A new large-cell superhard carbon allotrope: orthorhombic oC240.

A new carbon allotrope orthorhombic oC₂₄₀, is proposed by first principles calculation. The unit cell of oC₂₄₀ is composed of 240 carbon atoms. The calculated elastic constants and phonon spectra confirmed that oC₂₄₀ is mechanically and dynamically stable under ambient pressure. The calculated hardness is 56.3 GPa. The electronic structure calculations show that oC₂₄₀ is an indirect band gap semiconductor with the gap of 2.13 eV. [ABSTRACT FROM AUTHOR]