Diffusion of La and Mn in Ba0.5Sr0.5Co0.8Fe0.2O3−δpolycrystalline ceramics

Abstract

The diffusion of lanthanum and manganese cations in Ba0.5Sr0.5Co0.8Fe0.2O3−δ polycrystalline ceramics was measured in the temperature range 1123 ≤ T/K ≤ 1283 at the pO2 of 0.21 bar, and at 1173 K over the range of pO2 10−5 ≤ pO2/bar ≤ 1. The diffusion coefficients of La and Mn were similar at all temperatures investigated. In addition, the two species display the same small activation energy of (1.6 ± 0.2) eV. These observations indicate the two cations are moving by a complex diffusion mechanism in Ba0.5Sr0.5Co0.8Fe0.2O3–δ, and a coupled migration involving a defect complex was proposed. The diffusion coefficients of La and Mn displayed a pO2 dependence with a slope of ≈ ½, which also indicates that migration of the cations occurs by a vacancy mediated mechanism. However, no conventional defect model is able to reproduce the strong dependence. An alternative explanation requires the cation mobility to vary strongly with pO2, on account of large changes in the charge state of the transition metal cation. Grain boundary diffusion, 103–104 times faster than the corresponding bulk diffusion, was observed at all temperatures investigated. Grain boundary diffusion on the order 10−12 cm2 s−1 was observed at 1173 K over the entire pO2 range investigated. The implications of the results for the kinetic demixing of Ba0.5Sr0.5Co0.8Fe0.2O3–δ, especially when used in oxygen separation membrane applications, are also discussed.