Vc. Long et al., Far-infrared study of the Jahn-Teller-distorted C-60 monoanion in C-60-tetraphenylphosphoniumiodide, PHYS REV B, 58(21), 1998, pp. 14338-14348
We report high-resolution far-infrared transmission measurements on C-60-te
traphenylphosphoniumiodide as a function of temperature. In the spectral re
gion 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, indic
ating a D-5d or D-3d distorted ball. This result is consistent with a dynam
ic Jahn-Teller effect in the strong-coupling limit or with a static distort
ion stabilized by low-symmetry perturbations. The appearance of silent odd
modes is in keeping with symmetry reduction of the hall, while activation o
f even modes is attributed to interband electron-phonon coupling and orient
ational disorder in the fulleride salt. Temperature dependences reveal a we
ak transition in the region 125-150 K in both C-60(-) and counterion modes,
indicating a bulk, rather than solely molecular, effect. Anomalous softeni
ng (with decreasing temperature) in several modes may correlate with the ra
dial character of those vibrations. [S0163-1829(98)03245-7].