  Search of materials with C-N composition hold a great promise in creating
materials which would rival diamond in hardness due to the very strong and
relatively low-ionic C-N bond. Early experimental and theoretical works on C-N
compounds were based on structural similarity with binary A3B4 structural-
types; however, the synthesis of C3N4 remains elusive. Here we explored an
unbiased synthesis from the elemental materials at high pressures and
temperatures. Using in situ synchrotron X-ray diffraction and Raman
spectroscopy we demonstrate synthesis of highly incompressible Pnnm CN compound
with sp3 hybridized carbon is synthesized above 55 GPa and 7000 K. This result
is supported by first principles evolutionary search, which finds that Pnnm CN
is the most stable compound above 10.9 GPa. On pressure release below 6 GPa the
synthesized CN compound amorphizes reattaining its 1:1 stoichiometry as
confirmed by Energy-Dispersive X-ray Spectroscopy. This work underscores the
importance of understanding of novel high-pressure chemistry rules and it opens
a new route for synthesis of superhard materials.
