DOPING-INDUCED DISTORTIONS AND BONDING IN K6C60 AND RB6C60

Using the Car-Parrinello method we have investigated theoretically the properties of K6C60 and Rb6C60 crystals. Our results are in very good agreement with available experimental data. We find an interesting pattern of relaxations both in the C60 fullerene cage and in the position of the K ions with respect to an ideal configuration. The relaxation energy gain is almost-equal-to 1 eV and is due in comparable amounts to the C60 and the K's. These energy gains are achieved by relatively small ionic displacements. Analysis of the electronic charge density reveals non-rigid-band effects. Similarities and differences with graphite intercalation compounds are discussed.