Polymorphic phases of sp3-hybridized superhard CN

It is well established that carbon nitride (CN) is a potential superhard material as its bond in network structures is slightly shorter than the C-C bond in diamond. However, the structure of superhard CN materials is yet to be determined experimentally. We have performed an extensive structural search for the high pressure crystalline phases of CN using the particle swarm optimization technique; seven low-energy polymorphic structures of sp(3)-hybridized CN have been found in an unbiased search. Density-functional theory calculations indicate that, among the seven low-energy crystalline structures, Pnnm structure (8 atoms/cell) is energetically more favorable than the previously reported most stable crystalline structure with 1:1 stoichiometry. Furthermore, Pnnm possesses the highest hardness (62.3 GPa). Formation enthalpies demonstrate that this material can be synthesized at pressure 10.9 GPa, lower than needed by beta-C3N4 (14.1 GPa). (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4765324]