Examination of the superprotonic transition and dehydration behavior of Cs_(0.75)Rb_(0.25)H_2PO_4 by thermogravimetric and differential thermal analyses

Abstract

The dehydration and the superprotonic transition behavior of Cs_(0.75)Rb_(0.25)H_2PO_4 have been investigated via thermogravimetry and differential thermal analysis. To enable measurements under the high water partial pressures required, a high humidity system (p_(H_2O) = 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 Cs_(0.75)Rb_(0.25)H_2PO_4 is 231 °C, slightly higher than that of CsH_2PO_4. In contrast, dehydration of Cs_(0.75)Rb_(0.25)H_2PO_4 under p_(H_2O) = 0.35 atm occurs at 265 °C, a somewhat lower temperature than that at which CsH_2PO_4 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.