Structural, strength and fracture mechanisms of superconducting transition metal nitrides TM₃N₅ (TM = W and Mo)

Abstract

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 WN_<i>x</i>, 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.