Physico-chemical Studies of the Reversible and Irreversible Phase Transitions in Potassium Ferrocyanide Trihydrate and Its Deuterate Analogue

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Abstract
Phase transitions of K4Fe(CN)6·3H2O and its deuterate were studied through the measurements of differential thermal analysis curves, heat capacity, volume expansion and Raman spectra. Samples prepared by recrystallization from the aqueous solution were found to be in the metastable phase. The ferroelectric transition temperature between two stable monoclinic phases was determined to be 247.8 K and 252.7 K for the hydrate and deuterate, respectively. The enthalpy and entropy changes of the transition were estimated to be 2592 J mol−1 and 12.41 JK−1 mol−1, respectively, for the hydrate, and 3017 J mol−1 and 14.36 JK−1 mol−1, respectively, for the deuterate. The molecular mechanism of the transition was discussed in terms of entropy change. The transition was also concluded to be of the second order according to Ehrenfest’s criterion. Irreversible transformation of tetragonal to monoclinic phase was observed for the first time for the deuterate as a new type of phase change having a Martensitic nature. These irreversible transformations took place in the temperature region covering 20 K below ca. 232 K and ca. 237 K for the hydrate and deuterate, respectively. The anhydrous salt showed no transformation in the temperature region between 13 K and room temperature.