Exploring the Real Ground-State Structures of Molybdenum Dinitride

Molybdenum dinitride (MoN₂) was recently synthesized at a moderate pressure of 3.5 GPa, and a layered MoS₂-type structure has been proposed. However, our first-principles calculations of thermodynamic, mechanical and dynamical properties suggest that this layered <i>R</i>3<i>m</i> structure is unstable. Therefore, stable structures of MoN₂ 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 MoN₂ system is a pernitride structure with space group <i>P</i>6₃/<i>mmc</i> which transforms to a <i>P</i>4/<i>mbm</i> phase above 82 GPa. Chemical bonding analysis shows that one could assign MoN₂ as Mo⁴⁺(N₂^4–); i.e., Mo is formally a d² metal, in agreement with the experimental results of Wang et al. The presence of covalent N₂ dumbbells and strong bonding between Mo⁴⁺ and N₂^4– is the source of the superior mechanical properties of these predicted ultra-incompressible MoN₂ pernitrides