Insulating and conducting phases ofRbC60

Optical measurements were performed on thin films of ${\mathrm{Rb}}_{\mathit{x}}$ ${\mathrm{C}}_{60}$, identified by x-ray diffraction as mostly x=1 material. The samples were subjected 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 metastable insulating phase. Slow cooling results in a phase transition between two electrically conducting phases.