Effect of MgO Doping on the Phase Transformations of BaTiO3

The successive phase transformations in MgO-doped BaTiO₃ were studied. Upon MgO doping, dielectric anomalies corresponding to lower phase transformations were broadened and depressed, while an anomaly for a cubic–tetragonal transformation remained and shifted to a lower temperature. XRD peak splitting upon tetragonality of BaTiO₃ was decreased, and the peaks exhibited abnormally broadened profiles which are different from the one for cubic BaTiO₃ above T _c. Raman spectroscopy revealed the existence of orthorhombic phase at room temperature for the solid solution with 0.5 mol% or more MgO. The temperature dependence of the Raman spectrum showed that orthorhombic and rhombohedral phases in MgO-doped BaTiO₃ were stabilized at higher temperatures than pure BaTiO₃.     

Chemical Diffusivity of BaTiO3−δ: IV, Acceptor-Doped Case

The chemical diffusivity of 1.8 mol% aluminum-doped BaTiO_3−δ was measured on single-crystal specimens, as a function of ambient oxygen partial pressure, in the range 10⁻¹⁸ atm ≤ P _O2≤ 1 atm and at temperatures of 800°≤ T ≤ 1100°C, via a conductivity-relaxation technique. As in the polycrystalline, undoped BaTiO_3−δ described in Part II of this work, the chemical diffusivity exhibited a maximum, of thermodynamic origin, approximately at the stoichiometric composition (δ= 0). The measured diffusivity was analyzed, based on the defect structure proposed and Wagner's classic theory of chemical diffusion, and the mobilities of the electrons and holes, as well as all of the relevant defect-equilibrium constants, then were evaluated with no prior assumptions. The evaluated parameters were compared with those for the undoped BaTiO_3−δ.     

Electrical Properties of Cerium-Doped BaTiO3

The high-temperature equilibrium electrical conductivity of Ce-doped BaTiO₃ was studied in terms of oxygen partial pressure, P (O₂), and composition. In (Ba_1−xCe _x )TiO₃, the conductivity follows the −1/4 power dependence of P (O₂) at high oxygen activities, which supports the view that metal vacancies are created for the compensation of Ce donors, and is nearly independent of P (O₂) where electron compensation prevails at low P (O₂). When Ce is substituted for the normal Ti sites, there is no significant change in conductivity behavior compared with undoped BaTiO₃, indicating the substitution of Ce as Ce⁴⁺ for Ti⁴⁺ in Ba(Ti_1−yCe _y )O₃. The Curie temperature ( T _c) was systematically lowered when Ce³⁺ was incorporated into Ba²⁺ sites, whereas the substitution of Ce⁴⁺ for Ti⁴⁺ sites resulted in no change in this parameter.     

Solubility of Silver and Palladium in BaTiO3

Silver, palladium, and their alloys are frequently used as electrode materials for BaTiO₃ (BT) based dielectrics. However, the electrodes and dielectrics usually are cofired at high temperatures, and silver and palladium can dissolve into the BT during cofiring. In the present study, the solubility of silver and palladium into BT after cofiring was determined. Three measurement techniques were used to determine solubility: chemical analysis, structural analysis, and dielectric analysis. The solubility of the silver in the BT was low, 450 ppm, after cofiring at 1290°C for 2 h in air. The diffusion distance of the silver ions into the BT was >5 μm. The solubility of the palladium in the BT was even lower, 50 ppm at 1290°C, and the diffusion distance was ∼1 μm. The solubility of both the silver and the palladium in the BT decreased as the oxygen partial pressure of the sintering atmosphere decreased. These results demonstrated that both silver and palladium solutes act as acceptors for BT.     

Spectroscopic Properties of Er3+-Doped Na2O–La2O3–Al2O3–SiO2 Glasses

Er³⁺-doped sodium lanthanum aluminosilicate glasses with compositions of (90− x )(0.7SiO₂·0.3Al₂O₃)· x Na₂O·8.2La₂O₃· 0.6Er₂O₃·0.2Yb₂O₃·1Sb₂O₃ (in mol%) ( x = 12, 20, 24, 40, 60 mol%) were prepared and their spectroscopic properties were investigated. Judd–Ofelt analysis was used to calculate spectroscopic properties of all glasses. The Judd–Ofelt intensity parameter Ω _t ( t = 2, 4, 6) decreases with increasing Na₂O. Ω₂ decreases rapidly with increasing Na₂O while Ω₄ and Ω₆ decrease slowly. Both the fluorescent lifetime and the radiative transition rate increase with increasing Na₂O. Fluorescence spectra of the ⁴ I _13/2 to ⁴ I _15/2 transition have been measured and the change with Na₂O content is discussed. It is found that the full width at half-maximum decreases with increasing Na₂O.     

