Simulations were used to examine the relationship between oxide ion conductivity and dopant content in systems having the general formula, Ba1-xSrxCo1-yFeyO2.5, where x and y were varied between 0 and 1, and all B-site cations were assumed to be in the +3 state. Doping Ba-rich systems with iron initially caused a decrease in the ionic conductivity, which reached a minimum at around 50at%Fe. Above this concentration, the conductivity increased again to a maximum for y = 1. For Sr-rich systems (x = 1.0), the conductivity increased continuously as y was increased. Simulations using a mean field approximation for the short-range Co3+/Fe3+ interactions were also reported. Comparison with the discrete ion model results showed that the parabolic variation in the ionic conductivity was due to a mixing effect caused by the random distribution of dopant ions in the perovskite structure. Calculation of coordination numbers for each cationic species showed that anion vacancies tended to be strongly associated with Co3+ ions, and that the strongest bonding occurred for y = 0.5. Doping with Sr2+ was found to increase the conductivity. These results showed that compositional changes had a strong effect upon the oxide ion conductivity when the concentration of oxygen vacancies was held constant.

Oxide Ion Diffusion in Perovskite-Structured Ba1-xSrxCo1-yFeyO2.5: a Molecular Dynamics Study. Fisher, C.A.J., Yoshiya, M., Iwamoto, Y., Ishii, J., Asanuma, M., Yabuta, K.: Solid State Ionics, 2007, 177[39-40], 3425-31