First-principles investigation of the structural, mechanical and electronic properties of the NbO-structured 3d, 4d and 5d transition metal nitrides

We have performed ab initio calculations on 29 nitride phases of transition metals from the 3d, 4d and 5d rows, in NbO structure. We calculated cohesive energy, lattice constant and elastic constants C11, C12 and C44, and derived mechanical moduli, related ratios and hardness. For five out of the ten 3d transition metal nitrides, namely, CrN, MnN, FeN, CoN and NiN, cohesive energy in this new structure is similar to that of the same composition in the rocksalt structure. The lattice constant and bulk modulus were found to be anti-correlated. We observed the correlation between the shear modulus (G), Pugh's ratio (k) and derived Vickers hardness (HV). For identical metal element significant variations in the mechanical properties of nitrides are found between rocksalt and NbO structures. However for a fixed structure, 3d, 4d and 5d metal nitrides behave similarly. We computed Debye temperature and demonstrated its correlation with HV as proposed by Madelung, Einstein and Deus. The nitrides, CrN, MoN and WN in NbO structure show values of HV larger than 20 GPa. We showed systematically that C44, G, k and HV are anti-correlated with the number of electronic states around EF, leading to a semi-quantitative link of nitride electronic structure to mechanical instability and hardness. The local density of states demonstrating systematic evolution of the electronic structure with the number of d electrons in the metal atoms was studied. Bader charge transfer from metal to nitrogen atom was analyzed throughout the 29 nitrides showing comparison with rocksalt structure and experimental electronegativity data.