New phase transition in superprotonic phase of inorganic solid acid Cs 2 (HSO 4 )(H 2 PO 4 )

The crystal structure, the thermal analysis and the proton conduction in the superprotonic phase of Cs2(HSO4)(H2PO4) have been investigated from 113K to 423K. 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.