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 C 3 N 4. The hexagonal-C 3 N 4 structure, which has been the subject of earlier investigations , is reened, and its metastability is veriied. Two other metastable structures are identiied: a structure resembling zinc-blende CN with one C vacancy per cubic cell, and a structure resembling graphitic CN with one C vacancy per 4 N sites. Both the reened phase and the cubic phase have compressibilities comparable to that of diamond. The graphitic and the phases lie very close in energy and are slightly favored over the cubic phase. 1 Carbon nitrides have recently been proposed as candidates for new hard materials. 1 and an ab initio calculation. 2 The model indicated that short covalent bonds like those that form between C and N are favorable for achieving low compressibility. A local-density-approximation-based (LDA) calculation of the structural and electronic properties of a hypothetical C-N solid called-C 3 N 4 supported this view. The calculation predicted the bulk modulus of-C 3 N 4 to be very large and nearly equal to that of diamond. In addition, the cohesive energy of-C 3 N 4 was found to be moderately large, suggesting that it might be an energetically favorable phase in the C-N system. Motivated by these results, several groups have attempted to synthesize covalent C-N solids such as-C 3 N 4. 3{6 In one study, 3 amorphous carbon nitride lms were synthesized, and indentation tests found the N-rich lms to be extremely hard. In other studies, 4;5 small crystallites were found embedded in amorphous C-N lms. Electron diiraction indicates that-C 3 N 4 is a viable structure for these crystallites. As yet, however, adequate samples for quantitative measurements of the bulk modulus and hardness of the crystalline material have not been produced. In this paper, we expand the theoretical search for low-energy carbon nitride solids. We reene the-C 3 N 4 structure and examine its metastability. We also investigate two additional structural phases with composition C 3 N 4. In both forms, the network of short C-N bonds diiers considerably from that in the structure. The compressibilities and energetics of all three phases are compared, and the Born stability criterion is checked in each case to test for elastic stability. Throughout this work an ab initio molecular dynamics (MD) with variable-cell-shape (VCS) …