00-12U

We verify whether the previously developed model of deuterated KD$_2$PO$_4$ type crystals, with the shear strain $\varepsilon _6$ taken into account, is able to describe the longitudinal electric field $E_3$ influence on the KH$_2$PO$_4$ family ferroelectrics. Major effects of the strain $\varepsilon _6$ are splitting of the Slater energies of the short-range correlations and the effective field created by piezoelectric coupling. Calculated $T_{\rm C}-E_3$ phase diagrams, field dependences of polarization, susceptibility, and elastic constant of deuterated KD$_2$PO$_4$ and pure KH$_2$PO$_4$ well accord with the available experimental data. For a consistent description of all dielectric and piezoelectric characteristics of the crystals, phonon degrees of freedom must be taken into account.