Crystal structure and oxygen permeation properties of (La, Ba, Sr)(Co, Ta)O3-δ

To clarify the influence of Ta-substitution on oxygen permeability (JO2) as well as ion conductivity (σi) in a cobaltite perovskite, we investigated detailed crystal structures of (La0.4Sr0.6)(Co1-yTay)O3-δ(y = 0.0, 0.1, 0.2) and (La0.4BaxSr0.6-x)(Co0.8Ta0.2)O3-δ (x = 0.1, 0.2) at room temperature using synchrotron x-ray powder diffractometry (XRPD). The crystal structure of the (La0.4Sr0.6)(Co1-yTay)O3-δ was modified from cubic (Pm3̅m) to rhombohedral (R3̅c) by substituting Ta ions into perovskite B-sites because the tolerance factor deviates from unity with the Ta-substitutions. The tilting angle ϕ of the CoO6 octahedra and strain parameters η estimated from the refined structural parameters suggest that the crystal structures of the Ta-substituted samples are improved to cubic at elevated temperatures of 800-900 °C. Although the lattice volume, the lattice free volume, the O-O bond lengths, and the bottleneck areas gradually increased concomitantly with increasing Ta content, the activation energies estimated from both JO2 and σi increased with Ta-substitution. Results suggest that the electrostatic interaction between Ta5 + and O2 − has greater impact than the crystallographic factors for the activation energy increase. Symmetry of the (La0.4BaxSr0.6-x)(Co0.8Ta0.2)O3-δ was improved from rhombohedral (R3̅c) to cubic (Pm3̅m) by substituting Ba ions for the A-sites because the tolerance factor approaches unity. The lattice constant, the O-O bond length, and the bottleneck area increased with the Ba-substitution. The lattice free volume depends on amount ratio between Co3 + and Co4 + cations in the samples. Both JO2 and σi increased with the Ba-substitution. The activation energy Ei of the σi slightly increased with the Ba-substitution.