铁电相变的软模理论

本文综述了铁电相变研究的概况,重点是最近二十年中发展起来的“软模”理论。软模理论把晶格振动与晶体的结构变化联系在一起,使结构相变的物理图象变得清晰了,本文主要介绍软声子、赝自旋模、电子——振动相变和软模——中心模理论。     

铁电相变的软模理论

本文综述了铁电相变研究的概况,重点是最近二十年中发展起来的“软模”理论。软模理论把晶格振动与晶体的结构变化联系在一起,使结构相变的物理图象变得清晰了,本文主要介绍软声子、赝自旋模、电子——振动相变和软模——中心模理论。     

关于铁电相变的一个唯象讨论

从不可逆过程热力学的角度研究了铁电相变中的不可逆性.一级铁电相变中的热滞及铁电体的多畴结构,可以在最小熵产生原理的基础上得到说明.并得出结论,热滞并不是一级铁电相变体系的内禀性质,体系表面的有限性与热滞是有关的. 关键词： 热滞 畴构型 不可逆性 最小熵产生     

铌酸钡钠和钽酸锂铁电相变的实地观察

本文报道了用透射电子显微镜实地观察铌酸钡钠和钽酸锂铁电相变的实验结果。在室温电畴的尺寸为0.2—1μm。当温度增加到接近居里点时,形成准规则排列的微畴结构,其尺寸为几百埃,实验结果用稍微修正了的模型进行了解释。在钽酸锂中当接近铁电相变温度时,发现了强烈的辐照损伤,这可能跟相变的软模有关。 关键词：     

近化学计量比铌酸锂晶体的晶格振动和相变软模研究

在室温下,近化学计量比铌酸锂晶体(S-LN)的拉曼散射研究从理论和实验上揭示了晶格空位对于拉曼光谱的影响。高温下,随着温度的升高,软模(5的拉曼峰变化非常强烈。一个软光学声子模来源于在铌-氧平面内的三重简并对称弯曲振动。顺电-铁电结构相变发生在大约1170℃。     

冲击加载下PZT-95/5陶瓷铁电－反铁电相变实验研究

 用石英计测量了PZT-95/5陶瓷在冲击波作用下发生铁电－反铁电一级相变时产生的双波结构，相变起始压力约为0.5 GPa。对不同状态下的PZT-95/5陶瓷材料进行了扫描电镜电畴分析，结果表明，在该压力区域陶瓷发生了铁电－反铁电相变。     

用布里渊散射研究晶体相变

在研究相变和临界现象中,光散射实验起着十分重要的作用.在晶体的结构相变研究中,高分辨的喇曼和布里渊散射谱能够提供有关“软模”的许多知识.软模是一种对温度有着很强依赖性的晶格振动,其振动频率在相变点变为零.软模的凝聚导致自发对称破缺. 当晶体发生相变时,弹性性质通常表现出反常.由于布里渊散射实验能够探测长波声学模行为,因此在过去的十六年中,布里渊散射光谱学被广泛地应用来研究固体相变.它有力地补充了与此紧密相近的声共振和超声传播测量手段所获得资料.这些实验结果,与喇曼散射、非弹性中子散射和X射线测量结果综合起来,使…     

铁电材料中的极性拓扑结构

调控磁性材料中的自旋拓扑结构(流量闭合型、涡旋、半子(meron)、斯格明子(skyrmion)等自旋组态)可以改进材料的磁性和电磁性能,因而引起了学术界的广泛关注.最近研究表明,在尺寸效应、界面耦合及其相互作用、外延应变等作用下,铁电材料中也会出现自发的极性拓扑畴结构,同时表现出新的铁电相结构和丰富的物理性能.本文总结了铁电纳米结构、铁电薄膜和铁电超晶格中的极性拓扑畴结构类型及其形成机理,分析了这些极性拓扑结构与铁电、压电、介电、光电性能之间的关联,并分别讨论了铁电材料中极性拓扑结构的整体拓扑相变调控和单个极性拓扑结构的外场调控,最后展望了极性拓扑结构未来的可能研究方向.     

