Vicinity Of Ferroelectric Phase Transition Research Articles

Thermal expansion anomalies in polar and ferroelectric crystals

The physical nature of the phenomenon of negative thermal expansion coefficient seen in the polar crystals and ferroelectrics is considered. Usually it is observed at low temperatures but especially evident in vicinity of ferroelectric phase transitions. This phenomenon is associated with fundamental nature of non-centrosymmetric crystals, caused by electronegativity difference of adjoined ions forming polar crystal. Negative thermal expansion coefficient indicates a change in the nature of forces of the attraction at close proximity of ions forming polar bond. The thermal expansion minimum is specified by the configurational entropy, conditioned by polar bonds self-ordering in pyroelectrics and ferroelectrics.

Heat transfer in area of ferroelectric phase transitions

By measuring thermal conductivity and thermal diffusivity in the vicinity of ferroelectric phase transitions, the processes of polar bonds self-ordering and its forced-ordering by bias electrical field are studied. Thermal energy transfer in polar crystals is inhibited, since responsible for heat transfer shortwave acoustic phonons interact with optical phonons. But in the antiferroelectrics, it is found that optical-acoustic interaction may reinforce thermal diffusion, which is expressed as a large sharp maximum of diffusivity, because soft phonon frequency vanishes in the middle of Brillouin zone that promotes optical phonons participation in heat transfer together with acoustic phonons.

Slow and fast relaxation times of quantum lattice model with local multi-well potentials: phenomenological dynamics for Sn2P2S6 ferroelectric crystals

As a continuation of the previously published work [Velychko O. V., Stasyuk I. V., Phase Transitions, 2019, 92, 420], a phenomenological framework for the relaxation dynamics of quantum lattice model with multi-well potentials is given in the case of deformed Sn2P2S6 ferroelectric lattice. The framework is based on the combination of statistical equilibrium theory and irreversible thermodynamics. In order to study these dynamics in a connected way we assume that the dipole ordering or polarization (η) and volume deformation (u) can be treated as fluxes and forces in the sense of Onsager theory. From the linear relations between the forces and fluxes, the rate equations are derived and characterized by two relaxation times (τS , τF) which describe the irreversible process near the equilibrium states. The behaviors of τS and τF in the vicinity of ferroelectric phase transitions are studied.

Magnetoelectric effect in doped magnetic ferroelectrics

We propose a model of magneto-electric effect in doped magnetic ferroelectrics. This magneto-electric effect does not involve the spin-orbit coupling and is based purely on the Coulomb interaction. We calculate magnetic phase diagram of doped magnetic ferroelectrics. We show that magneto-electric coupling is pronounced only for ferroelectrics with low dielectric constant. We find that magneto-electric coupling leads to modification of magnetization temperature dependence in the vicinity of ferroelectric phase transition. A peak of magnetization appears. We find that magnetization of doped magnetic ferroelectrics strongly depends on applied electric field.

Study of ferroelectric phase transition in Pb5Ge3O11 by paramagnetic resonance of Gd3+ centres

The temperature dependence of the fine structure of trigonal paramagnetic Gd3+ centres in Pb5Ge3O11 was investigated in a wide temperature range in the vicinity of structural phase transition. The temperature dependence of the squared order parameter has been constructed based on the obtained data. It was shown that for the adequate description of its behaviour, the sixth-power term in polarization must be taken into account in the expansion of the thermodynamic potential. The orientational dependence of anomalous broadening of electron paramagnetic resonance signals in the vicinity of ferroelectric phase transition was studied. By comparison of the observed behaviour with the angular dependences of the line width that are characteristic of various broadening mechanisms, it was inferred that it can be attributed to defect induced statistical dispersion of fine structure triclinic parameters.

A study of disorder effects at ferroelectric phase transition in BaTiO 3

The total structure factor of BaTiO 3 obtained by neutron powder diffraction in the temperature range 360–430 K in the vicinity of ferroelectric phase transition has been modeled by the reverse Monte Carlo method. A presence of disordered displacements of Ti atoms due to dynamical tetragonal elongations of unit cells and off-centered sites of Ti atoms in paraelectric phase in the vicinity of the ferroelectric phase transition was observed. Both features result in the anomalous increase of mean square displacements of Ti atoms and reflect the coexistence of displacive and order–disorder components in the ferroelectric phase transition.

