Prediction of a novel carbon allotrope from first-principle calculations: A potential superhard material in monoclinic symmetry

A novel carbon allotrope, denoted P2/m C-54, is proposed in this work. The structural properties, mechanical properties, mechanical and dynamical stability, relative formation enthalpy, anisotropy in Young's modulus and electronic properties for P2/m C-54 are investigated in this work. First, P2/m C-54 is found to remain mechanically and dynamically stable below 100 GPa, with a relative formation enthalpy that is 0.729 eV/atom less than that of T carbon. The bulk modulus, shear modulus and Young's modulus of P2/m C-54 are calculated to be 345 GPa, 332 GPa and 754 GPa, respectively; the hardness of P2/m C-54 is calculated to be 54.1 GPa and 70.4 GPa using Chen's model and Lyakhov and Oganov's model, respectively. All these calculations suggest that P2/m C-54 is a potential superhard material. Furthermore, the band gap of P2/m C-54 is determined to be 4.95 eV; in other words, P2/m C-54 is an indirect band gap semiconductor material with a wide band gap at ambient pressure.