03-11U

We propose an extension of the proton tunneling model in order to study dielectric, piezoelectric, and elastic properties of the KDP family crystals related to the shear strain $u_6$. In the four-particle cluster approximation for the short-range interactions and the mean field approximation for the long-range interactsion we explore the phase transition and piezoelectric effect in the KH$_2$PO$_4$ ferroelectrics. In the paraelectric phase the dielectric, piezoelectric, and elastic characteristics of the NH$_4$H$_2$PO$_4$ antiferroelectrics are calculated. At the proper choice of the theory parameters a good agreement between theoretical and experimental results is obtained.