Face-centered-cubic to orthorhombic phase transition in single-crystalRbC60analyzed by Raman scattering

Single crystals of ${\mathrm{C}}_{60}$ were doped with rubidium to the phase ${\mathrm{RbC}}_{60}$ by a load and equilibrate technique. No matter which nonequilibrium state of doping was reached, the equilibration procedure always resulted in the Rb monofulleride phase. Raman scattering for the fcc and for the orthorhombic phase is reported and compared with spectra for ${\mathrm{KC}}_{60}$. For rapidly cooled crystals the latter are found to be identical with the spectra of the rudibium compound. A line at 344 ${\mathrm{cm}}^{\mathrm{-}1}$ was found to be most characteristic for the orthorhombic phase in both cases and, together with some other new and very strong spectral features, considered as an indication of covalent bonds in a polymeric chain. The phase transition for the Rb phase was observed to start at 397 K on cooling with a hysteresis of about 17 K between the cooling and the heating cycle. The continuous shift of the pinch mode line during the process of the phase transition is considered as evidence for a growing linear chain during the process of the phase transition.