Electron Paramagnetic Resonance Investigation of Acceptor Centers in Pb(Zr,Ti)O3 Ceramics

The nature of extrinsic point defects in lead zirconate titanate (PZT) ceramics of various compositions prepared by solution chemistry has been explored. Using electron paramagnetic resonance (EPR), several impurity sites have been identified in the as-received materials, which include Fe³⁺oxygen vacancy (V_O^‥) complex and isolated Cu²⁺ ions; both of these ions are incorporated into the lattice by replacing the Ti(Zr) ion. A Fe₃₊V_O^‥ complex serves as a sensitive probe of the local crystalline field environment of the ceramic; the symmetry of this defect is roughly correlated with its phase diagram as the composition is varied from PbTiO₃ to PbZrO₃. The Fe³⁺V_O^‥ complex experiences tetragonal, rhombic, or orthorhombic symmetry as the composition is varied from PbTiO₃ to PbZrO₃. As the composition of the Cu²⁺ ion is varied, it appears as though the addition of Zr, and not necessarily a change in phase, is largely responsible in determining the local environment of this acceptor impurity. Also, the Cu²⁺ resonance parameters weakly reflect the relative Ti–O(Zr–O) bond covalency in the perovskite lattice.     

Occurrence of Ti3+ and Fe2+ in Mullite

Electron paramagnetic studies showed that Ti³⁺ and Fe²⁺ occur in mullites taken from a refractory material which was fused-cast under a reducing atmosphere. Exposure of the mullite samples to temperatures >1600°C caused oxidation of Ti³⁺ and Fe²⁺ to Ti⁴⁺ and Fe³⁺, respectively.     

Behavior of Paramagnetic Iron during the Thermal Transformations of Kaolinite

Kaolinites with various degrees of structural order and iron content were heated and subsequently analyzed via electron paramagnetic resonance. Iron was present in two different states in the heated materials, either as dilute structural Fe³⁺ ions or in concentrated Fe³⁺ phases. During metakaolinization, the environment of dilute Fe³⁺ ions changed, following modifications of the Al³⁺ coordination, and the Fe³⁺ concentration increased. With the breakdown of metakaolinite, the diffusion of Fe³⁺ ions induced their exsolution in superparamagnetic iron-rich domains (Fe³⁺ clusters in γ-Al₂O₃ and/or Fe³⁺ oxide nanophases), which produced a decrease in the dilute Fe³⁺ concentration. The subsequent breakdown of γ-Al₂O₃ and the formation of mullite made the dilute Fe³⁺ concentration increase again, because of the incorporation of Fe³⁺ ions in the mullite structure.     

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.     

Polymorphism of BaTiO3 Acceptor Doped with Mn3+, Fe3+, and Ti3+

The effect of Mn doping on the cubic to hexagonal phase transition temperature in BaTiO₃ has been determined by quenching samples with different Mn contents from a range of temperatures. Under conditions of equilibrating samples in air over the range 1000°–1400°C, cubic solid solutions BaTi_{1− x} Mn _x O_3−δ form over the range 0≤ x ≤0.015(5), whereas hexagonal solid solutions form for x ≥0.02, depending on the temperature. The results are compared with those on doping BaTiO₃ with Fe³⁺ and observations made concerning acceptor doping with Ti³⁺.     

Structural and Electric Properties of Sol–Gel-Derived Ba0.9Sr0.1TiO3 Multilayers

Crack-free Ba_0.9Sr_0.1TiO₃ (BST) and Mn-doped Ba_0.9Sr_0.1TiO₃ (BSTM) multilayers with thickness over 2 μm have been prepared by chemical solution deposition based on one single precursor. Both multilayers exhibit good performance as Bragg reflectors. Mn doping tends to suppress the leakage current in BST multilayers effectively by smoothing the layers and the reduction of the charge carries. The Mn-doped BST multilayer displays an excellent ferroelectric property, with an average remnant polarization ( P _r⁺– P _r⁻)/2 of 12.69 μC/cm² and an average coercive field ( E ⁺ – E ⁻ )/2 of about 72.95 kV/cm under an applied field of 440 kV/cm.     

