THE TRANSITION FROM THE ORDERED TO THE MEROHEDRAL DISORDERED PHASE INOXYGENATED SOLID C-60

C-60 powder, pellets and single crystals were oxygenated at 0.11 kbar and then studied with MAS C-13 NMR, DSC and XRD. In agreement with rep orts by other authors, powder samples exhibited minor NMR resonances, indicative of blocking of fullerene rotation by interstitial oxygen, t he distribution of which had yet to attain the equilibrium state. Also , as reported by others, DSC showed two endothermic peaks near 260 K, and both, we found, corresponded to a discontinuous change in the latt ice parameter. The peak on the low-temperature side moved up in temper ature with elapsed time. Pellets, in which presumably oxygen diffused easily, displayed only one peak but shifted in the same manner. Single crystals, in contrast, had only the higher-temperature ''immobile'' p eak. We suggest that the shifting endotherm originates from the transi tion of oxygen-intercalated C-60 to a merohedral disordered phase, in which the rotation of fullerene molecules is partially impeded by oxyg en. A phenomenological model is proposed for this transition.