Necessary Conditions for the Formation of {111} Twins in Barium Titanate

The experimental conditions for {111} twin formation in BaTiO₃ were investigated. When BaTiO₃ compacts without excess TiO₂ were sintered either in an oxidizing atmosphere (air) or in a reducing atmosphere (95N₂–5H₂), no {111} twins formed within the BaTiO₃ grains and no abnormal grain growth occurred. In contrast, many {111} twins were present within the abnormally grown grains in the excess-TiO₂-containing BaTiO₃ samples sintered in air, while no twins were observed in the excess-TiO₂-containing samples sintered in 95N₂–5H₂. X-ray diffraction analysis showed that excess TiO₂ forms a Ba₆Ti₁₇O₄₀ phase during sintering with the space group A 2/ a in air and a Ba₆Ti₁₇O_{40− x} phase with the space group C in 95N₂–5H₂. It appears therefore that excess TiO₂ and an oxidizing atmosphere are necessary for {111} twin formation in BaTiO₃. These results may also indicate that the interface structure between BaTiO₃ and Ba₆Ti₁₇O₄₀ influences the twin formation.     

Texture Development in Barium Titanate and PMN–PT Using Hexabarium 17-Titanate Heterotemplates

Bulk BaTiO₃ ceramics with 〈111〉-texture have been prepared by the modified templated grain growth method, using platelike Ba₆Ti₁₇O₄₀ particles as templates, and the mechanism of texture development is examined. The Ba₆Ti₁₇O₄₀ particles induce the abnormal growth of BaTiO₃ grains, and a structure similarity between {001} of Ba₆Ti₁₇O₄₀ and {111} of BaTiO₃ gives 〈111〉-texture to abnormally grown BaTiO₃ grains. Thus, the 〈111〉-texture develops in the BaTiO₃ matrix. The use of platelike Ba₆Ti₁₇O₄₀ particles has been extended to a 0.65Pb(Mg_1/3Nb_2/3)O₃–0.35PbTiO₃ matrix, but the matrix phase is decomposed by extensive chemical reactions between the matrix and template phases.     

Genesis of the (111) Twin in Barium Titanate

On the basis of the topotaxy between BaTiO₃ and Ba₆Ti₁₇O₄₀ found recently, a model of a nonconservative (111) twin in TiO₂-rich BaTiO₃ was constructed. The model consists of several (001) layers of Ba₆Ti₁₇O₄₀ intergrown between (111) layers of BaTiO₃, the core of the twin being a slightly modified double layer of Ba₆Ti₁₇O₄₀ containing face-sharing octahedra. Using this model, anomalous grain growth below the eutectic temperature and preferential growth of (111) twins in a reducing atmosphere were explained, as well a nucleation of butterfly twins.     

Effect of Ba6Ti17O40/BaTiO3 Interface Structure on {111} Twin Formation and Abnormal Grain Growth in BaTiO3

A structural transition of Ba₆Ti₁₇O₄₀/BaTiO₃ interfaces from faceted to rough was induced by reducing oxygen partial pressure in the atmosphere. As the oxygen partial pressure decreased, the number densities of {111} twins and abnormal grain decreased. TEM observation showed that the twin formation was governed only by the faceting of the interface. Experimental evidence of {111} twin-assisted abnormal growth of faceted BaTiO₃ grains was also obtained.     

Effects of Ba6Ti17O40 on the Dielectric Properties of Nb-Doped BaTiO3 Ceramics

The dielectric properties, including the DC breakdown strength, of 1 mol% Nb⁵⁺-doped BaTiO₃ ceramics with different quantities of excess TiO₂ have been investigated. The breakdown strength was found to decrease with increasing TiO₂ content, but could not be readily explained by relative density and grain size effects. The decrease in the breakdown strength from a stoichiometric BaTiO₃ composition to samples with excess TiO₂ is believed to be due to the field enhancement effect (up to a factor of 1.40) at the BaTiO₃ matrix because of the presence of a Ba₆Ti₁₇O₄₀ second phase. The thermal expansion coefficient mismatch between the BaTiO₃ matrix phase and the Ba₆Ti₁₇O₄₀ phase may also result in a low breakdown strength. The dielectric properties of the pure Ba₆Ti₁₇O₄₀ phase were also investigated and are reported herein.     

Solubility of TiO2 in BaTiO3

Scanning electron microscopy and electron probe micro-analysis were used to investigate the microstructure of both slow-cooled and quenched polycrystalline BaTiO₃ specimens with a small excess of TiO₂ (Ba/Ti=0.995 to 0.999) or of BaO (Ba/Ti=1.002 and 1.005). The electron micrographs of polished and etched TiO₂-excess BaTiOs samples, and of fracture surfaces of quenched samples, showed a second phase in the grain boundaries and triple-point regions, whereas no second phase was observed in samples having Ba/Ti=1.000. Microprobe analysis of the second phase gave compositions near that of the reported adjacent phase of higher TiO₂ content, Ba₆Ti₁₇O₄₀. The results indicate that the solubility of TiO₂ in BaTiO₃ is <0.1 mol%.     

