Influence of the geometry of a porous network on the phase transition in a ferroelectric embedded in a porous matrix

A ferroelectric phase transition in a system of electrically interacting small particles is considered. The size effects in a single particle are described in terms of the Landau phenomenological model. The interaction between particles is reduced to a dipole-dipole interaction. It is shown that the interparticle interaction can lead to a substantial increase in the phase transition temperature as compared to the temperature of the ferroelectric transition in a single small particle.     

On the possible mechanism of increase in the phase transition temperature in a ferroelectric-insulator composite

The phase transition temperature of a small ferroelectric sample has been determined by solving the system of nonlinear equations of electrostatics and the equilibrium equation with a nonlinear boundary condition at the sample surface with the use of a method of finite elements. It has been shown that the phase transition temperature can be shifted to higher temperatures owing to the effect of the surface. The dependence of the phase transition temperature on the size and shape of the ferroelectric sample and on the properties of its surface has been found. The possibility of the experimental observation of this effect has been discussed.     

Desorption of carbon dioxide from small potassium niobate particles induced by the particles’ ferroelectric transition

The aim of this work is to understand surface properties of ferroelectric crystals related to gas adsorption. Various ferroelectric crystals involved in these studies readily adsorb carbon dioxide, thus our studies were centered on adsorption studies of this molecule. It has been claimed that a dipole moment is induced on carbon dioxide molecules that are near an oxide surface. Our experiments explored the possibility of a dipole-dipole interaction between the gas molecule and the ferroelectric oxide surface in order to explain its adsorption. We characterized the samples with scanning electron microscopy, X-ray diffraction and Raman spectroscopy. We determined the ferroelectric nature of the particles and studied the temperature-dependent phase transitions in small particles of KNbO₃ using Raman spectroscopy. We were able to correlate desorption of CO₂ from one surface state of KNbO₃ with the occurrence of the orthorhombic to tetragonal transition in KNbO₃ in particles of 1 μm size. This CO₂ surface site was not observed in KTaO₃, which does not show ferroelectricity at room temperature.     

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.     

钨青铜型结构铁电体的低温扩散相转变

对六种钨青铜结构的铌酸盐铁电晶体进行了从15 K 到室温范围的介电特性及热电特性的研究. 分析其介电特性和热电特性与极化电场的关系, 用x 射线粉末衍射进行佐证. 证实在50 一70 K 的范围内, SBN , PBN , KNSBN 三类铁电钨青铜铌酸盐晶体均存在着一个新的相变. 相变是由点群4mm铁电相到点群mm2 铁电相的转变. 铁电自发极化方向由四方晶胞的c轴方向转变到正交晶胞的b 轴方向.介电特性的高频及低频测量表明该相变具有扩散(或称弥散)型特征. 比热的实验结果证明相变是属于高于一阶相变的高阶相变. 对相变前后的晶胞结构提出了一个模型解释. 关键词：     

Long—Range Effects on the Pyroelectric Coefficient of Ferroelectric Superlattice

Long-range effects on the pyroelectric coefficient of a ferroelectric superlattice consisting of two different ferroelectric materials are investigated based on the transverse Ising model.The effects of the interfacial coupling and the thickness of one period on the pyroelectric coefficient of the ferroelectric superlattics are studied by taking into account the long-range interaction.It is found that with the increase of the strength of the long-range interaction,the pyroelectric coefficient decreases when the temperature is lower than the phase transition temperature;the mumber of the pyroelectric peaks decreases gradually and the phase transition temperature increases,It is also found that with the decrease of the interfacial coupling and the thickness of one period.the phase transition temperature and the number of the pyroelectric peaks decrease.     

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.     

Long-Range Effects on the Pyroelectric Coefficient of Ferroelectric Superlattice

Long-range effects on the pyroelectric coefficient of a ferroelectric superlattice consisting of two differentferroelectric materials are investigated based on the transverse Ising model. The effects of the interfacial coupling andthe thickness of one period on the pyroelectric coefficient of the ferroelectric superlattice are studied by taking intoaccount the long-range interaction. It is found that with the increase of the strength of the long-range interaction, thepyroelectric coefficient decreases when the temperature is lower than the phase transition temperature; the number ofthe pyroelectric peaks decreases gradually and the phase transition temperature increases. It is also found that with thedecrease of the interfacial coupling and the thickness of one period, the phase transition temperature and the number ofthe pyroelectric peaks decrease.     

钨青铜型结构铁电体的低温扩散相转变

对六种钨青铜结构的铌酸盐铁电晶体进行了从15K到室温范围的介电特性及热电特性的研究。分析其介电特性和热电特性与极化电场的关系,用X射线粉末衍射进行佐证。证实在50—70K的范围内,SBN,PBN,KNSBN三类铁电钨青铜型铌酸盐晶体均存在着一个新的相变。相变是由点群4mm铁电相到点群mm²铁电相的转变。铁电自发极化方向由四方晶胞的e轴方向转变到正交晶胞的b轴方向。介电特性的高频及低频测量表明该相变具有扩散(或称瀰散)型特征。比热的实验结果证明相变是属于高于一阶相变的高阶相变。对相变前 关键词：     

Investigation of the seeding-layer effect for a ferroelectric thin film with the transverse Ising model

The transition feature of a ferroelectric thin film with a seeding layer is studied based on the transverse Ising model. The influence of the seeding layer on the transition behavior of a ferroelectric thin film is investigated systemically, and the effect of the interaction parameters for the seeding layer on the phase diagram is also obtained. Meanwhile, the polarization and Curie temperature of the ferroelectric thin film are calculated for different seeding-layer structures. The results show that the polarization and Curie temperature of the film will be obviously modified on adding a seeding layer.     

