First-principles studies of superhard BC8N structures

Abstract

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.