GUYVAN H. Anisotropy of dielectric properties of KH2PO4, KD2PO4 and NаКС4H4O6*4H2O crystals under external pressure and electric field

The thesis is devoted to all-round studies of external factors (temperature, pressure, and electric field along different crystallographic directions) effect on the anisotropy of dielectric parameters of NаКС4H4O6*4H2O, KH2PO4, and KD2PO4 crystals. The regularities of temperature transformation of cross-sections of indicating surface of dielectric permittivity tensor for these crystals at phase transitions from the paraelectric to ferroelectric phase are obtained. The experimental results on the effect of electric field and pressure on the Rochelle salt NаКС4H4O6*4H2O dielectric permittivity are analyzed in the framework of phenomenological theory with the account of the phase transition closeness to a double critical point. Coefficients of thermodynamic potential expansion over the order parameter are determined as well as their pressure behaviour. Phase (sigma_i,T) and (E ,Т) diagrams of the Rochelle salt are built. sigma_1 and sigma_4 mechanical strains are shown to result in an expansion of the temperature interval of the ferroelectric phase existence while sigma_2 and sigma_3 lead to its shrinkage.