ENTHALPIES OF PHASE-TRANSFORMATIONS IN THE ALKALI FULLERIDE RBC60

Phase transformations in the alkali fulleride RbC60 between the polyme ric, dimeric and metastable fee phases are investigated by differentia l scanning calorimetry (DSC) and xray diffraction in the similar to 21 5 to 500 K range. The transformation enthalpies of the polymer-->monom er and dimer-->monomer transitions (25.8 and 10.5 kJ/mol, respectively ) are less than expected for covalent interfullerene bonding which sug gests substantial contributions from Madelung and deformation energies . Stability of the polymer phase depends on the temperature and durati on of polymerization. In samples quenched from 500 to 273 K a transfor mation from an fee phase with hindered rotation of the C-60 monomers t o another fee phase in which C-60(-) ions are freely rotating is obser ved at 290 K. The relative Gibbs free energies of the stable and metas table phases are estimated from the measured transformation temperatur es and enthalpies. The polymer is the most stable phase at low tempera tures.