C2/c-C32: A New Superhard sp^{3} Carbon Allotrope with Direct Band Gap

Abstract

A new monoclinic carbon crystal with the space group C2/c was uncovered; its unit cell comprises 32 carbon (C32) atoms. This full sp³-bonded carbon phase is dynamically and mechanically stable. The results show that C2/c-C32 has excellent mechanical properties, with its shear modulus and Vickers hardness being 343 and 56 GPa, respectively. Based on the elastic anisotropic calculations, C2/c-C32 is confirmed to be an anisotropic material. Notably, C2/c-C32 has a direct band gap structure with a band gap of 2.50 eV.