Barium Vacancies Research Articles

Relaxor-like and spectroscopic properties of niobium modified barium titanate

Dielectric susceptibility measurements on niobium-modified barium titanate ceramics for Nb compositions extending up to the solubility limit (18%) show that these materials display relaxor-like properties for compositions larger than ∼ 6% Nb. Raman spectra exhibit extra modes attributed to local responses related with the substitution of niobium for titanium ions. The behaviour of this barium titanate modification, in which Nb5+ aliovalent cations are substituted for Ti4+, is discussed by comparison with the zirconium-modified series, in which, oppositely, only a size difference characterizes the substitution on the B site. It is emphasized that electrical random fields created in the former case are more efficient in inducing a relaxor behaviour than elastic random fields created in the latter. The nature of the complex defects expected from the inherent high concentration of barium vacancies is discussed in connection with photoluminescence spectra.

Synthesis of Nanometer-Sized Barium Titanate Crystallites Using a Modified Low Temperature Direct Synthesis Method and Their Characterization.

A low temperature direct synthesis (LTDS) method is useful for preparing nm-sized barium titanate crystallites. However, the precise characterization of impurities in barium titanate crystallites is difficult because barium carbonate is formed as a by-product. In this study, a normal LTDS method was modified to prepare barium titanate crystallites under a CO2-free atmosphere, i.e., an N2 atmosphere. The barium titanate single phase was first prepared using this modified LTDS method. In the characterization of as-prepared barium titanate crystallites, only a hydroxyl group was detected as an impurity for ca. 11mass%. A chemical analysis of the as-prepared barium titanate crystallites determined that the Ba/Ti atomic ratio was 0.635±0.005, which means that a lot of barium vacancies formed in the crystallites. The average crystal structure, determined by XRD, was assigned to a cubic m3m symmetry, while the local structure, determined by Raman scattering spectroscopy, was assigned to a tetragonal 4mm symmetry, which suggested that in the crystal structure of as-prepared barium titanate crystallites, there existed a disorder system. Crystallites were also characterized after annealing at various temperatures.

Mechanism of improvement of resistance degradation in Y-doped BaTiO 3 based MLCCs with Ni electrodes under highly accelerated life testing

The mechanism of improvement of resistance degradation in Y-doped BaTiO 3 based multilayer ceramic capacitors (MLCCs) with Ni electrodes has been studied using electrical measurement techniques, minor phase identification and the measurement of oxygen vacancy concentration. Admittance spectroscopy and thermally stimulated current measurements show the relaxation due to barium vacancy states caused by Y doping. The minor phase identification by XRD indicated those Y 3+ substitutes for perovskite A-site. Oxygen vacancy concentration measurement indicated that Y decreases the oxygen ion vacancy concentration in BaTiO 3. These results suggest that Y 3+ acts as a donor and creates barium vacancies, which compensate for the oxygen vacancies and thereby improve the resistance degradation.

Effect of secondary-phase segregation on the positive temperature coefficient in resistance characteristics of n-BaTiO3 ceramics

Modifications in the positive temperature coefficient in resistance (PTCR) of n-BaTiO3 ceramics are brought about by specific additives such as Al2O3, B2O3 or SiO2, leading to the segregation of secondary phases such as BaAl6TiO12, BaB6TiO12 or BaTiSi3O9 at the grain boundaries. Segregation of barium aluminotitanates resulted in broad PTCR curves, whereas B2O3 addition gave rise to steeper jumps and SiO2 addition did not result in much broadening compared with donor-only doped samples. Microstructural studies clearly show the formation of a structurally coherent expitaxial second phase layer of barium aluminotitanate surrounding the BaTiO3 grains. Electron paramagnetic resonance investigations indicated barium vacancies, VBa, as the major electron trap centres which are activated across the tetragonal-to-cubic phase transition according to the process VXBa + e′ ⇋ V′Ba. The grain size dependence of the intensity of the V′Ba signal indicated the concentration of these trap centers in the grain-boundary layer (GBL) regions. Further, the charge occupancy of these centres is modified by the secondary phases formed through grain-boundary segregation layers. BaAl6TiO12 gave rise to Al-O− hole centres whereas no paramagnetic centres corresponding to boron could be detected on B2O3 addition. Such secondary phases, forming epitaxial layers over the BaTiO3 grains, modify the GBL region, rich in electron traps, surrounding the grain core. The complex impedance analyses support this three-layer structure, showing the corresponding contributions to the total resistance which can be assigned as Rg, Rgb and Rsecondary phase. The epitaxial second phase layers bring about inhomogeneity in the spatial distribution of acceptor states between the grain boundary and the grain bulk resulting in extended diffuse phase transition characteristics for the GBL regions in n-BaTiO3 ceramics. This can cause the GBL regions to have different transition temperatures from the grain bulk and a spread in energy levels of the associated GBL trap states, thus modifying the PTCR curves. An attempt has been made to explain the results based on the vibronic interactions applied to the mid-band-gap states in n-BaTiO3.

Effect of Y-Doping on Resistance Degradation of Multilayer Ceramic Capacitors with Ni Electrodes under the Highly Accelerated Life Test

The effect of Y-doping on the degradation behavior of insulation resistance for multilayer ceramic capacitors with Ni electrodes has been studied using electrical measurement techniques such as admittance spectroscopy, thermally stimulated current (TSC), and electrical conductivity measurements. TSC measurements show that a strong internal bias field remained after applying an electrical field at a high temperature, like during highly accelerated life testing. This internal bias field originated from the migration of positive carriers. Admittance spectroscopy indicated three types of relaxation, with activation energies of 0.4, 0.8, and 0.8–1.0 eV. The intensity of the 0.4 eV relaxation peak increased with Y3+ doping and disappeared at low oxygen partial pressure ( PO2) annealing. The existence of holes was also observed from the temperature dependence of insulation resistance. The 0.4 eV relaxation is considered to be due to the transition of electrons from the valence band to barium vacancy states. These results suggest that Y3+ acts as a donor and creates barium vacancies, which compensate for the oxygen vacancies and thereby improve the resistance to degradation.

Crystallographic Properties of Hydrothermal Barium Titanate Crystallites

The crystallographic properties and defects of hydrothermally grown barium titanate crystallites are reported. When a precursor with a Ba:Ti molar ratio of 1.0 was used, cubic barium titanate crystallites were formed. However, when the precursor Ba:Ti molar ratio was 3.0, the product was tetragonal. The defect concentration (including OH defects and barium vacancies) was higher in cubic crystallites than in tetragonal crystallites. Because of the defects in the crystallites, the displacement of Ba2+ cations and the deformation of the Ti‐O6 octahedrons was suppressed; thus, no phase transition from cubic to tetragonal occurred at the Curie point. When the defects were eliminated by thermal treatment, the normal crystallographic properties were obtained. A high reaction temperature and a high Ba:Ti molar ratio in the precursor were beneficial to limiting defect dormation in the crystallites; thus, tetragonal crystallites were obtained.

Complex impedance analyses of n-BaTiO3 ceramics showing positive temperature coefficient of resistance

The influence was studied of grain boundary layer modifiers on the impedance spectra of semiconducting BaTiO3 ceramics processed in various atmospheres. Ceramic samples of undoped BaTiO3 annealed in an N2+H2 atmosphere showed a single semicircle in the Cole–Cole plot when ohmic Ni electrodes were used. Impedance spectra of donor-doped specimens sintered in air showed overlapping semicircles which may indicate relaxational processes within the grains, e.g. electron trapping and detrapping at the deep traps. This is strongly supported by the minima shown by the grain resistance (Rg) around the Curie point (Tc), whereas the behaviour of the grain boundary resistance (Rgb) with temperature is similar to the d.c. resistance, i.e. a positive temperature coefficient of resistance (PTCR). Electron paramagnetic resonance (EPR) results indicate that the charge trapping is caused by the redistribution of electrons at native and extrinsic acceptor states during phase transitions. Thus, the neutral barium vacancies (VXBa) present in the tetragonal and orthorhombic phases change into singly ionized barium vacancies (V′Ba) in the cubic and rhombohedral phases. From the correlation between the resistivity changes and the charge redistribution at the trap states, it is envisaged that the PTCR effect is related to charge trapping at the acceptor states having higher concentration in the regions of the grain boundary layer. The impedance spectral data support this conclusion.

Analysis of lattice vibration in fine particles of barium titanate single crystal including the lattice hydroxyl group

Fine particles of barium titanate single crystal with an average particle size of about 66 nm were prepared by a hydrothermal method. In the as-prepared sample, there was a large amount of the hydroxyl group and barium vacancy, and its crystal structure was assigned to cubic with an expanded lattice using a Rietveld method. The hydroxyl group desorbed with increasing calcination temperature, especially desorbing remarkably in the range from 200 to 300 °C. Above 700 °C, there was no hydroxyl group in the particle, and the crystal structure of the sample treated at 800 °C was assigned to tetragonal with tetragonality of 1.001 using a Rietveld method. The state of the lattice vibration was measured by an infrared reflection method, and analyzed using a nonlinear least-squares method with a four-parameter semiquantum model. As the result, the crystal structure of the as-prepared sample estimated using Fourier transform infrared and Raman was assigned to tetragonal. On the O6 octahedra deformation mode, the resonance frequency was independent of the concentration of the lattice hydroxyl group while the damping factor decreased with decreasing concentration of the lattice hydroxyl group. The role of the lattice hydroxyl group on the O6 octahedra deformation mode is discussed.

Positron annihilation in the epitaxial superconducting thin-film GdBa2Cu3O7- delta studied by using a pulsed positron beam.

The positron lifetime as a function of implantation energy was measured on the epitaxial superconducting thin film GdBa2Cu3O7-delta at different temperatures. The coexistence of both shallow and deep positron trapping centers was observed in the film. The shallow trapping centers include the screw dislocations and twin boundaries. The binding energy of the shallow trapping centers was estimated to be 56 +/- 12 meV. The deep trapping centers are assigned the cation vacancies, especially barium vacancies. On the surface of the sample there are macroscopic free volume holes in which positronium could be formed.

The Effect of the Particle Sizes and the Correlational Sizes of Dipoles Introduced by the Lattice Defects on the Crystal Structure of Barium Titanate Fine Particles

Barium titanate single-crystal particles with various average particle sizes (20-100 nm) were prepared by a hydrothermal method. In the particles as prepared, there were a lot of hydroxyl group and barium vacancies as the lattice detects, and the concentration of the lattice defects increased with decreasing particle sizes. But most of the hydroxyl group in the particles desorbed by the thermal treatment at 600° C. The crystal structure of the barium titanate particles as prepared was an expanded cubic. (a∼4.02 Å), but after a thermal treatment above 600° C to remove the hydroxyl group, the cubic phase changed to tetragonal phase (c/a≪1.011). This phenomenon was observed in the particles with average particle sizes from 20 to 100 nm, and the crystal structure was independent of the particle sizes. This result could be explained using the following model, that as the physical quantity, with a finer dimension than the particle sizes, there are the correlational sizes of dipoles introduced by the lattice defects, and the correlational size can determine the stability of the lattice vibration and then the stability of the crystal structure.

The positive temperature coefficient of resistivity in barium titanate

Positive temperature coefficient of resistivity (PTCR) materials have become very important components, and among these materials barium titanate compounds make up the most important group. When properly processed these compounds show a high PTCR at the Curie temperature (the transition temperature from the ferroelectric tetragonal phase to the paraelectric cube phase). In the first half of this paper literature related to the resistivity-temperature behaviour is discussed. As explained by the well established Heywang model, the PTCR effect is caused by trapped electrons at the grain boundaries. From reviewing experimental results in the literature it is clear that the PTCR effect can not be explained by assuming only one kind of electron trap. It is concluded that as well as barium vacancies, adsorbed oxygen as 3d-elements can act as electron traps. In the second half of this paper, the influence of the processing parameters on the PTCR related properties is discussed. Special emphasis is placed on the phenomenon that the conductivity and grain size decrease abruptly with increasing donor concentration above ∼ 0.3 at%. Several models explaining this phenomenon are discussed and apparent discrepancies in experimental data are explained.

Nature of Cation Vacancies Formed to Compensate Donors during Oxidation of Barium Titanate

Oxidative cooling is a critical step in the processing of barrier layer electroceramics based on BaTiO3. While it has been proposed that barium vacancies are formed at the grain boundaries to compensate donors,1, 2 no direct evidence for this mechanism exists. On the other hand, literature data can be found to support the compensation of donors in the bulk by either barium or titanium vacancies. As a result the defect(s) formed at electrically active titanate grain boundaries during oxidation has remained uncertain. We explore this phenomenon by observing changes in the surface composition of donor‐doped BaTiO3 when cation vacancies are introduced during oxidation, using SAES (scanning Auger electron spectroscopy). Direct experimental support for the formation and in‐diffusion of barium vacancies during oxidative cooling is obtained in a composition containing 0.7% Nb. It is suggested that barium vacancy compensation constitutes a metastable defect equilibrium in BaTiO3. In a sample of lower concentration (0.3% Nb), results are inconclusive, perhaps because of slower oxidation limited by surface reaction kinetics.

