The transition from the ordered to the merohedral disordered phase in oxygenated solid C₆₀

C60powder, pellets and single crystals were oxygenated at 0.11 kbar and then studied with MAS13C NMR, DSC and XRD. In agreement with reports by other authors, powder samples exhibited minor NMR resonances, indicative of blocking of fullerene rotation by interstitial oxygen, the 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 lattice parameter. The peak on the low-temperature side moved up in temperature 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" peak. We suggest that the shifting endotherm originates from the transition of oxygen-intercalated C60to a merohedral disordered phase, in which the rotation of fullerene molecules is partially impeded by oxygen. A phenomenological model is proposed for this transition.