Crystal chemistry and DFT investigation of an ultrahard novel triclinic allotrope C ₈ with a structure close to that of diamond

In this communication the existence of a diamond-like novel triclinic allotrope tri -C 8 with space group <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <m:mrow> <m:mi>P</m:mi> <m:mover accent="true"> <m:mn>1</m:mn> <m:mo>̅</m:mo> </m:mover> </m:mrow> </m:math> P̅1 , No. 2, is proposed from crystal engineering of corner-sharing stacking of C 4 tetrahedra. DFT-based calculations of the ground-state structure show that tri -C 8 belongs to “ sie; deh3 ” topology characterizing the base-centered orthorhombic C 16 allotrope and considered thereupon as a distorted higher-symmetry structure. Energy-derived properties such as the cohesive energy, the mechanical, dynamic, thermodynamic characteristics, and the electron band structure show close relationship with diamond. Specifically, the high density ρ = 3.43 g cm −3 , versus ρ diamond = 3.53 g cm −3 , the ultra-hard behavior with Vickers hardness H V = 83 GPa versus, H V( diamond) = 95 GPa, the specific heat C V curve fitting well with diamond’s experimental values and the largely insulating electronic system.