The present study was carried out within the well-established quantum density functional theory DFT framework with the aim to highlight the ultra-hardness along with modern models of novel hexacarbon C 6 structures based on three BC 5 forms. In particular, the mechanical (elastic constants) and dynamical (phonons bands) stabilities of these phases were confirmed. Furthermore, they are identified with thermal properties alike diamond. Orthorhombic, tetragonal, and trigonal C 6 all have energies close to diamond's and are equally characterized by a stacking of C4 tetrahedra.