High-resolution calorimetric study of pb(mg_{1/3}nb_{2/3})o_{3} single crystal

Motivated by the long-standing unresolved enigma of the relaxor ferroelectric ground state, we performed a high-resolution heat capacity and polarization study of the field-induced phase transition in the relaxor ferroelectric single crystal Pb(Mg_{1/3}Nb_{2/3})O_{3} (PMN) oriented along the [110] direction. We show that the discontinuous evolution of polarization as a function of the electric field or temperature is a consequence of a true first order transition from a glassy to ferroelectric state, which is accompanied by an excess heat capacity anomaly and released latent heat. We also find that in a zero field there is no ferroelectric phase transition in bulk PMN at any temperature, indicating that the nonergodic dipolar glass phase persists down to the lowest temperatures.     

Bias field effects on the dielectric properties of 0.67Pb(Mg1/3Nb2/3)O3-0.33PbTiO3 single crystals with different orientations

The domain states and phase transitions in 0.67Pb(Mg_1/3Nb_2/3)O₃-0.33PbTiO₃ single crystals were investigated by studying their relative permittivity under various dc bias at constant heating and cooling rates. The orientation dependence of the bias field effect was revealed by examining the temperature dependence of relative permittivity as a function of crystal orientation (the 〈111〉, 〈011〉 and 〈001〉 directions) and dc bias field. The crystals basically have a macrodomain rhombohedral ferroelectric state in the ferroelectric phase under zero dc bias. External bias field could modulate the domain state and induce a stable macrodomain state in the crystals. Also, it is proposed that the dc bias applied along the 〈001〉 or 〈011〉 direction could induce a tetragonal ferroelectric phase or an orthorhombic ferroelectric phase, respectively, in an intermediate temperature range.     

LiNbO3晶体中缺陷的X射线形貌研究

用X射线透射扫描形貌方法观察了直拉法生长LiNbO₃晶体中各种类型点阵缺陷,诸如铁电畴壁、生长层、位错、亚晶界和胞状组织等;用不同衍射矢量对[001]和[210]方向生长的晶体的形貌消象规律,结合X射线铁电异常散射效应和光学显微观察,讨论了晶体中180°铁电畴和生长层的衬度及其分布,并研究了LiNbO₃晶体中180°畴壁形成及其相互关系。 关键词：     

利用压电力显微镜研究PMN-30%PT单晶体中铁电畴的结构及其演变(英文)

本文总结了我们近年来利用压电力显微镜(PFM)研究PMN 30%PT单晶体中铁电畴的结构及其演变的结果。选择PMN-30%PT晶体是因为该组分在超声传感器等应用方面具有最大的潜力。铁电畴的观察是基于反压电现象;具体来讲就是当交变电场通过原子力显微镜探针加到晶体表面时,会引起晶体表面的起伏振荡,而锁相放大器可以解出该振荡信号的振幅和相位角;其中振幅衬度反映了压电系数d_(33)的大小,而相位衬度则反映了铁电畴的极化方向。文中介绍了平面内以及垂直平面的PFM成像技术,并演示了影响畴的图像的一些因素,其中包括静电荷效应,表层效应和机械抛光的影响。本文还利用有限元模型对PFM成像原理进行了模拟分析。着重研究了晶体中铁电畴的尺寸分布,畴与晶体取向,时间和温度的相关性,以及畴的演变过程。     

Investigation of domain structure of TGS single crystal after a transverse electric field by piezoresponse force microscopy

An investigation of the influence of electric field transverse to the ferroelectric axis b_HOP and parallel to c_HOP axis of triglycine sulfate (TGS) single crystal on ferroelectric domain structure was performed by piezoresponse force microscopy. To check if the applied electric field changed the dielectric properties and ferroelectric domain structure the hysteresis loop measurements were carried out as well as observations of domain structure by the liquid crystal technique. The investigation revealed existence of blocked domains in the crystal modified by the electric field TGS.     

