Defect Chemistry of Y Doped BaTiO3

Defect chemistry of Y doped BaTiO₃ was investigated as a function of the Ba/Ti ratio. When the Ba/Ti ratio was greater than unity, Y^{3 +} was substituted for the normal Ti site and the equilibrium conductivity showed a strong evidence of acceptor-doped behavior. With the Ba/Ti ratio < 1, Y^{3 +} was substituted for the Ba site and the equilibrium conductivity showed donor-doped behavior. In the case excess Y₂O₃ was added to the stoichiometric BaTiO₃(Ba/Ti = 1), the conductivity profile showed a donor-doped behavior at low concentrations (< 1.0 mol%), whereas, at higher donor levels (> 2.0 mol%), the equilibrium conductivity minimum shifted toward lower Po₂, indicating acceptor doped behavior.     

P-Type Partial Conductivity of Donor(La)-Doped BaTiO3

P-type partial conductivity has been determined on donor (La _Ba )-doped BaTiO₃ in full thermodynamic equilibrium state at a fixed temperature of 1200°C: For the nominal compositions of Ba_0.99La_0.01Ti_0.9975O₃, Ba_0.99La_0.01TiO₃ and Ba_0.985La_0.01TiO₃, the p-type conductivity is found to vary with oxygen activity as _p = (_m/2)(a _{O 2}/a _{O 2}*)^+1/4 with _m 0.01 S cm^–1 and a _{O 2}* 32, 120, 310, respectively, in the a _{O 2} region where conventionally the electronic conductivity varies as a_{O 2} ^–1/4 and hence, the doped donors are believed to be compensated by cation vacancies (say, [La_Ba ] 4[VPrime;_Ti]). This experimental fact supports that in the vicinity of the stoichiometric composition of the system which falls approximately at a _{O 2} = a _{O 2}*, while cation vacancy concentration is fixed by the donor concentration, oxygen vacancy concentration in the minority is also essentially fixed, thus, keeping the activity of TiO₂ (or BaO) fixed. It is consequently suggested that donor-doped BaTiO₃ contains a second phase even in its stoichiometric regime.     

Investigation of Semiconducting Barium Titanate Ceramics by Oxygen Coulometry

Donor-doped BaTiO₃ ceramics (0–0.6 mol% Nb) were investigated during the sintering process at an oxygen partial pressure of 2.4 Pa and at a maximum temperature of 1430C. The occurring oxygen exchange with the ambient atmosphere was monitored quantitatively by oxygen coulometry. The coincidence between the grain growth behavior and the occurrence of distinct oxygen release peaks during the anomalous grain growth reveals that significant amounts of the donor are incorporated and charge-compensated by electrons only under the conditions of the anomalous grain growth. Quantitative analysis of the coulometric data shows that at a doping level of 0.2 mol% nearly all donors are charge-compensated by electrons. With increasing Nb concentration also Ti vacancies gradually begin to serve for compensation. At 0.5 mol% Nb their influence reaches 70 % of the electronic compensation.     

Cationic Surface Segregation in Donor-Doped SrTiO3 Under Oxidizing Conditions

The influence of high temperature oxygen annealing on (100) oriented donor-doped SrTiO₃ single crystals was studied. Crystalline precipitates were found on the optical scale on surfaces of lanthanum-doped as well as niobium-doped specimens with donor concentrations above 0.5 at.%. The amount of the secondary phase increases with the doping level, oxidation temperature and oxidation time. EDX analyses of the crystallites reveal a SrO_x composition.The formation of the observed secondary phase is discussed by means of the defect re-equilibration of the cation sub-lattice. In view of the point defect model for donor-doped perovskites, n-conducting SrTiO₃ changes its compensation mechanism during an oxidation treatment from electronic compensation (N _D = n) to self-compensation (N _D = 2[V _Sr]) by forming cation vacancies. Due to the favored Schottky-type disorder in perovskites, the formation of strontium vacancies is accompanied by a release of strontium from the regular lattice. Since the excess strontium is found to be situated at the surface in form of SrO-rich precipitates only, we propose the formation of strontium vacancies via a surface defect reaction and the chemical diffusion of strontium vacancies from the surface into the crystal as the most probable re-equilibration mechanism for the oxidation treatment of single crystals.The introduced mechanism is in contrast to an established model which proposes the formation of Ruddlesden-Popper intergrowth phases SrO·(SrTiO₃)_n in the interior of the crystal.     

