Defect Chemistry of Undoped and Acceptor-Doped BaPbO3

The equilibrium defect chemistry of polycrystalline, undoped, and acceptor-doped BaPbO₃ was studied by measurement of the equilibrium electrical conductivity as a function of temperature, 800°–900°C, and oxygen activity, 10⁻¹⁸–1 atm. Both equilibrium electrical conductivity data of undoped and acceptor-doped samples were quantitatively fit to a defect model involving only doubly ionized oxygen vacancies, lead vacancies, holes, and acceptor impurities. The results in low and midrange of oxygen activity are dominated by acceptor impurities, whether deliberately added or not. Only in the highly oxidized condition is the conductivity independent of impurity content, confirming that this region represents the intrinsic behavior of BaPbO₃.     

Ti-Rich Nonstoichiometric BaTiO3: I, High-Temperature Electrical Conductivity Measurements

Equilibrium electrical conductivity of nonstoichiometric poly-crystalline BaTiO₃ with varying Ba:Ti ratios was investigated at temperatures between 800° and 1200°C and P o₂ from 10⁻²² to I atm. A transition from p -type to n -type conductivity was observed. Although the electrical conductivity of different specimens varied slightly, these differences did not appear to be a function of the Ba:Ti ratio in the region investigated. An intrinsic band-gap energy of ∼3.1 eV was calculated from the temperature dependence of the minimum conductivity. The O₂ partial pressure dependence of the isothermal n -type conductivity cannot be described by simple defect models incorporating only singly or doubly ionized O vacancies. Likewise, simple defect models incorporating cation vacancies are not consistent with the observed pressure dependence of the p -type conductivity. More complex defect models which correspond to the observed behavior over the entire range of temperature and P o₂ will be discussed in a subsequent paper.     

Effect of Oxide Defect Structure on the Electrical Properties of ZrO2

The defect structure of monoclinic ZrO₂ was studied by measuring the transfer numbers and electrical conductivity as functions of O₂ pressure and temperature. The data suggest a defect structure of doubly ionized oxygen vacancies at low pressures, i.e. <10⁻¹⁹ atm, and singly ionized oxygen interstitials at pressures >10⁻⁹ atm. Zirconia is primarily an ionic conductor below #700°C and an electronic conductor at 700° to 1000°C for 10⁻²²≤P_o2≤1 atm.     

Defect Structure and Electrical Properties of Single-Crystal Ba0.03Sr0.97TiO3

The electrical conductivity and thermoelectric power of single-crystalline Ba_0.03Sr_0.97TiO₃ were measured over a wide temperature (800° to 1100°C) and oxygen partial pressure (10⁵ to 10^-15 Pa) range. Our experimental data, like those of previous workers on nominally undoped BaTiO₃ or SrTiO₃, support a defect model based on doubly ionized oxygen vacancies, electrons, holes, and accidental acceptor impurities. The simultaneous measurement of electrical conductivity and thermoelectric power, together with precise experimental data obtained with an advanced thermoectric power measurement technique, enabled us to determine for the first time reliable values for the preexponential factors and the activation energies which characterize the defect equilibrium constants. These calculated values, together with the defect model, were found to give an excellent fit to the experimental data, and were used to generate the boundaries, in P _o2-1/ T space, of the various defect regimes.     

Electrical Properties and Defect Structure of HfO2

The defect structure of high-purity, polycrystalline HfO₂ was investigated by measuring the oxygen partial pressure dependence of the electrical conductivity and the sample weight. From 1000° to 1500°C and above oxygen partial pressures of 10 ⁻⁶, the conductivity is electronic and proportional to p o₂^1/5. The predominant defect is completely ionized hafnium vacancies. At lower oxygen partial pressures a broad shallow minimum in the lower temperature conductivity isotherms indicates the presence of an oxygen pressure independent source of electronic charge carriers. By combining the weight change and conductivity data, mobility values were found to vary from 1.6 × 10⁻³ to 3 × 10⁻⁴ cm²/V-sec. The activation energies for the hole mobilities were calculated to be 0.2 ev above 1300° C and 0.7 ev below this temperature.     

