New phase transition in superprotonic phase of inorganic solid acid Cs2(HSO4)(H2PO4)

The crystal structure, the thermal analysis and the proton conduction in the superprotonic phase of Cs2(HSO4)(H2PO4) have been investigated from 113 K to 423 K. A new structure model has been proposed in terms of the temperature change of XRD patterns. A phase transition was found in the superprotonic phase on the basis of the XRD and the thermal analysis in the low-temperature region. The phase transition shows a glass-transition-like behavior. The temperature dependence of proton conductivity has been discussed in relation to this transition.