Author(s):      Krzysztof Kniaz, John E. Fischer, Andrew McGhie,
                Louis A. Girifalco, Robert Strongin and Amos Smith III

Title:          Fullerene Alloys

Source:         Solid State Communications, v 96, p 739, 1995

Abstract:       The results of differential scanning calorimetry (DSC)
                and powder X-ray diffraction (XRD) on C60(x) - C70(1-x) 
                fullerene alloys are presented.
                Samples have been prepared by solubilizing weighed amounts of
                C60 and C70 in toluene and then evaporating the solvent, and
                by subliming C60 - C70 powders under dynamic
                vacuum. Dilute alloys prepared from toluene solution undergo
                the orientational transition at lower temperatures than 
                pure C60 and C70, the depression of
                the transition temperature (measured by the DSC)
                depending on the alloy fraction.
                These samples also exhibit a 
                large miscibility gap for x bigger than 0.04 
                and less than 0.95.
                In contrast, alloys prepared by sublimation are 
                miscible in all
                proportions, at least in the high-temperature fcc phase. 
                The experimental solubilities are compared with the
                calculated binary phase diagram, in which 
                intermolecular interactions
                are treated by the Girifalco potential. 
                In the studied model C60-C70
                alloy is treated essentially as a material made up of
                huge "inert gas" molecules, interacting via Van der Waals
                potential. Although the internal energy of mixing
                for the system is positive ie fullerene alloy is not
                energetically stable with respect to pristine components,
                huge negative entropy of mixing makes the system stable.