Impedance spectroscopy study on the phase transition in Cs1 − x(NH4)xHSO4

Conductivity measurements above room temperature have been performed in polycrystalline Cs1 − x(NH4)xHSO4 (x = 0.22, 0.54, 0.82) by use of the impedance spectroscopy method. From a detailed analysis of the obtained conductivity, it was confirmed that the cesium-rich compound (x = 0.22) exhibits successive superprotonic phase transitions at 407 and 380 K in cooling cycle. The phase transition at 407 K appears to be contributed from the CsHSO4 lattice. On the other hand, each of the ammonium-rich compounds (x = 0.54 and 0.82) shows single superprotonic phase transition. It is suggested that the origin of these phase transitions in the ammonium-rich compounds as well as the additional one at 380 K in the cesium-rich compound is closely associated with the reorientational disorder of NH4+ dipolar group.