FE-57 MOSSBAUER, IR AND RAMAN-SPECTRA AND HEAT-CAPACITY OF THE MIXED-VALENCE COMPLEX, 1',1'''-DIETHYLBIFERROCENIUM TRIIODIDE, SHOWING FUSION TYPE AVERAGING PROCESS OF THE MIXED-VALENCE STATE

Variable temperature i.r. and Raman spectra of 1', 1"'-diethylbiferrocenium triiodide, which shows a fusion type averaging process of the mixed-valence state in the Fe-57 Mossbauer spectra, were recorded from 85 to 298 K. An unsymmetric triiodide anion at low temperatures changes to a symmetric form with increasing temperature. This structural change in the anion is closely connected with a valence-detrapping of the mixed-valence cation observed in Fe-57 Mossbauer spectra. The heat capacity of the same batch of sample was also measured with an adiabatic calorimeter between 14 and 315 K. A small heat capacity peak with the transition entropy of 2.9 J K-1 mol-1 was observed at 67.2 K but this is not directly connected with the averaging process of the mixed-valence state. Another broad anomaly was observed above approximately 200 K. This anomaly just corresponds to the temperature region where the Fe-57 Mossbauer spectra are altered from a trapped- to an average-valence state and the triiodide ion altered from an unsymmetric to a symmetric structure. This anomaly is interpreted in terms of a Schottky anomaly due to a tunneling-splitting whose energy difference is 750 cm-1. The tunneling-splitting may be caused by a coupling between two vibronic states and the asymmetric breathing mode of the cation.