b) Ground state solution for BCN Despite the relative stability of layered orthorhombic BCN, which is the first proposition of layered-like structure in the B-C-N ternary system, it remains a non-compact 2D-like structure. A template 3D model allowing a tetrahedral coordination for B with N and C and connected BC2N2 tetrahedra was needed. The crystal structure of hexacarbon C 6 proposed in 1994 (Fig. ) has the unique characteristics of possessing two different types of carbon: