PROTON ORDERING MODEL OF SUPERIONIC PHASE-TRANSITION IN (NH4)(3)H(SEO4)(2) CRYSTAL

The microscopic proton ordering model is developed for the description of the phase transition sequence from superionic phase II to ferroela stic phases III and IV in the (NH4)(3)H(SeO4)(2) crystal. Symmetry ana lysis of proton orderings in phases III and IV is performed using stru ctural data. The equilibrium states of the proton subsystem are studie d and the corresponding phase diagram determining the necessary condit ions for the existence of different proton orderings is constructed wi thin the frame of the mean-field approximation taking into account lon g-range proton interactions. Investigations of thermodynamic functions as well as the temperature dependence of proton average occupation nu mbers are performed numerically. The resulting picture corresponds to experimental data for the network of hydrogen bonds in these crystal p hases. The results are compared with the conclusions which follow from the phenomenological approach based on the Landau expansion.