A superhard orthorhombic carbon allotrope

An allotrope of carbon called C24-Q is proposed and its structure, electronic, mechanic properties and stability are investigated elaborately by means of density functional theory (DFT). The elastic constants and phonon spectrum of C24-Q indicate that C24-Q has mechanical stability and dy-namic stability, respectively. There are two kinds of carbon atoms in different positions of C24-Q in space, and the crystal structure of C24-Q shows that it has 4-, 6-, and 8-membered rings. The relative enthalpy of C24-Q is smaller than that of T-carbon. C24-Q has a larger Young's modulus and shear modulus than T-carbon and C24-D. C24-Q also shows mechanical anisotropy. Furthermore, the mechanical anisotropy in Young's modulus, shear modulus and Poisson's ratio of C24-Q in seven principal planes are also considered, and it was found that C24-Q has smaller anisotropy differences than C24-D and C24-I. The hardness of this proposed novel carbon structure is 50.31 GPa, which reveals that C24-Q has superhard characteristics. More importantly, it is derived from the electronic band structure of C24-Q that it is a semiconductor material with a quasi-direct band gap of 4.432 eV.