Carbon nitride: Ab initio investigation of carbon-rich phases

We have examined the potential energy hypersurfaces for the carbon-rich phases of carbon nitride, CN and C3N, and discovered low-energy structures different from those reported previously. Trends in the preferred local bonding environments have been analyzed as a function of nitrogen content. For each composition, several structures with similar energies were found, but they have very different equilibrium volumes; the structure produced during synthesis will strongly depend on the preparation conditions. When low densities are favored, conjugated planar-ring structures with sp(2) hybridized carbon are most likely to be formed. These structures are similar to those suggested as potential photocatalytic materials. At high pressures, the preferred structures contain three-coordinate nitrogen and sp(3) hybridized carbon, including the beta-InS structure, which we predict to be the thermodynamically preferred structure for CN under positive hydrostatic pressures. This structure has a moderately high bulk modulus with a lower formation energy than beta-C3N4 and so should be more easily synthesized.