Highly efficient acousto-optic diffraction in Sn2P2S6 crystals

We have studied the acousto-optic (AO) diffraction in Sn2P2S6 crystals and found that they manifest high values of an AO figure of merit. The above crystals may therefore be used as highly efficient materials in different AO applications.     

Pseudocapacitive Sodium Storage by Ferroelectric Sn2P2S6 with Layered Nanostructure

Sodium ion batteries (SIB) are considered promising alternative candidates for lithium ion batteries (LIB) because of the wide availability and low cost of sodium, therefore the development of alternative sodium storage materials with comparable performance to LIB is urgently desired. The sodium ions with larger sizes resist intercalation or alloying because of slow reaction kinetics. Most pseudocapacitive sodium storage materials are based on subtle nanomaterial engineering, which is difficult for large‐scale production. Here, ferroelectric Sn₂P₂S₆ with layered nanostructure is developed as sodium ion storage material. The ferroelectricity‐enhanced pseudocapacitance of sodium ion in the interlayer spacing makes the electrochemical reaction easier and faster, endowing the Sn₂P₂S₆ electrode with excellent rate capability and cycle stability. Furthermore, the facile solid state reaction synthesis and common electrode fabrication make the Sn₂P₂S₆ that becomes a promising anode material of SIB.     

Interferences in light deflection by ferroelastic domain walls

Deflection of light is studied in a crystal of glycine phosphite containing two twin walls. When the crystal is rotated in the incident laser beam, interferences are observed in both the direct beam and in the main deflected beam (A or B) for both polarizations of the incident light. The contrast is especially high, because the mutual tilt angle of the principal axes is close to 45 degrees in this twinned crystal. On this principle, fundamental-harmonic beam splitters could be built from as-grown twin crystals. Furthermore, the electrical modulation of the light deflected by ferroelectric-ferroelastic crystals can be now explained in terms of interference effects.     

Unexpectedly low barrier of ferroelectric switching in HfO2 via topological domain walls

Fluorite-structure ferroelectrics — in particular the orthorhombic phase of HfO₂ — are of paramount interest to academia and industry because they show unprecedented scalability down to 1-nm-thick size and are compatible with Si electronics. However, their polarization switching is believed to be limited by the intrinsically high energy barrier of ferroelectric domain wall (DW) motions. Here, by unveiling a new topological class of DWs, we establish an atomic-scale mechanism of polarization switching in orthorhombic HfO₂ that exhibits unexpectedly low energy barriers of DW motion (up to 35-fold lower than given by previous conjectures). These findings demonstrate that the nucleation-and-growth-based mechanism is feasible, challenging the commonly held view that the rapid growth of the oppositely polarized domain is impossible. Building on this insight, we describe a strategy to substantially reduce the coercive fields in HfO₂-based ferroelectric devices. Our work is a crucial step towards understanding the polarization switching of HfO₂, which could provide a means to solve the key problems associated with operation speed and endurance.     

Luminescence properties of Sn2P2Se6 crystals

A large family of Sn_2yPb_2(1−y)P₂S_6xSe_6(1−x) semiconductor-ferroelectric crystals were obtained by the Bridgman technique. The photoluminescence properties of the Sn_2yPb_2(1−y)P₂S_6xSe_6(1−x) family crystals strongly depend on their chemical composition, excitation energy and temperature. The influence of the Pb → Sn and S → Se isovalent substitutions on the luminescence properties of a crystal with the Sn₂P₂Se₆ basic composition was investigated. A broad emission band observed in the Sn₂P₂Se₆ crystal with a maximum roughly at 600 nm (at T = 8.6 K) was assigned to a band-to-band electron-hole recombination, whereas broad emission bands, peaked near 785 nm (at T = 8.6 K) and 1025 nm (at T = 44 K) were assigned to an electron-hole recombination from defect levels localised within the bandgap. Possible types of recombination defect centres and specific mechanisms of luminescence in the Sn₂P₂Se₆ semiconductor-ferroelectric crystals were considered and discussed on the basis of the obtained results and the referenced data.     

