In this dissertation the static and dynamic properties of ferroactive crystals with order-disorder type phase transitions are studied and the influence of electric fields, hydrostatic and uniaxial pressures and shear stresses on their physical characteristics is investigated. To do this, in the dissertation there is developed pseudospin models of deformed crystals KH2PO4, CsH2PO4, RbD2PO4, Rochelle salt, glycine phosphite (GPI) and RbHSO4, each of which takes into account the coupling of the pseudospin subsystem with lattice strains. The thermodynamic potential was calculated for each of the crystals, from the condition of the minimum of which a system of equations for the order parameters and lattice strains, as well as expressions for different thermodynamic characteristics were obtained. From the condition of the best agreement of theoretical results with experimental data, the optimal model parameters for these crystals were found.
In particular, the GPI model takes into account the dependence of the interaction parameters between pseudospins on the tensile and shear strains of the lattice. In the two-particle cluster approximation, the expressions for entropy, the component of the polarization vector and the dielectric constant tensor, and the piezoelectric and elastic characteristics were obtained. The dynamic dielectric constant and pseudospin relaxation times of GPI were calculated within the Glauber stochastic model. It is shown that hydrostatic and uniaxial pressures lower the Curie temperature in the GPI crystal, as well as increase the relaxation times and shift the dispersion region of the dielectric constant to lower frequencies. Shear stresses σ4 and σ6 in the absence of an electric field nonlinearly increase Tc. The transverse permittivity goes to infinity at the Tc point. On the other hand, when only the transverse field is applied to the GPI in the absence of shear stresses, the Curie temperature at weak fields decreases approximately by the quadratic law, and the transverse permittivity increases markedly in the ferroelectric phase. Application to the GPI of the transverse field simultaneously with the stresse σ4 or σ6 leads to smearing of the phase transition. In the GPI model, taking into account the dependence of the longitudinal effective dipole moments on the order parameters, the influence of the longitudinal and transverse fields is described, electrocaloric effect is calculated.
In the pseudospin models of CsH2PO4 and RbD2PO4 crystals the linear on the strains ε1, ε2, ε3, ε5 contributions to the energy of the pseudospin subsystem are taken into account. Within these models, in the two-particle cluster approximation it was studied the influence of hydrostatic and uniaxial pressures and longitudinal electric field on dielectric, piezoelectric, elastic and thermal characteristics of CsH2PO4, as well as the effect of hydrostatic pressure on the longitudinal dielectric constant of RbD2PO4. The phase transition to the antiferroelectric phase at high hydrostatic pressures in the CsH2PO4 is explained. There is studied the character of the smearing of the phase transition from the paraelectric to the ferroelectric phase in this crystal at low pressures, as well as the suppression of the antiferroelectric phase at high pressures under the action of the longitudinal field.
In the framework of the modified pseudospin model of KH2PO4, taking into account the linear on the lattice strains ε1, ε2, ε3, ε6 contribution to the energy of the pseudospin subsystem in the four-particle cluster approximation, its polarization, longitudinal dielectric permittivity and piezoelectric coefficients are calculated. It is shown that the hydrostatic pressure linearly lowers the temperature Tc. The dependence of the effective dipole moments on the order parameter is also taken into account, the electrocaloric and piezocaloric effects are calculated.
Within the modified pseudospin model of the Rochelle salt, taking into account the piezoelectric coupling of the pseudospin and lattice subsystems in the mean field approximation, it is shown that the field E2 narrows the temperature range of the ferroelectric phase and the field E3 widens it. The transverse fields can increase the transverse permittivity by several orders of magnitude near Tc.
In the pseudospin model of RbHSO4 there is taken into account the linear dependence of the parameters of the inter-pseudospin interaction on the shear and tensile strains of the lattice. In the mean field approximation, dielectric, piezoelectric, elastic and thermal characteristics under the action of mechanical stresses and longitudinal electric field are calculated. This model predicts a linearly increasing dependence of Tc temperature on hydrostatic and uniaxial p2, p3 pressures, as well as on shear stress σ5. Uniaxial pressure p1 and shear stress σ6 lower the Curie temperature. The electric field smears the phase transition.
