Structural response to Er doping in Ba2FeCoO5 oxygen-deficient perovskites

Er 3+ substitution in oxygen-deficient Ba 2 FeCoO 5 perovskites was investigated using nominal A-site Ba 2-x Er x FeCoO 5 , and B-site-targeted Ba 2 Fe 1-x/2 Co 1-x/2 Er x O 5 series (x = 0.05, 0.10, 0.20). Samples were prepared by solid-state reaction and characterized by X-ray diffraction, Rietveld refinement and scanning electron microscopy. Both series retained cubic Pm3m symmetry and showed progressive unit-cell expansion with Er content. This is counterintuitive for the nominal A-site series, where Ba 2+ /Er 3+ substitution should induce lattice contraction. The structural trends cannot be rationalized by simple A-site substitution and are consistent with a more complex defect-accommodation mechanism.