Vladimir V. Sereda

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.

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.