On the mechanism of the increasing phase transition point in ferroactive nanocomposites

The numerical analysis of a nonlinear equation set has revealed a possible increase in the ferroelectric phase transition point at the presence of the intermediate layer at the ferroelectric–dielectric interface. The order parameter distribution in the ferroelectric particles, as well as the effect of the intermediate layer thickness on the Curie temperature, has been studied.     

Ising model for a ferroelectric phase transition in a system of interacting small particles

An approach based on the Ising model has been proposed for describing a ferroelectric phase transition in a system of interacting identical small particles. It has been found that the shift of the phase transition temperature with respect to the transition point in a bulk sample is affected by both the size effects due to the smallness of the particles and their interaction with each other. The behavior of the dependence of the phase transition temperature on the distance between particles is determined by the nature of the interparticle interaction. An analysis has demonstrated that the interaction between small particles should be taken into account in the interpretation of the ferroelectric properties of nanocomposite materials.     

Effect of point defects on phase transitions in ferroelectric nanocrystals

The dependences of the phase transition temperature of ferroelectric nanocrystals in a dielectric matrix on the point charged defect concentration have been obtained. The influence of point defects on the nonlinear characteristics of ferroelectric nanocrystals has been studied as a function of the strength and direction of an external electric field with the adequate inclusion of depolarizing electric fields and nonlocal effects.     

Surface tension and Curie temperature in ferroelectric nanowires and nanodots

The effect of surface tension associated internal pressure on the Curie phase transition in ferroelectric nanowires and nanodots has been investigated using a modified Landau–Ginzburg–Devonshire phenomenological approach. Based on experimental data on the size- dependent phase transition in freely suspended single-crystalline ferroelectric nanocrystals, bulk surface tension coefficients for BaTiO₃ and PbTiO₃ have been determined to be of the order of 1–2 N/m. The present theoretical study reproduces the size dependence of the transition temperature experimentally acquired in individual BaTiO₃ single-crystalline nanowires. In the case of PbTiO₃ single-crystalline nanodots, however, in order to fit the theoretically calculated size-dependent ferroelectric transition with the experimental data, an effective surface tension coefficient has been introduced, which is size dependent and can be much higher than the bulk value. An erratum to this article can be found at     

The effect of hydrostatic pressure on the ferroelectric phase transition in [NH2(CH3)2]3[Sb2Cl9]

The effect of hydrostatic pressure on the ferroelectric phase transition temperature in [NH₂(CH₃)₂]₃[Sb₂Cl₉] (DMACA) has been studied by electric permittivity measurements at pressures up to 400 MPa. The pressure-temperature phase diagram is given. The phase transition temperature (T_c) increases with increasing pressure up to 150 MPa, passes through a maximum and then decreases with a further increase of pressure. The unexpected nonlinear decrease in T_c with pressure increasing above 150 MPa suggests that the mechanism of ferroelectric phase transition in DMACA is different from hitherto assumed.     

Structural phase transition in ferroelectric fluorine polymers: X-ray diffraction and infrared/Raman spectroscopic study

The structural phase transition has been investigated by X-ray diffraction and infrared and Raman spectroscopic measurements for ferroelectric fluorine polymers, including poly(vinylidene fluoride) and its copolymers with trifluoroethylene or tetrafluoroethylene. One of the most characteristic features of this ferroelectric transition is the large conformational change of the molecular chains between the trans and gauche rotational isomers, quite different from the structural change observed generally in the usual ionic ferroelectric materials. The crystallization and transition behaviors depend sensitively on the monomer composition in the copolymers as well as on the sample preparation conditions. The roles of the optic and acoustic phonons in the ferroelectric phase transition have been discussed based on the temperature dependences of the far-infrared spectra and the ultrasonic velocity.     

Effect of confined geometry on linear and nonlinear dielectric properties of triglycine sulfate near the phase transition

The temperature dependence of the linear permittivity and the third harmonic generation amplitude of nanocomposites representing nanoporous silica matrices (opal matrix and SBA-15) with triglycine sulfate embedded in pores has been studied in the region of the ferroelectric phase transition. A broadening of the phase transition and an increase its temperature in comparison with bulk triglycine sulfate have been revealed. The latter becomes more significant as the pore size decreases. It has been shown that the nonlinear dielectric properties of nanocomposites near the phase transition differ significantly from the properties of bulk triglycine sulfate.     

