New superhard carbon allotropes based on C20 fullerene

Three new carbon allotropes have been uncovered by compressing hexagonal C-20 solid based on density functional tight binding level simulations. They are identified to be energetically more favorable and stable than C-20 molecule, with the phase III structure being the energetically most favorable. The volume compression calculations suggest that the phase III has a very high anti-compressibility with a bulk modulus of 427 GPa. All of the predicted structures are semiconductors with varied bandgaps and the phase III has a direct band gap of 5.1 eV. Our results indicate that solid C-20 can be converted to a transparent superhard material under cold compression. (C) 2013 Elsevier Ltd. All rights reserved.