Sol–gel synthesis, phase relationships, and oxygen permeation properties of Sr4Fe6−xCoxO13+δ (0≤x≤3)

Sr4Fe6−xCoxO13+δ samples have been synthesized by a sol–gel procedure for 0≤x≤3 and the influence of firing temperature and procedure on the phase relationships has been investigated systematically. For a synthesis temperature of 1000°C, X-ray diffraction data reveal that the samples with x≤1.8 are single phase consisting of the intergrowth phase Sr4Fe6−xCoxO13+δ, while those with x>1.8 are composed of three phases – Sr4Fe6−xCoxO13+δ intergrowth phase, SrFe1−xCoxO3−δ perovskite phase, and Co3−xFexO4 spinel phase. Rietveld analysis of the X-ray diffraction data and thermogravimetric analysis of the samples reveal that the fraction of the perovskite phase increases as the synthesis temperature increases from 1000 to 1150°C for x>1.8. For a given synthesis/annealing temperature, the sol–gel procedure favors a higher fraction of and higher solubility of Co in the intergrowth phase compared to the conventional solid state synthesis. In the single phase samples of the intergrowth oxide Sr4Fe6−xCoxO13+δ, the oxygen permeation flux at 900°C decreases with increasing cobalt content x. On the other hand, the oxygen flux increases with increasing x in the three phase samples due to an increasing amount of the SrFe1−xCoxO3−δ perovskite phase.