Effect of thermal phenomena on a second-order phase transition in the Landau-Ginzburg model

The influence of electrocaloric and pyroelectric effects on a phase transition in a ferroelectric material has been studied. The difference in the parameters of the Landau model for isothermal and adiabatic processes has been indicated. The temperature dependence of the spontaneous polarization is described by a special function (the probability integral) that results in the disappearance of the second-order phase transition.     

Dielectric properties of the nanoporous MCM-41 matrix filled with the (NH4)2SO4 ferroelectric

Variations with temperature of the linear dielectric permittivity and amplitude of the third harmonic were studied for nanoporous MCM-41 matrices with 4.0-nm channel pores filled with the (NH₄)₂SO₄ ferroelectric, in comparison with bulk ammonium sulfate. The measurements were performed upon heating and cooling in the temperature range from 100 K to room temperature. A noticeable shift to low temperatures (by approximately 25 K) for the ferroelectric phase transition in the MCM-41/(NH₄)₂SO₄ nanocomposite as compared to bulk (NH₄)₂SO₄ was revealed. The temperature hysteresis observed at the phase transition in the nanocomposite was approximately 2 K which is close to that in bulk ammonium sulfate. The significant decrease of the transition temperature in nanostructured ammonium sulfate agrees with the theoretical predictions based on the Landau and Ising models of the size effect on the ferroelectric phase transition in isolated small particles.     

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.     

BKT phase transition in a 2D system with long-range dipole-dipole interaction

We consider phase transitions in 2D XY-like systems with long-range dipole-dipole interactions and demonstrate that BKT-type phase transition always occurs separating the ordered (ferroelectric) and the disordered (paraelectric) phases. The low-temperature phase corresponds to a thermal state with bound vortex-antivortex pairs characterized by linear attraction at large distances. Using the Maier-Schwabl topological charge model, we show that bound vortex pairs polarize and screen the vortex-antivortex interaction, leaving only the logarithmic attraction at sufficiently large separations between the vortices. At higher temperatures the pairs dissociate and the phase transition similar to BKT occurs, though at a larger temperature than in a system without the dipole-dipole interaction.     

A low-temperature tilter system and its application to the measurement of the anisotropy of optical rotation in K2Zncl4 in the vicinity of the phase transition at 145 k

The ferroelectric/ferroelastic phase transition of K₂ZnCl₄ at 145 K has been investigated employing the tilter method (Kaminsky and Glazer (1996) Ferroelectrics 183, 133) which was adapted to low-temperature experiments. We were able to observe the anisotropy of optical rotation in K₂ZnCl₄ which shows a distinct discontinuity at the transition temperature. A precursor-optical rotation in the ferroelectric phase is discussed in connection with a cluster ordering scheme suggested in an earlier X-ray study. The rotation of the optical indicatrix which accompanies the transition from the orthorhombic to the monoclinic system shows a Landau-type temperature dependence. Model calculations based on the dipole-dipole interaction between the atoms of the structures at different temperatures are in qualitative agreement with the experimental results.     

Ab-initio calculation of band structure and linear optical properties of SbSI in para- and ferroelectric phases

An ab-initio pseudopotential calculation has been performed by using density functional methods within the local density approximation (LDA) to investigate the band structure and optical properties of the ferroelectric-semiconductor SbSI in the para- and ferroelectric phases. It has been shown that SbSI has an indirect gap in both phases (1.45 eV and 1.49 eV in the para- and ferroelectric phases respectively) and that the smallest direct gap is at the S point of the Brillouin zone (1.56 eV and 1.58 eV in the para- and ferroelectric phases respectively). Furthermore, it is shown that first-order phase transition, from the paraelectric phase to the ferroelectric phase (the transiton temperature is about 22 °C), does not change the nature of the band gap. Moreover, the linear frequency dependent dielectric function, including self-energy effects, has been calculated along the c-polar axis in the para- and ferroelectric phases.      

Diffuse phase transition of Fe doped lead ytterbium tantalate ceramics

The effect of different concentration of Fe on the phase transition behavior of Lead ytterbium tantalate is investigated by dielectric and differential scanning calrimetry measurements. The samples are prepared through solid state reaction method and it has been found that the sintering temperature significantly lowered when the proportion of Pb(Fe_1/2Ta_1/2)O₃ increased. It has been observed that the doping in small amounts (0≤x≤0.2) of Fe could meliorate the dielectric and ferroelectric properties. The diffuseness in the mode of phase transition increases and the phase transition temperature decreases as a function of Fe content. It is revealed that the dielectric data and heat capacity data follow a similar trend in the variation of the mode of phase transition and phase transition temperatures. The phase transition temperature values obtained from the heat capacity measurement well agreed with the values obtained from dielectric measurement.     

Dielectric studies of nanoporous alumina films filled with the Rochelle salt

Dielectric studies of nanoparticles of the Rochelle salt embedded in pores of porous alumina have been performed in the temperature range from 80 K to the decomposition temperature of the bulk Rochelle salt. It has been revealed that the permittivity exhibits an anomaly corresponding to the lower structural transition to the paraelectric phase, whereas the upper ferroelectric transition is shifted above the decomposition temperature in agreement with the recently published data. The temperature of the lower transition for nanoparticles in pores is found to decrease by 10 K. Possible physical factors that can be responsible for the broadening of the region of existence of the ferroelectric phase have been discussed.     

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