Second-harmonic scattering by domains in RbH2PO4 ferroelectrics

Second-harmonic scattering (SHS) of light by laminar ferroelectric domains of the orthorhombic phase of RbH₂PO₄ below T_c=147 K is studied in detail. CCD images of the far-field SHS pattern reveal up to six off-axis distinct intensity maxima in a plane normal to the ferroelectric axis c. These scattering maxima are interpreted on the basis of the quasi-phase matching (QPM) conditions for the three sets of polarizations allowed by the nonvanishing elements of the second-order nonlinear susceptibility tensor. On this assumption scattering angles of the three SHS modes calculated from crystal optics are shown to fit well the experimental data for any incidence of the pump beam normal to c. Scattering intensities of the two most intense modes are also measured versus the component of the wave vector perpendicular to the modulated structure. A stochastic model giving the intensity as a function of the density probability of domain widths is developed and is shown to give a good fit of the scattered intensity. Moreover, it permits the evaluation of the mean and dispersion of the ferroelectric domain widths, which are found to be, respectively, 2.6 and 0.7 μm in the crystal under study. This is consistent with the domain size of the ferroelastic counterpart revealed by polarization microscopy.     

Ground state and properties of ferroelectric superlattices based on crystals of the perovskite family

The crystal structure of the ground state of ten free-standing ferroelectric superlattices based on crystals with the perovskite structure (BaTiO₃/SrTiO₃, PbTiO₃/SrTiO₃, PbTiO₃/PbZrO₃, SrZrO₃/SrTiO₃, PbZrO₃/BaZrO₃, BaTiO₃/BaZrO₃, PbTiO₃/BaTiO₃, BaTiO₃/CaTiO₃, KNbO₃/KTaO₃, and KNbO₃/NaNbO₃) was calculated from first principles within the density functional theory taking into account criteria for stability of the structures with respect to acoustic and optical distortions. It was shown that the ground state in all the considered superlattices corresponds to the ferroelectric phase. It was found that the polarization vector has a tendency toward a tilt to the plane of the superlattice layers, which makes it possible to decrease the electrostatic and elastic energy in the superlattices consisting of materials with different ferroelectric properties. The importance of the inclusion of structural distortions due to unstable phonons at the Brillouin zone boundary, which, in a number of cases, lead to significant changes in ferroelectric and dielectric properties of the superlattices, was demonstrated.     

PT/PZT/PT铁电薄膜的铁电畴和畴壁

用sol-gel法制备出了具有良好铁电性、纯钙钛矿结构的PbTiO₃/Pb(Zr_0.3Ti_0.7)O₃/PbTiO₃(PT/PZT/PT)新型夹心结构铁电薄膜.用扫描力显微镜(SFM)的压电响应模式获得了薄膜铁电畴的垂直于膜平面方向(OPP)、膜平面内(IPP)及OPP的相位和幅度图像，结合理论分析指出薄膜的电畴主要由c畴和偏离垂直于膜平面方向上的c畴构成，薄膜取向的复杂性导致了复杂的畴结构.对于［111］取向的薄膜，当偏离垂直于膜平面方向上的c畴在垂直膜平面方向和面内方向都相反时构成180°的畴壁，在垂直膜平面方向上相同、面内方向相反或由垂直膜平面方向上相反、面内方向相同时构成90°畴壁. 关键词： 铁电薄膜 PT/PZT/PT 电畴和畴壁 扫描力显微镜(SFM)     

Diffractive self-imaging based on selective etching of a ferroelectric domain inversion grating

A hexagonal array grating based on selective etching of a 2D ferroelectric domain inversion in a periodically poled Mg O-doped Li Nb O3 crystal is fabricated. The effects to the diffractive self-imaging as a function of diffraction distance for a fixed phase difference and array duty cycle of the grating is theoretically analyzed.The Talbot diffractive self-imaging properties after selective etching of a 2D ferroelectric domain inversion grating under a fixed phase difference are experimentally demonstrated. A good agreement between theoretical and experimental results is observed.     

利用压电力显微镜研究PMN-30%PT单晶体中铁电畴的结构及其演变

本文总结了我们近年米利用压电力显微镜(PFM)研究PMN-30%PT单晶体中铁电畴的结构及其演变的结果.选择PMN-30%PT品体是因为该组分在超声传感器等应用方面具有最大的潜力.铁电畴的观察是基于反压电现象;具体来讲就是当交变电场通过原子力显微镜探针加到晶体表面时,会引起品体表面的起伏振荡,而锁相放大器可以解出该振荡信号的振幅和相位角;其中振幅衬度反映了压电系数d33的大小,而相位衬度则反映了铁电畴的极化方向.文中介绍了平面内以及垂直平面的PFM成像技术,并演示了影响畴的图像的一些因素,其中包括静电倚效应,表层效应和机械抛光的影响.本文还利用有限无模型对PFM成像原理进行了模拟分析.着重研究了晶体中铁电畴的尺寸分布,畴与晶体取向,时间和温度的相关性,以及畴的演变过程.     

