Oxygen stoichiometry and isotope exchange of oxides Ba0.5Sr0.5Co0.8Fe0.2O3−δ doped with Ta, Nb, Mo or W

Abstract

This study focuses on the oxygen exchange kinetics and the oxygen diffusivity of the Ba0.5Sr0.5Co0.8Fe0.2O3−δ oxides doped with high-charged cations M = Ta, Nb, Mo or W and factors governing the oxygen vacancies mobility and surface exchange activity. The dependence of oxygen content in Nb-doped oxide has been investigated as the function of oxygen pressure and temperature within the ranges of − 4.5 ≤ log(Po2/P0o2) ≤ − 0.7 and 600–900 °C by means of the quasi-equilibrium oxygen release method. Based on «T − Po2 – δ» diagrams the modelling of point defect equilibrium has been performed. The composition and morphology of the surface of all the oxides have been studied by the X-ray photoelectron spectroscopy (XPS) and the scanning electron microscopy (SEM), respectively. The kinetics of interaction of gas phase oxygen with the oxides has been explored by the isotope exchange method with gas phase equilibration (IE-GPE) in the temperature range of 600–800 °C and the oxygen pressure at 6.7 mbar. All the oxides have been divided into two groups with respect to the type of rate-determining step of oxygen surface exchange process, while the strong dependence of oxygen vacancy mobility on dopant concentration has been found. The obtained experimental data and proposed models of point defect equilibrium allowed to reveal the factors governing oxygen diffusion and oxygen surface exchange mechanism in series of oxides.