Abstract
Abstract DSC and complex impedance studies of the protonic conductor (NH4)4H2(SeO4)3, which undergoes a superionic phase transition of first order at Ts = 378 K show that the activation energy of ionic conductivity d(lg σ)/dt and the ordering enthalpy ΔCp of the crystal are proportional: d(lg σ)/dT = XΔCp/RTs + const, as found for MAg4I5 crystals undergoing a second-order superionic phase transition. Thus the short-range order environment of the species involved in fast-ion transport plays the main role in the superionic phase transition. This is also supported by the value of the entropy change at Ts, ΔS = 43 J/mole·K. A new metastable phase was found to be induced on heating the (NH4)4H2(SeO4)3 crystal above Ts.
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