Does the monoclinic solid acid Rb3H(SeO4)2 truly undergo a physical transformation into a trigonal superprotonic-conducting phase?

Abstract We investigated the nature of the thermal anomaly observed in trirubidium hydrogen biselenate, Rb 3 H(SeO 4 ) 2 , at the characteristic temperature T sp , which lies between 173 and 183 °C, using impedance spectroscopy, modulated differential scanning calorimetry, simultaneous thermogravimetric and differential thermal analysis, and temperature-dependent X -ray diffraction. While the previous studies concluded that this anomaly corresponds to a first-order phase transition from a monoclinic, low-proton-conducting phase to a trigonal, superprotonic-conducting phase, our results challenge this interpretation. Instead, we provide evidence that the observed enthalpy change is associated with a chemical transformation rather than a physical one. Our findings indicate that when this salt is heated through T sp , the sample undergoes a surface chemical thermal decomposition rather than a structural bulk phase transition. This chemical transformation results in the formation of crystalline Rb 2 SeO 4 , along with liquid and gaseous phases. The increase in conductivity around T sp is a consequence of this surface chemical reaction. Therefore, we conclude that the previously reported trigonal superprotonic-conducting phase has been misinterpreted. To the best of our knowledge, no previous studies have questioned the existence of the superprotonic trigonal phase in Rb 3 H(SeO 4 ) 2 , underscoring the significance of these findings for understanding proton-conducting materials and their thermal behavior.