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.     

Phase transition and linear thermal expansion of (Pb1− x Ba x )ZrO3 ceramics

(Pb_{1? x} Ba _x )ZrO₃ ceramics for the composition range 0?≤?x?≤?0.30 were prepared by the mixed oxide solid state reaction method. Phase transition was studied by dielectric and dilatometric measurements. The ferroelectric to paraelectric phase transition temperature was progressively shifted to a lower temperature by replacing lead with barium. The x?=?0.20 sample showed the maximum dielectric constant of 16,300 at the transition temperature. For compositions 0?≤?x?≤?0.075, the antiferroelectric to ferroelectric phase transition exhibited a large linear thermal expansion. However, the antiferroelectric to ferroelectric phase transition did not exist for 0.10?≤?x?≤?0.30 samples. A phase diagram for PBZ ceramics prepared by the conventional mixed oxide method was also present.     

Dielectric behavior and low temperature phase transition in NH4IO3

The electrical properties namely ac conductivity σ(ω,?T) and the complex dielectric permittivity (ε*) are measured at selected frequencies (5–100?kHz) as function of temperature (95?K?T?4IO₃. The ferroelectric hysteresis loops and the X-ray diffraction pattern are also measured. The analysis of the data indicates that the compound undergoes a structural phase transition at ～103?K and the behavior of σ(ω,?T) obeys the power law. The trend of the temperature dependence of the angular frequency exponent s (0?s?4IO₃; (2) the data indicate that the compound undergoes a structural phase transition at 103?K; (3) the originality of this transition has been confirmed by X-ray diffraction; (4) no evidence for the existence of a ferroelectric transition at 103?K as mentioned earlier; and (5) the quantum mechanical tunneling is proposed as the main mechanism of the electric conduction.     

Ferroic phase transitions in β-LiNH4SO4

Abstract

The paper reviews the results of experimental and theoretical studies of ferroic phase transitions in β-LiNH₄SO₄ and its deuterated analogue. β-LiNH₄SO₄ undergoes succesive phase transitions: a paraelectric - ferroelectric phase transition at T₁ ? 462 K, a ferroelectric - ferroelastic phase transition at T₂ ? 283 K and a transition from one ferroelastic phase to the other at T₃ ? 28 K. Attention is focused on the influence of the order of phase transitions on the pattern of ferroelectric and ferroelastic domain structure, and also on the role played by the dynamics of molecular groups in the mechanism of transitions. The pre-transition effect connected with the ferroelectric-paraelectric transition: heterophase, capable of accounting for anomalies in different physical properties present 1-3 K below T₁ is shown. The anomalous temperature variation of spontaneous polarisation of the crystal is discussed within the framework of the phenomenological model of weak ferroelectrics.     

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.     

Ultrasonic propagation near the ferroelectric phase transition temperature in RbH2PO4

Ferroelectric phase transition in RbH₂PO₄ is studied by the propagation of longitudinal ultrasonic waves along the polar axis. The velocity data are analyzed to obtain the temperature dependence of the soft mode frequency in the ferroelectric phase. Unlike KH₂PO₄ and KD₂PO₄, the attenuation in RbH₂PO₄ does not exhibit a relaxation type behaviour.     

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.     

Successive phase transitions in the highly ordered complex perovskite PbLu1/2Nb1/2O3

The structural phase transitions and the electrical behaviour of the complex perovskite PbLu_1/2Nb_1/2O₃ have been investigated using X-ray powder diffraction, dielectric constant measurements, differential scanning calorimetry and measurement of the polarisation as a function of applied electric field. The high-temperature paraelectric phase is highly ordered. A first-order paraelectric-antiferroelectric phase transition occurs at 270°C and an antiferroelectric-ferroelectric phase transition, characterised by dispersion in the curves of dielectric constant as a function of temperature, occurs at ≈ 30°C. The antiferroelectric phase is isostructural with the orthorhombic form of PbYb1/₂Nb1/₂O₃. The low-temperature ferroelectric phase also has an orthorhombic crystal structure.     

