Fulleride ions in various crystal fields studied by infrared spectroscopy

We performed infrared measurements on Cs4C60, Rb4C60, K4C60 and Na2C60 to investigate the interplay between the molecular Jahn–Teller effect and the crystal field of the cations. We were able to tune the crystal field strength by varying the size of the cations and the temperature. Distortion patterns were derived from the observed splitting of the infrared-active C60 modes. We conclude that the dynamics of the distortion changes on weakening the crystal field in the following order: static distortion – constrained pseudorotation – free pseudorotation.