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 firstprinciples calculations of thermodynamic, mechanical and dynamical properties suggest that this layered R3m 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 P6 3 /mmc which transforms to a P4/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.

Chromium hydride is a prototype stoichiometric transition metal hydride. The phase diagram of Cr-H system at high pressures remains largely unexplored due to the challenges in dealing with the high activation barriers and complications in handing hydrogen under pressure. We have performed an extensive structural study on Cr-H system at pressure range 0 ∼ 300 GPa using an unbiased structure prediction method based on evolutionary algorithm. Upon compression, a number of hydrides are predicted to become stable in the excess hydrogen environment and these have compositions of Cr2Hn (n = 2–4, 6, 8, 16). Cr2H3, CrH2 and Cr2H5 structures are versions of the perfect anti-NiAs-type CrH with ordered tetrahedral interstitial sites filled by H atoms. CrH3 and CrH4 exhibit host-guest structural characteristics. In CrH8, H2 units are also identified. Our study unravels that CrH is a superconductor at atmospheric pressure with an estimated transition temperature (T c) of 10.6 K, and superconductivity in CrH3 is enhanced by the metallic hydrogen sublattice with T c of 37.1 K at 81 GPa, very similar to the extensively studied MgB2.