Superhard sp2-sp3 hybridized BC2N: A 3D crystal with 1D and 2D alternate metallicity

Abstract

A novel sp2-sp3 hybridized orthorhombic BC2N (o-BC2N) structure (space group: Pmm2, No. 25) is investigated using first-principles calculations. O-BC2N is constructed from multi-layers of C sandwiched between two layers of BN along the c axis; this structure contains sp2- and sp3-hybridized B-C, C-C, and C-N bonds. The structural stability of o-BC2N is confirmed based on the calculation results for elastic constants and phonon dispersions. On the basis of the semi-empirical microscopic model, we speculate that the o-BC2N compound is a potential superhard material with a Vickers hardness of 41.2 GPa. Calculated results for electronic band structures, density of states (DOS) and partial DOS (PDOS) show that the o-BC2N crystal is metallic. The conducting electrons at the Fermi level are mostly from the 2p orbits of sp2-hybridized B4, N1, and Ci (i = 2, 3, 4, 6, 7, 8) atoms, with slight contribution from the sp3-hybridizd B2 atoms. Furthermore, the calculated electron orbits of the o-BC2N crystal demonstrate that the 2p orbits of the sp2-hybridized atoms overlapped and formed π bonds. The electrons can conduct through the π bonds along the orientation parallel to the [100] and [010] directions in different layers, and the basal planes were formed by B2-C3-C4 blocks, indicating that the o-BC2N possesses the fascinating electronic property of linear-planar metallicity.