Numerous new carbon allotropes have been uncovered by compressing carbon nanotubes based on our computational investigation. The volume compression calculations suggest that these new phases have a very high anti-compressibility with a large bulk modulus （B0）. The predicted B0 of new phases is larger than that of c-BN （373 GPa） and smaller than that of diamond （453 GPa）. All of the predicted structures are superhard transparent materials with a larger band gap and possess the covalent characteristics with sp3-hybridized electronic states. The simulated results will help us better understand the structural phase transition of cold-compressed carbon nanotubes.
A new family of superhard carbon allotropes C48（2i ＋ 1 ） is constructed by alternating even 4 and 8 membered rings. These new carbon allotropes are of a spatially antisymmetrical structure, compared with the symmetrical structures of bet- C4, Z-carbon, and P-carbon. Our calculations show that bulk moduli of C48（2i ＋ 1 ） are larger than that of c-BN and smaller than that of cubic-diamond. C48（2i ＋ 1 ） are transparent superhard materials possessing large Vicker hardness comparable to diamoud. This work can help us understand the structural phase transformations of cold-compression graphite and carbon nanotubes.
