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

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.