01-11U

The Mitsui model for the order-disorder type ferroelectrics is studied within the mean field approximation. The phase diagram of the model is obtained, its dependence on the transverse field is studied. It is revealed that at non-zero tunneling, the system can undergo the same sequence of phase transitions as in NH$_4$HSO$_4$. A scheme of setting the values of the model parameters is proposed; all the dielectric characteristics of Rochelle salt are calculated with the found parameters. Within the Mitsui model we study the physical characteristics of RbHSO$_4$ type crystals; the found values of the model parameters provides a good description of both dielectric and thermodynamic properties of these compounds.