Optical properties of cubic and tetragonal KTa0.5Nb0.5O3 by density functional theory

Using density functional methods, optical properties of KTa_0.5Nb_0.5O₃ (KTN) are investigated. The imaginary part of dielectric function ε₂(ω), the optical absorption coefficient I(ω) and the reflectivity R(ω) of KTN in the cubic (paraelectric) and tetragonal (ferroelectric) phases are calculated. The origin of the different behavior of the optical spectra between the two phases is discussed.     

Ab initio calculation of the ferroelectric phase transition in disordered and ordered PbSc0.5Ta0.5O3 and PbSc0.5Nd0.5O3 solid solutions with the use of a model Hamiltonian

To describe the ferroelectric phase transition in ordered and disordered PbSc_0.5Ta_0.5O₃ (PST) and PbSc_0.5Nd_0.5O₃ (PSN) solid solutions, a model Hamiltonian is written in the local-mode approximation, in which long-range dipole-dipole and short-range interactions of local modes are taken into account. The Hamiltonian parameters are determined from the set of energies of a series of distorted structures, calculated within the nonempirical generalized Gordon-Kim model. The statistical mechanics of the systems with a model Hamiltonian is investigated by the Monte Carlo method. It is revealed that PSN solid solutions undergo a transition to the ferroelectric rhombohedral phase through an intermediate phase, existing in a narrow temperature range. PST solid solutions undergo a ferroelectric phase transition directly from the paraelectric cubic to the ferroelectric rhombohedral phase. Original Russian Text ? V.I. Zinenko, N.G. Zamkova, 2008, published in Zhurnal éksperimental’noĭ i Teoreticheskoĭ Fiziki, 2008, Vol. 133, No. 3, pp. 622–631.     

Ordering and diffuse phase transitions in Pb(Sc0.5Ta0.5)O3 ceramics

The ordering of the Sc³⁺ and Ta⁵⁺ ions in Pb (Sc_0.5Ta_0.5) O₃ can be varied by proper heat treatments (1000–1500°C). The ferroelectric → paraelectric transition is strongly influenced by this ordering. With decreasing order parameter Ω the phase transition becomes more diffuse and a number of properties show strong changes.     

Synthesis and characteristics of KTa0.5Nb0.5O3 thin films

KTa_0.5Nb_0.5O₃ thin films were synthesized via a metal-organic solution. Characteristics were measured, such as X-ray diffraction pattern, surface morphology and roughness, and electric properties. The synthesized films have a (100) preferential growth orientation on Si (100) substrate. The homogeneous microstructure and smooth surface benefit to the good electric properties of the thin films. The current density-voltage characteristic shows an unexpected feature of the transition from linear to nonlinear, which can be explained by the space-charge-limited mode. Dielectric constant and loss of the thin films decrease with the increase of frequency. The decrease of dielectric loss is related to the decrease of net polarization in material. The decrease of dielectric constant can be explained by Debye formula. The phase transition temperature T _c is about 102 °C for KTa_0.5Nb_0.5O₃ materials.     

First principles study on the ferroelectricity of the perovskite ABO3 ferroelectrics

In order to understand well the different ferroelectric behaviour of quantum paraelectrics and ferroelectrics and the origin of the ferroelectricity of the solid solution KTa0.5Nb0.5O3(KTN),we calculated the electronic structure of CaTiO3,BaTiO3 and KTN by first principles calculation.From total energy analysis,it is shown that,with increasing cell volume,the crystals (CaTiO3,SrTiO3) will have a ferroelectric instability.For BaTiO3,the ferroelectricity will disappear as the cell volume is decreased.From the density of states analysis,it is shown that the hybridization between B d and O p is very important for the ferroelectric stability of ABO3 perovskite ferroelectrics.This is consistent with the analysis of band structure.     

Phase transitions in lead scandoniobate PbSc0.5Nb0.5O3

Crystals of PbSc_0.5Nb_0.5O₃ are investigated by x-ray diffraction and optical spectroscopy. It is found that the phase transformations in PbSc_0.5Nb_0.5O₃ are governed by two mechanisms: displacive ferroelectric phase transitions and consistent rotations of oxygen octahedra.     

