96-24E

The microscopic proton ordering model is developed for the description of phase transitions sequence from superionic phase II to ferroelastic phases III and IV in the (NH$_4$)$_3$H(SeO$_4$)$_2$ crystal. Symmetry analysis of proton orderings in phases III and IV is performed using the structural data. The equilibrium states of proton subsystem are studied and the corresponding phase diagram determining the necessary conditions for the existence of different proton orderings is constructed within the frame of mean field approximation taking into account the long-range proton interactions. Investigations of thermodynamical functions as well as the temperature dependences of proton average occupation numbers are performed numerically. The obtained results are compared with the conclusions which follow from the phenomenological approach based on Landau expansion.