Investigation of tetragonal ReN2 and WN2 with high shear moduli from first-principles calculations

Two new transition metal dinitrides, ReN2 and WN2, with the P4/mmmP4/mmm structure are investigated by the first-principles calculations. The computed shear moduli of 327 GPa for ReN2 and 334 GPa for WN2 exceed those of all transition metal dinitrides previously reported. The estimated theoretical hardness are 46.3 GPa for ReN2 and 47.9 GPa for WN2, respectively. The calculated high shear moduli and hardness indicate that they are potential ultrahard materials. It is important to note that the computed hardness of the weakest bond are 34.7 GPa (W–N) for WN2 and 33.1 GPa (Re–N) for ReN2, much higher than that of 21.1 GPa (Re–B) for ReB2, which suggests that tetragonal ReN2 and WN2 are probably harder than ReB2. The total and partial electron density of states and the electron localization function for ReN2 and WN2 are analyzed. We attribute the high bulk modulus, shear modulus, and hardness to a three-dimensional covalently bonded framework in tetragonal ReN2 and WN2. Our calculations show that tetragonal ReN2 is expected to be synthesized above 62.7 GPa and tetragonal WN2 may be hard to be synthesized.