Acoustic and dielectric anomalies in the temperature range of a ferroelectric phase transition in glycine phosphite crystals

The acoustical and dielectric properties of glycine phosphite crystals are investigated in the temperature range of a ferroelectric phase transition. The acoustic anomalies for longitudinal waves along the X, Y, and Z crystallographic axes (where Y is the spontaneous polarization axis) are analyzed in the framework of the Landau theory. It is shown that pronounced acoustic anomalies of the velocity can be quantitatively described within the pseudoproper ferroelectric phase transition model with due regard for the long-range dipole-dipole interaction. For longitudinal acoustic waves propagating along the polar crystal axis, the striction anomaly of the velocity is only partly suppressed by the long-range dipole-dipole forces and an abrupt jump in the velocity is observed in the vicinity of the phase transition. The temperature coefficients of the velocity in the paraphase are determined. The striction contribution and the contributions biquadratic in the order parameter and in the strain to the velocity anomaly are separated.     

外延铁电薄膜相变温度的尺寸效应

考虑外延钙钛矿型铁电薄膜内的等效应力、表面晶格变化和表面电荷引起的退极化效应等机电耦合边界条件,利用铁电薄膜系统的动态金茨堡-朗道方程(DGL),系统分析和讨论了外延铁电薄膜相变温度与临界相变厚度的尺寸效应.结果表明,铁电薄膜相变温度与临界相变厚度完全依赖于各种与薄膜厚度相关的力电耦合边界条件.也给出了BaTiO₃外延铁电薄膜相变温度在各种边界条件下随厚度的变化,从结果看出,本文的分析与结论更符合实验数据. 关键词： 尺寸效应 外延铁电薄膜 相变温度 力电耦合边界     

Quasi-static loops of the dielectric hysteresis for a photosensitive relaxor ferroelectric crystal in the temperature range of the diffuse phase transition

This paper reports on the results of an investigation into the dielectric hysteresis loops of La-and Ce-doped barium strontium niobate, a photosensitive relaxor ferroelectric, both in the dark and under illumination with an intensity of 0.22 mW/cm². Measurements are performed in a quasi-static electric field at a frequency of 5 × 10^?4 Hz for different temperatures in the range of the diffuse phase transition. Illumination increases the hysteresis loop amplitude. The amplitude and the coercive field decrease under heating, and the difference between the loops measured for illuminated and unilluminated crystals disappears gradually as the temperature of the maximum in the permittivity is approached. The experimental results are not in conflict with the assumption that random internal relaxor fields are screened by photoinduced carriers.     

Stress-induced phase transition in ferroelectric domain walls of BaTiO3

The seminal paper by Zhirnov (1958 Zh. Eksp. Teor. Fiz. 35 1175-80) explained why the structure of domain walls in ferroelectrics and ferromagnets is drastically different. Here we show that the antiparallel ferroelectric walls in rhombohedral ferroelectric BaTiO(3) can be switched between the Ising-like state (typical for ferroelectrics) and a Bloch-like state (unusual for ferroelectric walls but typical for magnetic ones). Phase-field simulations using a Ginzburg-Landau-Devonshire model suggest that this symmetry-breaking transition can be induced by a compressive epitaxial stress. The strain-tunable chiral properties of these domain walls promise a range of novel phenomena in epitaxial ferroelectric thin films.     

New liquid crystalline material with a broad temperature range ferroelectric phase

A new series of chiral liquid crystals having cyano group in the mesogenic core has been synthesized and characterized. The synthesized compounds exhibit a broad ferroelectric chiral smectic C mesophase with a high spontaneous polarization.The work was supported by Grants No. 202/93/155 and No. 106/93/0839 from the Grant Agency of the Czech Republic and by Grant No. 19062 from the Grant Agency of the Academy of Sciences of the Czech Republic.     

Proton conductivity and ferroelectric phase transition in hydrogen-bonded ferroelectric NH4IO3

We report on the ac dielectric permittivity (ε) and the electric conductivity (σ_ω), as function of the temperature 300?K?T4IO₃. The main feature of our measured parameters is that, the compound undergoes a ferroelectric phase transition of an improper character, at (368?±?1)K from a high temperature paraelectric phase I (Pm2₁ b) to a low temperature ferroelectric phase II (Pc21n). The electric conduction seems to be protonic. The frequency dependent conductivity has a linear response following the universal power law (σ_{( ω )}?=?A(T)ω ^{s (T)}). The temperature dependence of the frequency exponent s suggests the existence of two types of conduction mechanisms.     

Summary data on ferroelectric PbTiO3 structure

The original data and the data available in literature on PbTiO₃ structure are considered and discussed. From the results of an X-ray diffraction study carried out at room temperature it is concluded that the displacive model is more relevant than the order-disorder one. At the same time it is stressed that the Debye-Waller factors of the atoms are very unusual in the region of the phase transition: B(Pb) > B(O) > B(Ti). This may be evidence of order—disorder phenomena taking place in this temperature region.     