Corrugation-type instability of the free motion of 180° domain walls in uniaxial ferromagnets

Investigation of the spectrum of oscillations of a 180° domain wall (DW) freely moving in a uniaxial ferromagnet shows that the one-dimensional structure becomes unstable with respect to surface distortions in the region of negative effective mass (negative differential mobility). The upper critical value of the perturbation wave vector, above which the DW corrugation is not developed and the wave vector of a perturbation mode with the maximum increment are determined.     

Driving forces on domain walls in ferroelectric materials and interaction with defects

The driving force on a domain wall in a piezoelectric material is identified by using a variational principle. In addition to the domain wall, point defects are considered. These defects are modeled as electrically charged centers of dilatation (or contraction). For representative numerical simulations, the finite element method (FEM) is used. The simulations allow the analysis of the interaction of point defects and domain walls. It is found that point defects have the ability to reduce the driving force on the domain wall. This can potentially lead to the hindering of the domain wall mobility which is suspected to be a mechanism in electric fatigue.     

Atomic-scale study of electric dipoles near charged and uncharged domain walls in ferroelectric films

Ferroelectrics are materials exhibiting spontaneous electric polarization due to dipoles formed by displacements of charged ions inside the crystal unit cell. Their exceptional properties are exploited in a variety of microelectronic applications. As ferroelectricity is strongly influenced by surfaces, interfaces and domain boundaries, there is great interest in exploring how the local atomic structure affects the electric properties. Here, using the negative spherical-aberration imaging technique in an aberration-corrected transmission electron microscope, we investigate the cation-oxygen dipoles near 180 degrees domain walls in epitaxial PbZr(0.2)Ti(0.8)O(3) thin films on the atomic scale. The width and dipole distortion across a transversal wall and a longitudinal wall are measured, and on this basis the local polarization is calculated. For the first time, a large difference in atomic details between charged and uncharged domain walls is reported.     

New scenarios of the nonlinear dynamic transformation of domain walls in uniaxial magnetic films with planar anisotropy

The Landau-Lifshits equation was solved by a direct numerical procedure within the framework of a two-dimensional model of the magnetization distribution in a film with the easy magnetization axis parallel to the surface. In a magnetic field with a strength above the critical value, new types of the nonlinear dynamic transformation of the domain walls were found in films with various thicknesses.     

Preparation of ferroelectric BaNb2O6 by the urea method

A precipitate was formed when an aqueous solution of BaCl₂, NbF₅ and urea in required ratio (the ratio of total metal cations to the urea is five) is heated on a water bath at 100 °C. This precipitate on decomposition at 600 °C yielded the nanocrystallites of BaNb₂O₆ (BN), as confirmed by X-ray diffraction study (XRD). This is a much lower temperature as compared to that prepared by traditional solid state method (1000° C) as reported for the formation of BaNb₂O₆. Transmission electron microscope (TEM) investigations revealed that the average particle size is 85 nm for the calcined powders. The room temperature dielectric constant at 1 kHz is found to be 900. The ferroelectric hysteresis loop parameters of these samples were also studied.     

Effect of germanium doping on pyroelectric and piezoelectric properties of Sn(2)P(2)S(6) single crystal

The results of the search for dopants to optimize the ferroelectric phase transition temperature and pyroelectric and piezoelectric properties of Sn(2)P(2)S(6) crystals are reported. Among all dopants only germanium causes a pronounced shift of the phase transition toward higher temperatures. The highest Curie temperature achieved by Ge doping is as high as 88 degrees C. The major advantage of the Ge-doped crystals as compared to pure Sn(2)P(2)S(6) is that the hydrostatic piezoelectric and pyroelectric response is kept high in a wider temperature range with much lower temperature dependence. The effects of the doping on pyroelectric and piezoelectric properties, characterizing the sensitivity of the material to thermal and hydroacoustic excitation, are discussed.     

