Direct enumeration of wurtzite BC2N configurations for structural stability and hardness evaluation

An algorithm to systematically search nonequivalent unit cells for hcp lattice has been developed and wurtzite BC2N (WZ-BC2N) alloy configurations with nonequivalent unit cells containing up to 12 atoms are examined. By using bond-counting rule, low-energy WZ-BC2N structures are selected to carry out first-principles calculations. It is proved that the lowest-energy WZ-BC2N structures are all (001) superlattice. The Vickers hardness of these structures is calculated and found to be higher than that of cubic BN, indicating that WZ-BC2N serves as a good candidate of superhard materials. By comparing WZ-BC2N with zincblende BC2N (ZB-BC2N), the zincblende structure is found to be energetically more stable than the wurtzite structure for BC2N, consistent with the phase stability for bulk diamond and BN. Since the formation energy for WZ-BC2N (001) superlattice gets smaller as the periodicity increases, experimental effort can be made in synthesizing WZ-BC2N superlattice with a relatively long periodicity if the phase transition from WZ-BC2N to ZB-BC2N can be effectively inhibited.