The effect of B‐site Y substitution on cubic phase stabilization in (Ba 0.5 Sr 0.5 )(Co 0.8 Fe 0.2 )O 3−δ

The cubic phase mixed ionic‐electronic conductor (Ba 0.5 Sr 0.5 )(Co 0.8 Fe 0.2 )O 3−δ ( BSCF ) 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 BSCF as oxygen transport membrane. B‐cation substitution by 1, 3 and 10 at.% Y was employed in this work for stabilization of the cubic BSCF 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 n+1 Co n O 3n+3 (Co 8 O 8 ) (n ≥ 2) and Co x O y phases which would otherwise act as nucleation centers for the highly undesirable hexagonal BSCF phase. This work validates for 10 at.% Y cation substitution perfect stabilization of the cubic BSCF phase at temperatures ≥800°C, while a negligible small volume fraction of the hexagonal BSCF 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.