Nuclear magnetic resonance study of potassium dihydrophosphate

A powder sample of potassium dihydrophosphate KH₂PO₄ has been studied by the ³¹P NMR method in a wide temperature range covering the ferroelectric phase transition. Changes in the position and shape of the resonance line at the transition to the ferroelectric phase have been revealed. The parameters of the chemical shift tensor of ³¹P (isotropic shift, anisotropy, and asymmetry) in the ferroelectric phase have been calculated from the experimental data. A sharp increase in the anisotropy of the tensor at the phase transition has been demonstrated. Dielectric measurements have also been carried out to verify the transition temperature.     

Off-center displacements and hydrostatic pressure induced phase transition in perovskites

Pressure has a profound effect on the paraelectric and ferroelectric properties of perovskite crystals. In this paper we theoretically investigate the effect of pressure on the cubic-to-tetragonal phase transition and on the soft mode dynamics of some classical perovskite crystals: BaTiO(3), PbTiO(3), and KNbO(3). We use a model consisting of three subsystems: electrons, phonons, and off-center displacements treated as spins. Experiments show that pressure has a large effect on the tunneling and hopping of the off-center displacements, that in turn strongly affect the pressure dependence of the transition temperature and the soft mode frequency. This model, with a very small number of adjustable parameters, accounts quantitatively for the experimentally measured nonlinear pressure dependence of the cubic-to-tetragonal phase transition temperature, up to the critical pressure where the transition temperature is zero. It also accounts quantitatively for the pressure dependence of the soft mode frequency, which is finite at the phase transition in spite of the fact that the phase transition at elevated pressures is second order, and for the pressure dependence of the electronic gap energy.     

Giant infrared intensity of the Peierls mode at the neutral-ionic phase transition

We present exact diagonalization results on a modified Peierls-Hubbard model for the neutral-ionic phase transition. The ground state potential energy surface and the infrared intensity of the Peierls mode point to a strong, nonlinear electron-phonon coupling, with effects that are dominated by the proximity to the electronic instability rather than by electronic correlations. The huge infrared intensity of the Peierls mode at the ferroelectric transition is related to the temperature dependence of the dielectric constant of mixed-stack organic crystals.     

Enhancement of the magnetoelectric effect in thin ferroelectric films

The enhancement of the nonlinear magnetoelectric effect in a magnetic field is predicted for a ferroelectric film in a system consisting of ferroelectric and paraelectric layers in the vicinity of the size-induced ferroelectric phase transition. This effect is assumed to be maximum in semiconductor ferroelectrics.     

Manifestation of a ferroelectric phase transition in ultrathin films of polyvinylidene fluoride

Temperature dependences of the dielectric properties of ultrathin polyvinylidene fluoride films prepared using the Langmuir-Blodgett method were studied by linear and nonlinear dielectric spectroscopy. It is shown that ultrathin Langmuir films of polyvinylidene fluoride exhibit a manifestation of a first-order ferroelectric phase transition, which can be assigned to the interaction between the spontaneous polarization and the surfaces bounding the film. As the film thickness increases, the effect of the surfaces decreases and the ferroelectric phase transition shifts to high temperatures to vanish altogether when the temperature region of the transition rises above the melting point.     

The role of the spatial distribution of local polarization perturbations in the posistor effect onset

The electrical resistance of polycrystalline ferroelectric semiconductors is defined by the potential barriers due to the existence of local charged surface states at crystallite boundaries. The barrier screening depends on the state of the ferroelectric system and is maximal during spontaneous-polarization switching. It is shown in this paper that the local perturbation of the ferroelectric system, resulting from the repolarization and appearing as a domain wall between the regions with different polarization directions, has a zigzag configuration. The electric field in the vicinity of the zigzag domain wall is stabilized and coincides with the coercive field, which provides low potential barriers in the ferroelectric phase compared with the paraelectric phase. The repolarization processes become inefficient in the potential barrier screening at the transition from the ferroelectric to the paraelectric phase. As a result, a sharp increase in the electrical resistance is observed at the ferroelectric-paraelectric phase transition, called the posistor effect.     

