A superhard carbon allotrope with cage structure: oS44-carbon

Abstract First-principles calculations are applied to investigate a new carbon allotrope, denoted oS44-carbon, with orthorhombic symmetry and Cmmm space group. This fully sp3-hybridized bonded carbon network resembles an orderly stack of hexagonal prisms and two types of cages. The total energy is lower, and the thermodynamic stability is better, for oS44-carbon than synthesized T-carbon and supercubane. The dynamic stability and mechanical stability of oS44-carbon under 0–60 ​GPa of pressure are demonstrated by its calculated elastic constants and phonon spectra. Ab initio molecular dynamics simulations show that oS44-carbon can withstand high temperatures up to 1000 ​K. By adopting Chen's model, the hardness of oS44-carbon is found to be 55.93 ​GPa, indicating that it is superhard. Its multicage structure gives it a notably low density (2.77 ​g/cm3) among sp3-bonded superhard carbon allotropes. Surprisingly, the band structure shows that oS44-carbon is an indirect ultrawide-bandgap semiconductor with a bandgap of 4.81 ​eV. Finally, we find that its indirect semiconducting feature can survive pressures from 0 to 60 ​GPa.