98-19E

Within the four-particle cluster approximation for the proton ordering model, we study effects of external pressures which do not lower the crystals symmetry on deuterated ferroelectrics and antiferroelectrics of KH$_2$PO$_4$\ family. The theory provides a satisfactory description of the available experimental data for transition temperature and static dielectric properties of the crystals and the experimentally observed universality of the transition temperature vs D-site distance dependence. Importance of the D-site distance in the phase transition and dielectric response of the hydrogen bonded crystals is shown.