Surface Decomposition of Strontium-Doped Soft PbZrO3–PbTiO3

Sintering temperature has a pronounced effect on perovskite phase stability at the surface of Pb_0.88Sr_0.12Zr_0.54Ti_0.44Sb_0.02O₃ (PSZT) soft piezoelectric ceramics ( d ₃₃≈ 600 pC/N). After sintering 4 h at 1070°C, XRD reveals only perovskite PSZT peaks in the bulk and at the surface. As sintering temperature increases, XRD from the ceramic surface reveals a second-phase peak at ∼27° (2θ), 0.316 nm ( d -spacing). After 4 h at 1280°C, further second-phase peaks are observed, confirming it to be monoclinic ZrO₂, accompanied by a strong increase in the degree of tetragonality of the perovskite phase. These observations are consistent with decomposition of the PSZT to ZrO₂ and tetragonal PZT (PbZrO₃–PbTiO₃) associated with PbO loss. SEM and cross-sectional TEM indicated that surface decomposition had progressed ∼0.5 mm into the sample after 4 h at 1280°C.     

Vibrational Spectra of Water- and Ammonium-Chloride-Related Residues and Upconversion Fluorescence of Er3+ in ZnCl2-Based Glasses

Chloride glasses of the ZnCl₂ and 20KCl-20BaCl₂-60ZnCl₂-0.5ErCl₃ systems were prepared using NH₄Cl as a dehydrating and chlorinating agent, under the melt-quenching method. Water- and ammonium-chloride-related residues in ZnCl₂ glasses were investigated by infrared and near-infrared absorption spectra. Water, Zn—OH, ClO, ClO₂⁻, Zn²⁺-coordinated water, free NH₃, NH₄⁺, and Zn²⁺-coordinated NH₃ were identified in ZnCl₂ glasses. 20KCl-20BaCl₂-60ZnCl₂-0.5ErCl₃ glasses prepared by melting at 500°C for 20 min, under reduced pressure, contained the smallest amounts of water, Zn—OH, and Zn²⁺-coordinated NH₃ and showed strong Er³⁺ upconversion fluorescence.     

Base-Metal-Electroded BaTiO3 Capacitor Materials with Duplex Microstructures

The effect of dopants and processing conditions on the dielectric properties of base-metal-electroded materials was investigated. BaTiO₃ materials simultaneously doped with MgO and Y₂O₃ additives can achieve small capacitance variation (Δ C / C ), which meets the X7R specification, when the proportion of additives is abundant enough and the materials are not over-fired. Presumably, small Δ C / C values of thus obtained materials are the result of the formation of core–shell structure, which requires stringent control of material processing conditions. In contrast, X7R-type materials can be obtained in a much wider processing window, when prepared by mixing two BaTiO₃ materials of suitable dielectric constant–temperature ( K – T ) characteristics. Duplexed materials prepared from these two end-point BaTiO₃ materials with ratios ranging from 3:1 to 1:2 exhibit K – T behavior within the X7R specification, provided that one of the components possesses flat K – T behavior. Moreover, the dielectric properties of these materials were simulated using a simplified microstructural model. Simulation results indicate that the effective dielectric constant of core–shell materials, ( K _e)_CS, varies significantly not only with the dielectric properties of cores and shells, but also with the shell-to-core thickness ratio, whereas the effective dielectric constant of duplexed materials, ( K _e)_D, can be maintained at a very small Δ C / C value for a wide range of end-point constituent ratios, which agrees very well with the measured K – T properties for the materials.     

