Direct enumeration of wurtzite BC_2N configurations for structural stability and hardness evaluation

摘要

An algorithm to systematically search nonequivalent unit cells for hcp lattice has been developed and wurtzite BC_2N (WZ-BC_2N) alloy configurations with nonequivalent unit cells containing up to 12 atoms are examined. By using bond-counting rule, low-energy WZ-BC_2N structures are selected to carry out first-principles calculations. It is proved that the lowest-energy WZ-BC_2N 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-BC_2N serves as a good candidate of superhard materials. By comparing WZ-BC_2N with zincblende BC_2N (ZB-BC_2N), the zincblende structure is found to be energetically more stable than the wurtzite structure for BC_2N, consistent with the phase stability for bulk diamond and BN. Since the formation energy for WZ-BC_2N (001) superlattice gets smaller as the periodicity increases, experimental effort can be made in synthesizing WZ-BC_2N superlattice with a relatively long periodicity if the phase transition from WZ-BC_2N to ZB-BC_2N can be effectively inhibited.