0.5Ba(Ti0.8Zr0.2)O3-0.5(Ba0.7Ca0.3)TiO3压电薄膜的摩擦、磨损性能

具有准同型相界组分的0.5Ba(Ti_0.8Zr_0.2)O₃-0.5(Ba_0.7Ca_0.3)TiO₃ (BZT-0.5BCT)陶瓷, 表现出优异的铁电、压电性能, 作为一种具有潜在应用前景的无铅压电材料得到广泛关注. 本文采用溶胶-凝胶方法在Si(100)基底上制备了BZT-0.5BCT压电薄膜. 使用原子力显微镜和扫描电子显微镜测量得到样品的形貌图, 形貌图表明该方法制备的无铅压电薄膜表面光滑, 晶粒大小均匀、呈半球形, 直径为80–100 nm, 厚度为1.7 μm, 膜的内部有气孔.摩擦力实验表明, 压电薄膜样品与硅针尖之间存在静电力的作用, 导致其摩擦力远大于硅针尖与SiO₂之间的摩擦力, 但是两者的摩擦系数基本相同.划痕实验表明, BZT-0.5BC薄膜具有很强的法向承载能力, 但是切向抗磨损能力差, 样品的平均弹性模量为23.64 GPa± 5 GPa, 其硬度为2.7–4 GPa, 两者均略低于压电陶瓷Pb(Zr, Ti)O₃材料的体态值. 关键词： BZT-BCT薄膜 纳米摩擦力 纳米压痕 纳米划痕     

A study of disorder in the arrangement of Pb atoms of the PbFe0.5Nb0.5O3 single crystals above the curie point

A precision X-ray diffraction study of PbFe_0.5Nb_0.5O₃ crystal in a paraelectric phase at 160°C has shown the presence of small disordered displacements of the Pb atoms from their sites in the ideal perovskite structure. Several models of such displacements were considered. The lowest discrepancy factor R is 0.0472.     

Evolution from relaxor-like dielectric to ferroelectric in Ba[(Fe0.5Nb0.5)1−xTix]O3 solid solutions

Ba[(Fe_0.5Nb_0.5)_1−xTi_x]O₃ (x=0.2,0.4,0.6,0.8,0.85,0.9 and 0.95) solid solutions were synthesized by a standard solid-state reaction technique. X-ray diffraction at room temperature and dielectric characteristics over a broad temperature and frequency range were evaluated systematically. The structure of Ba[(Fe_0.5Nb_0.5)_1−xTi_x]O₃ solid solutions changed from cubic to tetragonal with increasing x. A Debye-like dielectric relaxation following the Arrhenius law similar to that in Ba(Fe_0.5Nb_0.5)O₃ was observed at lower temperature in the composition range 0.2≤x≤0.8, while the relaxor ferroelectric, diffused ferroelectric and normal ferroelectric behavior were observed for x=0.85,0.9 and 0.95, respectively. The process of the evolution of relaxor-like dielectric to ferroelectric suggested the changing from dilute polar micro-domains to polar micro-domains, polar micro/macro-domains and then polar macro-domains in the present ceramics.     

High P–T Raman study of transitions in relaxor multiferroic Pb(Fe0.5Nb0.5)O3

The vibrational and structural properties of Pb(Fe_0.5Nb_0.5)O₃ have been investigated using Raman spectroscopy up to 40 GPa at 300 K and from 300 to 415 K at selected pressures. The measurements reveal three phase transitions, at 5.5, 8.7, and 24 GPa at room temperature. The temperature dependences of the spectra indicate transitions at 1.5 GPa, at 335 and 365 K. The results are consistent with the appearance of an intermediate tetragonal P4mm phase between the ferroelectric R3m and paraelectric Pm‐3m phases. A P–T phase diagram is proposed that allows further insight into the magnetoelectric coupling present in this material. Copyright © 2015 John Wiley & Sons, Ltd.     