Faceting of High-Angle Grain Boundaries in Titanium-Excess BaTiO3

The grain boundaries in BaTiO₃ with excess Ti of 0.5, 0.3, and 0.1 at.% sintered at 1300° or 1250°C have been examined by scanning electron microscopy (SEM), electron backscattered diffraction pattern (EBSP), and transmission electron microscopy (TEM). In the 0.1% Ti-excess specimen, large grains growing abnormally form high-angle grain boundaries when they impinge on each other as verified by EBSP. A large fraction of these grain boundaries are faceted with hill-and-valley shapes. In the 0.5% Ti-excess specimen, large grains growing abnormally are elongated in the directions of their {111} double twins. These grains often form flat grain boundaries parallel to their {111} planes with the fine matrix grains, and the grain-boundary segments between the large impinging grains with high misorientation angles are often also parallel to the {111} planes of one of the grains. These grain boundaries are expected to be singular. Most of the grain boundaries between the randomly oriented fine-matrix grains in the 0.3 at.% Ti-excess specimen are also faceted with hill-and-valley shapes at finer scales when observed under TEM. The facet planes are parallel to {111}, {011}, and {012} planes of one of the grain pairs and are also expected to be singular. These high-angle grain boundaries lying on low index planes of one of the grain pairs are similar to those observed in other oxides and metals.     

Energy Transfer and Population Inversion in Heavy Metal Oxide Glasses Doped with Tm3+ and Tb3+

Emission properties and energy transfer of PbO–Bi₂O₃–Ga₂O₃–GeO₂ glasses codoped with Tm³⁺ and Tb³⁺ ions were investigated. The 1.48-μm emission due to the Tm³⁺:³H₄→³F₄ transition can be used to amplify the S-band (1460–1530-nm) signal light. With Tb³⁺ addition, the lifetime and emission intensity of the Tm³⁺:³F₄ level decreased sharply via the Tm³⁺:³F₄→Tb³⁺:⁷F_0,1,2 energy transfer. Population densities of the ³F₄ and ³H₄ levels in Tm³⁺ calculated from rate equations clearly verified that population inversion in Tm³⁺ ions became possible with as little as 0.1 mol% of Tb³⁺ addition.     

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.     

Incorporation of Aliovalent Dopants into the Bismuth-Layered Perovskite-Like Structure of BaBi4Ti4O15

The solid solubility of the aliovalent dopants Fe³⁺ and Nb⁵⁺ in the BaBi₄Ti₄O₁₅ compound, a member of the family of Aurivillius bismuth-based layer-structure perovskites, has been studied using quantitative wavelength-dispersive spectroscopic microanalysis (SEM/EPMA) in combination with X-ray powder diffractometry (XRPD). The samples with nominal (starting) compositions corresponding to the chemical formulas BaBi₄Ti_{4–4 X} Fe_{4 X} O₁₅ and BaBi₄Ti_{4–4 X} Nb_{4 X} O₁₅ were prepared by hot forging a mixture of BaTiO₃ and Bi₄Ti₃O₁₂ with additions of Fe₂O₃ or Nb₂O₅ followed by a long annealing at 1100°C. The study showed that an excess charge introduced into the structure by the substitution of Ti⁴⁺ ions with aliovalent dopants was preferentially compensated by a change in the ratio of Ba²⁺ to Bi³⁺ ions in the host structure according to the general formulas of the solid solutions Ba_{1–4 X} Bi_{4+4 X} Ti_{4–4 X} Fe^'_{4 X} O₁₅ and Ba_{1+4 X} Bi_{4–4 X} Ti_{4–4 X} Nb^·_{4 X} O₁₅.     

Solid-State Preparation of BaTiO3-Based Dielectrics, Using Ultrafine Raw Materials

BaTiO₃ and Ba(Ti,Zr)O₃ dielectric powders have been prepared from submicrometer BaCO₃, TiO₂, and ZrO₂. By use of submicrometer BaCO₃ the intermediate formation of Ba₂TiO₄ second phase can be widely suppressed. Monophase perovskites of BaTiO₃ were already formed at 900°C and Ba(Ti,Zr)O₃ at 1050°C. Aggregates of very small subgrains could be easily disintegrated to particle sizes <0.5 μm.     

Insulating Properties of Lanthanum-Doped BaTiO3 Ceramics Prepared by Low-Temperature Synthesis

Samples of composition Ba_{1− x} La _x Ti_{1− x /4}O₃, x = 0, 0.003, 0.03, and 0.10, were prepared by an alkoxide sol–gel route with final firing of ceramics at 1100°C, 2 h in air. All samples showed bulk insulating behavior with no evidence of semiconductivity caused by either direct donor doping or oxygen loss.     

