A first-principles prediction of an sp³ carbon allotrope comprising four-, five-, six-, and eight-member rings

A superhard carbon phase with Pmmm (D_2h¹, 47) symmetry is predicted by using a recently developed particle swarm optimization method for searching for crystal structures. The carbon phase is an orthorhombic crystal system that contains 16 atoms per unit cell, named oC16, which has an all-sp³-hybridized bonding network and contains a large cavity. oC16 has a distinct topology, including zigzag four-, five-, six-, and eightfold carbon rings. The dynamic, elastic, and electronic properties of oC16 are investigated by first-principles calculations, and the results show that oC16 is more energetically stable than the experimentally synthesized T-carbon, BC8, and BC12. The phonon spectra and elastic constants confirm its dynamical and mechanical stability at zero pressure, respectively. The calculated bulk moduli and hardness indicate that oC16 is an ultra-incompressible and superhard material. Analyzing its electronic band structure reveals that oC16 has insulation characteristics with an indirect bandgap of 4.42 eV. Also investigated is how the elastic moduli of the oC16 phase depend on the crystal orientation. Because of its superhard and porous properties, the potential uses of oC16 include hydrogen storage, molecular sieves, coating, and tools for cutting, polishing, and grinding.