Nonstoichiometry in Acceptor-Doped BaTiO3

The effect of added Al, as an acceptor impurity, on the equilibrium electrical conductivity of large-grained, polycrystalline BaTiO₃, is consistent with a previously proposed defect model which involves only doubly ionized oxygen vacancies, electrons, holes, and acceptor impurities. The behavior is an extension of that of undoped BaTiO₃, in which an accidental net acceptor excess already plays an important role. Comparison of the derived active acceptor content with the amount of added Al indicates that Al is <50% effective in creating acceptor levels. The magnitude of a small Po₂-independent conductivity component, necessary to fit the observed conductivity minima, increases with added Al content. This is consistent with a contribution from extrinsic oxygen vacancy conduction.     

Effect of PO2 on Bulk and Grain Boundary Resistance of n-Type BaTiO3 at Cryogenic Temperatures

Complex impedance analysis at cryogenic temperatures has revealed that the bulk and grain boundary properties of BaTiO₃ polycrystals are very sensitive to the oxygen partial pressure during sintering. Polycrystals sintered at P _O2 as low as 10⁻¹⁵ atm were already electrically heterogeneous. The activation energy of the bulk conductivity in the rhombohedral phase was found to be close to that of the reduced undoped single crystal (i.e., 0.093 eV). The activation energy of the grain boundary conductivity increases with the temperature of the postsinter oxidation treatment from 0.064 to 0.113 eV. Analysis of polycrystalline BaTiO₃ sintered in reducing atmosphere and then annealed at P _O2= 0.2 atm has shown that the onset of the PTCR effect occurs at much higher temperatures than expected in the framework of the oxygen chemisorption model. The EPR intensity of barium and titanium vacancies increases after oxidation at T > 1000°C. A substantial PTCR effect is achieved only after prolonged annealing of the ceramic in air at temperatures as high as 1200–1250°C. This result suggests that the PTCR effect in polycrystalline BaTiO₃ is associated with interfacial segregation of cation vacancies during oxidation of the grain boundaries.     

Measurement of Oxygen Diffusion in Dy2O3 and Gd2O3 by Studying High-Temperature Oxidation of the Metals

Studies of the oxidation of Gd and Dy at P _O2's from 10^−0.3 to 10^−14.5 atm and temperatures from 727° to 1327°C indicate both semiconducting and ionic-conducting domains in the sesquioxides formed. At higher temperatures, where dense coarsegrained oxide layers developed, the rate of oxidation in the high- P ₀₂ semiconducting domain yielded oxygen diffusion coefficients in Dy₂O₃ in excellent agreement with literature values derived from oxidation of partially reduced oxide single crystals. Under the same conditions, the oxidation of Gd yielded oxygen diffusion coefficients in cubic Gd₂O₃ which are considerably below literature values for monoclinic single-crystal Gd₂O₃. At lower temperatures, porous scales were formed, and apparent diffusion coefficients derived from oxidation rates show a smaller temperature dependence than the high-temperature data. At low P _O2, the oxides behave as ionic conductors, and metal oxidation rates result in estimates of the electronic contribution to the electrical conductivity of the order of 10⁻⁶ to 10⁻⁷Ω⁻¹ cm⁻¹.     

High-Temperature Hall Measurements on BaSnO3Ceramics

Simultaneous Hall and conductivity measurements were performed in situ between 650° and 1050°C on n-type semiconducting BaSnO₃ceramics. The variation of the Hall mobility and the Hall carrier density as a function of oxygen partial pressure between 10² and 10⁵ Pa and of temperature was investigated. At temperatures below 900°C the conductivity exhibits a dependence on temperature and oxygen partial pressure which is mainly determined by variations of the Hall mobility. Above 900°C most of the significant dependence is due to a variation in carrier density. Furthermore, a simple defect model assuming doubly ionized oxygen vacancies and acceptor impurities is discussed for BaSnO₃.     

Reversible Weight Change of Acceptor-Doped BaTiO3

The oxygen vacancy concentration of BaTiO₃ doped with acceptors (Cr to Ni) is determined gravimetrically as a function of the O₂ partial pressure during and after annealing at 700° to 1300°C. The oxygen vacancy concentration of these materials is larger than that of undoped and donor-doped BaTiO₃. The oxygen vacancies are doubly ionized and they compensate the acceptors of lower valence. Both the vacancy concentration and the valence of the acceptor dopants depend on the annealing conditions. The electronic energy levels of the acceptors within the BaTiO₃ band gap are derived from the gravimetric measurements. The electrical properties of the acceptor-doped ceramics are favorable for base-metal-electrode multilayer capacitors, which require sintering in reducing atmospheres.     

