Pressure effect on charge-transfer phase transition in a mixed-valence iron complex, (n-C3H7)4N[FeIIFeIII(dto)3] (dto = C2O2S2)

We have investigated the hydrostatic pressure effect on the charge-transfer phase transition between Fe-II and Fe-III in a mixed-valence iron complex, (eta-C3H7)(4)N[(FeFeIII)-Fe-II(dto)(3)] (dto = C2O2S2) up to 0.9 GPa. The transition temperature of the char.-e-transfer phase transition increases almost linearly with increasing applied hydrostatic pressure (similar to100 K/GPa), while the ferromagnetic Curie temperature (T-c similar to 7 K) changes only slightly. The results suggest that the applied hydrostatic pressure stabilizes the low-temperature phase [Fe-II(S = 0)-Fe-III(S = 5/2)] through the compression of the lattice volume. A phenomenological model taking into account the elastic interactions as a molecular-field explains the characteristics of the charge-transfer phase transition. The analysis of the results of the model calculation suggests that the lattice volume of the high-temperature phase is only about 0.2% larger than that of the low-temperature phase, and a lattice deformation of [(FeFeIII)-Fe-II(dto)(3)] molecules without the lattice expansion plays an important role in the first-order character of the charge-transfer phase transition.