Heat Capacities and Phase Transitions of the Molecule-Based Mixed-Valence Complex NBu₄[Fe^IIFe^III(ox)₃] and the Mixed-Metal Complex NBu₄[Zn^IIFe^III(ox)₃]

抄録

Heat capacities of two molecule-based materials NBu₄[Fe^IIFe^III(ox)₃] and NBu₄[Zn^IIFe^III(ox)₃] (Bu=n-butyl, ox=oxalate) were measured by adiabatic calorimetery. In case of ferrimagnetic NBu₄[Fe^IIFe^III(ox)₃] two phase transitions were detected at 43.3 K (T₂) and 217 K (T₁) along with a small heat capacity anomaly at 16.3 K (T₃). The heat capacity anomaly at 43.3 K is due to the ferrimagnetic ordering, whereas that at 217 K has been assigned to an order-disorder phase transition due to increasing conformational change of the organic cation. The small heat capacity anomaly at T₃ seems to arise from a spin glass like state at low temperatures. The enthalpy and entropy gains from the heat capacity anomalies at T₁, T₂ and T₃ are 2.29 kJ mol^-1 and 11.47 J K^-1 mol^-1; 1.37 kJ mol^-1 and 22.65 J K^-1 mol^-1; 0.068 kJ mol^-1 and 4.89 J K^-1 mol^-1, respectively. The sum of the entropies ( = 27.54 J K^-1 mol^-1) corresponding to the heat capacity anomalies at T₂ and T₃ is very close to the expected magnetic entropy R ln30 ( = 28.27 J K^-1 mol^-1) for the spin multiplicity of high spin Fe^III and Fe^II ions. No phase transition around 38 K has been detected, as predicted from previous magnetic measurements. In case of paramagnetic NBu₄[Zn^IIFe^III(ox)₃] a heat capacity anomaly due to a similar order-disorder phase transition was observed at 227 K accompanied with the enthalpy and entropy gains of 2.91 kJ mol^-1 and 13.32 J K^-1 mol^-1, respectively. A model has been applied to realize the high temperature order-disorder phase transition.