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

Abstract

The diffusion of lanthanum and manganese cations in Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−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 Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3–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 Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3–δ, especially when used in oxygen separation membrane applications, are also discussed.