97-23U

Effects of external hydrostatic and uniaxial pressure on KH$_2$PO$_4$\ type ferroelectrics is performed. The previously proposed model of strained KD$_2$PO$_4$-type crystals is extended to undeuterated systems (quantum system with tunneling). With the help of a model potential of a hydrogen bond, we study the pressure dependence of the tunneling integral. The set of equations for the order parameter and lattice strains is derived and the equation for the temperature of phase transition as a function of hydrostatic and uniaxial $\sigma _3$ are obtained. The longitidunal static dielectric susceptibilty of the crystal is calculated.