08-19U

Within modified proton ordering model of KH$_{2}$PO$_{4}$ family crystals with taking into account linear on strain $\varepsilon _6 $ contribution into energy of proton subsystem, but without taking into account tunneling, within the four-particle cluster approximation we study phase transition, thermodynamic and longitudinal dielectric, piezoelectric, elastic and dynamic properties of ND$_4$D$_2$PO$_4$ type antiferroelectrics. Quadratic piezoelectric and elastic characteristics, electrostrictive constants sound velocity and sound attenuation in crystals are calculated. At the proper set of the parameters in paraelectric phase quantitative description of the experimental data for ND$_{4}$D$_{2}$PO$_{4}$ and NH$_{4}$H$_{2}$PO$_{4}$ is found.