An orthorhombic superhard carbon allotrope: Pmma C24

Abstract

First-principles calculations are used to establish a novel carbon phase with a purely sp3 bonded network and an orthorhombic unit cell of 24 atoms, called Pmma C24. Density functional theory is used to investigate the structural characteristics, elastic properties, and mechanical and electronic properties of this novel carbon structure at different pressures. The mechanical and dynamic stability of Pmma C24 under zero and high pressures is determined by the elastic parameters and phonon spectrum. Interestingly, compared to diamond, this novel carbon phase is less compressible along the c-axis and has a lower relative enthalpy (0.562 ​eV/atom) with a hardness of 63.5 ​GPa. Pmma C24 has higher bulk (347 ​GPa) and shear (246 ​GPa) moduli than C96, tP40 carbon, C72, T-carbon and oP72 carbon. The calculated ratio of the bulk and shear moduli shows that Pmma C24 exhibits brittle characteristics at 0 ​GPa and transforms to a ductile phase under compression forces above 30 ​GPa. Electronic band structure calculations suggest that the proposed novel carbon structure has an indirect band gap of 4.30 ​eV and an indirect and wide semiconductor character.