Equilibrium phases in alkali metal doped C60 films and single crystals from in situ IR-reflectivity

We present IR reflectivity measurements on in situ doped C60 thin films and single crystals in the MIR spectral range. By iterative doping and annealing single phase material for the main stoichiometric phases was obtained. These phases extended over the whole material in the case of the thin films and formed layers up to 4 μm in the case of the single crystals. In the spectral range of the four IR active F1u modes frequency shift versus charge relations were analysed and at least for two modes a charged phonon behavior was found. Particular attention is paid to the modes of the phase Rb1C60 as a function of temperature and of sample orientation. The fcc → orthorhombic phase transition was revealed by a splitting of the F1u modes into three lines independent of orientation within a temperature range of 2 degrees at 404415 K for a cooling/heating cycle, respectively. The reflectivity spectra remained unchanged after exposing the orthorhombic Rb1C60 to air and changed only weakly after heating the crystal beyond the phase transition up to 450 K. The conductivity of both phases shows a weakly activated temperature dependence.