Pretransitional effects at the superionic phase transition of Rb3H(SeO4)2 protonic conductor

Rb3H(SeO4)2 single crystal undergoes a transition from low-temperature ferroelastic phase to superionic, paraelastic one at TS≈456 K. The low-temperature monoclinic structure contains a zero-dimensional hydrogen bond network. Above TS, the reorienting HSeO4 groups form a dynamic hydrogen bond network in the (001) plane. The presented results of proton conductivity, ferroelastic domain pattern and differential scanning calorimetry (DSC) studies in the vicinity of TS show that the transition region is stretched into ∼10 K and the transition process proceeds in steps: the changes in the ferroelastic domain pattern close to TS are accompanied by DSC anomalies. The temperature dependence of the conductivity σ[100] on heating and cooling revealed ∼2 K hysteresis of the superionic phase transition. The experimental results support the suggestion of recent papers on theoretical studies of the behaviour of protonic conductivity in M3H(XO4)2 crystal group in the superionic phase transition region.