Theoretical study on tetragonal transition metal dinitrides from first principles calculations

Three new transition metal dinitrides TMN2 (TM = Ta, W, and Re) with the P4/mbm structure are investigated by the first principles calculations method based on the density functional theory. The elastic constants and phonons calculations have confirmed that these three compounds are all mechanical and dynamically stable at ambient pressure. The distributions of elastic moduli of these dinitrides have been systematically studied and the obtained results indicate that the (0 0 1) plane may be viewed as the cleavage plane for TaN2 and WN2 as well as (1 0 0) plane for ReN2. Moreover, TMN2 within this tetragonal structure are found to be ultra-incompressible and hard, among which WN2 exhibits the largest bulk modulus (389 GPa) and Vickers hardness (38.5 GPa). Density of states calculation revealed that the strong TM-N covalent bonding is the driving force for the high bulk and shear modulus as well as small Poisson's ratio of the studied dinitrides.