Charge transfer phase transition and ferromagnetism in a mixed-valence iron complex, (n-C3H7)4N[FeIIFeIII(dto)3] (dto=C2O2S2)

We have investigated the physical properties of (n-C3H7)4N[FeIIFeIII(dto)3] (dto=C2O2S2) by means of 57Fe Mِssbauer spectroscopy and magnetic susceptibility measurement. From the analysis of 57Fe Mِssbauer spectra and magnetic susceptibility, we have discovered a new type of first order phase transition around 120 K, where the charge transfer transition between FeII and FeIII occurs reversibly. In the higher temperature phase, the FeIII (S=1/2) and FeII (S=2) sites are coordinated by six S atoms and six O atoms, respectively. In the lower temperature phase, on the other hand, the FeIII (S=5/2) and FeII (S=0) sites are coordinated by six O atoms and six S atoms, respectively. Moreover, we have found the ferromagnetic phase transition at 6.5 K. The ferromagnetic order is presumably induced by the charge transfer interaction between the FeIII (S=5/2) and FeII (S=0) sites.