First-principles studies of superhard BC₈N structures

We report first-principles calculations of the electronic and mechanical properties of three monoclinic BC₈N structures (space group: Pm, No. 6), namely, BC₈N-1, BC₈N-2, and BC₈N-3, by employing a newly developed ab initio particle swarm optimization methodology for crystal structure prediction. The mechanical stability and dynamical stability of these BC₈N structures are confirmed based on the calculated results of elastic constants and phonon dispersions. Among the three proposed BC₈N phases, BC₈N-3 has a negative formation energy of −0.002 eV/atom, indicating that it may be synthesized from diamond and cubic boron nitride. Investigation of their electronic properties shows that all three BC₈N phases are semiconductors with an indirect bandgap ranging from 2.52 eV to 4.61 eV. Using a semiempirical microscopic hardness theoretical model, we estimate that the three BC₈N phases are potential superhard materials with the Vickers hardness of 75.72, 77.21, and 78.43 GPa. Based on the Pugh criterion, the B/G ratios of the proposed BC₈N are 0.92, 0.92, and 0.90, which are all higher than that of diamond (0.83), implying their ductile nature. These multiple excellent properties enable BC₈N materials to have wide potential applications as optical and electronic device absorbents, cutting tools, coatings, and so on.