New-type Phase Transition Coupled with Spin and Charge in Iron Mixed-Valence System

Abstract

We have investigated the physical properties of (n-C n H 2n+1 ) 4 N[Fe II Fe III (dto) 3 ](dto = C 2 O 2 S 2 ) by means of 57 Fe Mössbauer spectroscopy, magnetic susceptibility, and ESR. From the analysis of 57 Fe Mössbauer spectra and ESR signal, we have discovered a new type of first order phase transition around 120 K for (n-C n H 2n+1 ) 4 N[Fe II Fe III (dto) 3 ](n = 3, 4), where the charge transfer transition between 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 and six O atoms, respectively. In the lower temperature phase, on the other hand, 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 ferromagnetic phase transition in this system. The ferromagnetic order is induced by the charge transfer interaction between the Fe III ( S = 5/2) and Fe II ( S = 0) sites. spin crossover ferromagnetism charge transfer phase transition mixed valence