Stabilization of the cubic perovskiteBSCFphase byY‐doping (2016)

Among complex oxides Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3−d (BSCF) exhibits excellent oxygen permeability due to its high oxygen non‐stoichiometry. However, the favoured cubic perovskite phase tends to partly decompose in the desired operation temperature between 700 and 900 °C. Various secondary phases with high Co‐content and low O‐non‐stoichiometry are formed close to grain boundaries [1]. The decomposition can be correlated with thermally activated O‐vacancies. By lowering the temperature (from e.g. 1000 °C), the O‐vacancy concentration decreases forcing the multivalent transition metal atoms (mainly Co) to increase their valence state to maintain charge neutrality. This leads to the collapse of the cubic phase because the tolerance factor shifts into the hexagonal regime [2]. The formation of secondary phases leads to a substantial reduction in the O‐permeation [2]. To overcome this issue, doping was suggested which proved to be beneficial for several different cations (e.g. Zr, Y) [3,4]. Among these candidates, Y was chosen due to its monovalent character and large ionic radius which are both beneficial for stabilizing the cubic phase. Transmission/scanning electron microscopy (TEM/SEM) investigations were carried out on 1% (BSCF1Y) to 10% Y (BSCF10Y) B‐site doped samples to analyse the phase constitution. O‐permeation measurements were performed to assess the long‐time permeability. Due to the intermediate ionic radius of Y the lattice position was checked using atom location by channeling enhanced microanalysis (ALCHEMI) [5]. BSCF tends to form a variety of secondary phases. Especially at high temperatures Co tends to diffuse out of the cubic lattice. This happens during the sintering process at high temperatures of 1050…1150 °C when CoO grains form close to grain boundaries. At lower temperatures partial phase decomposition to Ba n+1 Co n O 3n+3 (Co 8 O 8 ) (n ≥ 2, BCO), Co 3 O 4 and Ba 0.5+x Sr 0.5−x CoO 3 (hexagonal) phases was demonstrated. Valence‐state analysis of Co employing the Co‐L 2,3 white‐line distance method [7] revealed that secondary phases with higher Co‐valence state tend to be more stable at lower temperatures. Permeation measurements of BSCF10Y show that the degradation of O‐permeation is reduced compared to undoped BSCF. This can be understood by the suppression of secondary phase formation in BSCF10Y at temperatures ≥800 °C (cf. Fig. 1 and Fig. 2). Secondary phases like BCO (cf. Fig. 3) and Co x O y are completely supressed by ≥ 3 at% Y‐doping. The surfaces of permeation pellets show only minor amounts of BaSO 4 which can be attributed to sulphur impurities in the feed gas. At lower temperatures (~700 °C) small volume fractions of the hexagonal phase are formed even in BSCF10Y (cf. Fig. 4). However, the volume fraction was negligible compared to the amount of secondary phases in undoped BSCF revealing a stabilizing effect on the cubic BSCF phase even at 700 °C. ALCHEMI experiments showed unintended partial Y‐occupation of up to 55 % on the A‐site [8]. Therefore, Y doping is expected to generate B‐site vacancies. These can contribute to the increased stability of the cubic phase because BSCF seems to tend towards B‐site deficiency. To verify this hypothesis BSCF with 5% B‐site deficiency was intentionally prepared which indeed showed less Co‐outdiffusion and less pronounced secondary phase formation.