96-18E

We performed experimental investigations of deuteration and uniaxial $p=-\sigma _3$ and $p=-\sigma _1$ mechanical stress influence on temperature and wavelength dependences of the birefringence and transition temperature of K(H$_x$D$_{1-x})_2$PO$_4$\ crystals. On the basis of the previously proposed model for a strained KD$_2$PO$_4$ crystal, we study the effects of uniaxial $p=-\sigma _3$ and $p=-\sigma _1=-\sigma _2$ pressures on the phase transition, thermal, dielectric and elastic properties of highly deuterated K(H$_x$D$_{1-x})_2$PO$_4$\ crystals. The obtained results are compared with available experimental data and earlier theoretical calculations, concerning the hydrostatic pressure effects.