弛豫型铁电单晶0.67Pb(Mg_(1/3)Nb_(2/3))O_3-0.33PbTiO_3的拉曼散射研究

本文采用拉曼散射技术在温度从374到-196℃的范围内,研究了弛豫型铁电单晶0.67Pb(Mg1/3Nb2/3)O3-0.33PbTiO3(PMNT)。观察了在不同温度下拉曼光谱的变化,经分析这些变化反映了该晶体经历了两个相变:第一个相变温度发生在120℃,从顺电立方相到铁电四方相的相变,780 cm-1处模式在VH偏振下的改变标志了这一相变;第二个相变温度发生在34℃,是从铁电四方相到铁电三方相的相变,软模的出现代表了这一相变。     

铁性体中畴结构产生的热力学描述

在批判平衡态热力学局限性的基础上,采用不可逆过程热力学对铁性体中畴结构的产生做了动态描述.注意到了铁电与铁磁两种情况下的相似与区别.在铁弹情况下,出于客观的必要性,类比地引入了铁弹相变“分子场”的概念.明确了体系的有限性、过程的不可逆性与畴结构的产生的内在关联. 关键词： 铁性体 畴结构 不可逆性 对称性     

Narrow Waveguide Based on Ferroelectric Domain Wall

Ferroelectric materials are spontaneous symmetry breaking systems that are characterized by ordered electric polarizations.Similar to its ferromagnetic counterpart,a ferroelectric domain wall can be regarded as a soft interface separating two different ferroelectric domains.Here we show that two bound state excitations of electric polarization(polar wave),or the vibration and breathing modes,can be hosted and propagate within the ferroelectric domain wall.In particular,the vibration polar wave has zero frequency gap,thus is constricted deeply inside ferroelectric domain wall,and can even propagate in the presence of local pinnings.The ferroelectric domain wall waveguide as demonstrated here offers a new paradigm in developing ferroelectric information processing units.     

Ferroelectric domains and the condensed soft mode in gadolinium molybdate

Results of lattice dynamical calculations of the high temperature phase of gadolinium molybdate (GMO) support the view that the phase transition in GMO arises from a softening of a doubly degenerate zone edge mode. We find a close correlation between the displacements associated with these soft modes and the difference in the structure of the two phases, suggesting that, at the transition one or other set of soft mode displacements is ‘condensed’, or ‘frozen in’ to the structure. On this basis we can explain the ferroelectric switching.     

Observation of soft modes in the Raman spectrum of ferroelectric Cr3B7O13Cl

Raman spectroscopy has been used to establish the existence of two soft modes in the orthorhombic phase of the improper ferroelectric chromium chlorine boracite. The orthorhombic-cubic transition at 263 K is sharply first-order and no “soft” feature was seen in the Raman spectrum of the paraelectric phase. These results confirm the proposed microscopic mechanism for the transition.     

Anomalous neutron scattering and ferroelectric soft modes

It is suggested that anomalous neutron scattering could prove a powerful experimental tool in studying ferroelectric phase transition, the sublattice displacements of the soft modes as well as their symmetry characteristics.     

压应力场下PLZT铁电陶瓷的原位Raman观测

铁电陶瓷材料在外场加载下的畴变所引起的材料结构变化,是导致材料性能衰变和破坏的主要原因,Raman光谱技术是一种研究铁电材料畴变和微结构变化的无损伤性及原位微区的观测方法。采用传统固相法合成Zr/Ti原子比为53/478的掺镧锆钛酸铅(PLZT)铁电陶瓷材料 ,采用X射线衍射仪和扫描电子显微镜及Precision_LC铁电测试系统分别对试样进行结构形貌表征和铁电物理性能测试,利用自制的应力加载装置与Raman光谱仪联用,实现不同压应力场作用下试样的原位Raman谱测试,考察和分析Raman谱软模E(2TO)和E(3TO+2LO)+B1的峰强和峰位随散射偏振方向的变化规律。结果表明,不同压应力场下Raman软模E(2TO)和E(3TO+2LO)+B1的峰强均随散射偏振角度呈现正弦式的变化规律,在60°偏振角度上软模峰强最大,在150°偏振角度上软模峰强最小。随着压应力场的增加,在0°和60°偏振角度获得的软模峰强随应力场的增加呈现明显的下降趋势,而在90°和150°偏振角度获得的软模峰强基本不变。压应力场变化对PLZT陶瓷的Raman软模E(2TO)和E(3TO+2LO)+B1的峰位均不产生影响。     

On the theory of ferroelectric transition in the crystal LiNbO3

A simple model for ferroelectric transition in the crystal LiNbO₃ is proposed. It is considered that one of two optical branches of vibrations of the system of crystal planes in the direction of trigonal c-axis (on which the ions Li⁺, Nb⁵⁺ and O²⁻ are distributed) plays the role of the “soft mode” at the ferroelectric transition in LiNbO₃. The dynamic problem of these plane vibrations is solved. For frequencies of optical modes the analytical expressions depending on coordinates, charges and weights of ions are obtained. It is shown, that the frequency of one of these two optical modes, under a certain condition, tends to zero (soft mode). This condition enables one to find with high accuracy the dispositions of ions in both ferroelectric and paraelectric phases and gives an opportunity to follow the changes of these dispositions at the phase transition. Then, it is assumed that the reason of a ferroelectric transition is the thermal expansion of crystal. On the basis of this assumption the thermal shifts of ions and the Curie-Weiss law for the square of the frequency of SM are obtained.     