Modified Phase Diagram for the Barium Oxide–Titanium Dioxide System for the Ferroelectric Barium Titanate

The ferroelectric phase transition behavior in BaTiO₃ was investigated for various annealing times, temperatures, and Ba/Ti ratios by means of a differential scanning calorimeter. Coupling these observations with powder X-ray diffraction and transmission electron microscopy allowed new insights into the barium oxide (BaO)–titanium dioxide (TiO₂) phase diagram. The transition temperature was varied systematically with the Ba/Ti ratio at annealing temperatures from 1200° to 1400°C in air. The transition temperature decreased with increasing concentrations of BaO and TiO₂ partial Schottky defects, and showed a discontinuous change at the phase boundaries. Beyond the solubility region, two peritectoid reactions were confirmed and revised; first around 1150°C for Ba_1.054Ti_0.946O_2.946→Ba₂TiO₄+BaTiO₃ and second 1250°C for BaTi₂O₅→Ba₆Ti₁₇O₄₀+BaTiO₃, respectively. All other regimes of the BaO–TiO₂ were found to be consistent with the reported diagrams in the literature.     

Reactive-Templated Grain Growth of Bi1/2(Na,K)1/2TiO3: Effects of Formulation on Texture Development

In this paper we report the effects of formulation on texture development for the "reactive-templated grain growth" (RTGG) of Bi_1/2(Na,K)_1/2TiO₃ (BNKT). The solids formulation for BNKT was systematically varied by prereacting to well—defined alkali and bismuth titanates (Na₂Ti₃O₇ (N₂T₃), K₂Ti₂O₅ (K₂T₂), and Bi₂Ti₄O₁₁ (B₂T₄)). Use of these precursors in different BNKT formulations determined that the amount of expansion associated with reacting dry-pressed compacts at 600−800°C could be influenced by formulation. Lotgering factors ( F _{00 l} ) derived from Θ/2Θ X-ray diffraction scans indicated that the formulation route strongly affected the {00 l } texture development in tape-cast and sintered specimens. Prereacting alkali carbonates with TiO₂ to form N₂T₃ and K₂T₂ inhibited texture development in RTGG-processsed BNKT. However, when Bi₂O₃ was prereacted to form B₂T₄, the measured F _{00 l} increased from 0.5 to 0.7.     

Phase Equilibria in the TiO2-Rich Region of the System BaO-TiO2

Phase relations in the system BaO-TiO₂ from 67 to 100 mol% TiO₂ were investigated at 1200° to 1450°C in O₂. Data were obtained by microstructural, X-ray, and thermal analyses. The existence of the stable compounds Ba₆Ti₁₇O₄₀, Ba₄Ti₁₃O₃₀, BaTi₄O₉, and Ba₂Ti₉O₂₀ was confirmed. The compound BaTi₂O₅ is unstable and either forms as a reaction intermediate below the solidus or crystallizes from the melt. The compounds Ba₆Ti₁₇O₄₀ and Ba₄Ti₁₃O₃₀ decompose in peritectic reactions, and BaTiO₃ and Ba₆Ti₁₇O₄₀ react to form a eutectic. Special conditions are required for the formation of Ba₂Ti₉O₂₀, which decomposes in a peritectoid reaction at 1420°C. The new phase diagram is presented.     

Effects of B2O3 on the Phase Stability of Ba2Ti9O20 Microwave Ceramic

Preparation of dense and phase-pure Ba₂Ti₉O₂₀ is generally difficult using solid-state reaction, since there are several thermodynamically stable compounds in the vicinity of the desired composition and a curvature of Ba₂Ti₉O₂₀ equilibrium phase boundary in the BaO–TiO₂ system at high temperatures. In this study, the effects of B₂O₃ on the densification, microstructural evolution, and phase stability of Ba₂Ti₉O₂₀ were investigated. It was found that the densification of Ba₂Ti₉O₂₀ sintered with B₂O₃ was promoted by the transient liquid phase formed at 840°C. At sintering temperatures higher than 1100°C, the solid-state sintering became dominant because of the evaporation of B₂O₃. With the addition of 5 wt% B₂O₃, the ceramic yielded a pure Ba₂Ti₉O₂₀ phase at sintering temperatures as low as 900°C, without any solid solution additive such as SnO₂ or ZrO₂. The facilities of B₂O₃ addition to the stability of Ba₂Ti₉O₂₀ are apparently due to the eutectic liquid phase which accelerates the migration of reactant species.     

