Surface and bulk thermodynamic factors of Ba0.5Sr0.5(Co0.8Fe0.2)1−xMexO3−δ (Me = Ta, W) oxides

Objectives. In this work, we consider the relationship between the tracer ( k * ) and chemical ( k δ ) oxygen exchange coefficients for Ba 0.5 Sr 0.5 (Co 0.8 Fe 0.2 ) 1− x Me x O 3− δ (Me = Ta, W) oxides. The aim is to analyze the experimental dependencies of the chemical ( k δ ) and tracer ( k * ) coefficients of oxygen exchange, evaluate the surface thermodynamic factor w 0 | x =± L , and compare its value with the bulk thermodynamic factor w 0 | x =0 determined from the dependence of oxygen content in oxides on the temperature and partial pressure of oxygen. Possible reasons for the discrepancy between these two thermodynamic factors are discussed. Methods. The oxygen exchange kinetics between the gas phase and the surface of oxide materials under nonequilibrium conditions was studied using the method of oxygen pressure relaxation. The surface thermodynamic factor was calculated based on data obtained under both equilibrium and nonequilibrium conditions. Results. Comparison of the tracer ( k * ) and chemical ( k δ ) oxygen exchange coefficients allowed the w 0 | x =± L surface thermodynamic factor to be estimated by the k δ = k * w 0 | x =± L equation. Conclusions. The surface thermodynamic factor was found to differ from the bulk thermodynamic factor of the oxide material, w 0 = [1 ∂ln ( p O2 )] / [2 ∂ln (3−δ)], which can be calculated from the dependence of oxygen content in oxides on the temperature and partial pressure of oxygen. This difference can be explained by the difference in the defect structure of the surface layers of oxide materials.