Ferroelectric elastance of sodium niobate-lithium niobate solid solutions in relation to A-O bond covalence

The ferroelectric elastance of (Na, Li, A _0.5 ^′ )NbO₃ solid solutions was found to vary directly with the total electronegativity of elements A (EN _{∑ A} ), that is, with the A-O bond covalence at B=const. Using EN _{∑ A} values and the position on the phase diagram, one can determine the solid solution parameters optimum for use in high-frequency (A′=Sr, Pb), medium-frequency (A′=Cd), and elevated-temperature (A′=Cd) devices.     

Structural studies of the P-T phase diagram of sodium niobate

The crystal structure of sodium niobate (NaNbO₃) has been investigated by energy-dispersive X-ray diffraction at high pressures (up to 4.3 GPa) in the temperature range 300–1050 K. At normal conditions, NaNbO₃ has an orthorhombic structure with Pbcm symmetry (antiferroelectric P phase). Upon heating, sodium niobate undergoes a series of consecutive transitions between structural modulated phases P-R-S-T(1)-T(2)-U; these transitions manifest themselves as anomalies in the temperature dependences of the positions and widths of diffraction peaks. Application of high pressure leads to a decrease in the temperatures of the structural transitions to the R, S, T(1), T(2), and U phases with different baric coefficients. A phase diagram for sodium niobate has been build in the pressure range 0–4.3 GPa and the temperature range 300–1050 K. The dependences of the unit-cell parameters and volume on pressure and temperature have been obtained. The bulk modulus and the volume coefficients of thermal expansion have been calculated for different structural modulated phases of sodium niobate. A phase transition (presumably, from the antiferroelectric orthorhombic P phase to the ferroelectric rhombohedral N phase) has been observed at high pressure (P = 1.6 GPa) and room temperature.     

Thermal and physical properties of sodium niobate ceramics over a wide temperature range

The temperature dependences of the heat capacity C _p (T) and thermal expansion coefficient α(T) of NaNbO₃ ceramic samples have been investigated in the temperature range from 2 to 800 K. In addition to the anomalies associated with the known phase transitions at temperatures T ₆ ≈ 265 K, T ₅ ≈ 638 K, T ₄ ≈ 760 K, and T ₃ ≈ 793 K, anomalies in the behavior of C _p (T) and α(T) have been observed near T _5″ ≈ 500 K and T _5′ ≈ 600 K. It has been found that all the observed structural transformations, according to the values of the entropy change, are not related to the ordering of structural elements. It has been shown that, with an increase in the temperature, the unit cell volume during the phase transitions near 265, 515, 604, and 638 K decreases. The specific features of the transition to the phase R3c have been examined. Two possible scenarios of the sequence of phase transformations in the temperature range between T ₅ and T ₆ have been analyzed.     

Preparation and piezoelectric properties of potassium sodium niobate glass ceramics

This paper describes the preparation of a piezoelectric glass ceramic material from potassium sodium niobate(K0.5Na0.5Nb O3;KNN) using a novel melting method.The effects of the subsequent heat-treatment on the optical,thermal,electrical,and mechanical properties of the material are carefully examined,and its crystal structure and surface morphology are characterized respectively by x-ray diffraction and scanning electron microscopy.This new material has a much higher piezoelectric coefficient(163 p C·N-1) than traditional piezoelectric ceramics(131 p C·N-1).On this basis therefore,a strategy for the future study and development of lead-free KNN-based piezoelectric glass ceramics is proposed.     

Phase states and dielectric properties of sodium-potassium niobate solid solutions

The temperature dependence of dielectric constant ? for single crystals of Na_1?xK_xNbO₃ (0.04?x?0.15) is studied for the first time. From the shape of the ?(T) anomalies corresponding to rotational phase transitions, the type of interaction between the order parameters and the polarization is determined. A phenomenological model is developed which adequately describes the experimentally observed sequence of high-temperature (T>300°C) phase transitions, the dielectric anomalies associated with these transitions, and the changes in the phase states under the action of external factors (pressure, electric field).     

Phase diagram and critical properties of the asymmetric Mattis model

We extend the well-known Mattis model to the case of asymmetric bond distributions. Although the partition function is identical with that of the pure ferromagnetic Ising model (FIM) when the external field is absent, the response to the external field is nontrivial even at zero field. There are some exact relations between the present model and the FIM in the correlation functions, from which the phase diagram and critical exponents can be determined. Multicritical behavior and some other interesting phenomena typical for a random system are demonstrated by this model.     

