X-ray diffraction, thermogravimetric (TGA), differential scanning calorimetric (DSC), and impedance analysis were used to study the reported high-temperature phase transitions at 107, 149, 230, and 256 °C in crystals of cesium dihydrogen phosphate, CsH2PO4 (CDP). Our results show strong evidence that at all these temperatures, the observed DSC or differential thermal analysis (DTA) endothermic effects appear only as a consequence of a dehydration process starting on the surface of the crystal. Our results thus show that the reported transition at 230 °C is not a polymorphic transition. This means that the monoclinic symmetry, stable at room temperature, with space group P21/m–C2k2, is maintained up to the final decomposition. Moreover, since we have not found any evidence for the existence of a superprotonic high-temperature phase above 230 °C, the high conductivity above 230 °C is thus only a consequence of the dehydration of the crystal surface.

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The sequence of previously found phase transitions in CsHSO4 at around 60, 122, and 141 °C was carefully examined by using thermo-gravimetric analysis (TGA), mass spectroscopy (MS), differential thermal analysis, ac calorimetry, impedance spectroscopy, and x-ray diffraction (XRD). Our results show evidence that at all these transition temperatures, the dehydration processes take place at or very near the surface of the crystal. As a consequence, our results support the phase of CsHSO4 above 141 °C being not a superprotonic-conducting phase but rather a mixture of CsHSO4 and Cs2S2O7 caused by a partial thermal decomposition that includes the break-up of hydrogen bonds and the formation of H2O molecules.

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