Prediction of a Superhard Carbon-Rich C-N Compound Comparable to Diamond

Until now, it has been a challenge both in experiment and in theory to design new superhard materials with high hardness values that are comparable to that of diamond. Here, by using first-principles calculations, we have introduced two new phases for a carbon-rich CN compound with stoichiometry C3N, which is predicted to be energetically stable or metastable with respect to graphite and solid N-2 at ambient pressure. It is found that C3N has a layered structure containing graphitic layers sandwiched with freely rotated N-2 molecules. The layer-structured C3N is calculated to transform into a three-dimensional C222(1) structure at 9 GPa with sp(3)-hybridized C atoms and sp(2)-hybridized N atoms. Phonon dispersion and elastic constant calculations reveal the dynamical and mechanical stability of the C222(11) phase of C3N at ambient pressure. Significantly, first-principles ideal strength calculations indicate that the C222(1) phase of C3N is a superhard material with an estimated Vickers hardness (similar to 76 GPa) comparable to that of diamond (60120 GPa). The present results shed strong light on designing new superhard materials in the CN system.