Phase behaviour of the solid proton conductor CsHSO4

The sequence of previously found phase transitions in CsHSO4 at around 60, 122, and 141 \ub0C 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 \ub0C 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