Far-infrared study of the Jahn-Teller-distortedC60monoanion inC60-tetraphenylphosphoniumiodide

Abstract

We report high-resolution far-infrared transmission measurements on ${\mathrm{C}}_{60}$-tetraphenylphosphoniumiodide as a function of temperature. In the spectral region investigated $(20--650{\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}),$ we assign intramolecular modes of the ${\mathrm{C}}_{60}$ monoanion and identify low-frequency combination modes. The well-known ${F}_{1u}(1)$ and ${F}_{1u}(2)$ modes are split into doublets 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 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 ${\mathrm{C}}_{60}^{\mathrm{\ensuremath{-}}}$ 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.