Molecular orientational ordering in solid C-60

Single-crystal X-ray diffraction studies of the orientational ordering in solid C-60 are reviewed. The temperature dependence of selected Bragg reflections was carefully examined, allowing to obtain original information on the first-order orientational ordering transition at T-o = 259K, the unusual behavior of both fundamental and superstructure reflections below T-o and the freezing in of the C-60 reorientations at T-g similar or equal to 85K. The diffuse intensity due to orientational disorder of the C-60 molecules at room temperature is found to be strongly modulated, both radially and azimuthally, showing that the molecular orientations are indeed correlated. The corresponding intensity distribution has been calculated within a mean-field theory for different microscopic models of intermolecular interactions. It should allow a better understanding of these interactions.