Description:
<jats:title>Abstract</jats:title><jats:p>The cubic phase mixed ionic‐electronic conductor (Ba<jats:sub>0.5</jats:sub>Sr<jats:sub>0.5</jats:sub>)(Co<jats:sub>0.8</jats:sub>Fe<jats:sub>0.2</jats:sub>)O<jats:sub>3−δ</jats:sub> (<jats:styled-content style="fixed-case">BSCF</jats:styled-content>) is well‐known for its excellent oxygen ion conductivity and high catalytic activity. However, formation of secondary phases impedes oxygen ion transport and consequentially a widespread application of <jats:styled-content style="fixed-case">BSCF</jats:styled-content> as oxygen transport membrane. B‐cation substitution by 1, 3 and 10 at.% Y was employed in this work for stabilization of the cubic <jats:styled-content style="fixed-case">BSCF</jats:styled-content> phase. Secondary phase formation was quantified on bulk and powder samples exposed to temperatures between 640 and 1100°C with annealing time up to 44 days. The phase composition, cation valence states, and chemical composition of all samples were analyzed by high‐resolution analytical electron microscopic techniques. Y doping effectively suppresses the formation of Ba<jats:sub>n+1</jats:sub>Co<jats:sub>n</jats:sub>O<jats:sub>3n+3</jats:sub>(Co<jats:sub>8</jats:sub>O<jats:sub>8</jats:sub>) (n ≥ 2) and Co<jats:sub>x</jats:sub>O<jats:sub>y</jats:sub> phases which would otherwise act as nucleation centers for the highly undesirable hexagonal <jats:styled-content style="fixed-case">BSCF</jats:styled-content> phase. This work validates for 10 at.% Y cation substitution perfect stabilization of the cubic <jats:styled-content style="fixed-case">BSCF</jats:styled-content> phase at temperatures ≥800°C, while a negligible small volume fraction of the hexagonal <jats:styled-content style="fixed-case">BSCF</jats:styled-content> phase was found at lower temperatures. A newly developed model describes the effect of Y doping on the formation of secondary phases and their effective suppression with increasing Y concentration.</jats:p>