Monoclinic C2/m-20 carbon: a novel superhard sp3 carbon allotrope

Based on density function theory with the ultrasoft pseudopotential scheme in the frame of the generalized gradient approximation, the structural, mechanical, and electronic properties of C2/m-20 carbon have been systematically investigated in this paper. The elastic constants and phonon spectra calculations show that C2/m-20 carbon is mechanically stable and dynamically stable at 0 GPa and 100 GPa. From our calculations, we found that C2/m-20 carbon has a larger bulk modulus of 412 GPa, a larger shear modulus of 463 GPa, a larger Young's modulus of 1010 GPa, and a hardness of 70.6 GPa, which means that it is a superhard material with potential technological and industrial applications. C2/m-20 carbon exhibits greater anisotropy than C2/m-16 carbon and M-carbon using Young's modulus and the universal anisotropic index. In addition, C2/m-20 carbon is an indirect and wide semiconductor with a band gap of 5.10 eV. The most extraordinary thing is that the band gap increases with increasing pressure.