Phase transitions and domain structures in thin ferroelectric films

Recent advances in technology made available high quality thin ferroelectric films and ferroelectric–paraelectric multilayers. But understanding of the properties of these systems is far from being complete. In particular, it is not clear why various anomalies observed at phase transitions here are different from those in high quality bulk systems. The aim of the paper is to discuss some specific features of ferroelectric phase transitions in thin films and multilayers which are taken into account only partially by the theory. The discussion is limited to revealing the character of ferroelectric state forming just after the transition, i.e. if it is single-domain or multi-domain, if it is single-phase or two-phase. First, thin films with electrodes and on a substrate are discussed. The role of non-ideality of electrodes in realization of one of the first pairs of possibilities is emphasized. A more recent idea is that even for ideal electrodes a domain formation is possible when some conditions on the electrode–ferroelectric interface and the material constants of the material are met. This seems to be quite possible for the considered systems. Clamping by the substrate may lead to formation of a two-phase state which is practically unexplored for very thin films on substrates. Second, ferroelectric–paraelectric multilayers are considered which are more challenging for theoretical study than the thin films. Indeed, despite periodicity in the composition the domain structures are almost never periodic along the multilayer. The non-ideality of electrodes seems to lead to practical impossibility of single-domain ferroelectric phase transition in the multilayers of the considered type.     

Ferroelectric phenomena in CdSnO<Subscript>3</Subscript>: A first-principles studies

The phonon spectrum of cubic cadmium metastannate and parameters of the crystal structure of its distorted phases were calculated from first principles within the density functional theory. It is shown that the phonon spectrum and the energy spectrum of the distorted phases in α-CdSnO₃ resemble surprisingly the corresponding characteristics of CdTiO₃. The ground state of α-CdSnO₃ is the ferroelectric Pbn2₁ phase, the energy gain from the phase transition to this phase from the nonpolar phase Pbnm is ∼30 meV, and the spontaneous polarization is 0.25 C/m². The analysis of the eigenvector of the ferroelectric mode in α-CdSnO₃ and the partial densities of states indicates that the ferroelectric instability in this crystal, which does not contain transition d-element atoms, is associated with the formation of a covalent bonding between Cd and O atoms.     

Analysis of polarization switching in ferroelectric crystals in the injection mode

We report on the results of experiments on polarization switching in a ferroelectric TGS crystal during injection of electron beams from a scanning electron microscope under a surface layer. A series of models reflecting the polarization switching dynamics of a ferroelectric crystal under the action of an injected charge is constructed. The implementation of these models is based on the principles of evolution of domain structures taking into account analysis of possible polarization switching mechanisms for ferroelectric samples. A mathematical model developed using these principles demonstrates qualitative similarity of model current pulses and those obtained experimentally in the injection mode.     

YMnO_3薄膜的铁电行为及其纳米尺度铁电畴的研究

六方YMnO_3是一种特殊的多铁性材料,因其具有介电常数低、单一极化轴、无挥发性元素等特点,在磁电领域具有独特的优势,但目前关于YMnO_3薄膜的铁电性特别是畴结构的研究相对较少.本文采用溶胶-凝胶法在Si(100)基片上制备了多铁性YMnO_3薄膜,利用掠入射X-射线衍射、原子力显微镜对薄膜的结构及表面形貌进行了分析,用压力显微镜(PFM)技术研究了纳米尺度畴结构及微区电滞行为,并通过I-V,P-E曲线进一步研究了薄膜的漏电流和宏观电滞行为.结果表明,该薄膜为六方钙钛矿结构,YMnO_3晶粒大小均匀并且结晶性较好,薄膜表面粗糙度为7.209 nm.PFM图显示出清晰的电畴结构,结合典型的微区振幅蝴蝶曲线和相位电滞回线,证实该YMnO_3薄膜具有较好的铁电性.由于受内建电场的作用,振幅曲线和相位曲线都向正向偏移,表现出非对称特征.该薄膜的漏电流密度低于10~(-6)A·cm~(-2),因而其电滞回线基本能够达到饱和.     

