Abstract
Two novel superhard 3D sp3 hybridized carbon allotropes (Cmmm-C32 and P6/mmm-C54) are proposed in this work through first-principles calculations. Both Cmmm-C32 and P6/mmm-C54 have a honeycomb structure. P6/mmm-C54 shows a wide direct bandgap (3.564 eV), while Cmmm-C32 shows an ultrawide indirect bandgap (5.051 eV), as determined using the HSE06 hybrid functional. The hardnesses of Cmmm-C32 and P6/mmm-C54 are 83.72 and 54.01 GPa, respectively, and their calculated ideal strengths further confirm their superhardness characteristics. P6/mmm-C54 is isotropic in the (001) plane. The minimum thermal conductivities of Cmmm-C32 and P6/mmm-C54 are larger than those of diamond-C, illustrating that Cmmm-C32 and P6/mmm-C54 are potential semiconductor device materials.
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