Electronic Conductivity, Seebeck Coefficient, and Defect Structure of LaFeO3

Electronic conductivity and Seebeck coefficients of LaFeO₃ were measured as a function of temperature (1000° to 1400°C) and P ( O ₂) (10⁵ to 10⁻¹³ Pa). Electronic conduction was found to be n-type in the lower P ( O₂ ) range, and p -type in a higher P(O₂) range. The calculated carrier mobilities suggest a hopping-type conduction mechanism. The carrier concentrations were calculated as a function of P ( O₂ ) and the defect structure was described. It was found that the electrical properties of LaFeO₃ are determined not only by the concentration of oxygen vacancies, but also by the La/Fe ratio.     

Electrical Properties and Defect Structure of Y2O3

Guarded measurements of the electrical conductivity of high-purity, polycrystalline Y₂O₃ in thermodynamic equilibrium with the gas phase were made under controlled temperature and oxygen partial pressure conditions. Data are presented as isobars from 1200° to 1600°C, and as isotherms from oxygen partial pressures of 10⁻¹ to 10⁻¹⁷ atm. The ionic contribution to the total conductivity, determined by the blocking electrode polarization technique, was less than 1% over the entire range of temperatures and oxygen partial pressures studied. Yttria is shown to be an amphoteric semiconductor with the region of predominant hole conduction shifting to higher pressures at higher temperatures. In the region of p -type conduction, the conductivity is represented by the expression σ= 1.3 × 10³ p O₂^3/16 exp (-1.94/kT). The observed pressure dependence is attributed to the predominance of fully ionized yttrium vacancies. Yttria is shown to be a mixed conductor below 900°C.     

Calcium as an Acceptor Impurity in BaTiO3

The equilibrium electrical conductivity of polycrystalline, calcium-doped BaTiO₃ was studied over the oxygen partial pressure range 10^-13 to 10⁵ Pa and the temperature range 800° to 1000°C. There is little effect if CaO is substituted for a corresponding amount of BaO, i.e., Ba, _1-xCa_xTiO₃. If CaO is substituted for a corresponding amount of the TiO₂ content, i.e., BaTi_1-xCa_xO_3-x, the equilibrium conductivity shows strong evidence of acceptor-doped behavior. If the corresponding amount of excess CaO is added to stoichiometric BaTiO₃, i.e., BaCa_xTiO_3+x, the conductivity profiles are very close to those for samples with TiO₂ replaced by CaO, and show highly acceptor-doped behavior. This is in agreement with the replacement of a small amount of Ti by Ca²⁺ on the octahedral B-sites of BaTiO₃, where it acts as an acceptor center, Ca_T     

Strength of Hot Isostatically Pressed Si3N4 After Heat Treatment in Ar-O2 and H2-H2O

The effects of heat treatment in Ar-O₂ and H₂-H₂O atmospheres on the flexural strength of hot isostatically pressed Si₃N₄ were investigated. Increases in room-temperature strength, to values significantly above that of the aspolished material, were observed when the Si₃N₄ was exposed at 1400°C to (1) H₂ with water vapor pressure ( P _H2O) greater than 1 × 10⁻⁴ MPa or (2) Ar with oxygen partial pressure ( P _O2) of between 7 × 10⁻⁶ and 1.5 × 10⁻⁵ MPa. However, the strength of the material was degraded when the P _H2O in H₂ was lower than 1 × 10⁻⁴ MPa, and essentially unaffected when the P _O2 in Ar was higher than 1.5 × 10⁻⁵ MPa. We suggest that the observed strength increases are the result of strength-limiting surface flaws being healed by a Y₂Si₂O₇ layer formed during exposure.     

Electrical Conductivity of Y2O3 as a Function of Oxygen Partial Pressure in Wet and Dry Atmospheres