Effect of thermal stresses on the phase transition temperature in a ferroelectric-dielectric nanocomposite

The mechanism of an increase in the phase transition temperature of a ferroelectric-dielectric nanocomposite above the corresponding temperature of an unbounded homogeneous ferroelectric has been revealed. The significant thermal stresses induced in the composite material due to the difference in the thermal and elastic characteristics of its components affect the polarized state of the ferroelectric component via the electrostrictive coupling and can lead to an increase in the phase transition temperature of the composite.     

关于铁电相变的一个唯象讨论

从不可逆过程热力学的角度研究了铁电相变中的不可逆性.一级铁电相变中的热滞及铁电体的多畴结构,可以在最小熵产生原理的基础上得到说明.并得出结论,热滞并不是一级铁电相变体系的内禀性质,体系表面的有限性与热滞是有关的. 关键词： 热滞 畴构型 不可逆性 最小熵产生     

X-ray study of the phase transition in RbHSeO4

The ferroelectric phase transition in RbHSeO₄ has been examined by precise measurements of the temperature dependence of the unit-cell parameters using the Bond technique. The transition, at T_c = 370.6 K, is of first order and the triclinic unit cell of the ferroelectric phase transforms to an equitranslational monoclinic cell in the paraelectric phase. In a temperature region of several K below T_c reflections belonging to both phases have been observed.     

Electret states and the phase transition in a surface layer of the TlGaSe2 ferroelectric semiconductor

The electret polarization is investigated in the TlGaSe₂ ferroelectric semiconductor. It is proved for the first time that stable internal electric fields associated with residual electret polarization are induced in crystals of the TlGaSe₂ ferroelectric semiconductor at temperatures T < 200 K. It is experimentally established that the peak of the pyroelectric current measured in the vicinity of the phase transition to the ferroelectric polar phase depends substantially on the temperature at which the external electric field is switched off when the TlGaSe₂ ferroelectric crystal under investigation is preliminarily cooled from room temperature. The results obtained are discussed in the framework of a model according to which internal electret fields are induced by charges localized at different levels in the bulk and on the surface of the TlGaSe₂ ferroelectric crystal. These fields drastically change at temperatures in a narrow range near 135 K. The inference is made that a phase transition occurs in the surface layer of the TlGaSe₂ crystal at a temperature close to ～135 K.     

Specific features of the formation of the ferroelectric phase in polytypes of TlGaSe2 crystals

The polytypism of layered crystals of thallium gallium diselenide TlGaSe₂ has been found to substantially affect the temperature of phase transformations and the mechanism of formation of the polar state in these ferroelectrics. In particular, it is shown that the phase transition observed in the C-TlGaSe₂ polytype is an improper ferroelectric phase transition occurring at a temperature T _c ≈ 108 K, whereas the phase transition observed in the 2C-TlGaSe₂ polytype is a proper ferroelectric phase transition occurring at a higher temperature T _c ≈ 111 K. It is concluded that the elucidation of the polytype of a particular sample is a necessary stage of investigation of the TlGaSe₂ crystals.     

Ferroelectricity, nonlinear dynamics, and relaxation effects in monoclinic Sn2P2S6

An ab initio-based model of the temperature-induced ferroelectric phase transition in Sn2P2S6 (SPS) as a prototype of an unconventional ferroelectric is developed. The order parameter in SPS is found as the valley line on a total-energy surface of the zone-center fully symmetrical Ag and polar Bu distortions. Significant nonlinear coupling between order parameter and strain is observed. Monte Carlo simulations describe the additional low-temperature rearrangement in polar structure, which appears in domain boundaries, and describe the relaxation phenomena near the ferroelectric phase transition.     

Study of new rare earth family Pb1.6K1.2R0.2Nb5O15 (R=La, Nd, Sm, Eu and Gd) of tetragonal tungsten bronze-type ferroelectrics

A new ferroelectric rare earth family Pb_1.6K_1.2R_0.2Nb₅O₁₅ with R=La, Nd, Sm, Eu and Gd (PKRN) of tetragonal tungsten bronze type ferroelectrics was synthesized. The ferroelectric transition with nonuniform distribution of critical temperature over a ceramic sample was found from dielectric measurements. According to X-ray diffraction measurements, the ferroelectric phase has an orthorhombic symmetry. The transition temperature was shown to decrease weakly with increasing radius of the rare earth ion R.