R. R. Levitskii

Thermodynamic and Dielectric Properties of K1-x(NH4)xH2PO4 Mixed Crystal

We used the cluster pseudospin model for the K1−x(NH4)xH2PO4 type proton glasses, which takes into account the energy levels of protons near a PO4 groups (within the cluster approximation), the long-range interactions between the hydrogen bonds, and an internal random deformation field. We obtained a qualitative description of the temperature behavior of the order parameters, static dielectric permittivity, and the phase diagram of the K1−x(NH4)xH2PO4 compounds.

Microscopic Theory of Rb1 - x(NH4)xH2PO4 Type Compounds

We propose a pseudospin model for the Rb 1 −x (NH 4 )xH 2 PO 4 type proton glasses, which takes into account energy levels of hydrogens near a PO 4 group (within a cluster approximation), long-range interactions between the hydrogen bonds, and an internal random deformational field. Relaxation is explored within the framework of the Glauber dynamics. We obtain a qualitative description of temperature behavior of the state parameters and dielectric permittivities of Rb 1 −x (ND 4 ) x D 2 PO 4 compounds at different frequencies.

Longitudinal field influence on phase transition and physical properties of the KH2PO4 family ferroelectrics

Abstract We verify whether the previously developed model of a KD2PO4 crystal, with the shear strain varepsilon6 taken into account, is able to describe the longitudinal electric field E 3 influence on the KH2PO4 family ferroelectrics. Major effects of the strain varepsilon6 are splitting of the Slater energies of the short-range correlations and the effective field created by piezoelectric coupling. Calculated T C-E 3 phase diagrams, field dependences of polarization, susceptibility, and elastic constant of deuterated KD2PO4 and pure KH2PO4 well accord with the available experimental data. For a consistent description of all dielectric and piezoelectric characteristics of the crystals, phonon degrees of freedom must be taken into account.We verify whether the previously developed model of a KD2PO4 crystal, with the shear strain eps6 taken into account, is able to describe the longitudinal electric field E3 influence on the KH2PO4 family ferroelectrics. Major effects of the strain eps6 are splitting of the Slater energies of the short-range correlations and the effective field created by piezoelectric coupling. Calculated TC-E3 phase diagrams, field dependences of polarization, susceptibility, and elastic constant of deuterated KD2PO4 and pure KH2PO4 well accord with the available experimental data. For a consistent description of all dielectric and piezoelectric characteristics of the crystals, phonon degrees of freedom must be taken into account.

Features of the behavior of longitudinal static and dynamic dielectric, piezoelectric, and elastic characteristics of KH2PO4 and NH4H2PO4 crystals

Along with their electromechanical coupling coefficients, the longitudinal dielectric, piezoelectric and elastic characteristics in ferroelectric KH2PO4 and antiferroelectric NH4H2PO4 crystals are calculated using a modified proton ordering model that considers piezoelectric coupling and the four-particle cluster approximation. The possibility of detecting piezoactivity in solid solutions of K1 − x(NH4)xH2PO4 is substantiated.

Mitsui model with diagonal strains: A unified description of external pressure effect and thermal expansion of Rochelle salt NaKC_4H_4O_6·4H_2O

We elaborate a modification of the deformable two-sublattice Mitsui model of [Levitskii R.R. et al., Phys. Rev. B. 2003, 67, 174112] and [Levitskii R.R. et al., Condens. Matter Phys., 2005, 8, 881] that consistently takes into account diagonal components of the strain tensor, arising either due to external pressures or due to thermal expansion. We calculate the related to those strains thermal, piezoelectric, and elastic characteristics of the system. Using the developed fitting procedure, a set of the model parameters is found for the case of Rochelle salt crystals, providing a satisfactory agreement with the available experimental data for the hydrostatic and uniaxial pressure dependences of the Curie temperatures, temperature dependences of spontaneous diagonal strains, linear thermal expansion coefficients, elastic constants c_ij^E and c_i4^E, piezoelectric coefficients d_1i and g_1i (i=1,2,3). The hydrostatic pressure variation of dielectric permittivity is described using a derived expression for the permittivity of a partially clamped crystal. The dipole moments and the asymmetry parameter of Rochelle salt are found to increase with hydrostatic pressure.

Monoclinic elastic and piezoelectric properties of Rochelle salt. Description within the modified Mitsui model

We propose a modification of the two-sublattice Mitsui model that takes into account a coupling with diagonal components of the strain tensor. Within the developed model, we calculate the piezoelectric and elastic characteristics of Rochelle salt related to those strains. In the approximation of zero thermal expansion, we find a set of the model parameters providing a satisfactory agreement with the available experimental data for the elastic constants cEij and c E i4 and for the piezoelectric constants d1j and g1j .

Dielectric, Elastic and Electromechanical Properties of K1-x(NH4)xH2PO4 Solid Solutions in Paraelectric Phase

Temperature dependences of dielectric permittivity ϵ33, piezoelectric constant d36 and elastic compliance S66 of K1-x(NH4)xH2PO4 ferro – antiferroelectric (FE - AFE) mixed crystals have been studied within temperature range 30 – 300 К. It was found a considerable contribution of the strain component x6 to polarization P3 near the FE transition temperature due to piezoelectric effect. Piezoelectric contribution to dielectric response in the vicinity AFE phase transition is weaker. It leads, in particular, to difference in the both shapes and positions of ϵ33 and S66 anomalies for K0,25(NH4)0,75H2PO4 single crystal near the transition temperature.

Dynamics of the Rochelle salt NaKC_{4}H_{4}O_{6}·4H_{2}O crystal studied within the Mitsui model extended by piezoelectric interaction and transverse field

We calculated dynamic dielectric permittivity of the Rochelle salt within the Mitsui model, extended by piezoelectric interaction and transverse eld. Calculations were based on the parameters derived earlier within the study of thermodynamic properties of Rochelle salt. The study of dynamic properties was performed within the Bloch equation method. We showed that taking transverse eld into account allows for better description of the Rochelle salt relaxation dynamics. Furthermore, we showed that taking transverse eld into account results in the appearance of a resonant component in dynamic permittivity like it is observed in experiment. However, in accordance with the calculations, resonant response reveals itself within infrared frequency range, whereas in experiment it is observed within submillimeter spectral region.

Role of piezoelectricity in dielectric response of Rochelle salt type crystals

Comparing the spontaneous polarizations, static and dynamic dielectric properties of piezoelectric Rochelle salt and non-piezoelectric RbHSO4 we explore the role played by piezoelectric coupling in forming the dielectric response of the crystals of this type. The calculations for crystals of both types are performed within the Mitsui model, modified for the case of Rochelle salt by including the terms related to piezoelectric coupling with spontaneous strain ε4. It is shown that such a modification improves the agreement between theory and experiment for spontaneous polarization and yields a correct temperature behavior of relaxation times and dynamic dielectric permittivity of Rochelle salt in the vicinity of the transition points.

Dielectric, Electromechanical, and Elastic Properties of K1−x(NH4)xH2PO4 Compounds

We present the results of the thermodynamic theory of piezoelectric ferroelectrics and of the phenomenological theory of thermodynamic characteristics of the KH2PO4 type ferroelectrics. Available experimental data for the dielectric, piezoelectric, and elastic characteristics of the K1−x(NH4)xH2PO4 type systems are described using the proposed microscopic theory of these crystals.