01-38U

Within the framework of the proton model we study thermodynamic and static dielectric properties of the ADP type antiferroelectrics taking into account tunneling effects of protons on hydrogen bonds. In the four-particle cluster approximation for short-range interaction and in the mean field approximation for long-range interaction we calculate the free energy, entropy and components of static dielectric susceptibility, we also obtain 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 obtain a good agreement between the theoretical results and the corresponding experimental data for the temperature dependences of the spontaneous sublattice polarization, specific heat, longitudinal and transverse static dielectric permittivities of the crystals.