Orientational ordering transition in solidC60

Synchrotron-x-ray powder-diffraction and differential-scanning-calorimetry measurements on solid ${\mathrm{C}}_{60}$ 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.