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Novel Ultra-hard tetragonal Octacarbon C8 close to Diamond from First Principles

Novel ultra-hard carbon allotrope is proposed with mechanical, dynamic, and thermal properties like diamond. Based on energy criteria from computations within density functional theory DFT, tetragonal C8 stoichiometry is identified as a cohesive network of corner sharing C4 tetrahedra illustrated by charge density projections exhibiting sp3-like carbon hybridization. The new allotrope is mechanically (elastic constants) and dynamically (phonons) stable, exhibiting thermal properties (heat capacity CV) in close agreement with experimental data of diamond from the literature. From the used models to evaluate Vickers hardness, a larger magnitude with respect diamond is hypothesized for the new allotrope. Electronic band structure calculations show insulating behavior with large band gap of 5 eV like diamond.