Solid-state Carbon Nitrides

Whether crystalline or amorphous sp3‐bonded carbon nitrides containing substantial amounts of nitrogen can be synthesized remains an open question. Various deposition techniques and high pressure syntheses to 40 GPa have not been successful thus far. The use of appropriately designed molecular or solid‐state precursors and low enough synthesis temperatures to insure kinetic control of reaction products appears to be a promising direction for future efforts. In addition, synthesis at higher pressures, which can effect larger shifts in free energies of reaction should be pursued. The importance of careful characterization to obtain structural (by means of electron or X‐ray diffraction), compositional (by means of EELS, EDX, or RBS), and, especially, bonding (by means of EELS) information on single‐phase material cannot be overemphasized. Because few examples are known of covalently bonded networks containing carbon, synthesis of a sp3‐bonded carbon nitride would be a spectacular achievement. The synthesis of sp3‐bonded solid‐state carbon nitrides such as β‐C3N4, a potentially technologically important hard material, remains a challenge. The stability, characterization, and synthesis of both sp2‐ and sp3‐bonded carbon nitrides is reviewed. A discussion of the alternative structures that have been proposed for C3N4 based on local density approximation (LDA) calculations is included. Recommendations are also made as to the directions that should be pursued for success in this area of research.