Two novel superhard carbon allotropes with honeycomb structures

Two novel superhard 3D sp(3) hybridized carbon allotropes (Cmmm-C-32 and P6/mmm-C-54) are proposed in this work through first-principles calculations. Both Cmmm-C-32 and P6/mmm-C-54 have a honeycomb structure. P6/mmm-C-54 shows a wide direct bandgap (3.564 eV), while Cmmm-C-32 shows an ultrawide indirect bandgap (5.051 eV), as determined using the HSE06 hybrid functional. The hardnesses of Cmmm-C-32 and P6/mmm-C-54 are 83.72 and 54.01 GPa, respectively, and their calculated ideal strengths further confirm their superhardness characteristics. P6/mmm-C-54 is isotropic in the (001) plane. The minimum thermal conductivities of Cmmm-C-32 and P6/mmm-C-54 are larger than those of diamond-C, illustrating that Cmmm-C-32 and P6/mmm-C-54 are potential semiconductor device materials. Published under license by AIP Publishing.