03-03U

Within the four-particle cluster approximation for the proton ordering model we study the influence of shear stress $\sigma _{6}$ on the phase transition, static dielectric, elastic and thermal properties of deuterated NH$_4$H$_2$PO$_4$ type ferroelectrics. Thermodynamic potentials of the crystals in the presence of stress $\sigma _{6}$ are calculated. Numerical analysis of the obtained results in paraelectric phase is performed, and the set of the theory parameters providing the best fit to the available experimental data is found.