Electronic structure study of KxC60 compounds with x > 6

We present C 1s X-ray absorption and photoemission results for KxC60 with x > 6, the so-called ‘superfullerides’. These are the first measurements of the unoccupied valence band for these systems and confirm that more than six electrons can be transferred to each fullerene. We observe saturation of the K-to-C60 charge transfer, with part of the t1g LUMO + 1 orbital remaining unoccupied, so that fewer than 12 electrons are accepted per C60 molecule. This fullerene state exhibits metallic behaviour, while a transition to insulating behaviour is observed as lower charge states are formed by adding more C60. The superfullerides appear to follow the pattern of strongly correlated solids.