Preparation of Barium Titanate Fine Particles by Hydrothermal Method and Their Characterization

Barium titanate fine particles with average particle sizes from 20 to 90 nm were prepared by a hydrothermal method using titanium tetrahydroxide gel and barium hydroxide. The particle size was inversely proportional to the Ba/Ti atomic ratio in the starting material, while it was independent of temperature in the narrow range from 70 to 150°C. For the explanation of these findings, we proposed the following mechanism for the formation of barium titanate fine particles, i.e., the nuclei of barium titanate form via the reaction of titanium tetrahydroxide gel with barium hydroxide and then grow. In these particles, there were many lattice defects in the form of hydroxyl groups and barium vacancies, and their concentration increased with decreasing particle size. We explained this result using the model that a particle can be divided into two parts: a defective surface layer with many defects and the bulk with fewer defects. The crystal structure of barium titanate particles was cubic with an expanded lattice (a-0.402nm) independent of particle size. This result indicated the possibility that the lattice defects influenced the crystal structure.

Transition of Compensating Defect Mode in Niobium‐Doped Barium Titanate

The influence of Nb‐doping from 1.0 to 8.0 at.% on grain size, ferroelectric phase transition, lattice strain, and electrical properties of BaTiO3 Cerumics has been studied. A change in the Nb‐doping effect was observed at compositions around 3.5–4.5 at.% Nb. The impedance behavior revealed that specimens of low Nb‐doping had an activation energy of 1.85 eV for carrier conduction either in bulk or at grain boundaries, but specimens having a high Nb‐doping content showed an energy of 1.30 eV in bulk and 1.85 eV at grain boundaries. The capacitance–voltage relation also disclosed different influences of Nb‐doping on the potential barrier height at grain boundaries. The above results are explained by the transition of a compensating defect mode from pure barium vacancies to a combination of titanium vacancies in grain interiors and barium vacancies at grain boundaries as the Nb‐doping content in BaTiO3 is increased.

Crystal chemistry of new triple-perovskite-slab intergrowth phases: non-stoichiometric barium–lead–copper mixed oxides Ba4–xPb3–xCuxO10–δ(x⩽0.33; δ⩽0.60)

The partial substitution of Cu2+ for Pb4+, rather than occurring in the BaPbO3 perovskite as claimed previously, has been identified in the triple-perovskite-slab Intergrowth plumbate Ba4Pb3O10. New non-stoichiometric phases, barium–copper–lead mixed oxides Ba4–xPb3–xCuxO10–δ(x⩽0.33; δ⩽0.67), were prepared at 1243 K in flowing oxygen. They exhibit both barium and oxygen vacancies and Pb4+/Cu2+ ordering in the external perovskite layers of the triple slab, as deduced from X-ray powder structure calculations. Magnetic susceptibility measurements in the range 77–400 K point to the existence of weak ferromagnetic interactions within the perovskite slab. From the value of µ, 30% of the total Cu content is assumed to be oxidized to Cu3+, in agreement with the structural data. Water intercalation is characterized from crystallographic and thermal analysis. The intercalation–deintercalation process is shown to be irreversible. A similar trend with respect to water intercalation is evidenced for the double-perovskite-slab Intergrowth plumbate Ba3Pb2O7.

Spontaneous Polarization Screening Effect and Trap‐State Density at Grain Boundaries of Semiconducting Barium Titanate Ceramics

The grain‐boundary trap‐state density and the polarization screening effect were studied for a series of semiconducting PTCR barium titanate ceramic samples with different manganese (Mn) additives and different thermal treatments. The grain‐boundary resistance and capacitance data were measured by the ac complex impedance method. The grain‐boundary data obtained were analyzed using a simple double‐depletion‐layer model and an absolute‐zero‐temperature approach for the Fermi distribution function for the boundary trap states. The energy density distributions of the boundary trap states were found to be V‐shaped for the energy range studied, from 0.35 to 1.35 eV, as measured from the conduction band downward. The “neutral” Fermi level at the grain boundary is taken as 1.35 eV and the bulk Fermi level is taken as 0.15 eV from the conduction band. For the samples without the Mn additive, the PTCR effect is controlled by the trap densities located near 0.35 eV. The trap centers are believed to be barium and oxygen vacancies, or chemisorped oxygens. For the samples with Mn additives, the trap densities increase dramatically near 1.35 eV and play a dominant role in the PTCR effect. These trap centers are believed to be Mn4+ ions at the titanium sites. The charge‐compensation effect of spontaneous polarizations on the trap charges was found to be linearly proportional to the total amount of trap charges at that temperature.