Unusual behavior of the ferroelectric polarization in PbTiO3/SrTiO3 superlattices

Artificial PbTiO3/SrTiO3 superlattices were constructed using off-axis rf magnetron sputtering. X-ray diffraction and piezoelectric atomic force microscopy were used to study the evolution of the ferroelectric polarization as the ratio of PbTiO3 to SrTiO3 was changed. For PbTiO3 layer thicknesses larger than the 3-unit cell SrTiO3 thickness used in the structure, the polarization is found to be reduced as the thickness is decreased. This observation confirms the primary role of the depolarization field in the polarization reduction in thin films. For the samples with ratios of PbTiO3 to SrTiO3 of less than one, a surprising recovery of ferroelectricity that cannot be explained by electrostatic considerations was observed.     

Thermal-wave investigation of the ferroelectric phase transition in colemanite

Thermal waves with penetration depths ranging from 150 m to 3 m and amplitudes smaller than 0.1 mK were used to investigate the ferroelectric phase transition in colemanite. The critical behavior of the pyroelectric current is found to be independent of the penetration depth. It agrees well with the prediction from mean field theory for the temperature derivative of the bulk order parameter. The results are discussed in terms of a possible application of the thermal wave method to the investigation of surface critical phenomena.     

Logarithmic temperature variations of the elastic constant of barium titanate near the ferroelectric phase transition

The temperature variations of the elastic constant C₁₁ and the corresponding acoustic attenuation coefficient of barium titanate single crystals grown by two different methods in the paraelectric phase were obtained in a wide temperature by means of Brillouin light scattering. Two phenomenological approaches were adopted to attempt to explain the anomalous change in C₁₁ (denoted as ΔC₁₁) in the paraelectric phase. ΔC₁₁ exhibited a logarithmic variation as log[(T ? T₀)/T₀] in a certain temperature range from T_c to about T_c + 80 °C instead of the other variation of (T ? T₀)^ζ predicted by the mean-field approach. This temperature range was almost the same as the range where precursor dynamics were theoretically predicted to set in. The fact that the logarithmic variation was proposed for uniaxial systems might indicate that the correlated deformation of precursor polar clusters is tetragonal, as was suggested from nuclear magnetic resonance study [7].     

Structural phase transition in ferroelectric NH4103

The structural phase transformation between the ferroelectric and paraelectric form of NH₄IO, near 83°C is dominated by tilt motions of IO₆-octahedra within a perovskite-like framework. Simultaneously, small displacements of NH₄ along the polar axis give rise to a switchable spontaneous polarization at low temperatures. As the transition mechanism is governed by the tilting of octahedra and not by the appearance of ferroelectricity, NH₄IO₃ is characterised as an improper ferroelectric material. Although the space group of the high-temperature β-phase is polar, the polarization moment is zero. Raman-active thermal soft modes appear in the ferroelectric phase. The phase transition is first order with anomalies of dielectric, piezoelectric and elastic constants.     

Studying the current-voltage characteristics of liquid-crystal langmuir films in the region of the ferroelectric phase transition

The electrophysical properties of hyperfine films of liquid crystals are studied. At temperatures near the phase transition, singularities in the curves of the temperature dependence of the sample capacitance are found. Hypotheses regarding the nature of the phase transition are proposed. A change in the electrical conductivity of the samples based on a TDOB AM BCC liquid crystal is detected upon the ferroelectric phase transition. Hypotheses regarding the causes of the influence of the phase transition on the electrophysical properties of the films are put forward.     

Caloric curves and energy fluctuations in the microcanonical liquid-Gas phase transition

In this paper we study a microcanonical lattice gas model with a constrained average volume. We show that the caloric curve explicitly depends on the considered transformation of the volume with the excitation energy and so does not bear direct information on the characteristics of the phase transition. Conversely, partial energy fluctuations are demonstrated to be a direct measure of the equation of state. Since the heat capacity has a negative branch in the phase transition region, the presence of abnormally large kinetic energy fluctuations is a signal of the liquid-gas phase transition.     

Effect of interstitial hydrogen impurities on ferroelectric polarization in PbTiO3

We investigate the electronic and atomic structure of interstitial hydrogen impurities in PbTiO3 and investigate their effect on ferroelectric polarization through first-principles total-energy calculations. Interstitial H in PbTiO3 is found to bond to oxygen and to act as a shallow donor impurity. At equilibrium, H does not much affect nearby Ti-O bonds and the H-O dipole increases the polarization. The barrier for reversing the defect dipole can give rise to imprint (extra stabilization of one of the two polarization modes).     

EPR study of the ferroelectric phase transition in chromium-doped DMAAS

X-band electron paramagnetic resonance (EPR) investigations of single crystals of Cr³⁺-doped dimethylammonium aluminium sulphate hexahydrate are presented from 100 K to room temperature. The crystal undergoes a phase transition at 152 K from the ferroelastic to the ferroelectric phase. The spin-Hamiltonian parameters have been determined for both phases. The spin-Hamiltonian parameters in the ferroelectric phase are:g=1.980±0.003,b ₂ ⁰ =(1140±15)·10^?4 cm^?1,b ₂ ² =(214±10)·10^?4 cm^?1. Remarkable EPR line width changes confirm the order-disorder character of the ferroelectric phase transition on a microscopic level and demonstrate that the dimethylammonium reorientation freezing-out is the prime reason for this transition.