Heat capacity of mixed-valence 1′,1′″-diiodobiferrocenium triiodide in the 0.080–25 K range: direct observation of tunnel splitting

Abstract The heat capacity of the mixed-valence complex 1′,1′″-diiodobiferrocenium triiodide, [(C 5 H 4 IFe) 2 (C 10 H 8 )] + I − 3 , has been measured by adiabatic calorimetry between 80 mK and 25 K. A Schottky anomaly centered around 0.6 K was found and interpreted in terms of a tunnel splitting of the vibronic ground state of the cation, which leads to a molecular quantum paraelectric (MQP). The splitting was determined to be ≈ 1 cm −1 which corresponds to an intermolecular electron transfer rate of ≈ 0.7 × 10 9 s −1 . The mean field theory by Kambara and Sasaki has been applied to determine the phase diagram. The MQP state is stable unless the cation—anion interaction exceeds ≈ 12.3 cm −1 . The possibility of a phase transition in the MQP under some external fields has been discussed.