X-RAY DIFFUSE-SCATTERING STUDY OF THE ORIENTATIONAL ORDERING IN SINGLE-CRYSTAL C-60

Single crystal X-ray diffuse scattering photography has been used to provide insights into the orientational ordering of the C60 molecules. The observation of anisotropic modulations of the diffuse scattering intensity in reciprocal space demonstrates the existence of orientational correlations at room temperature. A new structural transition is observed at T2 = 225 +/- 2 K, below the face-centered cubic (f.c.c.) to simple cubic orientational ordering phase transition at T1 = 259 +/- 1 K. The transition at T2 leads to a low temperature super-structure, presumably f.c.c., with a doubling of the cell constants. Fluctuations associated to both transitions are seen at room temperature while some anisotropic diffuse scattering is still present at 20 K.