Effects of Composition on the Reaction Kinetics of Hydrothermally Derived Barium Strontium Titanate

Barium strontium titanate (BST, Ba _x Sr_{1− x} TiO₃) powders were fabricated by reacting nanocrystalline TiO₂ with aqueous alkaline solutions containing Ba and Sr at 80°C. Measurements of reaction kinetics showed that Ba-rich BST compositions exhibited more rapid reaction rates compared with Sr-rich BST compositions, and the reaction rate increased monotonically with increasing Ba content. The average particle size also increased with increasing Ba content, with the particle growth rate of BaTiO₃ being approximately a factor of 10 greater than SrTiO₃. The increase in growth rate from Sr-rich to Ba-rich BST corresponded to a morphological transition from 20 to 30 nm cuboidal particles to 80 nm raspberry-like particles, respectively.     

Interaction between Barium Titanate and Binary Glasses

Interactions between BaTiO₃, and three binary glasses were studied through the reaction of BaTiO₃, powder with glass powder. For PbO–B₂O₃ and PbO–SiO₂ glasses, the reaction led to stable compound formation, the substitution of Pb in the BaTiO₃ structure, and noticeable grain growth of BaTiO₃. The interaction phenomena for these two glass systems were very similar. The substitution of Pb into BaTiO₃ is assisted by chemical reactions in which BaB₂O₄ or Ba₂SiO₄ is formed. The substitution into BaTiO3 also seems to be closely related to the grain growth of BaTiO₃. On the other hand, only compound formation was observed during the processing of BaTiO₃ with Bi₂O₃–B₂O₃ glass. Neither BaTiO₃ grain growth nor Bi substitution took place with the Bi₂O₃–B₂O₃ glass system. Based on the observed reactions and the glass viscosity, several sintering aids for BaTiO₃ ceramic products are suggested in this paper.     

The {111} Growth Twins in Tetragonal Barium Titanate

The {1 1 1} twins frequently observed in pressureless-sintered BaTiO₃ ceramics have been analyzed by the X-ray diffractometry, scanning and transmission electron microscopy. Both the single twins and double lamellar twins are growth (or annealing) twins. The twins lying in the {1 1 1} mirror planes, which is not one of the symmetry elements of the (basic) crystal lattice's but that of the superlattice's, are therefore superlattice twins. The {1 1 1} twins, particularly the double twins, were found more frequently from samples sintered in an Ar atmosphere of lower oxygen partial pressure (pO₂). Further decreasing of pO₂ using the Ar-5% H₂ mixture has rendered the sintered samples entirely of hexagonal BaTiO₃, the 6H-polytype. The formation of such twins is attributed to changing of the corner-sharing TiO₆ octahedra to Ti₂O₉ face-sharing octahedra, which accommodates for local oxygen deficiency in tetragonal BaTiO₃. The stacking sequence alters accordingly from c-layer (constituting the 3C-polytype, treating tetragonal pseudo-cubic) to h-layer (as in (chc)₁(chc)₂ of 6H).     

Phase Equilibria in the System Barium Titanate—Strontium Titanate

Phase equilibria in the system BaTiO₃–SrTiO₃ (with 0 to 7 mol% SrTiO₃) were studied at temperatures above 1600°C in air. Quenching experiments were performed using high-purity starting materials, and run products were examined by X-ray diffraction and differential scanning calorimetry to determine phase composition and Sr concentration. Melting involves a binary loop intersected by the invariant reaction hexagonal (Ba, Sr)TiO₃( ss ) ⇌ cubic (Ba, Sr)TiO₃( ss ) + liquid. In contrast with earlier work, these results indicate that there is no depression of the melting point with Sr addition and no congruent melting point in this compositional range.     

Tunable Microwave Properties of Barium Titanate-Based Ferroelectric Glass-Ceramics

BaTiO₃-based ferroelectric glass-ceramics with the composition 0.65(Ba_{1− X} Sr _X )TiO₃·0.27SiO₂·0.08Al₂O₃ ( X = 0.2–0.6) were fabricated, and their tunable dielectric properties were measured at microwave frequency. Major crystalline phases that precipitated during thermal treatment up to 1000°C were (Ba,Sr)TiO₃, Ba₂TiSi₂O₈, and BaAl₂Si₂O₈. The Curie temperatures of the heated bulk samples were successfully tuned near room temperature at the composition between X = 0.2 and 0.3. A thick-film sample with X = 0.3 showed 27% tunability at 5 GHz under 10 kV/cm bias voltage.     

