Following Jahn-Teller Distortions in Fulleride Salts by Optical Spectroscopy

C-60 salts represent perfect model systems for the Jahn-Teller effect, in particular for the interplay between the molecular dynamics and the distorting crystal field. In this paper, after a brief introduction to the theoretical background, we review experimental results on salts with fulleride anions containing different charge states in the solid state. Mid-infrared (MIR) and near infrared (NIR) spectroscopic measurements and their conclusions are reported in detail, while the results obtained by nuclear magnetic resonance (NMR), electron spin resonance (ESR) and X-ray diffraction are briefly summarized. The following questions are addressed: Are fulleride ions distorted in various solids? Is the distortion dominated by the molecular Jahn-Teller effect or by the potential field of the environment? What is the shape of the distortion? Is the distortion static or dynamic, is there a pseudorotation, are there transitions between static and dynamic JT states? How do these effects manifest themselves in vibrational and electronic excitations? The experimental difficulties one has to face when studying Jahn-Teller distortions in solids are also discussed. These limitations originate not only in the performance of the spectroscopic methods used, but also in the chemistry of some of the compounds, which can lead to segregation and polymerization.