About the mechanism of the solid-way synthesis of barium metatitanate. Industrial consequences

Barium metatitanate BaTiO 3 is the main component of the dielectric material of class 2 ceramic capacitors. The solid state reaction between BaCO 3 and TiO 2 is still one of the main industrial ways for the BaTiO 3 preparation. The aim of the paper is to propose a coherent set of interfacial reactions in order to explain all the experimental results previously published about this well known solid state reaction. The paper gives at first a summary of the main experimental results. Then all the interfacial reactions at the various stages of the advancement are proposed. All these reactions involve the same main point defects in BaTiO 3 which are barium and oxygen vacancies. It has been supposed that they are also the involved defects in the intermediate phase Ba 2TiO 4. Improvements of the BaTiO 3 solid way synthesis are examined.

Grain‐Boundary Chemistry of Barium Titanate and Strontium Titanate: II, Origin of Electrical Barriers in Positive‐Temperature‐Coefficient Thermistors

Scanning transmission electron microscopy (STEM) of positive‐temperature‐coefficient (PTC) BaTiO3 thermistors shows that the grain‐boundary oxygen content in as‐received (oxidatively cooled) materials is slightly enriched compared to quenched samples, and the acceptor‐rich space‐charge present at high temperatures is retained upon cooling. The defect density of the space charge is approximately equal to the acceptor state density at PTC boundaries determined by electrical measurements. Accordingly, it is proposed that the electrical barrier forms when acceptor defects already segregated in the ionic space charge at high temperature become active interface states when compensating donor defects in the grain‐boundary core are oxidized. These acceptor defects appear to be primarily barium vacancies, but need not form upon cooling in the manner proposed by Daniels and Wernicke. Acceptor solutes when present can also contribute to barrier formation through space‐charge segregation; the increase in interface state density upon addition of Mn is consistent with the magnitude of the expected segregation.

Grain boundary layer ceramic capacitors based on donor-doped Ba(Ti 1 − xSn x)O 3

Grain boundary layer capacitors are processed from Ba(Ti 1 − xSn x)O 3 solid solutions (0 < × < 0.25), doped simultaneously with donor and acceptor impurities. The starting powders are prepared by the hydrothermal method. Dielectric properties of these ceramics are strongly dependent on the concentration of donor as well as acceptor dopants, the ceramic microstructure and phase contents. It is found that the 3d n configuration of acceptor impurities also influences the dielectric properties. Fine-grained (less than or equal to 3 μm) ceramics show low loss (less than 5%), low ϵ r (6 × 10 3) and high resistivity (greater than or equal to 2.6 × 10 6 Ω cm), whereas, the coarse-grained (greater than 25 μm) ceramics show high loss (greater than 100%), high ϵ r (8.5 × 10 6) and low resistivity (less than or equal to 3 × 10 4 Ω cm). Energy-dispersive X-ray analysis shows uniform distribution of donor and acceptor impurities in these ceramics. Phase content analysis by electron paramagnetic resonance reveals partial coexistence of ferroelectric orthorhombic and tetragonal phases along with the paraelectric cubic phase which directly convert to the rhombohedral phase. Current-voltage characteristics and the frequency effect on dielectric properties are also investigated. The insulting behaviour of the grain boundary layers is explained on the basis of the higher concentration of barium vacancies in these layers and the vibronic activation of the acceptor states in various symmetry configurations of a lattice that undergoes diffuse phase transitions.

Defects restoration during cooling and annealing in PTC type barium titanate ceramics

The PTC-type BaTiO3 ceramics of similar microstructure are obtained by careful control of a sintering scheme. The defect chemistry of them is modified by varying the cooling rate and annealing conditons. In addition to applying Heywang and Jonker models for explaining the resistivity anomaly of these samples, the outward diffusion of oxygen vacancies (VO), which left excess barium vacancies (VBa) behind, is proposed to be the formation mechanism of surface states. The formation of defect complex consisting of a V O .. - Ba ″ pair is assumed to be the cause of small diffusivity of VO in these materials. The phenomena, in which the slower cooling rate raises the resistivity of samples, is ascribed to the higher concentration of excess barium vacancies (VBa) contained in these samples. However, the maximum potential barrier height (φmax) of the samples is the same, irrespective of the amount of surface state concentration (N s) and is estimated to be φmax = 0.66eV, from the In (ϱmax)-1/T max plot.