BaTiO3−δ: Defect Structure, Electrical Conductivity, Chemical Diffusivity, Thermoelectric Power, and Oxygen Nonstoichiometry

Electrical conductivity, thermoelectric power, and chemical diffusivity are the most typical, charge-and-mass transport properties of a mixed ionic electronic conductor oxide which are essentially governed by its defect structure, and the oxygen nonstoichiometry is a direct measure of its overall defect concentration. For the system of BaTiO_3–, the total electrical conductivity has been the most extensively and systematically studied as a function of oxygen partial pressure at elevated temperatures. The other properties have also been studied, but much less extensively and systematically. The electrical conductivity and thermopower were occasionally measured together on the same specimens so that mutual compatibility or consistency might be secured. But, the rest were all determined separately on the specimens of differing quality, consequently lacking in mutual consistency. It, thus, has remained hard to evaluate the canonical, defect-chemical parameters which are consistent with each and every of these defect structure-sensitive properties that were observed. Very recently the authors have determined the total conductivity, chemical diffusivity and thermoelectric power altogether on the same specimens of BaTiO_3–, and the nonstoichiometry on the same-quality specimens at temperatures of 1073 T/K 1373 over wide enough a range of oxygen partial pressure (normally, 10^–16 Po₂/atm 1) that encloses an electron/hole/ion mixed regime. In this article, we will compile all the literature data on these defect-structure-sensitive properties and extract from the authors' own, without using any ad hoc assumptions regarding, e.g., the electronic carrier mobilities and effective density of states, the basic defect-chemical parameters including defect-equilibrium constants, carrier mobilities and densities, and electronic heats of transport, which are the most consistent with the properties observed. Compared to the conventional picture of the defect structure of undoped BaTiO₃, thus, some new insights into the defect chemical nature of BaTiO_3– are provided.     

Microwave Dielectric Properties of Ferroelectric BaTiO3 Thin Film

The dielectric properties of c-axis epitaxial BaTiO₃ thin film on LaAlO₃ are investigated at frequencies of 0.5–30 GHz. For the measurements, interdigital capacitors with the Au/Ti electrode configurations of five fingers pairs that are 15 m wide and spaced 2 m apart are prepared by photolithography and lift-off patterning. Finger length varies from 20 to 80 m. The capacitance of epitaxial BaTiO₃ films exhibited no frequency dependence up to 10 GHz with the exception of slightly upward tendency of capacitance in BaTiO₃ film with a finger length of 80 m due to the self resonant frequency at 20 GHz. The Q-factors of the capacitors, defined as Q = 1/CR, are decreased up to 10 GHz with increased frequency. At 10 GHz, the BaTiO₃ film has a tunability [defined as k(V) = [C(0)–C(V)]C(0)] of 1.5% at 15 V, a loss tangent of 0.2 at room temperature. The small tunability can be interpreted as a result of in-plane compressive stress of BaTiO₃ film exhibiting large dielectric anisotropy. For the improvement of tunability and dielectric loss in the interdigital BaTiO₃ capacitor, the tetragonality (c/a) of epitaxial BaTiO₃ film and design of interdigital capacitor should be modified.     

Co-Doping Effect of Mn and Y on Charge and Mass Transport Properties of BaTiO3

BaTiO₃-based multilayer-ceramic capacitors (MLCC) using base metal (Ni) electrodes normally contains Mn and Y each approximately on the order of 0.5 mol%. It is only empirically known that the co-doping of Y and Mn facilitates sintering with the base-metal electrodes as well as improves the device performance and life time. In order to understand the effect of the co-doping, we have measured the electrical conductivity and chemical diffusivity on polycrystalline BaTiO₃ that is co-doped with Y and Mn each by 0.5 mol% against oxygen partial pressure at elevated temperatures. It is found that while the n-type conductivity in reducing atmospheres (e.g., Po₂ < 10^{– 6} atm at 1000C) remains similar to that of undoped or acceptor-doped BaTiO₃, its p-type conductivity in oxidizing atmospheres (e.g., Po₂ > 10^{– 6}at 1000C) is remarkably suppressed compared to the latter. The chemical diffusivity is also similar to that of the latter in magnitude (e.g., 10^{– 2}–10^{– 5} cm²/s at 1000C), but its trend of variation with oxygen partial pressure is rather opposite. These variations of the conductivity and chemical diffusivity are mainly attributed to Mn ions changing their valence from +2 to +3 to +4 with increasing oxygen partial pressure. It is explained from a defect-chemical view why the codoping of fixed-valent donor (Y) and variable-valent acceptor (Mn) has been practiced in MLCC processing.     

