AB-INITIO INVESTIGATION OF THE VIBRATIONAL AND GEOMETRICAL PROPERTIES OF SOLID C-60 AND K3C60

We have used a first-principles density-functional-based method to investigate the changes in the vibrational and geometrical properties of solid C60 arising from doping with potassium. Ab initio results for all intramolecular modes of C60 and K3C60 at the zone center and for selected external modes of the doped compound are presented. Estimates for the equilibrium structure of the C60 molecule for both compounds are derived. We find a doping-induced softening of the vibrational spectrum, which is especially strong for high-energy modes, and a reduction of the bond length alternation of the C-C bonds by 30%. The calculations further indicate an instability connected with the potassium ion sitting at the octahedral site. © 1995 The American Physical Society.