Within the proton ordering model with taking into account the short-range and long-range interactions as well as tunnelling effects, in the framework of the cluster approximation, we study the thermodynamic and static dielectric properties of the KH 2 PO 4 type ferroelectrics. Theoretical results are compared with the corresponding experimental data and with the results of theoretical calculations of other sources. We show that under the proper choice of the theory parameters, a very good quantitative description of the available experimental data is obtained for the temperature dependence of spontaneous polarization, specific heat, longitudinal and transverse static dielectric permittivities, as well for the Curie-Weiss temperature and constant. Suitability of the model for the description of experimental data for the crystals KH 2 PO 4 , RbH 2 PO 4 , KH 2 AsO 4 , RbH 2 AsO 4 without taking into account striction, fluctuations, and other effects is discussed.

Within the framework of the proton model we study thermodynamic and static dielectric properties of the ADP type antiferroelectrics with taking into account the tunnelling effects of protons on hydrogen bonds. In the fourparticle cluster approximation for the short-range interactions and in the mean field approximation for the long-range interactions we calculate the free energy, entropy, and components of static dielectric susceptibility of the crystals. We also get the system of equations for the sublattice proton order parameter and an equation for the phase transition temperature. We find the values of the theory parameters for the NH 4 H 2 PO 4 and NH 4 H 2 AsO 4 crystals and get a good agreement between the theoretical results and the corresponding experimental data for the temperature dependences of the spontaneous sublattice polarization, proton specific heat, longitudinal and transverse static dielectric permittivities of the crystals.

The research of thermodynamic properties of partially deuterated K(H 1−x Dx) 2 PO 4 crystals was performed within the framework of proton ordering model with tunnelling within the four-particle cluster approximation for short-range interaction and within the molecular field approximation for long-range interaction. The KD 2 PO 4 's model parameters were derived for the case of acceptable description of the experimental data for crystal thermodynamic characteristics. Based on the introduced procedure of deriving parameters for K(H 1−x Dx) 2 PO 4 crystals, the thermodynamic and dielectric properties of these crystals were calculated and the results were compared with the experimental ones. The problem of applying the mean crystal approximation to the analysis of physical characteristics of K(H 1−x Dx) 2 PO 4 crystals was discussed.