Single-crystal X-ray diffraction studies of the orientational ordering in solid C60are reviewed. The temperature dependence of selected Bragg reflections was carefully examined, allowing to obtain original information on the first-order orientational ordering transition atTo= 259K, the unusual behavior of both fundamental and superstructure reflections belowToand the freezing in of the C60reorientations atTg≈ 85K. The diffuse intensity due to orientational disorder of the C60molecules 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 inter molecular interactions. It should allow a better understanding of these interactions.
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