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

We have investigated the physical properties of (n-C3H7)(4)N[(FeFeIII)-Fe-I I(dto)(3)] (dto = C2O2S2) by means of Fe-57 Mossbauer spectroscopy and magn etic susceptibility measurement. From the analysis of Fe-57 Mossbauer spect ra and magnetic susceptibility, we have discovered a new type of first orde r phase transition around 120 K, where the charge transfer transition betwe en Fe-II and Fe-III occurs reversibly. In the higher temperature phase, the Fe-III (S = 1/2) and Fe-II (S = 2) sites are coordinated by six S atoms an d six O atoms, respectively. In the lower temperature phase, on the other h and, the Fe-III (S = 5/2) and Fe-II (S = 0) sites are coordinated by six O atoms and six S atoms, respectively. Moreover, we have found the ferromagne tic phase transition at 6.5 K. The ferromagnetic order is presumably induce d by the charge transfer interaction between the Fe-III (S = 5/2) and Fe-II (S = 0) sites. (C) 2001 Elsevier Science Ltd. All rights reserved.