Incommensurate phases in lead potassium niobate and lead sodium niobate,homologues of barium sodium niobate.

Electron diffraction and high resolution electron microscopy reveal the presence of incommensurate structures in lead potassium niobate, lead sodium niobate and in mixed crystals. The incommensurability can be described either as being due to the presence of quasi-periodic “diffuse” anti-phase boundaries or as being due to the presence of deformation waves, the deformation mode being shearing of the NbO₆ octahedra. The incommensurability depends on composition as well as on temperature.     

Phase diagram of a two-dimensional electron system

The melting curve of the two-dimensional electron system is interpolated between the known classical and ground state limits. The coexistence curve encloses a finite solid-phase domain, as in the three-dimensional case.     

Synthesis and dielectric properties of modified potassium sodium niobate solid solutions

In the present work processing microstructure and dielectric properties of lead free KNN based ceramics have been studied. Compositions with a stoichiometric formula (1 ? x)(K_0.5Na_0.5)Nb_{1 ? y} Sb _y O₃-xBaTiO₃ (x = 0.01, 0.02, 0.04; y = 0.04, 0.07) were produced by solid state sintering method. The addition of manganese oxide MnO₂ after synthesis promoted the sintering and densification of ceramic samples. The influence of BaTiO₃ on the microstructure, density and electrical properties was investigated. X-ray diffraction analysis confirmed that obtained samples had a pure perovskite structure with no traces of secondary phase. Phase transition peak at T _c is broad indicating the diffuse phase transition.     

Ca2+掺杂铌酸锶钡晶体的光折射变化特性研究

基于Ca2+掺杂铌酸锶钡晶体的透射特性,探讨了Ca2+在晶体中的电子行为机制,分析了Ca2+掺杂而引起的光致折射率变化特性.采用Michelson干涉装置测得了样品折射率随时间变化的特性曲线.实验结果分析表明,适当的ca2+掺杂可以有效改善铌酸锶钡晶体的光折射特性.     

Phase diagram and interfacial characteristics in a binary system

Physics of the Solid State - The size effects on the composition of coexisting phases, the interfacial layer between them, and the interfacial tension in a binary system composed of a matrix and...     

铁电钾钠铌酸锶钡薄膜电光性能的研究

采用溶胶-凝胶法在氧化镁单晶衬底上制备了符合化学计量比的完全填充型铁电钾钠铌酸锶钡(KNSBN)薄膜，通过X射线衍射，摇摆曲线，X射线Φ扫描，扫描电子显微镜等方法研究了薄膜的微结构，采用Adachi法研究了薄膜的电光特性. 实验发现，KNSBN薄膜在氧化镁(001)单晶衬底上沿c轴外延生长，K⁺，Na⁺的引入有效地提高了薄膜的横向电光系数r₅₁. 成分为K_0.2Na_0.2Sr_0.24Ba_0.56Nb₂O₆, K_0.2Na_0.2Sr_0.6Ba_0.2Nb₂O₆, K_0.2Na_0.2Sr_0.72Ba_0.08Nb₂O₆的三种KNSBN薄膜的r₅₁值分别为108.52pm/V, 119.98pm/V, 126.96pm/V，r₅₁的数值随Sr²⁺含量增加而增大. 关键词： 横向电光系数 钾钠铌酸锶钡 外延生长     

Phase diagram of the Bi-Sn system in the liquid and solid states

Results are given for the specific resistance, thermo-emf, and magnetic susceptibility in the range from room temperature up to 500 °C. The methods of measurement have previously been reported [1]. Each property is presented as a function of composition; the results are compared with ones for the system Bi-Cd. The temperature dependence of the three quantities is calculated for the alloy of eutectic composition.     

Properties of nonstoichiometric sodium niobate

Stoichiometric and nonstoichiometric sodium niobate of the composition Na_1−x NbO_3−x/2 (0≤x≤0.2) is obtained by the two-stage solid-phase reaction technique. X-ray diffraction study reveals the homogeneity region for compositions below x=0.1. At higher x, the samples become two-phase. The phase transition is found to occur at x=0.04. It is accompanied by a change in the multiplicity of the monoclinic subcell of the sodium niobate rhombic unit cell, as well as by the anomalous behavior of its structural and electrophysical characteristics. From the study of the dielectric and piezoelectric properties of the material, it is concluded that the stability of the antiferroelectric phase is enhanced, whereas the ferroelectric phase becomes less stable, as the nonstoichiometry in terms of Na₂O increases.