Role of Donor and Acceptor Ions in the Dielectric Loss Tangent of (Zr0.8Sn0.2)TiO4 Dielectric Resonator Material

The role of donor and acceptor ions in the dielectric loss tangent of (Zr,Sn)TiO₄ was investigated at frequencies from 0.1 kHz to 9 GHz. When a specimen was doped with Fe₂O₃, the loss tangent increased drastically at microwave frequencies. This increase is explained by a large concentration of Fe³⁺ ions and oxygen vacancies in the grains. When the specimen was doped with Ta₂O₅ and then annealed in a reducing atmosphere, electrical resistivity decreased and tan δ increased extremely at low frequencies but trivially at microwave frequencies. The high microwave Q value of Tadoped specimens is explained by assuming the Ta ions in the crystal to be tetravalent. Dielectric loss is determined predominantly by space charge polarization at low frequencies and by the disordered charge distribution at microwave frequencies.     

Effect of Rare-Earth Ions on Electrical Conductivity of BaTiO3 Ceramics

Lattice constants, grain size, electrical conductivity, and luminescence were measured for sintered BaTiO₃ ceramics doped with 0 to 1.2 at.% rare-earth ions. BaTiO₃ doped with low levels of rare-earth ions contains grains 10 μm in size and has lattice constants nearly equal to those of undoped ceramics. In this case, rare-earth ions occupy Ba²⁺ sites and yield donors. When grain growth is inhibited by high doping levels or by insufficient sintering, the lattice constants change, the rare-earth ions occupy both Ba²⁺ and Ti⁴⁺ sites, and, consequently, BaTiO₃ becomes insulative because of charge compensation.     

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.     

Weakly Coupled Relaxor Behavior of BaTiO3–BiScO3 Ceramics

The structural and dielectric properties of (1− x )BaTiO₃– x BiScO₃ ( x =0–0.5) ceramics were investigated to acquire a better understanding of the binary system, including determination of the symmetry of the phases, the associated dielectric properties, and the differences in the roles of Bi₂O₃ and BiScO₃ substitutions in a BaTiO₃ solid solution. The solubility limit for BiScO₃ into the BaTiO₃ perovskite structure was determined to be about x =0.4. A systematic structural change from the ferroelectric tetragonal phase to a pseudo-cubic one was observed at about x =0.05–0.075 at room temperature. Dielectric measurements revealed a gradual change from proper ferroelectric behavior in pure BaTiO₃ to highly diffusive and dispersive relaxor-like characteristics from 10 to 40 mol% BiScO₃. Several of the compositions showed high relative permittivities with low-temperature coefficients of capacitance over a wide range of temperature. Quantification of the relaxation behavior was obtained through the Vogel–Fulcher model, which yielded an activation energy of 0.2–0.3 eV. The attempt characteristic frequency was 10¹³ Hz and the freezing temperature, T _f, ranged from −177° to −93°C as a function of composition. The high coercive fields, low remanent polarization, and high activation energies suggest that in the BiScO₃–BaTiO₃ solid solutions, the polarization in nanopolar regions is weakly coupled from region to region, limiting the ability to obtain long-range dipole ordering in these relaxors under field-cooled conditions.     

Redox Equilibria of Multivalent Ions in Silicate Glasses

Experimental studies were made on the compositional dependence of the redox equilibrium of Eu in synthetic silicate liquids, together with an empirical model describing the observed compositional dependence. Electron paramagnetic resonance (EPR) was used to measure the concentration ratio of Eu²⁺ to Eu³⁺ in various glasses formed by rapidly quenching silicate liquids. The compositional field studied comprised mixtures of SiO₂, TiO₂, Al₂O₃, CaO, MgO, and Na₂O. The proposed model describes the Eu²⁺/Eu³⁺ ratio over the entire compositional field in terms of parameters easily related to each glass composition. The general applicability and utility of the model is further demonstrated by its application to the Fe²⁺-Fe³⁺, Ce³⁺-Ce⁴⁺, and Cr³⁺-Cr⁶⁺ redox reactions in binary alkali oxide silicate glasses of Li, Na, and K.     