Ferroelectric phase transitions in Pb0.96Ba0.04Sc0.5Nb0.5O3 and Pb0.94Ba0.06Sc0.5Nb0.5O3 solid-solution single crystals

The optical properties of Pb_0.96Ba_0.04Sc_0.5Nb_0.5O₃ (PBSN-4) and Pb_0.94Ba_0.06Sc_0.5Nb_0.5O₃ (PBSN-6) single-crystal solid solutions were studied for the first time. It was shown that the spontaneous phase transition occurring in PBSN-4 with no electric field present is accompanied by a sharp minimum in optical transmission, which indicates the percolation nature of the transition. No sharp changes were observed in the temperature dependence of optical transmission in PBSN-6 single crystals with no electric field applied. However, a very weak electric field, ～0.4 kV/cm, is sufficient to induce the ferroelectric state in PBSN-6 single crystals. It was shown that the destruction of the induced ferroelectric state is a first-order phase transition which is accompanied by an anomalously narrow peak in the small-angle light scattering intensity (or by a minimum in optical transmission) and occurs through the percolation mechanism.     

Raman Spectra Study on LiTa0.9Nb0.1O3 Single Crystal

In this letter, Raman spectra of LiTa_0.9Nb_0.1O₃ single crystal in both of its ferroelectric phase and paraelectric phase are presented for the first time. Comparing with LiTaO₃ crystal, we find that the distribution and Raman shifts of normal vibrational mades are similar, but the line-shape and relative intensities of these modes have changed.     

Microstructure and piezoelectric properties of Bi0.5(Na0.82K0.18)0.5TiO3−NaSbO3 ceramics

Lead-free piezoelectric ceramics (1−x)Bi_0.5(Na_0.82K_0.18)_0.5TiO₃−xNaSbO₃ have been prepared by a conventional ceramics technique, and their microstructure and electrical properties have been investigated. The addition of NaSbO₃ has no remarkable effect on the crystal structure within the studied doping content; however, an obvious change in microstructure took place. With increase in NaSbO₃ content, the temperature from a ferroelectric to antiferroelectric phase transition increases, and the temperature for a transition from antiferroelectric phases to paraelectric phases changes insignificantly. Simultaneously, the temperature range between the rhombohedral phase transition point and the Curie temperature point becomes smaller. The piezoelectric properties significantly increase with increase in NaSbO₃ content and the piezoelectric constant and electromechanical coupling factor attain maximum values of d₃₃=160 pC/N and k_p=0.333 at x=0.01. The results indicate that (1−x)Bi_0.5(Na_0.82K_0.18)_0.5TiO₃−xNaSbO₃ ceramic is a promising lead-free piezoelectric candidate material.     

Ba0.5Sr0.5TiO3有序构型的第一性原理研究

采用第一性原理计算了Ba_0.5Sr_0.5TiO₃三种有序构型的晶格结构和对应的电子结构,晶格结构的详细分析结果表明BST{100}有序构型为四方相,Ti-O八面体中Ti原子和Ba-Sr平面上的O原子沿[100]方向分别偏心位移0.040 Å和0.065 Å,八面体畸变导致反平行自发极化出现,构型处于反铁电态. BST{110}构型也是四方相,并且(110)和(1 关键词： 钛酸锶钡 第一性原理 有序结构 铁电性     

The effect of some transition metal oxides on the physical properties of K0.5Na0.5Nb0.95Ta0.05O3 ceramics

Abstract

Both K_0.5Na_0.5Nb_0.95Ta_0.05O₃ (KNNTO) and (K_0.5Na_0.5Nb_0.95Ta_0.05O₃)_0.99-M_0.01, M = Co₃O₄ and Mn₂O₃ (M/KNNTO) Ferromagnetic behaviour was observed for some M/KNNTO compounds. The hardness and compressive strength of all investigated samples are given. Comparisons with similar materials are discussed. Ceramics were synthesised using a solid-state reaction method. X-ray diffraction patterns of all samples revealed that the crystal structure is orthorhombic. Field-emission scanning electron microscopy was performed. Polarisation hysteresis curves indicated a disruption of ferroelectric order with the addition of M into KNNTO ceramics. The dielectric properties of the investigated ceramics have been studied as a function of frequency and temperature.     

