Hydrostatic pressure‐induced shifts in the phase transition temperatures of monocrystalline thiourea are studied. The transition occurring at 202 K under atmospheric pressure is found to shift linearly downwards. The other transitions behave likewise at pressures up to 0.9 kbar. At 0.9 kbar the phase transitions, lying in the temperature interval from 169 to 180 K under atmospheric pressure, vanish. From hysteresis loop observations, the hydrostatic pressure‐induced change in spontaneous polarization in the region of phases I and III is determined.

The dielectric permittivity E of BaTiO, single crystals versus the temperature 1 ' at different hydrostatic pressure u, and ~( u ) a t different T , was measured. The highest (i achieved was 12800 kg/cm2, between 22 "C and 180 "C. At u < 3600 kg/cm2, the Curie point T,:decreases a t a rate of -4.3 x "Ccm2/kg which ceases to be constant for a > 3500 kg/cm2.The Curie-Weiss temperature falls with growing u. At higher u, the Curie constant rises with growing u. The ~( u ) graphs a t constant T below 120 "C exhibit maxima related to an induced phase transition. Above 120 "C, E decreases with growing u. ,,Memory of pressure'' is observed on removal of high hydrostatic pressure.On a mesure la constante diblectrjque E des monocristaux de BaTiO, en fonction de la temperature T pour diffhentes valeurs de la pression hydrostatiqne u, ainsi que la dkpendance ~( u ) pour differentes T. La plus haute pression realisBe fut de 12800 kg/cm2, entre 22 "C et 180 "C. A des valeurs de u < 3500 kg/cm2, on a trouv6 un abaissement de la temperature Curie T, de -4,3 ."C cm2/kg qui cesse d'$tre linBaire pour u > 3500 kg/cm2.La temperature Curie-Weiss dberoit et la constante Curie monte pour des IS considbrables, lorsque u augmente. Au dessous de 120 "C, les courbes de ~( u )A T constante prbsentent dcs maxima lies A une transition phasique induite. Au dessus de 120 "C, E dbcroit lorsque IS augmente. Une ,,m&moire de pression" apparait a p r h Blimination de la haute pression hydrostatique.

were the first to study the effect of hydrostatic pressure on the phase transition of TGS. They found a linear upward shift in Curie point with -3 growing pressure at a rate of dT / d d = 2 . 5~1 0 et al. (2) followed the effect of pressure on T and extended the result of (1) t o 3.3 kbar. Upward of this pressure, a deviation from linearity was found up to pressures of 5 kbar. Measurements by Zheludev et al. (3) resulted in a linear increase in T at a rate of dT / d d = 1 . 8~1 0 -~d e g / b a r up to 12 kbar. Samara (4) studied the effect of hydrostatic pressure on the electric permittivity of a TGS monocrystal up t o 17 kbar. In this paper, we report results on the influence of hydrostatic pressure up to 12 kbar on the phase transition of a TGS monocrystal. Moreover, in a modified thermodynamical theory taking into account the temperature and pressure dependence of the electromechanical coefficient, we determine the pressure-induced shift of the Curie point. deg/bar UP to 2.7 kbar. Leonidova C C C C The electric permittivity of the TGS monocrystal was measured a s a function of temperature at various values of the hydrostatic pressure, and a s a function of press u r e at various temperatures. The electric permittivity measurements were performed both during heating and cooling. The characteristic permittivity curves were observed to shift towards higher temperatures with growing pressure. At transition from the paraelectric phase to the ferroelectric phase the permittivity values observed in the Curie point are larger than those found for the ferropara transition.Permittivity versus pressure measurements performed at various temperatures showed that the higher the temperature the larger was the pressure required to obtain a phase transition. Fig. 1 shows a linear rise in Curie temperature a s a function of pressure up to 4 kbar, at a rate of dT / d d = deg/bar; at still higher pressures the dependence C