The critical point indices of ferroelectric and ferromagnetic phase transitions

Magnetic transitions are described by the critical indices0,1/3,4/3 while some ferroelectric transitions appear to give0,R~1/2,1. It is pointed out that these two sets of values for the critical indices are allowed by the scaling laws and stability conditions near the phase transitions.The authors thank Prof. R. S. Krishnan for his encouragement and Mr. B. Viswanathan for some discussions. The financial assistance from DAE and CSIR is also acknowledged.     

Phase transitions in ultrathin ferroelectric polymer films and nanocrystals

The dielectric dispersion of nanocrystals of vinylidene fluoride—trifluoroethylene copolymer of 70/30 composition was investigated. A phase transition was revealed in ferroelectric nanocrystals at T = 100°C. A wide temperature hysteresis was observed, indicating a first-order phase transition.     

外加电场对铁电薄膜相变的影响

采用热力学非线性理论,研究了外加电场对立方基底Pb(Zr_0.3Ti_0.7)O₃(PZT)铁电薄膜相变的影响.通过数值计算,得到了"失配应变-外加电场"相图,及外加电场与极化强度的关系.当外加电场达到186 kV/cm时,能使生长在SrTiO₃ 基底上PZT铁电薄膜从单斜r相转变为c相.在实验上,采用扫描探针显微镜通过对PZT薄膜施加不同的极化电场来研究了它的电畴翻转.从得到的压电响应相图可以看出,绝大多数的电畴是清晰可 关键词： 铁电薄膜 相变 扫描探针显微镜 失配应变     

Structural phase transitions in niobium oxide nanocrystals

Niobium oxide nanocrystals were successfully synthesized employing the green synthesis method. Phase formation, microstructure and compositional properties of 1, 4 and 7 days incubation treated samples after calcinations at 450 °C were examined using X-ray diffraction, Raman, photoluminescence (PL), infrared, X-ray photoelectron spectra and transmission electron microscopic characterizations. It was observed that phase formation of Nb₂O₅ nanocrystals was dependent upon the incubation period required to form stable metal oxides. The characteristic results clearly revealed that with increasing incubation and aging, the transformation of cubic, orthorhombic and monoclinic phases were observed. The uniform heating at room temperature (32 °C) and the ligation of niobium atoms due to higher phenolic constituents of utilized rambutan during aging processing plays a vital role in structural phase transitions in niobium oxide nanocrystals. The defects over a period of incubation and the intensities of the PL spectra changing over a period of aging were related to the amount of the defects induced by the phase transition.     

Influence of disorder in crystal structure on ferroelectric phase transitions

A new model of ferroelectric phase transitions in disordered crystals is developed. The model takes into account the nonequivalence of the structural environment of identical ions, which alters the balance of forces governing ferroelectric structural instability. In contrast with its predecessors, the new model can be used for qualitative and, in many cases, quantitative predictions of the temperature range of the diffuse ferroelectric phase transition and the value of the disorder-induced shift of the average transition temperature as a function of the degree of disorder and chemical composition. This conclusion is confirmed by a comparison of the theoretical results with an abundance of known experimental facts. Zh. éksp. Teor. Fiz. 111, 1817–1832 (May 1997)     

Far infrared and phase transitions in ferroelectric materials

Abstract

A number of far infrared absorption bands are found in molecular and hydrogen-bonded crystals when there are several formula units in the primitive cell. They are very sensitive to the crystal structure and constitute a convenient probe for looking at small changes in the structure, especially at low temperatures where there is a lack of X-Ray data. As well known examples, the case of LTT (Lithium Thallium Tartrate) where the absorption band observed at 21 cm^?1 at room temperature softens down to 8 cm^?1 at the Curie temperature (T_c = 12 K), and LiNH₄SO₄ which has a phase transition at 26 K, will be considered.

