Investigation of tetragonal ReN 2 and WN 2 with high shear moduli from first-principles calculations

Abstract

Two new transition metal dinitrides, ReN 2 and WN 2, with the P 4 / mmm structure are investigated by the first-principles calculations. The computed shear moduli of 327 GPa for ReN 2 and 334 GPa for WN 2 exceed those of all transition metal dinitrides previously reported. The estimated theoretical hardness are 46.3 GPa for ReN 2 and 47.9 GPa for WN 2, 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 WN 2 and 33.1 GPa (Re–N) for ReN 2, much higher than that of 21.1 GPa (Re–B) for ReB 2, which suggests that tetragonal ReN 2 and WN 2 are probably harder than ReB 2. The total and partial electron density of states and the electron localization function for ReN 2 and WN 2 are analyzed. We attribute the high bulk modulus, shear modulus, and hardness to a three-dimensional covalently bonded framework in tetragonal ReN 2 and WN 2. Our calculations show that tetragonal ReN 2 is expected to be synthesized above 62.7 GPa and tetragonal WN 2 may be hard to be synthesized.