Raman study of a structural phase transition in C60 crystals

Raman spectra of purified C60 single crystals are measured over a wide temperature range. It is found that above 400 K the high-frequency Ag mode (pentagonal pinch mode) changes in frequency and becomes sharp compared with the Ag mode at room temperature. This spectral change is reversible. The results indicate that structural change occurs and the intermolecular interaction for each C60 molecule is isotropic above 400 K. A reversible spectral change is observed as the intensity of the excitation laser varies. The similarity between the spectral change at 400 K and the spectral change at high laser intensity levels suggests that the photoinduced spectral change observed in the present study is due to the thermal effects of laser irradiation