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     

New type of far-infrared soft mode in ferroelectric gadolinium molybdate

A new type of soft mode has been observed in far infrared transmission spectra of ferroelectric Gd₂(MoO₄)₃ in 40–50 cm^-1 region. The soft mode is split and its oscillator strength decreases towards the phase transition. Its contribution to the static dielectric constant is essentially temperature independent in the ferroelectric phase and amounts only to 0.034 ± 0.008.     

Soft modes and phonon interactions in studied by neutron scattering

The low frequency lattice dynamics and its relationship to the second order paraelectric-to-ferroelectric transition in Sn₂P₂S₆ is studied. The dispersion branches of the acoustic and lowest lying optical phonons in the a^*-c^* plane have been obtained in the ferroelectric phase, for x-polarized phonons. Close to the phase transition a considerable softening is found for the lowest optical mode (P_x), comparable to the behaviour observed in previous Raman investigations. As found previously in Sn₂P₂Se₆, a strong coupling between the TO(P_x) and TA(u_xz) phonons is observed, although, apparently, not strong enough to lead to an incommensurate phase. The soft TO(P_x) mode at the zone center is observed. The temperature dependence of its frequency and damping shows that the transition is not entirely displacive. At low temperatures an unusual apparent negative LO-TO splitting is observed which is shown to arise from the coupling of the x-polarized soft mode to the nearby z-polarized optical phonon. For comparison, the soft TO(P_x) dispersion in the a^*-b^* plane is measured in both the paraelectric and ferroelectric phases. Consistent frequency changes and LO-TO splitting are observed, revealing a significant interaction between the TA(u_yx) and LA(u_xx) acoustics branches and the TO and LO soft optic branches, respectively. In contrast, the nearby y-polarized optic branch shows almost no temperature dependence. Finally, the influence of piezoelectric effects on the limiting acoustic slopes in the ferroelectric phase is discussed. Received: 11 May 1998 / Revised and Accepted: 15 June 1998     

Anomalous light scattering at the ferroelectric phase transition of KH2PO4

The temperature-dependent scattered light intensity accompanying the ferroelectric phase transition of KH₂PO₄ is presented as an isofrequency-temperature scan. The temperature dependence of the data is shown to be better fit to the coupled harmonic oscillator model than to the single overdamped oscillator. This mode of data presentation also displays the anomalous light scattering near the phase transition.     

Persistence of tetragonal raman lines in cubic KNbO3

The temperature dependence of the vibrationnal modes in KNbO₃ around the cubic-tetragonal phase transition is studied by Raman scattering measurements. An unexpected intense spectrum is observed above the transition. In addition to broad bands probably due to second order process, it consists of forbidden lines which are characteristic of the tetragonal phase. In particular the presence of the hard component of the soft ferroelectric cubic mode until 30° above T_c can be attributed to the existence of a precursor order near the transition.     

Phase transitions in barium–strontium titanate films on MgO substrates with various orientations

A comparative study of the lattice dynamic upon phase transitions in a polycrystalline Ba_0.8Sr_0.2TiO₃ (BST) film on a Pt substrate and in epitaxial BST films grown on various sections of an MgO substrate has been performed by Raman spectroscopy. It has been found that different sequences of phase transitions take place in these films. The BST/Pt films demonstrate the same sequence of phase transitions that is observed in the bulk ceramics. The hardening of a soft mode in BST/(001)MgO and BST/Pt films shows that the transition from the tetragonal ferroelectric phase to the paraelectric phase has features of the displacement-type phase transition and also the order–disorder phase transition. When approaching the ferroelectric transition temperature, the soft mode in the BST/(111)MgO film is softened, following the Cockran law, which indicates the displacement-type phase transition.     

