Kazuo Fueki

Nonstoichiometry of the perovskite-type oxides La1−xSrxCoO3−δ

Abstract In order to clarify the extent of oxygen nonstoichiometry and defect equilibrium in the oxide solid solution La1−xSrxCoO3−δ (x = 0, 0.1, 0.2, 0.3, 0.5, and 0.7), thermogravimetric measurements were made in the range 10 −5 ≤ P( O 2 ) atm ≤ 1 and 300 ≤ T °C ≤ 1000 , where the solid solution was stable as a single-phase perovskite-type oxide. The nonstoichiometry, δ, increases with decreasing P(O2), increasing Sr content, x, and increasing temperature, T. At temperatures below 800°C, δ of LaCoO3−δ could not be detected. For La0.3Sr0.7CoO3−δ, a large δ was observed at temperatures as low as 300°C. The observed δ for each La1−xSrxCoO3−δ ranges between 0 and values slightly in excess of x 2 . Using the Gibbs-Helmholtz equation, the partial molar enthalpy and entropy of oxygen, (hO - hO°) and (sO - sO°) respectively, were calculated as a function of x and δ. The value of (hO - hO°) decreases linearly with increasing δ. The δ dependence of (sO - sO°) is determined by the configurational entropy of V··O and OxO on the oxygen sublattice, while no noticeable contribution was detected from the configurational entropy due to electronic states. This is consistent with the metallic character of the electronic conduction in La1−xSrxCoO3−δ.

Nonstoichiometry and defect structure of the perovskite-type oxides La1−xSrxFeO3−°

In order to elucidate the defect structure of the perovskite-type oxide solid solution La1−xSrxFeO3−δ (x = 0.0, 0.1, 0.25, 0.4, and 0.6), the nonstoichiometry, δ, was measured as a function of oxygen partial pressure, PO2, at temperatures up to 1200°C by means of the thermogravimetric method. Below 200°C and in an atmosphere of PO2 ≥ 0.13 atm, δ in La1−xSrxFeO3−δ was found to be close to 0. With decreasing log PO2, δ increased and asymptotically reached x2. The log(PO2atm) value corresponding to δ = x2 was about −10 at 1000°C. With further decrease in log PO2, δ slightly increased. For LaFeO3−δ, the observed δ values were as small as <0.015. It was found that the relation between δ and log PO2 is interpreted on the basis of the defect equilibrium among Sr′La (or V‴La for the case of LaFeO3−δ), V··O, Fe′Fe, and Fe·Fe. Calculations were made for the equilibrium constants Kox of the reaction 12O2(g) + V··o + 2FexFe = Oxo + 2Fe·Fe and Ki for the reaction 2FexFe = Fe′Fe + Fe·Fe· Using these constants, the defect concentrations were calculated as functions of PO2, temperature, and composition x. The present results are discussed with respect to previously reported results of conductivity measurements.

Diffusion of oxide ion vacancies in perovskite-type oxides

Abstract In order to elucidate the diffusion of oxide ion vacancies in perovskite-type oxides, we determined the tracer diffusion coefficient of oxide ions, D∗ O , in La 1− x Sr x CoO 3− δ ( x = 0.1) and La 1− x Sr x FeO 3− δ ( x = 0.1, 0.25, and 0.4) single crystals. The correlation factor, f , for a vacancy diffusion mechanism in a perovskite-type anion sublattice was calculated to be f = 0.69. Using this value and the nonstoichiometry data, the diffusion coefficient of oxide ion vacancies, D V , was estimated. It was found that D V in La 1− x Sr x M O 3− δ ( M = Co, Fe) take similar values in their magnitude and activation energy. It was concluded that a point defect model holds for the vacancy diffusion in these oxides. The diffusivity of oxide ions in the perovskite-type oxides was comparable to that in fluorite-type oxides.