Mn-doped BaTiO3: Electrical Transport Properties in Equilibrium State

The electrical conductivity and thermoelectric power of Mn-doped BaTiO₃ (1 mole%) and undoped BaTiO₃ have been measured as functions of oxygen partial pressure (in the range of 10^-16 to 1 atm) and temperatures (in the range of 900 to 1200°C), and compared with each other to differentiate the effect of the Mn-addition. It is found that the isothermal conductivity of Mn-doped BaTiO₃ varies with increasing Po₂ as to to , unlike previously reported. This behavior is well explained by the shift of the ionization equilibrium, . The corresponding equilibrium constant, K_A, is determined from the Po₂ values demarcating those three different Po₂ regions as =3.19×10²² exp(–1.69 eV/kT). Basic parameters involving carrier density and mobility, and defect structure of Mn-doped BaTiO₃ are discussed in comparison with those of undoped BaTiO₃.     

Thermoelectricity of BaTiO3+δ

Thermoelectricity of mixed ionic electronic conductor BaTiO₃₊ is thermodynamically analyzed, and measured across the mixed n/p regime of both undoped and 1.8 m/o A1-doped BaTiO₃ at elevated temperatures. There can be 4 different measurement conditions with respect to the nonstoichiometry () redistribution and types of atmosphere gases used to control the surrounding oxygen potential, which lead to differences in information content of the thermopower. Experimental thermopower isotherms are exhaustively analyzed to find that the ionic contribution is evident in the mixed n/p regime and that the heats of transport of electrons and holes are about the same as their migration enthalpies.     

Hydrothermal Preparation and Characterization of Ultra-Fine BaTiO3 Powders from Amorphous Peroxo-Hydroxide Precursor

Ultra-fine BaTiO₃ powders were hydrothermally prepared by using Ba Ti-peroxo-hydroxide precursor. Amorphous Ba Ti-peroxo-hydroxide precursor were prepared by coprecipitation of Ba(NO₃)₂ and TiCl₄ aqueous solution adding in NH₄OH aqueous solution. The phase-pure BaTiO₃ powders with a cubic perovskite structure were synthesized at temperature as low as 110_C and in the pH range of 10–12. This processing method provides a simple low temperature route for producing BaTiO₃ nanoparticles. Under a TEM image and a SAD pattern analysis, it is evident that BaTiO₃ powders had spherical shape and single crystal nature. The BaTiO₃ ceramic sintered at 1200_C for 1 h had 97% of theoretical density and a relatively high dielectric constant ( _r = 3500).     

Electrical Properties of Acceptor Doped BaTiO3

Electrical properties of acceptor (Mn, Mg or Mn+Mg)-doped BaTiO₃ ceramic have been studied in terms of oxygen vacancy concentration, various doping levels and electrical degradation behaviors. The solubility limit of Mn on Ti sites was confirmed to be close to or less than 1.0 mol%. Oxygen vacancy concentration of Ba(Ti_{0.995 –x}Mg_0.005Mn_x)O_{2.995 –y} (x = 0, 0.005, 0.01) was estimated to be 50 times greater than that of the un-doped BaTiO₃. The leakage current of 0.5 mol% Mn-doped BaTiO₃ was stable with time, which was much lower than that of the un-doped BaTiO₃. The BaTiO₃ specimen co-doped with 0.5 mol% Mg and 1.0 mol% Mn showed the lowest leakage current below 10^{– 10}A. It was confirmed that leakage currents of Mg-doped and un-doped BaTiO₃ under dc field are effectively suppressed by Mn co-doping as long as the Mn doping level is greater than Mg contents.     

