  Transition metal nitrides have been suggested to have both high hardness and
good thermal stability with large potential application value, but so far
stable superhard transition metal nitrides have not been synthesized. Here,
with our newly developed machine-learning accelerated crystal structure
searching method, we designed a superhard tungsten nitride, h-WN6, which can be
synthesized at pressure around 65 GPa and quenchable to ambient pressure. This
h-WN6 is constructed with single-bonded N6 rings and presents ionic-like
features, which can be formulated as W2.4+N62.4-. It has a band gap of 1.6 eV
at 0 GPa and exhibits an abnormal gap broadening behavior under pressure.
Excitingly, this h-WN6 is found to be the hardest among transition metal
nitrides known so far (Vickers hardness around 57 GPa) and also has a very high
melting temperature (around 1900 K). These predictions support the designing
rules and may stimulate future experiments to synthesize superhard material.
