Enthalpies of phase transformations in the alkali fulleride RbC60

Phase transformations in the alkali fulleride RbC60 between the polymeric, dimeric and metastable fee phases are investigated by differential scanning calorimetry (DSC) and xray diffraction in the similar to 215 to 500 K range. The transformation enthalpies of the polymer-->monomer and dimer-->monomer transitions (25.8 and 10.5 kJ/mol, respectively) are less than expected for covalent interfullerene bonding which suggests substantial contributions from Madelung and deformation energies. Stability of the polymer phase depends on the temperature and duration of polymerization. In samples quenched from 500 to 273 K a transformation from an fee phase with hindered rotation of the C-60 monomers to another fee phase in which C-60(-) ions are freely rotating is observed at 290 K. The relative Gibbs free energies of the stable and metastable phases are estimated from the measured transformation temperatures and enthalpies. The polymer is the most stable phase at low temperatures.