Synthesis and optical properties of tetragonal KTa0.6Nb0.4O3 nanoparticles

Tetragonal phase KTa_0.6Nb_0.4O₃ (KTN) nanoparticles have been prepared by hydrothermal method. The obtained particles were characterized by X-ray powder diffraction, transmission electron microscopy, energy dispersive X-ray spectroscopy and UV-vis absorption spectrum techniques. A systematic change in crystal structure from cubic to tetragonal is observed with the increase of reaction temperature and KOH concentration. Room temperature UV-vis absorption spectrums of KTN particles show that the band gap changes from 3.24 to 3.34 eV with grain size diminished, which reveals the existence of blue-shift phenomenon of absorption bands.     

Ferroelectric-relaxor behavior of (Na0.5K0.5)NbO3-based ceramics

We report that ferroelectric-relaxor behavior is induced by doping of SrO and TiO₂, or BaO and TiO₂ into classic ferroelectric (Na_0.5K_0.5)NbO₃. It is found that [(Na_0.5K_0.5)_0.9Sr_0.1](Nb_0.9Ti_0.1)O₃ ceramics exhibit a pronounced ferroelectric-relaxor behavior, comparable to that of [(Na_0.5K_0.5)_0.9Ba_0.1](Nb_0.9Ti_0.1)O₃ ceramics. Our results indicate that the relaxor behavior is closely related to the appearance of micropolar regions in these systems. The relaxor behavior should arise from the dynamic response of micropolar clusters. Raman spectra of [(Na_0.5K_0.5)_1−xSr_x](Nb_1−xTi_x)O₃ ceramics measured in the wavenumber range from 100 to 1200 cm⁻¹ confirm that the first order scattering is dominant in phonon bands should result from both short-range ordered region (micropolar regions) and disordered matrix. The frequency dependence of dielectric permittivity measurements show that the relaxor behavior of SrO and TiO₂, or BaO and TiO₂ doped (Na_0.5K_0.5)NbO₃ ceramics is not a Debye type in the radio frequency range.     

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.     

Microstructure and electrical properties of Ba0.5Sr0.5TiO3 thin films prepared on copper foils with La2O3 buffer layers

Ba_0.5Sr_0.5TiO₃ (BST) thin films were deposited on copper foils via sol-gel method with La₂O₃ as a buffer layer. The films were processed in almost inert atmosphere so that the substrate oxidation was avoided while allowing the perovskite film phase to crystallize. The existence of a La₂O₃ buffer layer between the BST thin film and Cu foil improved the dielectric constant and reduced the leakage current density of the BST thin film. Meanwhile, the BST thin film exhibited ferroelectric character at room temperature, which was contrast to the para-electric behavior of the film without the buffer layer. Effects of La₂O₃ buffer layer on the crystallizability and microstructure of BST thin films were also investigated.     

Low-temperature phase transformations in weakly doped quantum paraelectrics: novel features and quantum reentrant dipolar glass state in KTa0.982Nb0.018O3

An unusual sequence of phase transitions (PT) and reentrant dipole glass-like phase formation at low temperatures was found recently in KTaO₃ weakly doped with Li and Nb (K_0.9986Li_0.0014Ta_0.976Nb_0.024O₃) [Phys. Rev. B 63 (2001) 172]. We report on detailed low frequency (100 Hz-1 MHz) permittivity and Raman light scattering studies of similar composition, but without Li admixture, KTa_1−xNb_xO₃ with x=0.018 (KTN1.8). The aim of the study is to answer the question if the reentrant dipole glass-like phase exists in KTN1.8 and what is the microscopic origin of this phase. A detailed study of the sharp low-temperature PT observed at T_C∼27 K revealed properties inherent to the reentrant glass-type state at lower temperatures. The substitution of Nb for Ta influences the TO₁ soft lattice mode and leads to PT with the long-range ferroelectric ordering. A crossover to an order-disorder polar microregion dynamics with a non-standard ε′(T) behaviour and dipole glass-like formation were found below T_C (at ∼15 K), which is attributed to the randomness of the Nb distribution. A crossover to the long-range order was found under a dc bias field.