Dielectric Properties of Acceptor-Doped (Ba,Ca)(Ti,Zr)O3 Ceramics

The dielectric properties of (Ba,Ca)(Ti,Zr)O₃ ceramics containing various acceptor and donor dopants on the B sites have been studied. Formation of charge compensating complexes between acceptors, donors and oxygen vacancies has been observed. All acceptors cause a reduction of the Curie point. The maximum dielectric constant increases with the average grain size almost independent of the acceptor incorporated. Ceramics fired in reducive atmospheres reveal pronounced changes of the dielectric properties after annealing in oxidizing atmosphere. In particular the losses and the maximum dielectric constants are significantly affected by the annealing time, temperature and partial pressure of oxygen.     

Electrical Properties of 6H-BaTi0.95M0.05O3−δ Ceramics where M = Mn, Fe, Co and Ni

Hexagonal BaTiO₃ materials have been stabilised at room temperature according to the formula BaTi_0.95M_0.05 O_3– where M = Mn, Fe, Co and Ni. Dense ceramics (> 96% of the theoretical X-ray density) were sintered at 1450C in flowing O₂ gas from calcined powders prepared by the mixed oxide route at 1300C. All samples were single-phase and the bulk conductivity, _b, measured by Impedance Spectroscopy and Q.f measured by microwave dielectric resonance methods showed a strong dependence on the type of dopant. _b at 300C was 10^–7, 10^–5.5, 10^–5.5 and 10^–4 Scm^–1 for M = Mn, Fe, Ni and Co, respectively and Q.f at 5 GHz was 7790, 6670, 2442 and 1291 GHz, for M = Mn, Fe, Ni and Co, respectively. The correlation between _b and Q.f is attributed to the presence of oxygen vacancies and/or mixed valency of the dopant ions.     

Charge Compensation Mechanisms in La-Doped BaTiO3

The mechanism of doping BaTiO₃ with La has been investigated by a combination of X-ray diffraction, electron probe microanalysis, scanning and transmission electron microscopy and impedance measurements. Phase diagram results confirm that the principal doping mechanism involves ionic compensation through the creation of titanium vacancies. All samples heated in oxygen at 1350–1400°C are electrical insulators, consistent with an ionic compensation mechanism. Samples heated in air or atmospheres of low oxygen partial pressure, at similar temperatures, lose a small amount of oxygen and this gives rise to a second, electronic compensation mechanism in addition to the main, ionic compensation mechanism; as a result, samples are dark-coloured and semiconducting. The change from insulating to semiconducting behaviour is reversible, by changing the atmosphere on heating at 1350–1400°C. We find no evidence for any changes in cationic composition of the BaTiO₃ solid solutions arising from changes in oxygen content.     

Microstructure of X7R Type Base-Metal-Electroded BaTiO3 Capacitor Materials Co-Doped with MgO/Y2O3 Additives

Detailed microstructure of MgO/Y₂O₃ co-doped BaTiO₃ materials were examined using transmission electron microscopy (TEM). For the 1250_C-sintered BaTiO₃ samples possessing flat K-T characteristics, which meet the X7R specification, the granular structure is complicated. Most of the grains are very small ( 150 nm) and are highly strained. The small grains contain large proportion of Y₂O₃ species and are paraelectric, whereas the large grains contain Y₂O₃ species unevenly distributed and are of core-shell structure. In contrast, for the 1300C-sintered BaTiO₃ samples, which have K-T properties slightly off the X7R specification, the grains grew larger to around 300 nm. The core-shell structured grains are seldom observed. Apparently, it is the existence of such a non-equilibrium core-shell microstructure, which renders the dielectric properties of the BaTiO₃ materials extremely sensitive to the processing parameters.     

