12-03U

We proposed modified proton ordering model for quasi-one-dimensional monoclinic RbD$_2$PO$_4$, that takes into account linear over strains $\varepsilon _{i}$ and $\varepsilon _{5}$  contributions to the proton subsystem energy. Within the framework of the model, using the two-particle cluster approximation we obtain a system of equations for the strains $\varepsilon _{i}$ and for the order parameter. Using these equations we  calculate the static dielectric permittivity of the crystal. The dynamic dielectric permittivity of the crystal is calculated within the D.N.Zubarev nonequilibrium statistical operator method. The available experimental data for static and dynamic permittivities, is described. The effect of hydrostatic pressure on the physical characteristics of the crystal is explored.