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 proton model with taking into account the piezoelectric interaction with the shear strain ε 6 , a dynamic dielectric response of KD 2 PO 4 type ferroelectrics is considered. Experimentally observed phenomena of crystal clamping by high frequency electric field, piezoelectric resonance and microwave dispersion are described. Ultrasound velocity and attenuation are calculated, peculiarities of their temperature dependence at the Curie points are described. Existence of a cut-off frequency in the frequency dependence of attenuation is predicted.

The effect of external factors, such as dessicating/wetting, thermal annealing, uniaxial and hydrostatic pressure, on the dielectric permittivity of Rochelle salt crystals is investigated. The obtained results are compared with the available literature data and analyzed within the phenomenological Landau approach. A significant effect of the internal polar point defects in crystals and storage conditions on the dielectric permittivity is shown.

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.

Within the framework of the proposed unified proton ordering model for the ferroelectric compounds of the KH 2 PO 4 family, in the four-particle cluster approximation for the short-range interactions and mean field approximation for the long-range interactions, we calculate thermodynamic and longitudinal dynamic characteristics of the KD 2 PO 4 type ferroelectrics and ND 4 D 2 PO 4 type antiferroelectrics. Calculations for partially deuterated K(H 1−x Dx) 2 PO 4 type ferroelectrics and N(H 1−x Dx) 4 (H 1−x Dx) 2 PO 4 type antiferroelectrics are performed within the mean crystal approximation. It is shown that at the proper choice of the theory parameters, a good quantitative description of experimental data for the KH 2 PO 4 family crystals is obtained.