Interfacial Segregation in Perovskites: II, Experimental Evidence

Experimental evidence determined by scanning transmission electron microscopy, scanning Auger electron spectroscopy, and lattice parameter measurements is reported for interfacial segregation in pure and doped perovskite materials, e.g., BaTiO₃ and SrTiO₃. The segregation behavior for isovalent dopants (e.g., Sr²⁺, Ca²⁺, or Pb²⁺ at a Ba²⁺ site) is mainly associated with a strain energy contribution from ionic misfit. However, both strain energy and space charge contributions are important in determining the segregation behavior of aliovalent ions (e.g., La³⁺ or Nd³⁺ at a Ba²⁺ site and Fe³⁺ or Ni³⁺ at a Ti⁴⁺ site). Segregation depths for aliovlent dopants are typically 15 to 20 nm thick. Besides dopants, constituent alkaline-earth ions also segregate to interfaces. Their segregation behavior is promoted by high temperatures, low partial pressures of oxygen, and the presence of acceptors. The results are explained in terms of a proposed theory of segregation presented in part I of the present series.     

Evaluation of Lattice Site and Valence of Mn and Fe in Polycrystalline PbTiO3 by Electron Spin Resonance and Thermogravimetry

The oxides of Mn and Fe or combinations of La₂O₃ with these oxides were incorporated into PbTiO₃ under defined thermodynamic conditions. The substitution reaction of these additives was examined gravimetrically. At low partial pressures of O₂ and PbO vapor, Mn²⁺ and Fe²⁺ are formed at the A sites, and Mn³⁺ and Fe³⁺ enter both A and B sites. At higher partial pressures of PbO, Mn⁴⁺ is most probably formed at the B sites.
The Mn²⁺, Mn⁴⁺, and Fe³⁺ were determined by ESR.     

Effect of Fe4+ in the System SrFeO3-SrTiO3

Perovskites of the system SrFeO₃-SrTiO₃ were prepared, and measurements were made of their magnetic and electrical behavior. Chemical analysis showed that the percentage of Fe⁴⁺ varied from 72.5% for SrFeO_2.86 to about zero for Sr(Fe_0.1Ti_0.9)O_2.95; the remainder of the iron was in the Fe³⁺ state and electrical balance was achieved by oxygen loss. Sr(Fe_{1- x} Ti _x )O₃ was antiferromagnetic between x = 0 and x = 0.9, with a Néel temperature below 60°K. A parasitic ferromagnetic component developed when these compounds were cooled in a magnetic field, the magnitude of this component being dependent on the cooling field. The conductivity of these perovskites ranged from 10⁻⁸ ohm⁻¹ cm⁻¹ for x = 1.0 to 10⁻² for x = 0.0 and showed a marked change at x = 0.8. The break corresponded to a change in slope of the lattice parameter and the disappearance of Fe⁴⁺. The Fe⁴⁺ content depended on the heat treatment and atmosphere during formation.     

Interaction between Dissolved Ba2+ and PAA-NH4 Dispersant in Aqueous Barium Titanate Suspensions

The interaction between dissolved Ba²⁺ and dissociated ammonium salt of poly(acrylic acid) (PAA-NH₄) in aqueous suspension has been studied. The dissolved Ba²⁺ causes flocculation of dissociated PAA-NH₄, thus degrading its dispersing effectiveness in aqueous BaTiO₃ suspensions. The concentration of PAA-NH₄ required to stabilize aqueous BaTiO₃ suspension increases with increasing Ba²⁺ concentration at a given pH. A stability map, which is determined by a rheological study, is constructed to describe the amount of PAA-NH₄ required to have well-dispersed aqueous BaTiO₃ suspensions as a function of Ba²⁺ concentrations at different pH values.     

Microstructural Changes during the Reduction/Reoxidation Process in Donor-Doped BaTiO3 Ceramics

Microstructural changes occurring during oxidation of the reduced form of donor-doped BaTiO₃ (Ba_{1− X} D _X ^.Ti_{1− X} ⁴⁺Ti _X ³⁺O₃) and during reduction of the oxidized form of donor-doped BaTiO₃ (Ba_{1− X} D _X ^.Ti_{1− X /4}⁴⁺( V _Ti^) _{X /4}O₃) were studied using TEM. Samples of both types of solid solutions, containing different La concentrations (from 2 to 20 mol% La), were prepared by sintering under reducing conditions and in air, respectively. The reduced form of donor-doped BaTiO₃ was oxidized by annealing at high temperatures (1150° and 1350°C) in air, while the oxidized form was reduced by annealing under reducing conditions. Because of oxidation of the reduced phase of donor-doped BaTiO₃, the Ti-rich phases Ba₆Ti₁₇O₄₀ and BaLa₂Ti₄O₁₂ were precipitated. Reduction of the oxidized form caused precipitation of the Ba-rich phase Ba₂TiO₄ preferentially inside the matrix grains. All precipitates had well-defined orientational relationships with the perovskite matrix.     