It is less known that some ferroelectric crystals can be studied as very thick single crystal plates (thickness t up to 10 mm) at 4 K, and they show in some cases a far IR transmission that is much higher than expected from the study of thin plates (i.e. t = 20 μm). In fact the transmission does not decrease very much when thickness is higher than some specific value t ₀/2. The crystal is not homogeneous at 4 K. A model with a far IR absorbing surface layer (thickness t ₀/2) and a transparent bulk is a good first approximation.

At some temperature T ₀ located between liquid helium and liquid nitrogen temperatures, the center of the bulk undergoes a phase transition from the absorbing phase into the transparent one. When temperature is still lowered, the transparent phase is extended towards the surface with an incommensurate phase between the surface and the bulk.     

Diffuse ferroelectric phase transitions in cubically sabilized perovskites

Typical examples of ferroelectrics with diffuse phase transitions (relaxor ferroelectrics), like Pb(Mg_1/3Nb_2/3)O₃ are actually non transforming. The paraelectric phase is fully stablized against a ferroelectric phase transition in this case. A phase transition can be induced, however, by an electric field with appropriate orientation below the temperature of the dielectric constant maximum. The analogy with stress-induced martensitic phase transitions in metallic alloys is pointed out. Pecularities of the properties and of the polarization reversal of such systems are demonstrated. Actual diffuse ferroelectric phase transitions in disordered solid solutions and mixed compounds with a partially stabilized parent phase are compared with athermal martensitic transformations. With particular regard to technical ceramics based on PZT, the influence of interfaces between transformed regions and remnants of the parent phase which have to distinguished from domain walls, and of the reduced stability of the ferroelectric phase on the properties of these systems is discussed. Some effects usually explained solely by domain processes may be understood also from this point of view.

Stabilization of a parent phase against an order-disorder-type phase transition is supposed to be caused by glass-like freezing caused by inelastic cooperative interactions between disordered molecular groups.     

High-temperature phase transitions in the improper ferroelectric KIO3

The ac conductivity (σ_ac) and dielectric permittivity (?) are determined in the temperature range 300?K?T3 compound. The results indicated that the compound behaves as an improper ferroelectric and undergoes a ferroelectric phase transition from a high temperature rhombohedral phase I to a low temperature monoclinic phase II at T _c?=?(486?±?1)?K. A second structural phase transition was observed around 345?K. The conductivity varies with temperature range and for T?>?428?K intrinsic conduction prevails. Different activation energies in the different temperature regions were calculated. The frequency dependence of σ(ω) was found to follow the universal dynamic response [σ(ω)∝(ω) ^s(T)]. The thermal behaviour of the frequency exponent s(T) suggests the hopping over the barrier model rather than the quantum mechanical tunneling model for the conduction mechanism.     

铁电液晶MBOPDOB相变的光散射研究

我们对铁电液晶 MBOPDOB 进行了温度有关的拉曼散射研究。在低频范围记录了三条拉曼谱线(15,49,93cm~(-1))。其中15和49cm~(-1)谱线消失于 C-Sm C相变温度,没有检测到任何频移现象。93cm~(-1)拉曼线产生部分频移,消失于各向同性相,联系着偶极-偶极互作用。高频范围四个振动模(1178和1116cm~(-1),1710和1738cm~(-1))之间能量转移归于由电子转移引起的 C=0键硬化和 C-0健的弱化。同时也讨论了该液晶 Sm C-Sm A 相变的二级连续型特征。     

Strain-induced ferroelectric phase transitions in incipient ferroelectric rutile TiO2

Uniaxial strain induced ferroelectric phase transitions in rutile TiO₂ are investigated by first-principles calculations. The calculated results show that the in-plane tensile strain induces rutile TiO₂, paraelectric phase with P_4-2/mnm (D_4h) space group, to a ferroelectric phase with P_m (C_s) space group,driven by the softening behaviour of the E_u1 mode. In addition, the out-of-plane tensile strain, vertical to the ab plane, leads to a ferroelectric phase with P_42nm (C_4v) space group, driven by the softening behaviour of the A_2u mode. The critical tensile strains are 3.7% in-plane and 4.0% out-of-plane, respectively. In addition, the in-plane compression strain, which has the same structure variation as out-of-plane tensile strain due to Poisson effect, leads the paraelectric rutile TiO₂ to a paraelectric phase with P_nnm (D_2h) space group driven by the softening behaviour of the B_1g mode. These results indicate that the sequence ferroelectric (or paraelectric) phase depends on the strain applied. The origin of ferroelectric stabilization in rutile TiO₂ is also discussed briefly in terms of strain induced Born effective charge transfer.     