Determination of Oxygen Nonstoichiometry in a High-Tc Superconductor Ba2YCu3O7-δ

In order to elucidate the correlations between composition, structure and electrical properties of a high-Tc superconductor Ba-Y-Cu-O system, thermogravimetric measurement and chemical analysis of oxygen nonstoichiometry were made at a temperature range of 350 to 1000°C and under the oxygen partial pressure of 10-4 to 1 atm. Within a stability range of the composition Ba2YCu3O7-δ, the oxygen deficiency, δ, was found to vary approximately from zero to 0.9, with associated mean valence of copper ions varying from 2.33 to 1.73. At a possible phase boundary region of tetragonal to orthorhombic crystal structures, no sign of discontinuity in δ was observed, suggesting its phase transition was of higher than a first order.

Electrode reaction at Pt, O2(g)/stabilized zirconia interfaces. Part I: Theoretical consideration of reaction model

Abstract This study aims to make clear the reaction kinetics at Pt, O 2 (g)/zirconia electrodes in the oxygen partial pressure, P O 2 , of ∼ 10 −4 − 1 atm at ∼ 400–∼800°C. By a critical review on the preceding studies, problems were pointed out in the application of the Langmuir adsorption isotherm to the P O 2 dependence of electrode conductance, in the assumption of electric double layer at the electrode interface, and of the inconsistency between the recent reaction model of surface diffusion controlled kinetics and the absolute rate theory. It was shown that the charge transfer kinetics cannot be the rate determining step (RDS) of the electrode reaction. The possible RDS was concluded to be either (i) dissociative adsorption of oxygen molecules on the Pt surface or (ii) surface diffusion of O ad atoms on the Pt surface to the Pt/zirconia contact. The diffusion of O ad atoms on the Pt surface was considered to be proportional to θ(1−θ)(∂μ O /∂ x ), where θ is the occupancy of O ad atoms on Pt and μ O is the oxygen chemical potential on the Pt surface. The rate equation, current-potential relationship, and the electrode conductivity, σ E , were calculated for the cases the RDS is (i) and (ii), respectively. By comparing the calculated σ E versus log P O 2 relationship with the reported ones, it was shown that the RDS is (i) for T ≲ 500°C and is (ii) for T ≳ 600°C. In the former case, σ E is essentially constant irrespective to P O 2 , and in the latter case, σ E maximum appears on the σ E versus log P O 2 relations.

Nonstoichiometry of the perovskite-type oxide La1−xSrxCrO3−δ

Abstract In order to elucidate the defect structure of the perovskite-type oxide solid solution La 1− x Sr x CrO 3− δ , the nonstoichiometry δ was measured as a function of oxygen partial pressure P O 2 at 1000–1300°C, for compositions of x =0.1–0.3 by means of a thermogravimetric method. In the region of P O 2 >10 −5 atm, δ was found to be close to 0. With decreasing P O 2 , δ increased and asymptotically reached x 2 . From the δ- T - P O 2 relation, the partial molar enthalpy h O and the partial molar entropy s O of oxygen in La 1− x Sr x CrO 3− δ were calculated as a function of δ and x . The conductivity data by Sasamoto and his coinvestigators was well interpreted. Assuming the reaction 1 2 O 2 ( g )+ V O ⋯ +2 Cr x Cr = O x O +2 Cr Cr , the standard enthalpy change Δ H 0 and the standard entropy change Δ S 0 of the reaction and the configurational entropy Δ S conf were calculated. It was found that h O depended on δ and x , and Δ H 0 and Δ S 0 depended on x . These dependences suggested a strong interaction between the doped strontium and oxygen sublattice.

Ionic conduction of the perovskite-type halides

Abstract The ionic conduction in the perovskite-type halides, CsPbCl3, CsPbCl3 and KMnCl3, was studied. Measurements were made of ac conductivity at temperatures from 150°C to the melting point, and of ionic transport number using the Tubandt, EMF and ion-blocking methods. The effects of impurity doping on the ionic conductivity of CsPbCl3 were also studied using the samples of composition, CsPb0.99M0.01Cl2.99 (M = Li, Na, K, Ag). It was concluded that these materials are halide-ion condcutors. The ionic conductivities of CsPbCl3 and CsPbBr3 are close to those of the well known halide-ion conductors, PbCl2 and PbBr2. The ionic transport numbers were found to be > 0.9 for CsPbCl3 and CsPbBr3, and about 0.99 for KMnCl3. The conduction was considered to be caused by the migration of halide-ion vacancies V X (X = Cl, Br). The activation energies for the migration of V X were 0.29 eV for CsPbCl3, 0.25 eV for CsPbBr3 and 0.39 eV for KMnCl3. The vacancy diffusion coefficients of these materials were found to be verylarge. However, the impurity doping did not increase the ionic conductivity markedly because of small dopant solubility.