Numerical analysis of ferroelectric/ferroelastic domain switching in ferroelectric ceramics

A numerical approach predicting the behavior of ferroelectric ceramics under electric field and mechanical loading is proposed in this paper. In the model, macroscopic properties of ferroelectric ceramics are determined by microscopic structures. Ferroelectric ceramics are seen to be composed of many domains with different orientations, and domain switching is the source of the nonlinear constitutive behavior of the ferroelectric ceramics. Numerical calculations based on the model were carried out, and the computational results are compared with the experimental results, which shows the two sets of results consist with each other. The calculation approach can provide a guidance for the ceramics component design.     

Preparation of nanocrystalline ferroelectric BaNb2O6 by citrate gel method

A gel was formed when a aqueous solution of BaCl₂, NbF₅ and citric acid in stoichiometric ratio is heated on a water bath. This gel on decomposition at 600°C yielded the nano crystallites of BaNb₂O₆, as confirmed by X-ray diffraction study (XRD). This is a much lower temperature as compared to that prepared by traditional solid state method (1000°C) as reported for the formation of BaNb₂O₆. Transmission electron microscopic (TEM) investigations revealed that the average particle size is 50 nm for the calcined powders. The room temperature dielectric constant at 1 kHz is found to be 1000. The ferroelectric hysteresis loop parameters of these samples were also studied.     

粗大晶粒PZT陶瓷中电畴的结构

应用扫描力显微镜(SFM)的压电响应模式观测未经抛光处理的PZT陶瓷片的电畴结构,用纵向压电响应信号和侧向压电响应信号获得PZT陶瓷材料三维电畴结构.结果表明,将样品晶粒的微形貌与SFM的纵向和侧向压电响应信号相结合,能准确表征粗大晶粒样品的三维电畴结构.用SFM可观测表面不经任何处理的陶瓷样品的电畴,不会引入表面应力等影响因素,能得到样品的原生畴结构.对原生畴结构的观察表明,对于受应力较大的晶粒,成畴的主要原因是降低应变能,而受应力较小的晶粒成畴的主要原因是降低退极化能.     

Energetic analysis of ferroelectric domain patterns by equivalent inclusion method

The formation of domain configuration in ferroelectrics is a consequence of energy minimization, and critically depends on their transformation strain and spontaneous polarization. In this article, we develop an energetic analysis on ferroelectric domain patterns using equivalent inclusion method, treating ferroelectric domain as an ellipsoidal inhomogeneous inclusion in a ferroelectric matrix. The potential energy of the domain is calculated in terms of its orientation and shape, and the energy minimizing configurations have been identified. Both tetragonal and rhombohedral crystals have been analyzed, and the lamellar domain configurations as predicted by the compatibility analysis have been recovered. Additional energy minimizing states have also been revealed, including needle type of domains and charged domains. Different contributions of strain compatibility and polarization compatibility have also been analyzed.     

Large electric-field-induced strain in ferroelectric crystals by point-defect-mediated reversible domain switching

Ferroelectric crystals are characterized by their asymmetric or polar structures. In an electric field, ions undergo asymmetric displacement and result in a small change in crystal dimension, which is proportional to the applied field. Such electric-field-induced strain (or piezoelectricity) has found extensive applications in actuators and sensors. However, the effect is generally very small and thus limits its usefulness. Here I show that with a different mechanism, an aged BaTiO(3) single crystal can generate a large recoverable nonlinear strain of 0.75% at a low field of 200 V mm(-1). At the same field this value is about 40 times higher than piezoelectric Pb(Zr, Ti)O(3) (PZT) ceramics and more than 10 times higher than the high-strain Pb(Zn(1/3)Nb(2/3))O(3)-PbTiO(3) (PZN-PT) single crystals. This large electro-strain stems from an unusual reversible domain switching (most importantly the switching of non-180 degrees domains) in which the restoring force is provided by a general symmetry-conforming property of point defects. This mechanism provides a general method to achieve large electro-strain effect in a wide range of ferroelectric systems and the effect may lead to novel applications in ultra-large stroke and nonlinear actuators.