12-17U

Within modified proton ordering model for KH$_2$PO$_4$ type ferroelectrics with hydrogen bonds with taking into account linear on strains  $\varepsilon_i$ and $\varepsilon_6$ contributions into energy of proton system within the four-particle cluster approximation the effect of hydrostatic pressure on phase transition and physical characteristics of K(H$_{1-x}$D$_x)_2$PO$_4$ ferroelectrics are studied. At the proper set of the parameters good quantitative description of the corresponding experimental data for those crystals is obtained.