Rotational ordering transition in single-crystalC60studied by Raman spectroscopy

Raman spectroscopic data are presented on the rotational ordering fcc→sc transition in single-crystal ${\mathrm{C}}_{60}$. Splittings and activation of new modes reveal the influence of the cubic structure, and are in agreement with the crystal symmetry in both phases. The temperature dependence of the spectra is consistent with a sharp first-order phase transition near 252 K. A simple model is proposed for the explanation of the temperature dependence of the spectra, involving a large vibrational-rotational coupling in the fcc phase, and essentially no rotational disorder in the sc phase.