A new superhard carbon allotrope: tetragonal C64

A novel superhard carbon allotrope C64 is predicted which is composed of C28 cages. It is porous and exhibits distinct topologies including zigzag 5, 6, 8, 10 and 12-fold carbon rings. The elastic constants and phonon calculations reveal that C64 is mechanically and dynamically stable at ambient pressure. The hardness of C64 is 60.2 GPa. The tensile and shear strength calculations indicate that the lowest tensile and shear strengths have the almost same value of 48.1 GPa. For the electronic properties, the band structure calculations show that C64 is a quasi-direct band gap semiconductor with a band gap of 1.32 eV.