Effect of Fe – substitution on phase transformation,optical, electrical and dielectrical properties of BaTiO<Subscript>3</Subscript> nanoceramics synthesized by sol-gel auto combustion method

The structural, microstructural, optical, electrical and dielectrical properties of nanocrystalline Fe substituted BaTiO₃ synthesized by sol-gel auto combustion have been investigated. The X-ray diffraction (XRD) analysis revealed the existence of the tetragonal phase for lower Fe content (x = 0.0–0.3) whereas, coexistence of the tetragonal and hexagonal structure of higher Fe content (x = 0.4 and 0.5). The lattice constant (a and c) and unit cell volume (V) increases with increase in Fe content; and an average crystallite size (t) was recorded in the range of ~14–20 nm. The surface morphology as examined using field emission scanning electron microscopy (FESEM) and the compositional stoichiometry was confirmed by energy dispersive spectrum (EDS) analysis. The UV-Vis spectra showed that the band gap energy sensitively depends on the Fe concentration x. DC-electrical conductivity (σ) was recorded in the temperature range of 333–714 K which was found to be increases with increasing temperature and Fe concentration; indicating that an electrical conduction was a thermally activated process. The type of temperature dependent DC conductivity indicates that the electrical conduction in the material is a thermally activated process. The dependencies of the conductivity contributions were predicted from the simple defect model presented, in which oxygen vacancies charge compensate Fe substitution of Ti. Dielectrical property was measured as a function of frequency in the range 50 Hz - 5 MHz at room temperature which was found to be higher at lower frequencies. Dielectric constant (ε’) and loss tangent (tan δ) shows strong compositional as well as frequency dependences.     

Mixed Electronic-Ionic Conduction in Ru-Doped SrTiO3 at High Temperature

The electronic and ionic conduction behavior of Ru-doped SrTiO₃ at high temperature was investigated. The conductivity increased significantly with increasing Ru content. SrTi_0.80Ru_0.20O_3– exhibits fairly high conductivities, e.g., 3 S cm^–1 at 1000°C, and 2 S cm^–1 at 600°C. The conductivity had only a slight dependence on the partial pressure of oxygen over a wide range and was largely attributed to n-type electronic conduction. Ru-doped SrTiO₃ showed mixed oxide-ionic and electronic conduction under reducing atmospheres. The mechanism of the electronic and ionic conduction is discussed.     

Protonic Conduction in SrY0.5Ta0.5O3-Based Ceramics at Elevated Temperatures

The perovskite-type-oxide solid solution SrY_0.5+x Ta_0.5+x O_3– was synthesized and its properties were investigated. The single phase character of the samples was confirmed by X-ray diffraction when 0 x 0.02, while lines from the impurity phase SrY₂O₄ appeared in patterns of x 0.03. The conductivities of SrY_0.52Ta_0.48O_3– were about an order of magnitude higher than those of SrY_0.50Ta_0.50O₃. The results of electrochemical measurements such as emf measurements of gas concentration cells, isotope effect in conductivity, and oxygen partial pressure dependence of conductivity showed that SrY_0.52Ta_0.48O_3– exhibited pure protonic conduction in reducing atmospheres and p-type electronic conduction under high oxygen partial pressure conditions.     

Donor-Acceptor Charge Complex Formation in Barium Titanate Ceramics: Role of Firing Atmosphere

BaTiO₃ ceramics containing Mn acceptors and various Mo, W and Nb donor dopants have been fired in reducing atmosphere and re-oxidized in N₂/O₂. In single Mn-doped ceramics, Mn²⁺ is completely oxidized to Mn³⁺; in N₂ containing 50 ppm O₂ at T > 800°C. Changes of the Curie point and sample length under reduction and re-oxidation have been detected using dielectric and thermomechanical measurements. Charge compensation and complex formation between acceptors and donors have been observed. In donor-acceptor charge complexes, Mn²⁺ cannot be oxidized.     

Crystal Growth and Dielectric Properties of New Ferroelectric Barium Titanate: BaTi2O5

Single crystals of the ferroelectric BaTi₂O₅ and BaTiO₃ were prepared from a solution of 33-mol% BaO and 67-mol% TiO₂ by a rapid cooling method. The dielectric constant () and dielectric loss tangent (tan) were measured in a wide temperature range of 10–860 K and in a frequency range of 0.1–3,000 kHz. The along the b-axis of the BaTi₂O₅ crystal, prepared in air, shows a sharp dielectric anomaly reaching 30,000 at the ferroelectric Curie temperature of T_C = 752 K. By contrast, the crystal prepared in a reducing atmosphere shows a diffuse phase transition near T_C = 703 K. The values of and tan are compared between these three crystals consisting of two kinds of BaTi₂O₅ and one BaTiO₃.