On the Jahn-Teller effect in ReF6

New experimental and theoretical work has led to a re-evaluation of the Jahn-Teller effect in ReF6. Low temperature neat and mixed crystal Raman and absorption spectra have been taken. The theory applicable to two Jahn-Teller active vibrations and to a Jahn-Teller molecule in a low symmetry crystalline field is discussed. The main conclusions of the study are that: 1) the standard linear Jahn-Teller treatment of vibronically active modes as independent motions characterized by a given vibronic angular momentum is in general inadequate; 2) Renner-Teller (quadratic) or higher order vibronic coupling terms are important; 3) crystal field splitting of the vibronic levels is not observed; and 4) vibrational exciton band widths of the Jahn-Teller active vibrations are substantially reduced.