No abstract

The in®uence of the sample orientation on the e¬ective value of the hydrostatic piezoelectric coe¯ cients d (i) h of Sn 2 P 2 S 6 crystals has been studied. The hydrostatic piezoelectric coe¯ cients d (1) h and d' (3) h , were measured, d (1) h = (244 § 3) pC/N and d' (3) h = (92 § 1) pC/N. The hydrostatic piezoelectric coe¯ cient d (3) h for orthogonal axis system was calculated to be d (3) h = (87 § 2) pC/N. The optimal orientation of the sample has been found as (XYl)-20¸-cut. Maximal value of the e¬ective hydrostatic piezoelectric coe¯ cient d (1) h equals 260 pC/N. Double rotated samples were also studied. The orientation of the samples insensitive to the pressure has been found. The theoretical mean value of hydrostatic piezoelectric coe¯ cient (d h) mean corresponding to randomly oriented Sn 2 P 2 S 6 grains in a poled composite has been calculated to be (d h) mean = 136 pC/N.

The temperature-frequency studies of dielectric susceptibility in Sn 2 P 2 S 6 , Sn 2 P 2 Se 6 , (Pb y Sn 1−y ) 2 P 2 Se 6 and crystals at low temperatures are reported. The freezing relaxation dynamics has been observed in a temperature-frequency dependence of the dielectric properties in the crystals which are in a polydomain ferroelectric state as well as in the crystals (Pb y Sn 1−y ) 2 P 2 Se 6 with y > 0.4 which at low temperatures are in the incommensurate phase. The nature of the effect and its interrelation with dynamics of ferroelectric domain walls and incommensurate structure is discussed.