STABILITY OF CARBON NITRIDE SOLIDS

An ab initio variable-cell-shape molecular-dynamics algorithm is used to investigate the stability Of three carbon nitrides with composition C3N4. The hexagonal beta-C3N4 structure, which has been the subject of earlier investigations, is refined and its metastability is verified. Two other metastable structures are identified: a structure resembling zinc-blende CN with one C vacancy per cubic cell and a structure resembling graphitic CN with one C vacancy per four N sites. Both the refined beta phase and the cubic phase have compressibilities comparable to that of diamond. The graphitic and the beta phases lie very close in energy and are slightly favored over the cubic phase.