Study of ferroelectric phase transitions in solid solutions based on bismuth vanadate

The structural, thermal, nonlinear optical, and conducting properties of Bi₄(V_{1 ? x} Me _x )₂O_{11 ? y} solid solutions (Me = Zr, Ga, Fe) have been investigated. The effect of domain wall pinning, accompanying the α ? β ferroelectric phase transition, is revealed. It is found that the conductivity in all studied series of solid solutions increases with an increase in x.     

Symmetry breaking at the phase transitions in ferroelectric pyridinium salts

The phase transitions in ferroelectric pyridinium tetrafluoroborate (PyBF₄) and pyridinium perchlorate (PyClO₄) have been characterized by structural studies. The continuous ferroelectric phase transition at 238.7 K in PyBF₄ appears to be a unique case for multiaxial ferroelectrics, while the first order phase transition in PyClO₄ at 248 K is consistent with the Landau theorem for deducing the character of phase transitions from symmetry considerations. The phase transition in both materials is caused by ordering of the pyridinium cation as well as tetrahedral anions.     

Structural phase transitions in nanosized ferroelectric barium strontium titanate films

The lattice parameters of epitaxial barium strontium titanate films with various thicknesses (from 6 to 960 nm) were measured as a function of temperature in the normal and tangential directions with respect to the film plane using x-ray diffraction. The films were grown through the layer-by-layer mechanism by rf cathode sputtering under elevated oxygen pressure. A critical film thickness (～ 50 nm) was found to exist, below and above which the films are subjected to compressive and tensile stresses, respectively. As the temperature varies from 780 to 100 K, the films undergo two diffuse structural phase transitions of the second order over the entire thickness range. The transitions in the films under tensile stresses are likely to be transformations from the paraelectric tetragonal to aa phase and then to r phase, whereas the transitions under compressive stresses are transformations from the tetragonal paraelectric to ferroelectric c phase and then, with further decreasing temperature, to r phase.     

Isotropic to ferroelectric and antiferroelectric liquid crystals phase transitions

This review is an attempt towards a unified picture of the direct transitions from the isotropic liquid to ferroelectric and antiferroelectric liquid crystalline phases formed by rod-like and bent-core molecules. The Landau–Ginzburg theories of the phase transitions between the isotropic liquid to ferroelectric and antiferroelectric liquid crystalline phases in compounds composed of chiral rod-like molecules and achiral bent-core molecules are presented. This includes a discussion of the nature of the order parameters and the nature of the various types of phase transitions. The various predictions are compared with the available experimental results.     

Strong disorder fixed point in absorbing-state phase transitions

The effect of quenched disorder on nonequilibrium phase transitions in the directed percolation universality class is studied by a strong disorder renormalization group approach and by density matrix renormalization group calculations. We show that for sufficiently strong disorder the critical behavior is controlled by a strong disorder fixed point and in one dimension the critical exponents are conjectured to be exact: beta=(3-sqrt[5])/2 and nu( perpendicular )=2. For disorder strengths outside the attractive region of this fixed point, disorder dependent critical exponents are detected. Existing numerical results in two dimensions can be interpreted within a similar scenario.     

Properties of ferroelectric thin films with a first-order phase transitions

A Green's function technique is used to investigate the properties of ferroelectric thin films with a first-order phase transitions on the basis of the transverse Ising model. Taking into account the four-spin interactions beside the two-spin interactions the dependence of the polarization on film thickness and temperature and the thickness dependence of the Curie temperature become more complicated.