Far-infrared study of the Jahn-Teller-distorted C60 monoanion in C60-tetraphenylphosphoniumiodide

We report high-resolution far-infrared transmission measurements on C-60-tetraphenylphosphoniumiodide as a function of temperature. In the spectral region investigated (20-650 cm(-1)), we assign intramolecular modes of the C-60 monoanion and identify low-frequency combination modes. The well-known F-1u(1) and F-1u(2) modes are split into doublers at room temperature, indicating a D-5d or D-3d distorted ball. This result is consistent with a dynamic Jahn-Teller effect in the strong-coupling limit or with a static distortion stabilized by low-symmetry perturbations. The appearance of silent odd modes is in keeping with symmetry reduction of the hall, while activation of even modes is attributed to interband electron-phonon coupling and orientational disorder in the fulleride salt. Temperature dependences reveal a weak transition in the region 125-150 K in both C-60(-) and counterion modes, indicating a bulk, rather than solely molecular, effect. Anomalous softening (with decreasing temperature) in several modes may correlate with the radial character of those vibrations. [S0163-1829(98)03245-7].