A novel carbon allotrope of C-centered orthorhombic ${\mathrm{C}}_8$ ($\mathrm{Cco}\mathrm{\text{−}}{\mathrm{C}}_8$) is predicted by using a recently developed particle-swarm optimization method on structural search. $\mathrm{Cco}\mathrm{\text{−}}{\mathrm{C}}_8$ adopts a $s{p}^3$ 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, $\mathrm{bct}\mathrm{\text{−}}{\mathrm{C}}_4$, $W$ carbon, and chiral ${\mathrm{C}}_6$) over a wide range of pressures studied (0–100 GPa). The simulated x-ray diffraction pattern, density, and bulk modulus of $\mathrm{Cco}\mathrm{\text{−}}{\mathrm{C}}_8$ 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 $\mathrm{Cco}\mathrm{\text{−}}{\mathrm{C}}_8$ can reach a remarkably high value of 95.1 GPa, such that it is capable of cracking diamond.

• Received 10 June 2011

DOI:https://doi.org/10.1103/PhysRevLett.107.215502