P. A. Heiney, J. E. Fischer, A. R. McGhie, W. J. Romanow, A. M. Denenstein, J. P. McCauley Jr., A. B. Smith III, and D. E. Cox, Phys. Rev. Lett. 66, 2911-2914 (1991).
Synchrotron x-ray powder diffraction and differential scanning calorimetry measurements on solid C60 reveal a first-order phase transition from a low-temperature simple cubic structure with a four-molecule basis to a face-centered cubic structure at 249 K. The free energy change at the transition is approximately 6.7 J/g. Model fits to the diffraction intensities are consistent with complete orientational disorder at room temperature, and with the development of orientational order rather than molecular displacements or distortions at low temperature.
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