Roles of Ba/Ti Ratios in the Dielectric Properties of BaTiO3 Ceramics

The effect of the Ba/Ti ratio on microstructure, dielectric/ferroelectric properties, and domain width was studied using optical microscopy, ɛ( T ) curves, D – E hysteresis, and transmission electron microscopy. Although Ti-excess samples showed abnormal grain growth and a decrease of room-temperature permittivity due to a liquid phase at grain boundaries, its ferroelectric properties were similar to those of stoichiometric BaTiO₃ ceramics. However, in Ba-excess samples, an increase of permittivity and ferroelectric properties different from those of stoichiometry were found. Changes in domain width and ferroelectric transition behavior indicated that the variation of dielectric properties was related to the internal stress. It is proposed that this internal stress originated from differences in the thermal expansion coefficient between the matrix and the second phase.     

Dislocation Loop Formation in Nonstoichiometric (Ba,Ca)TiO3 and BaTiO3 Ceramics

Dislocation loops observed in nonstoichiometric and stoichiometric (Ba,Ca)TiO₃, and in stoichiometric BaTiO₃ sintered in a reducing atmosphere, were characterized by conventional transmission electron microscopy (TEM) under two-beam conditions and high-resolution TEM atomic structure analysis. Dislocation loops mostly lay on {100} planes with Burgers vectors of type 〈100〉. The dynamic behavior of these dislocation loops during the electron beam irradiation (EBI), however, was classified into two different types of dislocation loops: in A-site-excess (Ba,Ca)TiO₃, contrasts of dislocation loops faded completely away; in BaTiO₃ and B-site-excess (Ba,Ca)TiO₃, fine-line contrasts remained. Dislocation loops with Burgers vectors of type 1/2〈100〉 and the resultant crystallographic shear (CS) structure with a displacement vector of type 1/2〈110〉 after EBI were proposed to interpret residual line images. Disappearance of these line images in A-site-excess (Ba,Ca)TiO₃ strongly suggests preferential Ca ion site occupancy at the CS structure.     

Influence of Barium Dissolution on the Electrokinetic Properties of Colloidal BaTiO3 in an Aqueous Medium

Hysteresis in the electrokinetic behavior of colloidal hydrothermal BaTiO₃ occurs during sequential acid and base titrations. Ba dissolution during acid titration results in an oxide-rich surface. When the acid-treated BaTiO₃ is titrated back to pH 10, dissolved Ba is specifically adsorbed and/or precipitated onto the particle surface. The combined effects of dissolution and subsequent adsorption–precipitation results in titration hysteresis. Most of the labile Ba can be removed by multiple acid treatments, which result in a TiO₂-like surface layer composition. Barium dissolution increases with decreasing pH but levels off below pH 4 due to diffusion through the surface oxide layer as predicted previously. A phenomenological model is offered to explain the electrokinetic behavior as a function of pH. It is suggested that inherent BaCO₃ contamination is not the primary source of dissolved Ba from hydrothermal BaTiO₃ in acidic solution.     

π-Rotation Faults in Hexagonal BaTiO3

Planar defects in the metastably retained h-BaTiO₃ exhibiting α-fringe pattern have been characterized via transmission electron microscopy (TEM). The eligible fault vectors were determined by adopting the invisibility criteria of 2πg·R = 0 or 2 n π augmented by high-resolution imaging. Three stacking faults, F₁, F₂, and F₃, of the extrinsic nature have been fully analyzed. The eligible fault vectors for faults F₁ and F₃ contained a basal component respectively of ⅓[0001] and ⅙[0001] and a common prismatic component of ⅓〈10[1-macr]0〉. However, only three of the 〈10[1-macr]0〉 vectors are the eligible prismatic component for the fault vectors R_F1=⅓[0[1-macr]11], ⅓[10[1-macr]1], and ⅓[[1-macr]101], and R_F3=⅙[02[2-macr]1], ⅙[2[2-macr]01], and ⅙[[2-macr]021] that have fulfilled the invisibility criteria. On the other hand, all fault vectors R_F21=⅙〈[4-macr]223〉 for fault F₂, containing six vectors of the 〈[2-macr]110〉 family, is eligible. Unlike the faults of πR_F=⅙〈[2-macr]203〉 found in the D0₁₉ intermetallics of Ni₃Sn and Co₃W, neither fault F₁ nor F₃ is the π-rotation type. Fault F₂, however, is a π-rotation fault since a 60°-rotation clockwise about [0001] has produced another eligible fault vector.