Preparation and properties of Ba1−xCaxTiO3 nanopowders obtained by mechanochemical synthesis

ABSTRACT

The calcium-substituted barium titanate nanopowders Ba_1?xCa_xTiO₃ (0.2 ≤ x ≤ 0.3) have been obtained at room temperature by mechanochemical synthesis. The formation of the perovskite phase was controlled by X-ray diffraction studies at various milling duration. The powders possess the perovskite crystallographic structure directly after milling longer than 10 h. The dielectric properties of the ceramics obtained by sintering of the nanopowders were investigated in the temperature range between 300 and 500 K. The temperature dependence of permittivity exhibited a single anomaly, which corresponds to the ferroelectric–paraelectric phase transition.     

On the order parameter of RbH3(SeO3)2

Rubidium trihydrogen selenite RbH₃(SeO₃)₂ undergoes an improper ferro-electric phase transition due to the condensation of a doubly degenerate soft mode at the zone boundary. Superlattice reflections have been measured using neutron diffraction, and after taking domains into account, it is shown that the measured reflections scale onto the same curve as a function of temperature. The consequences of this scaling are explained in terms of the temperature dependence of the order parameter of the transition.     

Thermal expansion of Sn2P2S6 crystals

We present the results for thermal expansion coefficients of Sn₂P₂S₆ crystals determined both in the crystallographic system and the system based on eigenvectors of thermal expansion tensor. Peculiarities of temperature evolution of the indicative surface of thermal expansion tensor for Sn₂P₂S₆ are discussed, including the region of their ferroelectric phase transition.     

等静压下0.75Pb（Mg1/3Nb2/3）O3-0.25PbTiO3陶瓷的介电性能研究

研究了等静压对0.75Pb（Mg_1/3Nb_2/3）O₃-0.25PbTiO₃（PMN-25PT）陶瓷介电温谱的影响,PMN-25PT剩余极化随等静压变化和等静压压致相变.结果表明,随着压力增加,PMN-25PT的介电峰值温度T_m降低,/+{dT_m}/-{dP}≈-4℃/kbar,极化弛豫增强;剩余极化随压力增加连续减小;介电常数对压力的依赖关系与对温度场的依赖相似,压力诱导PMN-25PT发生弛豫铁电—顺电相变,相变为宽化的渐变过程,频率色散和极化弛豫更加强烈和普遍. 关键词： 铌镁酸铅-钛酸铅 等静压 介电弛豫 压致相变     

Soft vibrational mode in LiNbO3 and LiTaO3

The temperature dependence of the far-infrared reflectivity as obtained with a scanning interferometer for the A₁- and E-type modes of both LiNbO₃ and LiTaO₃ is reported in the ferroelectric phase. Results of a Kramers-Kronig analysis are compared with Raman and neutron scattering data which are controversial about the existence of a soft vibrational mode. For LiTaO₃, spectra are obtained 300 K above the Curie temperature, in the paraelectric phase. The lowest-frequency A₁ (TO) mode is unambiguously found as soft and becomes rapidly overdamped.     

First-order character of the displacive structural transition in BaWO4

Nearly all displacive transitions have been considered to be continuous or second order, and the rigid unit mode （RUM） provides a natural candidate for the soft mode. However, in-situ X-ray diffraction and Raman measurements show clearly the first-order evidences for the scheelite-to-fergusonite displacive transition in SaWO4： a 1.6% volume collapse, coexistence of phases, and hysteresis on release of pressure. Such first-order signatures are found to be the same as the soft modes in BaWO4, which indicates the scheelite-to-fergusonite displacive phase transition hides a deeper physical mechanism. By the refinement of atomic displacement parameters, we further show that the first-order character of this phase transition stems from a coupling of large compression of soft BaOs polyhedrons to the small displacive distortion of rigid WO4 tetrahedrons. Such a coupling will lead to a deeper physical insight in the phase transition of the common scheelite-structured compounds.