Solid fullerenes under compression

This chapter reviews the properties and phase transitions of C₆₀ and C₂₀ solid fullerenes under high pressures through density functional tight binding simulations, augmented by an empiri cal van der Waals force. The geometric structures of C₆₀ molecules situated in the solid fcc phase are considerably changed under compression due to variations in the type of hybridization of carbon atoms, and the HOMO-LUMO gap dramatically decreases in response to the overlap of n orbitals from neighboring C₆₀ molecules. For C₂₀ solid fullerene, three new carbon allotropes have been revealed by cold compression. The three predicted structures are shown to be ener getically more favorable and stable than C₂₀ molecule. The volume compression calculations suggest that phase III has a large bulk modulus (427 GPa) and a bandgap of 5.1 eV. The results indicate that C₂₀ solid fullerene is a potential building block for transparent superhard carbon with more advantages than graphite. We believe that the high-pressure study of small fullerenes will continue to be a rewarding field of study in the future as the advance in synthesis technology of small fullerene.