We have made dilatometric studies of phase transitions in Sn 2 P 2 S 6 ferroelectric under high hydrostatic pressures. We investigated the pressure dependence of the coefficients in an expansion of the thermodynamic potential in the order parameter in the vicinity of the Lifshitz point. Key words: pressure, ferroelectrics, compressibility PACS: 64.60.FrThe Sn 2 P 2 S 6 monoclinic crystals are proper ferroelectrics, whose unit cell parameters under standard conditions are a=9.378Å, b=7.488Å, c=6.513Å and β=91.150. Under atmospheric pressure p = p atm a temperature increase induces a phase transition (PT) T 0 = 337K into a nonpolar phase with a change of symmetry from P c to P 2 1 /c. By means of a uniform compression (p = 180MPa) we can also realize the Sn 2 P 2 S 6 crystal transition from the ferroelectric state into the paraelectric state at temperature T=296K.The investigation of the p,T -diagram of the Sn 2 P 2 S 6 ferroelectric showed that an external hydrostatic pressure induces a high pressure incommensurate phase in this crystal. The multicritical point of p,T -diagram, where a splitting of the commensurate p,T -line T 0 (p) into a line T i (p) and a line T c (p) bordering the incommensurate phase region occurs, is the Lifshitz point.For the purpose of further PT and critical behaviour investigation of Sn 2 P 2 S 6 crystal in the vicinity of Lifshitz point, induced by external pressure, we have made pressure behaviour studies and analysis of pressure dependencies for linear and volume compressibilities in the present paper.The linear compressibilities under uniform compression were determined by the experimental pressure dependencies of the linear deformations, which were obtained by means of optical interference technique [3] proposed previously. In this case, the directions of measurements coincided with crystallographic directions [100], [010], and [001]. The test specimens for experiments were prepared from Sn 2 P 2 S 6 crystals grown by Bridgeman method. The studies were performed in an optical presc A.G.Slivka, E.I.Gerzanich, P.P.Guranich, V.S.Shusta, V.M.Kedyulich 415

Temperature dependences of longitudinal and transverse dielectric permeability of KDP and DKDP crystals are studied at different values of hydrostatic pressure in order to determine the pressure behaviour of the isotropic point for these crystals. The isotropic point temperature in KDP crystals at atmospheric pressure is T i = 186 K at the measuring field frequency 1 kHz and T i = 176 K at the frequency of 1 MHz. In DKDP crystals the isotropic point is achieved at the temperature T i = 300 K ( 1 kHz) and T i = 253 K ( 1 MHz). The hydrostatic pressure increase results in the reduction of the isotropic point temperature with the pressure coefficients ∂T i /∂p = −4.3 K/kbar for KDP and ∂T i /∂p = −2.9 K/kbar for DKDP. The analysis of the experimental results in the framework of the Blinc-Žekš pseudospin formalism has shown a good agreement between the calculated and the experimentally obtained temperature of the isotropic point for KDP crystals.

The investigations on the temperature change in dielectric permeability of Sn 2 P 2 S 6 crystal at different misaligning electric fields and hydrostatic pressures have been carried out to establish the form of p, T, E-diagram for this crystal. Based on the investigations on dielectric permeability anomalies, the E, T -diagrams at different magnitudes of hydrostatic pressure and p, T, E-diagram of Sn 2 P 2 S 6 crystal have been constructed. The magnitudes of coefficients in the thermodynamic potential expansion have been estimated and coordinates of critical points in E, T -diagrams have been defined. : 77.80.Bh, 64.70.Rh It is known that under the action of external hydrostatic pressure an incommensurate (IC) phase and, respectively, Lifshitz point (LP) [1] are induced in the proper Sn 2 P 2 S 6 ferroelectric. To obtain additional information on the character of PT and the peculiarities of a pressure behaviour of the thermodynamic potential expansion coefficients in this crystal it was of great importance to investigate the effect of external electric fields on phase transitions along p,T -diagram. In the given work the results of experimental investigations on the effect of electric field and hydrostatic pressure on temperature dependences of the dielectric permeability of Sn 2 P 2 S 6 crystal are given which were conducted to establish the form of p,T ,E-diagram of this crystal. Key words: ferroelectrics, phase transitions, electric field, hydrostatic pressure PACSIn figure 1 the E,T -diagrams of Sn 2 P 2 S 6 crystal at different hydrostatic pressures are given. The temperature of phase transition polar-incommensurate phase T c was defined from the maximum of temperature dependence of the dielectric permeability and the temperature of phase transition symmetric-IC phase T i was defined from the break in the dependence ε −1

Temperature dependences of transverse dielectric permeability of KH 2 PO 4 crystals under different values of hydrostatic pressure have been investigated. Within the framework of a tunnelling mode model, the dipole interaction energy and the tunnelling energy are suggested to be determined using the temperature position of the dielectric permeability kinking point. From the temperature dependences of the dielectric permeability ε a (T ) under various pressures, the pressure dependences of these values are derived.