Structural, strength and fracture mechanisms of superconducting transition metal nitrides TMN (TM = W and Mo).

Transition metal (TM) nitrides are recognized for their outstanding and highly desirable properties, categorizing them as a class of multifunctional materials with diverse industrial applications. In particular, the newly synthesized WN is notable for its exceptional ultra-incompressibility (406 GPa for bulk modulus), remarkable hardness (34 GPa), and superconductivity (9.4 K), positioning it as a potential ultra-hard superconductor. We performed a comprehensive study of the structural, electronic, and mechanical properties of TMN (TM = W and Mo), emphasizing their behavior under shear deformation and lattice instability. The distinct ionic TM-N and covalent N-N bonding characteristics in TMN were characterized through a topological analysis of charge density. Compared to WN, the superconducting transition temperature of MoN at ambient pressure was estimated to be 14.8 K. Both compounds demonstrate impressive uniaxial compressive strengths of -265.7 GPa for WN and -216.5 GPa for MoN, which are comparable to that of diamond (-223.1 GPa) along the [100] direction. The superior mechanical strength of TMN, especially in WN, was manifested by the calculated ideal tensile strengths exceeding 40 GPa along the main crystal axes of [100], [010], [001], and [101]. However, WN shows a considerably lower Vickers indentation shear strength of 16.2 GPa along the (110)[11̄0] direction when compared to the well-known WN, indicating a limitation in its shear fracture resistance and hardness, as suggested by the determined Vickers hardness of 22.0-22.5 GPa. Finally, the lattice instability and fracture mechanisms of WN under indentation shear deformation were clarified through in-depth analyses of atomic bonding and electronic structure evolutions.