Infrared Spectroscopic Study of Phase Transisions in A1c60 Compounds (a=K,Rb,Cs)

Optical measurements on doped C$_{60}$ films provide evidence for a transition from a high temperature conducting phase to a low temperature insulating phase in quenched Rb$_1$C$_{60}$ and K$_1$C$_{60}$ compounds, while Cs$_1$C$_{60}$ did not exhibit this behavior. For slow cooled samples our study confirms earlier results indicating phase separation of K$_1$C$_{60}$ to K$_3$C$_{60}$ and C$_{60}$, and the formation of a new Rb$_1$C$_{60}$ phase at low temperatures. Upon slow cooling Cs$_1$C$_{60}$ behaves similar to Rb$_1$C$_{60}$.