Predicted crystal structures of superhard diamond-like boron carbides. (a) Tetragonal BC₂.^[106] (b–d) BC₃, (e–h) BC₅, and (i–l) BC₇ structures, respectively.^{[54,100–102,104,105]} The BC₃ and BC₅ structures have been suggested as reasonable solutions for the experimentally synthesized superhard boron carbide phases.^[7,9,52] As seen from Table 4, these structures have high theoretical hardness values, which are above 40 GPa, and are commonly metallic due to the addition or doping of boron. Notably, the structures in (b), (e), (f), (h), and (i) are also superconductive. There is no obvious dependence between T_c and the concentration of B; however, hardness has significantly increased with increased C content. C and B atoms are painted in brown and green colors, respectively.