The Solid Solubility of Holmium in BaTiO3 Under Reducing Conditions

The solid solubility of Ho³⁺ in BaTiO₃ fired under the reducing conditions of a nitrogen atmosphere containing 10% of hydrogen was studied by quantitative electron-probe microanalysis using wavelength-dispersive spectroscopy. The solubility was found to depend mainly on the starting composition. In the TiO₂-rich samples, the solubility of the Ho³⁺ donors at the Ba lattice sites was measured to be ∼4 mol% Ho_Ba (Ba_0.96Ho_0.04TiO_3−δ at 1400°C), which is much higher than after sintering in air. In the BaO-rich compositions, the solubility of the Ho³⁺ acceptors at the Ti sites (solubility limit     at 1500°C) decreased compared with that in air. Like with sintering in air, under reducing conditions, the highest solubility of holmium in BaTiO₃ was determined when it was incorporated at both lattice sites (solubility limit     ).     

A Magnetic Resonance Investigation of the Process of Corundum Formation Starting from Sol–Gel Precursors

²⁷Al–MAS (magic angle spinning)–NMR and Fe³⁺ EPR measurements have been performed to follow the local process of corundum formation starting from xerogel on the Al³⁺ and Fe³⁺ sites yielding complementary information. Different heat treatments have been applied to the samples: an isochronous procedure and thermoanalytical measurements stopped by quenching. Despite the different mechanisms of the phase transitions deduced, both seeding and Fe³⁺ doping under the conditions of isochronous procedure favor the formation of α-Al₂O₃ at remarkably low temperatures. The first observed temperature of corundum formation following the isochronous procedure with iron-doped samples is as low as 750°C. The transition alumina, which could be clearly evidenced, is the γ-Al₂O₃ phase (four- and six-fold Al coordination). In undoped or unseeded samples, intermediate Fe species could be detected by ESR and evidence for θ-Al₂O₃ was obtained from ²⁷Al–NMR spectroscopy.     

Glass Formation in the System SiO2–PbO Containing Transition-Metal Oxides

Glass-forming regions, valence states, and viscosities in SiO₂–PbO systems containing various transition-metal oxides as a third component were investigated. The glasses were prepared by melting in an open atmosphere. The glass-forming regions ranged as follows: MnO≡ZnO > FeO_1.5>NiO. The ratios Fe²⁺/(Fe²⁺+ Fe³⁺) and Mn³⁺/ (Mn³⁺+ Mn²⁺) in the glasses were determined by chemical analysis. The Fe²⁺/ (Fe²⁺+ Fe³⁺) ratio in SiO₂–PbO–FeO_1.5 glasses ranged from 0.016 to 0.050. The Mn³⁺/ (Mn³⁺+ Mn²⁺) ratio in SiO₂–PbO–MnO glasses ranged from 0.056 to 0.30. The fraction of manganese (III) ions in the glasses varies considerably with the glass composition. The effects of transitionmetal oxides on the viscosity are discussed.     

Effect of Reoxidation on the Grain-Boundary Acceptor-State Density of Reduced BaTiO3 Ceramics

To investigate the effect of reoxidation on the grain-boundary acceptor-state density of reduced barium titanate, n -doped BaTiO₃ ceramics are sintered in a reducing atmosphere (2% H₂+ 98% N₂) and then annealed in oxygen. After annealing at 1150°C for different times, the experimental results show a relationship between temperature-averaged acceptor-state density and annealing time as N _s= N _so Bt ^1/n with n between 2 and 3. An inherent acceptorstate density N _so= 4.25 × 10¹² cm⁻² is obtained with an increase rate B = 4.8 × 10¹² cm⁻². min^−1/3, when n reaches 3. The inherent grain-boundary acceptor states in the reduced n -doped BaTiO₃ ceramics are believed not due to adsorbed oxygen ions.