  A novel carbon allotrope, hexagonal C12, is proposed from crystal chemistry
and quantum density functional theory DFT calculations of ground state and
physical properties. The structure exhibits corner sharing distorted tetrahedra
with the presence of C3 triangular cyclopropane-like moiety connecting planar
carbon. C12 allotrope is found cohesive and stable both mechanically (elastic
constants and their combinations) and dynamically -- phonons band structures --
and presents ultra-hardness with Vickers number of 70 GPa. The temperature
dependence of the heat capacity CV shows close magnitudes to experimental
results of diamond. The electronic band structure reveals closely insulating
behavior with 2.5 eV band gap, half smaller than in diamond.
