99-25E

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 KD$_2$PO$_4$ type ferroelectrics. Thermodynamic potentials and physical characteristics of the crystals in the presence of stress $\sigma _6$ are calculated. Numerical analysis of the obtained results is performed, and the set of the theory parameters providing the best fit to the available experimental data is found. The $T_{\rm C}-\sigma _6$ phase diagram is constructed; temperature dependences of the calculated quantities are studied, possible stress dependences are indicated. Influence of $\sigma _6$ stress is analogous to that of electric field conjugate to the order parameter.