Solid Solubility of Holmium, Yttrium, and Dysprosium in BaTiO3

The solid solubility of R ions (R = Ho³⁺, Dy³⁺, and Y³⁺) in the BaTiO₃ perovskite structure was studied by quantitative electron-probe microanalysis (EPMA) using wavelength-dispersive spectroscopy (WDS), scanning electron microscopy (SEM), and X-ray diffractometry (XRD). Highly doped BaTiO₃ samples were prepared using mixed-oxide technology including equilibration at 1400° and 1500°C in ambient air. The solubility was found to depend mainly on the starting composition. In the TiO₂-rich samples a relatively low concentration of R incorporated preferentially at the Ba²⁺ lattice sites (solubility limit ∼Ba_0.986R_0.014Ti_0.9965(V^"_Ti^")_0.0035O₃at 1400°C). In BaO-rich samples a high concentration of R entered the BaTiO₃ structure at the Ti⁴⁺ lattice sites (solubility limit ∼BaTi_0.85R_0.15O_2.925(V_O^••)_0.075at 1500°C). Ho³⁺, Dy³⁺, and Y³⁺incorporated preferentially at the Ti⁴⁺ lattice sites stabilize the hexagonal polymorph of BaTiO₃. The phase equilibria of the Ho³⁺–BaTiO₃ solid solutions were presented in a BaO–Ho₂O₃–TiO₂phase diagram.     

The Systems BaO-SrO-TiO2, BaO-CaO-TiO2, and SrO-CaO-TiO2

The solid phases formed at 1400°C. in air in the three-component systems BaO-SrO-TiO₂, BaO-CaO-TiO₂, and SrO-CaO-TiO₂ are described. Besides solid solutions of components with known structures, some new ternary compounds have been studied. The dielectric constants and loss factors of a number of specimens are given. Crystallographic data of the compounds BaCaTiO₄, Ba₃Ca₂Ti₂O₉, and Ca₃Ti₂O₇ and of the solid solution series (Ba, Sr), TiO₄ are presented. The preparation of the new compounds is described in detail.     

Effect of pH on the Chemistry of the Barium Titanium Citrate Gel and Its Thermal Decomposition Behavior

In this investigation, X-ray diffractometry and ¹³C NMR spectroscopy were used to characterize the effect of pH on the formation of gel and its thermal decomposition by the Pechini process. It was found that the major effect of pH was to destroy the esterification between citric acid (CA) and ethylene glycol, which in turn could influence Ba species and the formation of a mixed-metal CA complex. It was also found that barium carbonate was derived from the Ba species and was not related to the instability of Ba₂Ti₂O₅CO₃, but Ba₂Ti₂O₅CO₃ was formed by the thermal decomposition of a mixed-metal CA complex. Moreover, BaTiO₃ formed via two routes, the reaction of BaCO₃ with Ti species and thermal decomposition of Ba₂Ti₂O₅CO₃.     

Existence of the Ba2Ti5O12 Phase in Ca-Doped TiO2-Rich BaTiO3

A TEM investigation was conducted on the structure of a second phase precipitated between the grains of a polycrystalline TiO₂-rich BaTiO₃ which was doped with 8 mol% Ca. This phase was identified as Ca-stabilized Ba₂Ti₅O₁₂ with a 10-layer orthorhombic structure and unit-cell parameters a=0.990, b=1.131, and c=2.330 nm.     

Synthesis and Characterization of Barium Lanthanum Titanates

Ternary compounds in the system BaO—TiO₂—La₂O₃ were prepared by the solid-state reaction technique at temperatures between 1300° and 1400°C using precursor oxides as the starting materials. In an alternative processing technique, BaTiO₃ was reacted with appropriate proportions of prefabricated lanthanum titanates at 1350°C to obtain the compounds. Two compounds were identified in the TiO₂-rich region of the system. The X-ray powder diffraction pattern of a compound with a chemical composition BaLa₂Ti₃O₁₀ (BaO·La₂O₃·3TiO₂) is indexed on the basis of an orthorhombic unit cell with a = 7.665 × 10⁻¹ nm, b = 28.524 × 10⁻¹ nm, and c = 3.876 × 10⁻¹ nm. The other compound, which has a chemical composition Ba₄La₈Ti₁₇O₅₀ (BaO·La₂O₃·4.25TiO₂) occurs in a narrow homogeneity range within the system. The X-ray powder diffraction pattern of the compound is indexed on the basis of an orthorhombic unit cell with a = 12.317 × 10⁻¹ nm, b = 22.394 × 10⁻¹ nm, and c = 3.881 × 10⁻¹ nm. Both the compounds are compatible with BaTiO₃ and form pseudobinary joins with BaTiO₃ in the system BaO—TiO₂—La₂O₃.