Phase transitions sequence in pyrochlore Cd2Nb2O7 studied by IR reflectivity

The infrared reflectivity of Cd₂Nb₂O₇ single crystal was studied in the temperature interval of 10-540 K, together with complementary dielectric measurements. A ferroelectric soft mode was revealed above the ferroelectric phase transition at T _c = 196 K coupled with a central-mode type dispersion in the near-millimetre range. This proves the mixed displacive and order-disorder nature of the transition. Below T _c many new modes were detected due to lowering of the symmetry, especially below the previously suggested incommensurate transition at 85 K. Discussion of the possible phase transitions based on symmetry considerations is presented with the conclusion that the ferroelectric transition is proper with the F_1u symmetry of the order parameter, whereas the intermediate ferroelastic transition is improper and triggered by the coupling with the ferroelectric order parameter. Received 17 July 2000     

Comments on the paper ‘The origin of the ferroelectric phase transition in ammonium sulphate’

The ‘soft mode’ theory of ferroelectric phase transition in ammonium sulphate reported recently by Sawada and co-workers has been criticised on the basis of experimental and theoretical facts known to-date about the crystal. It has been pointed out that the ‘soft mode’ responsible for the phase transition should have appropriate mixing of the internal modes of SO^2-₄ ion.     

Inelastic Light Scattering Near the Ferroelectric Phase-Transition Point in Bismuth Vanadate Crystals

The Raman and Brillouin spectra in bismuth vanadate crystals near the ferroelectric phase-transition point are studied. The intensity maximum corresponding to the soft mode in light scattering spectra is found. This mode is responsible for the instability of the bismuth vanadate crystal lattice in the vicinity of the ferroelectric phase-transition point. Strong interaction of the soft optical mode with an acoustic mode of the corresponding symmetry is revealed by an analysis of the Raman and Brillouin spectra observed. The theory of light scattering by coupled lattice modes based on the model of two strongly coupled oscillators is developed. The introduction of an additional oscillator strongly interacting with the fundamental soft mode is shown to result in a temperature shift of the structural phase transition to higher temperatures. The results obtained confirm the possibility of changing the phase-transition temperature by modification of the vibrational spectrum through introduction of additional degrees of freedom or comminution of the macroscopic sample with formation of the ordered superdispersed structure (globular crystal). These results are general and can be subsequently used to increase the phase-transition temperature in ferroelastics, ferroelectrics, and superconductors.     

Investigation of specific features of the lattice dynamics and the ferroelectric transition in perovskite crystals

The specific features of diffuse X-ray scattering in BaTiO₃, KNbO₃, and PbTiP₃ perovskite crystals have been investigated. The former two perovskite compounds in cubic, tetragonal, and orthorhombic phases exhibit anomalous sheets due to diffuse X-ray scattering, whereas no similar sheets are observed in the case of diffuse X-ray scattering in PbTiO₃. For these compounds, the phonon spectra are calculated in the quasi-harmonic approximation within the polarizable-shell model, and the mechanism of stabilization of the soft mode above the temperature of the phase transition to the ferroelectric state is considered. It is demonstrated that, in the cubic phase of BaTiO₃ and KNbO₃ crystals, there exist quasi-one-dimensional “soft” modes of vibrations of ions in M-O-M-O- chains, where M = Ti or Nb. In PbTiO₃, this feature of the soft mode has not been revealed. The pair correlation functions of simultaneous atomic displacements in BaTiO₃, KNbO₃, and PbTiO₃ are determined and used to calculate the intensity of diffuse X-ray scattering. The results obtained are in good agreement with experimental data. This is a strong argument in support of the hypothesis that the specific features of diffuse scattering are associated with the existence of quasi-one-dimensional correlations of atomic displacements in the soft optical mode and that the ferroelectric transition in perovskites is a displacive ferroelectric phase transition. The possible influence of the specific features revealed in the phonon spectra of the perovskite crystals on the processes of nuclear magnetic resonance and X-ray absorption (extended X-ray absorption fine structure spectra) is briefly discussed.