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Spin-entropy driven charge transfer phase transition in iron mixed-valence system

Kojima N., Itoi M., Ono Y., Okubo M., Enmoto M.

Materials Science

2003

Vol. 21, No. 2

181-189

We have synthesized iron mixed-valence complexes, (n-CnH2n+1)4N[FeIIFeIIIX3] (X = mto(C2O3S), dto(C2O2S2), tto(C2OS3)) and have investigated their physical properties by means of 57Fe Mössbauer spectroscopy, magnetic susceptibility and electrical resistivity measurements. From the analysis of 57Fe Mössbauer spectra, magnetic susceptibility and electrical resistivity, we have discovered a new type of first order phase transition around 120°K for (n-CnH2n+1)4N[FeIIFeIII(dto)3](n = 3, 4), 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 in this system. The ferromagnetic order is induced by the charge transfer interaction between the FeIII and FeII sites. Moreover, we propose various multifunctional properties for (n-CnH2n+1)4N[FeIIFeIII(mto)3] and (n-CnH2n+1)4N[FeIIFeIII(tto)3].