Low-frequency Raman scattering in the orientationally disordered phase of a C₆₀ crystal

The low-frequency Raman spectra of the orientationally disordered phase of C₆₀ crystals are studied in the temperature range of 295–673 K. It was found that the low-frequency spectrum is the first-order Raman scattering from vibrational excitations. A model for the low-frequency Raman scattering in the high-temperature phase of C₆₀ crystal is suggested. This model implies that due to the orientational disorder THz acoustic excitations are not well-defined plane waves and its density of states is reflected in Raman scattering. In the framework of the model, the dynamic correlation length of the local orientational order was estimated to be 4 nm which corresponds well to the magnitude found earlier by Pintschovius et al. [Phys. Rev. Lett. 69, 2662 (1992)] from quasielastic neutron scattering.