Novel Superhard Carbon: C-Centered Orthorhombic C8

A novel carbon allotrope of C-centered orthorhombic C-8 (Cco-C-8) is predicted by using a recently developed particle-swarm optimization method on structural search. Cco-C-8 adopts a sp(3) three-dimensional bonding network that can be viewed as interconnected (2, 2) carbon nanotubes through 4- and 6-member rings and is energetically more favorable than earlier proposed carbon polymorphs (e.g., M carbon, bct-C-4, W carbon, and chiral C-6) over a wide range of pressures studied (0-100 GPa). The simulated x-ray diffraction pattern, density, and bulk modulus of Cco-C-8 are in good accordance with the experimental data on structurally undetermined superhard carbon recovered from cold compression of carbon nanotube bundles. The simulated hardness of Cco-C-8 can reach a remarkably high value of 95.1 GPa, such that it is capable of cracking diamond.