INSULATING AND CONDUCTING PHASES OF RBC60

Optical measurements were performed on thin films of RbxC60, identifie d by x-ray diffraction as mostly x = 1 material. The samples were subj ected to various heat treatments, including quenching and slow cooling from 400 K. The dramatic increase in the transmission of the quenched samples, and the relaxation towards the transmission observed in slow -cooled samples, provides direct evidence for the existence of a metas table insulating phase. Slow cooling results in a phase transition bet ween two electrically conducting phases.