Does the monoclinic solid acid Rb3H(SeO4)2 truly undergo a physical transformation into a trigonal superprotonic-conducting phase?
摘要
We investigated the nature of the thermal anomaly observed in trirubidium hydrogen biselenate, Rb3H(SeO4)2, at the characteristic temperature Tsp, 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 Tsp, the sample undergoes a surface chemical thermal decomposition rather than a structural bulk phase transition. This chemical transformation results in the formation of crystalline Rb2SeO4, along with liquid and gaseous phases. The increase in conductivity around Tsp 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 Rb3H(SeO4)2, underscoring the significance of these findings for understanding proton-conducting materials and their thermal behavior.