  We verify whether the previously developed model of a KD2PO4 crystal, with
the shear strain eps6 taken into account, is able to describe the longitudinal
electric field E3 influence on the KH2PO4 family ferroelectrics. Major effects
of the strain eps6 are splitting of the Slater energies of the short-range
correlations and the effective field created by piezoelectric coupling.
Calculated TC-E3 phase diagrams, field dependences of polarization,
susceptibility, and elastic constant of deuterated KD2PO4 and pure KH2PO4 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.
