Pressure Effect on Charge-Transfer Phase Transition in a Mixed-Valence Iron Complex, (n-C₃H₇)₄N[Fe^IIFe^III(dto)₃] (dto = C₂O₂S₂)

Abstract

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, (n-C₃H₇)₄N[Fe^IIFe^III(dto)₃] (dto = C₂O₂S₂) up to 0.9 GPa. The transition temperature of the charge-transfer phase transition increases almost linearly with increasing applied hydrostatic pressure (∼100 K/GPa), while the ferromagnetic Curie temperature (T_c ∼ 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 [Fe^IIFe^III(dto)₃] molecules without the lattice expansion plays an important role in the first-order character of the charge-transfer phase transition.