Synthesis, Crystal Structures, Mechanical Properties, and Formation Mechanisms of Cubic Tungsten Nitrides

Abstract

Cubic tungsten nitrides with high elastic stiffness are promising replacements for metal carbides used in tool applications to achieve enhanced working efficiency. However, due to the difficulties in preparing these nitrides at ambient pressure, their crystal structures and mechanical properties remain largely elusive, which have limited the functionality of these materials. Here, we report a comprehensive study of cubic tungsten nitrides synthesized by a high-pressure method, leading to definitive structural identifications of rocksalt cF8-WN and NbO-type cP6-WN involving atomic deficiencies. Combined with calculations, we find that the structural stabilities of both nitrides are closely related to the atomic deficiency that prevents the filling of unfavorable W: 5d-t2g bands. The disordered N vacancies are decisive for stabilizing cF8-WN, while the ordering of W and N vacancies occurs at 3 GPa and relatively low temperatures and leads to the formation of nearly stoichiometric cP6-WN, rather than previously misassigned cP7-W3N4. Both nitrides exhibit similar excellent mechanical and thermal properties, rivaling and even exceeding WC. Besides, their formation mechanisms are also explored to be associated with atomic vacancies, shedding light on the rational design of functional nitrides by defect chemistry.