Electrical Conductivity and Seebeck Coefficient of Nonstoichiometric La1 − x Sr x CoO3 − δ

This paper reports the conductivity and Seebeck coefficient of the perovskite-type oxides La{sub 1 {minus} x}Sr{sub x}CoO{sub 3 {minus} {delta}}(x = 0-0.7) measured in 10{sup {minus} 5}-1 atm O{sub 2} gas at temperatures 25{degrees}-1000{degrees}C. The results are discussed in relation to the lattice-parameter and oxygen nonstoichiometry. Close relationships were found between the temperature dependence of the conductivity and the rhombohedral angle, {alpha}. For La{sub 1 {minus} x}Sr{sub x}CoO{sub 3 {minus} {delta}} {lt} 60.3{degrees}-60.4{degrees} (LaCoO{sub 3 {minus} {delta}} above 800{degrees}C and La{sub 1 {minus} x}Sr{sub x}CoO{sub 3 {minus} {delta}} with x {gt} 0.5 at room temperature), the conductivity decreases with temperature, suggesting metallic conduction. For La{sub 1 {minus} x}Sr{sub x}CoO{sub 3 {minus} {delta}} with {alpha} {gt} 60.4{degrees}, the conductivity increases with temperature like semiconductors. In the oxides with metallic conduction, the conductivity was found to decrease with increase in the oxygen vacancy concentration. Because the conduction band is composed of the Co-O-Co network, it is considered that the band is distributed by the formation of oxygen vacancies and becomes narrower, resulting in the decrease in conductivity.

Nonstoichiometry and phase relationship of the SrFeO2.5SrFeO3 system at high temperature

Abstract In order to make clear the phase relationships and nonstoichiometry of the SrFeO2.5 SrFeO3 system at elevated temperatures, measurements were made using high-temperature gravimetry and high-temperature powder X-ray diffraction (XRD) analysis under the controlled oxygen partial pressure of 1–10−5 atm at temperatures above 400°C. At 400–900°C, the system was found to consist of the SrFeO2.5+δ phase of the brownmillerite-type structure with small oxygen excess-type nonstoichiometry and the SrFeO3−δ phase of the cubic perovskite-type structure with large oxygen deficient-type nonstoichiometry. At temperatures above 900°C, the brownmillerite phase transformed to the perovskite-type and the system was composed of only the cubic perovskite-type phase.

Electrode reaction at Pt, O2(g)/stabilized zirconia interfaces. Part II: Electrochemical measurements and analysis

Abstract In order to make clear the reaction kinetics at the Pt, O 2 (g)/stabilized zirconia electrodes, measurements were made on the dc electrode conductivity, σ E , between 370 and 800°C and anodic and cathodic polarization at 700°C in the P O 2 range of 10 −4 −1 atm. Above 600°C, the log σ E versus log P O 2 plot showed maximum. Below 500°C, σ E was essentially constant irrespective of P O 2 . At 700°C, when the P O 2 was high, the limiting current appeared for anodic polarization. In the lower P O 2 , the limiting current was observed at the cathodic polarization. The results were analyzed based on the reaction model proposed in Part I of the present paper. Above 600°C, the diffusion of adsorbed oxygen atoms on the Pt surface was confirmed to be the rate determining step. The activation energy of the surface diffusion of O ad atoms on Pt was found to be 41 ± 2.5 Kcal/mol, and the heat of adsorption of O ad on Pt was 53 ± 4 Kcal/mol. Below 500°C, the reaction rate was determined by the dissociative adsorption of oxygen molecules on the Pt surface near the Pt/zirconia boundary. The activation energy of this reaction was about 37 ± 4 Kcal/mol.