D. S. Tsvetkov

Oxygen nonstoichiometry, defect structure and related properties of LaNi0.6Fe0.4O3−δ

Experimental results on oxygen nonstoichiometry (δ), thermal and chemical expansion (ΔL/L0), total electrical conductivity (σ) and Seebeck coefficient (Q) as functions of the oxygen partial pressure (pO2) and temperature for LaNi0.6Fe0.4O3−δ are presented. The defect structure model of LaNi0.6Fe0.4O3−δ based on the localized nature of the electronic defects was proposed and successfully verified using the measured δ = f(pO2, T) dependences. On the basis of the model proposed the concentrations of the point defects were calculated as functions of the T and pO2. These concentrations were then employed in the model of chemical expansion and that of Seebeck coefficient. It was shown that both models coincide completely with the corresponding experimental data. The chemical expansion coefficients (βc) and mobilities of the charge carriers (electrons and holes) as functions of T and pO2 were calculated as a result.

Oxygen Nonstoichiometry and Electrochemical Properties of GdBaCo2−xFexO6−δ Double Perovskite Cathodes

Mixed ionic- and electronic-conducting perovskite-type oxides are the state-of-the-art materials for high-temperature solid-state electrochemical devices such as solid oxide fuel cells (SOFCs), oxygen membranes, and sensors. Many of such materials are cobaltite-based oxides. Recently, double perovskites REBaCo2O5.5±δ, where RE is a trivalent rare earth and the oxygen content δ varies in wide range, have received a great attention as attractive materials for such application. Many interesting phenomena, such as giant magnetoresistance, charge ordering, and metal-insulator transition, have been observed in these compounds. Powder samples of GdBaCo2−xFexO6−δ(x=0;0.2) were synthesized by glycerol-nitrate method. Oxygen nonstoichiomentry of oxides GdBaCo2−xFexO6−δ(x=0;0.2) was measured by the thermogravimetric (TG) method as a function of temperature in the range of 25–1100°C in air. Total conductivity of aforementioned oxides was studied by the four-probe dc-method as a function of temperature in the range of 25–1100°C in air. Polarization resistance of double perovskite cathodes was investigated by impedance spectroscopy in symmetrical cell of the type electrode|electrolyte|electrode. “Metal-insulator” transition was found at 80°C in GdBaCo2O6−δ, whereas it was not observed in iron-doped sample GdBaCo1.8Fe0.2O6−δ due to the increase in oxygen content upon Fe-doping. At high temperatures, both double perovskites have almost the same total conductivity. Chemical interaction was found to decrease the performance of GdBaCo2−xFexO6−δ cathodes in YSZ-based SOFCs due to the chemical interaction between electrolyte and cathode materials, which significantly increases their polarization resistance. Behavior of total conductivity of oxides GdBaCo2−xFexO6−δ(x=0;0.2) with temperature was explained by assuming small polaron charge transfer. The particularity of the latter is larger mobility of electron holes as compared with that of electrons. Increase in cathode performance was shown in the case of YSZ covered by the Ce0.8Sm0.2O2 layer in comparison with pure Zr0.9Y0.1O2 electrolyte.

Conventional Methods for Measurements of Chemo-Mechanical Coupling

Energy-related oxide materials are often used at elevated temperatures under both oxidizing and reducing conditions and, therefore, their chemical composition with respect to oxygen may change.

Oxygen Content and Thermodynamic Stability of YBaCo2O6−δ Double Perovskite

The thermodynamic stability of the double perovskite YBaCo2O6−δ was studied using the coulometric titration technique and verified by measurements of the overall conductivity depending on oxygen partial pressure at a given temperature. As a result, the stability diagram of YBaCo2O6−δ was plotted. YBaCo2O6−δ was found to be thermodynamically stable in air at 850°C and higher temperatures, whereas its thermodynamic stability at 900°C is limited by the range of oxygen partial pressures −3.56 ≤ log(pO2/atm) ≤ −0.14. Oxygen content in YBaCo2O6−δ slightly decreases at 900°C from 5.035 at log(pO2/atm) = −0.14 to 4.989 in the atmosphere with log(pO2/atm) = −3.565 indicating a crucial role which variation of Co+3/Co+2 ratio plays in its stability. YBaCo2O6−δ decomposes into the mixture of YCoO3 and BaCoO3−z at the high pO2 stability limit, whereas YBaCo4O7, BaCo1−xYxO3−γ, and Y2O3 were identified as the products of its decomposition at the low pO2 one.

In Situ and ex Situ Study of Cubic La0.5Ba0.5CoO3−δ to Double Perovskite LaBaCo2O6−δ Transition and Formation of Domain Textured Phases with Fast Oxygen Exchange Capability

The disordered La0.5Ba0.5CoO3-δ ↔ ordered LaBaCo2O6-δ transition was studied in detail using several complementary in situ (X-ray diffraction, thermogravimetry, and coulometric titration) and ex situ (transmission electron microscopy) techniques. This transition was found to proceed through the formation of complex domain textured intermediate products. They were shown to have strong affinity to oxygen and exhibit its fast absorption from ambient atmosphere (oxygen partial pressure ( pO2) 0.21 atm) at a temperature as low as 70 °C. The thermodynamic stability limits of the cubic and double perovskites were determined by coulometric titration. The stability diagram of the LaBaCo2O6-δ - La0.5Ba0.5CoO3-δ system was plotted as a result. Oxygen nonstoichiometry of the thermodynamically stable cubic perovskite La0.5Ba0.5CoO3-δ was measured as a function of pO2 in the temperature range between 1000 and 1100 °C using a coulometric titration technique.

Synthesis, Single Crystal Growth, and Properties of Cobalt Deficient Double Perovskite EuBaCo2−xO6−δ (x = 0–0.1)

The cobalt deficient double perovskites EuBaCo2−xO6−δ with were obtained both as powders and as single crystal. Formation of cobalt vacancies in their crystal lattice was shown to be accompanied by the formation of oxygen ones. Chemical lattice strain caused by this cooperative disordering of cobalt and oxygen sublattices was found to be isotropic contrary to that caused by the formation of oxygen vacancies only. Cobalt deficiency was also shown to lead to lowering overall conductivity and Seebeck coefficient of EuBaCo2−xO6−δ double perovskites as a result of simultaneous decrease of charge carriers’ concentration and their mobility as well as number of sites available for electrons and holes transfer. Strong anisotropy of the overall conductivity of the single crystal double perovskites EuBaCo2−xO6−δ was found and explained on the basis of preferential location of oxygen vacancies in the rare-earth-oxygen- (REO-) planes.