Infrared-absorption studies of thin fullerene films as a function of temperature reveal a splitting of an absorption line at 245±3 K. Another absorption line narrows and its frequency shifts without splitting. These observations support earlier neutron- and x-ray-diffraction findings of an orientational phase transformation at 250 K in bulk samples. We provide an explanation based on an accepted crystal structure of ${\mathrm{C}}_{60}$.

• Received 31 January 1992

DOI:https://doi.org/10.1103/PhysRevB.46.2591