Synthesis of Ultra-incompressible $\mathrm{sp^{3}}$-Hybridized Carbon Nitride with 1:1 Stoichiometry

The search of compounds with C$_x$N$_y$ composition holds great promise for creating materials which would rival diamond in hardness due to the very strong covalent C–N bond. Early theoretical and experimental works on C$_x$N$_y$ compounds were based on the hypothetical structural similarity of predicted C$_3$N$_4$ phases with known binary A$_3$B$_4$ structural types; however, the synthesis of C$_3$N$_4$ other than g-C$_3$N$_4$ remains elusive. Here, we explore an “elemental synthesis” at high pressures and temperatures in which the compositional limitations due to the use of precursors in the early works are substantially lifted. Using in situ synchrotron X-ray diffraction and Raman spectroscopy, we demonstrate the synthesis of a highly incompressible Pnnm CN compound (x = y = 1) with sp$^3$-hybridized carbon above 55 GPa and 7000 K. This result is supported by first-principles evolutionary search, which finds that CN is the most stable compound above 14 GPa. On pressure release below 6 GPa, the synthesized CN compound amorphizes, maintaining its 1:1 stoichiometry as confirmed by energy-dispersive X-ray spectroscopy. This work underscores the importance of understanding the novel high-pressure chemistry laws that promote extended 3D C-N structures, never observed at ambient conditions. Moreover, it opens a new route for synthesis of superhard materials based on novel stoichiometrie