Crystal chemistry and DFT investigation of ultrahard novel triclinic C8 close to Diamond

In this Letter a novel triclinic tri-C8 with low symmetric space group P-1 No. 2 is proposed from crystal engineering of corner sharing C4 tetrahedra stacking. DFT-based calculations of the ground state structure show that tri-C8 belongs to “sie; deh3” topology characterizing base-center orthorhombic space group C16 allotrope and considered thereupon as a distorted structure from higher symmetry. Energy-derived properties such as the cohesive energy, the mechanical, dynamical, thermodynamic and electrons band structure show close relationship with Diamond especially regarding the high density of 3.44 g/cm3, the ultra-hard behavior with Vickers hardness HV=83 GPa and the largely insulating electronic system