Pressure-induced transition from the dynamic to static Jahn-Teller effect in (Ph_4P)_2IC_(60)

摘要

High-pressure infrared transmission measurements on (Ph_4P)_2IC_(60) were performed up to 9 GPa over a broad frequency range (200-20000 cm~(-1)) to monitor the vibrational and electronic/vibronic excitations under pressure. The four fundamental T_(1u) modes of C_(60) are split into doublets already at the lowest applied pressure and harden with increasing pressure. Several cation modes and fullerene-related modes split into doublets at around 2 GPa, the most prominent one being the G_(1u) mode. The splitting of the vibrational modes can be attributed to the transition from the dynamic to static Jahn-Teller effect, caused by steric crowding at high pressure. Four absorption bands are observed in the NIR-visible frequency range. They are discussed in terms of transitions between LUMO electronic states in C_(60)~-, which are split because of the Jahn-Teller distortion and can be coupled with vibrational modes. Various distortions and the corresponding symmetry lowering are discussed. The observed redshift of the absorption bands indicates that the splitting of the LUMO electronic states is reduced upon pressure application.