We report high-resolution far-infrared transmission measurements on (Formula presented)-tetraphenylphosphoniumiodide as a function of temperature. In the spectral region investigated (Formula presented) we assign intramolecular modes of the (Formula presented) monoanion and identify low-frequency combination modes. The well-known (Formula presented) and (Formula presented) modes are split into doublets at room temperature, indicating a (Formula presented) or (Formula presented) 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 ball, 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 (Formula presented) 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.
|Number of pages||11|
|Journal||Physical Review B - Condensed Matter and Materials Physics|
|Publication status||Published - Jan 1 1998|
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics