Examination of the superprotonic transition and dehydration behavior of Cs0.75Rb0.25H2PO4 by thermogravimetric and differential thermal analyses

The dehydration and the superprotonic transition behavior of Cs0.75Rb0.25H2PO4 have been investigated via thermogravimetry and differential thermal analysis. To enable measurements under the high water partial pressures required, a high humidity system (pH2O = 0.02–0.8 atm) capable of weight change detection at 0.7% resolution and thermal events with 0.01 °C resolution was assembled. It has been found that the superprotonic phase transition temperature of Cs0.75Rb0.25H2PO4 is 231 °C, slightly higher than that of CsH2PO4. In contrast, dehydration of Cs0.75Rb0.25H2PO4 under pH2O = 0.35 atm occurs at 265 °C, a somewhat lower temperature than that at which CsH2PO4 undergoes dehydration. Analysis of the temporal evolution of both transformations suggests a mechanism of continuous nucleation of the high temperature phase at a constant nucleation rate combined with three-dimensional growth.