  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.

Materials Science