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

Hao, Jian, Liu, Hanyu, Lei, Weiwei, Tang, Xiao, Lu, Jian, Liu, Dan, Li, Yinwei
The Journal of Physical Chemistry C 2015 v.119 no.51 pp. 28614-28619
graphene, hardness, light intensity, nitrogen, physical chemistry, prediction, stoichiometry
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 C–N compound with stoichiometry C₃N, which is predicted to be energetically stable or metastable with respect to graphite and solid N₂ at ambient pressure. It is found that C₃N has a layered structure containing graphitic layers sandwiched with freely rotated N₂ molecules. The layer-structured C₃N is calculated to transform into a three-dimensional C222₁ structure at 9 GPa with sp³-hybridized C atoms and sp²-hybridized N atoms. Phonon dispersion and elastic constant calculations reveal the dynamical and mechanical stability of the C222₁ phase of C₃N at ambient pressure. Significantly, first-principles ideal strength calculations indicate that the C222₁ phase of C₃N is a superhard material with an estimated Vickers hardness (∼76 GPa) comparable to that of diamond (60–120 GPa). The present results shed strong light on designing new superhard materials in the C–N system.