The electrical conductivity of polycrystalline Y₂O₃ has been studied as a function of the partial pressure of oxygen (10^–14 to 10⁵ Pa) at 900° to 1500°C in atmospheres saturated with water vapor at 12°C or dried with P₂O₅. Yttria is a p -conductor at high oxygen activities. The p -conductivity increases with increasing P _O2 and decreases with increasing P_H2O. At low oxygen activities the oxide is a mixed ionic/electronic conductor. The ionic conductivity is approximately independent of P _O2 and increases with increasing P _H2O. In the Y₂O₃ samples, excesses of lower-valent cation impurities (in the 10 to 100 mol-ppm range) are the dominating negatively charged defects, and in the presence of water vapor they are compensated by interstitial protons. At high P _H2O levels additional protons are probably compensated by interstitial oxygen ions. At high temperatures (±1100°C) and for high P _O2 and low P _H2O, the protons are no longer dominant, and the lower-valent cations are mainly compensated by electron holes. The electrical conductivity exhibits hysteresis-like effects which are interpreted in terms of segregation/desegregation of impurities at grain boundaries. The mobility of electron holes in yttria at 1500°C is estimated to be of the order of magnitude of 0.05 cm². s^–1. V^–1     

Gravimetric Determination of Defect Concentrations in NiO

Thermogravimetric measurements were made on NiO from 800° to 1100°C over the oxygen pressure range 10⁻¹ to 10⁻⁴ atm. On the basis of complementary conductivity measurements showing a P (O₂)^1/5 oxygen pressure dependence, it is proposed that the predominant defects are described by an electroneutrality condition involving doubly ionized metal vacancies, impurities, and electron holes, 2[ V"_M ]=[ F_M ]+ p. It is shown that for this defect model, the weight change relative to a low oxygen pressure reference weight is a measure of the effective vacancy concentration, defined as [ V"_M ]_eff≡[ V"_M ]-≡[ F_M ], and therefore has the same oxygen pressure dependence as the electron hole concentration followed in the conductivity measurements. The expression [ V"_M ]_eff=0.168 P (O₂)^1/5 exp (−0.86±0.15/ kT ) is derived to express the effective vacancy concentration in NiO. The probable effective impurity content of the specimens used is calculated.     

High-Temperature Defect Structure of Acceptor-Doped Strontium Titanate

The defect structure of acceptor-doped (Fe, Al, Cr) polycrystalline strontium titanate was investigated by measuring the equilibrium electrical conductivity as a function of oxygen activity (10⁻²¹≤a_o2≤1) and temperature (85°C≤ T ≤ 1050°C). The electrical conductivity was n type with ∼−1/4 dependence on a ₀₂ for a₀₂<10⁻¹⁰. For a₀₂>10⁻⁸, the observed data for Fe- or Al-doped samples were proportional to ∼1/4.5 power of a₀₂. In this region for Cr-doped samples, the value of m in.σp αa₀₂^+1/m varies from ∼4.80 to ∼9.00 as the concentration of Cr is increased from 460 to 10 000 ppm. The onset of p -type conductivity depends on the amount. of acceptor impurity added to the sample. The absolute values of the conductivity in the acceptor-doped samples were lower in the n-type region than those for undoped SrTiO₃. The conductivity minima shift toward lower oxygen activity with increasing acceptor concentration. For the entire oxygen activity rangae used in this study, the defect structure of SrTiO₃ is dominated by the added acceptor impurities.     

Electrical Charge Transport in Single-Crystal CaWO4

A combination of ac and dc electrical conductivity measurements was used to characterize the charge transport in single crystals of CaW0₄ which were equilibrated with H₂-H₂O-Ar gas mixtures. Measurements were made from 900° to 1300°C and at P_H2O/P_H2 ratios from 0.02 to 3.0. The ac conductivity at 1000°C varied from 51.4×10⁻⁶ to 5.89×10⁻⁶ mho/cm for P_H2O/P_H2=0.02 and 2.0, respectively; the dc conductivity changed from 51.0±10⁻⁶ to 5.42×10⁻⁶ mho/cm under the same experimental conditions. The partial logarithmic derivative of dc conductivity with respect to P_H2O/P_H2 was −1/2 between 1000° and 1300°C. The results may be described by a paired-defect model of oxygen vacancies and oxygen interstitials (majority defects and minority electrons).     

Defect Structure of TiTa2O7 at Elevated Temperatures

The electrical conductivity of polycrystalline TiTa₂O₇ was measured at 1000° and 1050°C as a function of oxygen partial pressure from 10⁻¹ to 10⁻²¹ MPa. In the apparent n-type region, the value of m in σ_n∝PO₂^−1/m was found to be ∼6 in the region >10⁻¹⁷ MPa and ∼4from 10⁻¹⁶ to 10⁻⁹ MPa. The conductivity appeared to be p-type for P₀₂<10⁻⁵ MPa. The measured data are explained on the basis of the presence of small amounts of acceptor impurities in the undoped sample.     

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.