Electronic and elastic properties of two new B-C-N superhard materials from first principles

In this paper, we reported two potential superhard monoclinic BC2N structures, namely, Cm-BC2N-1, and CmBC2N-2 are proposed. The structural properties, mechanical and dynamic stability, formation energy, Young's modulus, elastic anisotropy, and electronic properties under pressure are systematically studied via first principles calculations. The phonon dispersion curves and elastic constants show that monoclinic BC2N are dynamically and mechanically stable at pressures ranging from 0 GPa to 100 GPa. The positive formation energy of monoclinic BC2N indicates that they are metastable. The calculated electronic properties indicate that the CmBC2N-1 and Cm-BC2N-2 are indirect semiconductors with band gaps of 1.753 eV and 1.718 eV, respectively. Chen's model was used to calculate the Vickers hardness of the two Cm-BC2N structures as 62.4 GPa and 61.4 GPa respectively. The hardness has exceeded that of many B-N compounds and B-C-N compounds that have been found. Pugh criterion and Poisson's ratio indicate that the monoclinic BC2N present good ductility and high stability in the process of resisting shear strain. In addition, the Young's modulus, elastic anisotropy, and the minimum thermal conductivity of the monoclinic BC2N are also studied. This work provides insights for exploring and analyzing the properties of new superhard and incompressible B-C-N ternary compounds.