Dielectric properties of kdp-adp mixed crystals in the vicinity of ferroelectric phase transitions

Abstract

Linear birefringence in ferroelectric glycine phosphite and deuterated glycine phosphite

Linear birefringence changes were measured with the rotating analyzer method for the glycine phosphite crystal and its deuterated analogue in the vicinity of ferroelectric phase transition. Continuous character of the phase transition was assured. Power rule was found as fulfilled only in close vicinity of Tc. Evident isotope effect was denoted: deuteration changes Tc from 225 to 324K.

Dielectric dispersion in [N(CH3)3H]3Sb2(1-X)Bi2XCl9 mixed crystals

The dielectric response of pure [N(CH3)3H]3Sb2Cl9 (TMACA) and the mixed [N(CH3)3H]3Sb2(1-X)Bi2xCl9 (TMACAB, × = 0.07, 0.33, 0.42) crystals in the frequency range between 75 kHz and 1 GHz was investigated in the vicinity of ferroelectric phase transition. The substitution of Sb by Bi in TMACAB (x < 0.33) caused the shift of the ferroelectric phase transitions point towards the lower temperatures and weakly influenced the dynamic dielectric properties of mixed crystals being isomorphous with pure antimony analogue. The mixed crystals with × = 0.42 appeared to crystallise in different symmetry in comparison to the pure antimony analogue. A clear low frequency relaxation process was found in these last crystals between 300 and 360 K

Critical Slowing Down in Ferroelectric Pyridinium Tetrafluoroborate

Complex permittivity of a ferroelectric single crystal of PyBF4 was measured in the frequency range from 1 kHz to 13 MHz. In the vicinity of ferroelectric phase transition, the phenomenon of critical slowing down was detected. The dielectric relaxation in that crystal can be described by the Debye relaxation equation with a single relaxation time. The results suggest the order–disorder nature of the continuous ferroelectric phase transition.

Ferroelectric phase transition in deuterated glycinium phosphite crystals

The dielectric permittivity between 500 Hz and 1 GHz and pyroelectric properties of (${\mathrm{ND}}_{3}$ ${\mathrm{CH}}_{2}$ COOD)${\mathrm{D}}_{2}$ ${\mathrm{PO}}_{3}$ (DGPI) single crystals in the vicinity of ferroelectric phase transition (322 K) are measured. The spontaneous polarization along the b axis is found to be 5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}3}$ C/${\mathrm{m}}^{2}$ at 290 K. DGPI shows the critical slowing down near 322 K. The results confirm the order-disorder nature of the ferroelectric phase transition of the second order type.

Investigations of the elastic behaviour in the vicinity of ferroelectric phase transitions

Experimental methods are briefly summarized. A survey of anomalies of elastic coefficients in the vicinity of ferroelectric phase transitions and of the acoustic attenuation is given. New trends in research concerning cluster- and glass-like behaviour are also considered.

Dielectric and pyroelectric properties of (CH3NH3)3Me2Br9(Me = Sb, Bi) crystals in the ferroelectric phase transition regions

Abstract Dielectric and pyroelectric properties of isomorphous crystals (CH3NH3)3Me2Br9 (Me = Sb, Bi) in the low temperature region have been compared. The observed dielectric dispersion in the vicinity of ferroelectric phase transition of both crystals revealed the complex nature. The ferroelastic domain structure in all low temperature phases of these two salts exhibits considerable similarity.

Dielectric and pyroelectric properties of (CH3NH3)3Bi2Br9crystals in the ferroelectric phase transition region

Abstract In order to elucidate the nature of the spontaneous polarization the temperature measurements down to 10 K in (CH3NH3)3Bi2Br9 have been performed. The low frequency dielectric relaxation along the a-axis in the vicinity of ferroelectric phase transition clearly indicates the complex nature of dielectric response below T c3. The measurements showed that both the migration of charges and ferroelectric domains contribute to the electric permittivity in the ferroelectric phase.