05-13U

This research studies phase transition in crystals NaKC$_4$H$_4$O$_6\cdot $4H$_2$O, RbHSO$_4$, NH$_4$HSO$_4$ within the framework of the Mitsui model taking into account tunnelling effects. The procedure for receiving the optimal set of model theory parameters was proposed for the above mentioned crystals. Physical characteristics of researched crystals were calculated on the base of received parameters and thus compared to the experimental ones. Basing on the result of camparison certain conclusions related to the model conformity with researched crystals were made.