Title: Superhard lon C5 and derived carbon nitrides: C4N and C2N2. Crystal chemistry and first principles DFT studies

Abstract: Super-hard C5 with lon topology (lon: Lonsdaleite hexagonal diamond) and characterized by the presence of sp3 and sp2 -like carbon sites is devised from crystal chemistry and used as template matrix structure for identifying original carbonitrides C4N and C2N2 with lon topology except for the equiatomic belonging to a new topology (3,4L147). The steric effect of N(2s2) lone pair is highlighted in C2N2 in inducing an original structure of largely separated two-layered stacking of tetrahedra. The investigations based on crystal chemistry were backed by computations within the quantum density functional theory DFT. All systems were found cohesive and both mechanically (elastic constants) and dynamically (phonons band structures) stable. Super hardness characterizes the carbon allotrope C5 and the nitrides C4N and C2N2. Metallic-like conductivities and insulating characters were identified.