Pressure effect on phase transition in ferroelectic polymers

Pressure effects of phase transition behaviour in two kinds of ferroelectric polymers of poly(vinylidene fluoride), PVDF, and copolymers of vinylidene fluoride and trifluoroethylene, (VDF/TrFE), are discussed. In the case of PVDF, several high-pressure treatments including a high-pressure annealing and a uniaxial compression were shown to induce a crystal transformation from a non-polar Form II crystal to a polar Form I crystal, which has ferroelectric characteristics and high piezoelectric activity. In addition, substantial pressure effects on ferroelectric phase transition points as well as crystal structures were observed for (VDF/TrFE) copolymers with different VDF contents. The most significant pressure effects were observed for copolymer samples with unstable ferroelectric structures at atmospheric pressure. From high-pressure X-ray and Raman scattering studies, these pressure effects were suggested to originate from the pressure-induced conformational transition from gauche to trans in the molecular chains.     

量子顺电EuTiO_3材料基态磁性的第一性原理研究

EuTiO_3是钙钛矿结构的量子顺电体,实验发现其基态具有平面各向异性G类反铁磁结构,本文运用基于密度泛函理论的第一性原理计算研究了EuTiO_3处于量子顺电相和应力作用下处于铁电四方相时可能的自旋取向和自旋交换耦合作用,分析了自旋耦合作用的路径,探讨了应力对磁性交换路径的作用,结果发现:当体系自由时,EuTiO_3具有自旋沿[110]方向平面内单轴各向异性的G类反铁磁结构,该结构下Eu离子4f电子自旋通过处于面心位置的O 2p实现自旋反铁磁性的超交换耦合,而在外加应力诱导的铁电四方结构下,由于自旋交换路径中Eu—O—Eu键角改变,Eu 4f电子自旋实现了[110]方向的铁磁交换耦合。     

The role of the spatial distribution of local polarization perturbations in the posistor effect onset

The electrical resistance of polycrystalline ferroelectric semiconductors is defined by the potential barriers due to the existence of local charged surface states at crystallite boundaries. The barrier screening depends on the state of the ferroelectric system and is maximal during spontaneous-polarization switching. It is shown in this paper that the local perturbation of the ferroelectric system, resulting from the repolarization and appearing as a domain wall between the regions with different polarization directions, has a zigzag configuration. The electric field in the vicinity of the zigzag domain wall is stabilized and coincides with the coercive field, which provides low potential barriers in the ferroelectric phase compared with the paraelectric phase. The repolarization processes become inefficient in the potential barrier screening at the transition from the ferroelectric to the paraelectric phase. As a result, a sharp increase in the electrical resistance is observed at the ferroelectric-paraelectric phase transition, called the posistor effect.     

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.     

Domain wall thickness variations of ferroelectric BaMgF4 single crystals in the tip fields of an atomic force microscope

The ferroelectric domain wall thickness of a fluoride BaMgF₄ single crystal was investigated by piezoresponse force microscopy. It was found that the domain wall thickness shows a strong spatial variation in the as‐grown crystal and the polarization reversal process. The original wall thickness is greater (about two to seven times) than that switched by the tip fields of the atomic force microscope. A significantly narrower domain wall was obtained in the higher tip‐field. The trapped defects at the domain wall play an important role in the spatial variation of the polarization width of 180° domain wall in the BaMgF₄ single crystal. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

极性相变的介电谱研究

用时域和频域方法研究了(Ba_1-xSr_x)TiO₃陶瓷的极性相变.类似于单晶的铁电相变所观察到的晶格软化,屏蔽电荷激发和畴运动讯号外,在陶瓷中还出现空间电荷运动的强讯号.空间电荷和电畴属于凝聚态物质的高级结构,它提供慢极化效应.慢效应使频域测量结果不能得到确定值.从统计观点给出高级结构的历史记忆效应对频域测量结果的影响;解释了频域介电常量的倒数在一级结构的转变中发生线性软化,而在高级结构的转变中发生抛物型软化. 关键词：