98-27U

Effects of external hydrostatic and uniaxial pressure on NH$_4$H$_2$PO$_4$ type antiferroelectrics is performed. The previously proposed model of strained ND$_4$D$_2$PO$_4$-type crystals is extended to undeuterated systems (quantum system with tunneling). 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 $p=-\sigma _3$ are obtained. Components of static dielectric susceptibilty tensor of the strained crystal are calculated.