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1.
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. 相似文献
2.
G. Rujijanagul 《Phase Transitions》2013,86(3):209-215
(Pb1? 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. 相似文献
3.
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?280?K) for polycrystalline samples of NH4IO3. 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?1) suggests that the quantum mechanical tunneling model is the most likely one that describes the conduction mechanism. The core results of the article are: (1) the low temperature ac electrical parameters are measured for NH4IO3; (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. 相似文献
4.
Maria Połska 《Phase Transitions》2013,86(4):409-468
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. 相似文献
5.
We report on the ac dielectric permittivity (ε) and the electric conductivity (σω), as function of the temperature 300?K?T400?K and frequency 0.5–100?kHz for polycrystalline samples of hydrogen-bonded ferroelectric ammonium iodate, NH4IO3. 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. 相似文献
6.
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. 相似文献
7.
The ac conductivity (σac) and dielectric permittivity (?) are determined in the temperature range 300?K?T520?K at some selected frequencies (0.5–10?kHz) for the polycrystalline samples of KIO3 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. 相似文献
8.
The structural phase transitions and the electrical behaviour of the complex perovskite PbLu1/2Nb1/2O3 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. 相似文献
9.
Wenhui Ma 《Applied Physics A: Materials Science & Processing》2009,96(4):915-920
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 相似文献
10.
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. 相似文献
11.
S.W.H. Eijt R. Currat J.E. Lorenzo P. Saint-Grégoire B. Hennion Y.M. Vysochanskii 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,5(2):169-178
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 (Px), comparable to the behaviour observed in previous Raman investigations. As found previously in Sn2P2Se6, a strong coupling between the TO(Px) and TA(uxz) phonons is observed, although, apparently, not strong enough to lead to an incommensurate phase. The soft TO(Px) 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(Px) 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(uyx) and LA(uxx) 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 相似文献
12.
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. 相似文献
13.
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 Tc can be attributed to the existence of a precursor order near the transition. 相似文献
14.
A. S. Anokhin A. G. Razumnaya Yu. I. Yuzyuk Yu. I. Golovko V. M. Mukhortov 《Physics of the Solid State》2016,58(10):2027-2034
A comparative study of the lattice dynamic upon phase transitions in a polycrystalline Ba0.8Sr0.2TiO3 (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. 相似文献
15.
ABSTRACTThe calcium-substituted barium titanate nanopowders Ba1?xCaxTiO3 (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. 相似文献
16.
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. 相似文献
17.
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. 相似文献
18.
研究了等静压对0.75Pb(Mg1/3Nb2/3)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Tm降低,/+{dTm}/-{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 相似文献
19.
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. 相似文献
20.
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. 相似文献