Structural, strength and fracture mechanisms of superconducting transition metal nitrides TM3N5 (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 W3N5 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 TM3N5 (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 TM3N5 were characterized through a topological analysis of charge density. Compared to W3N5, the superconducting transition temperature of Mo3N5 at ambient pressure was estimated to be 14.8 K. Both compounds demonstrate impressive uniaxial compressive strengths of -265.7 GPa for W3N5 and -216.5 GPa for Mo3N5, which are comparable to that of diamond (-223.1 GPa) along the [100] direction. The superior mechanical strength of TM3N5, especially in W3N5, was manifested by the calculated ideal tensile strengths exceeding 40 GPa along the main crystal axes of [100], [010], [001], and [101]. However, W3N5 shows a considerably lower Vickers indentation shear strength of 16.2 GPa along the (110)[11̄0] direction when compared to the well-known WNx, 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 W3N5 under indentation shear deformation were clarified through in-depth analyses of atomic bonding and electronic structure evolutions.