodology USPEX for crystal structure prediction. We find that the ground state of the MoN 2 system is a pernitride structure with space group P 6 3 / mmc which transforms to a P 4/ mbm phase above 82 GPa. Chemical bonding analysis shows that one could assign MoN 2 as Mo 4+ (N 2 4– ); i.e., Mo is formally a d 2 metal, in agreement with the experimental results of Wang et al. The presence of covalent N 2 dumbbells and strong bonding between Mo 4+ and N 2 4– is the source of the superior mechanical properties of these predicted ultra-incompressible MoN 2 pernitrides.