  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 obtained. 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 RbHSO4 type crystals; the found values
of the model parameters provide a good description of both dielectric and
thermodynamic properties of these compounds.
