Uncertainty of oxygen content in highly nonstoichiometric oxides from neutron diffraction data: example of perovskite-type Ba0.5Sr0.5Co0.8Fe0.2O3−δ

Neutron powder diffraction is a powerful technique for probing oxygen in nonstoichiometric oxides, but often gives lower oxygen occupancy compared to traditional characterization methods. In this work, the oxygen nonstoichiometry of a model oxide system, cubic perovskite-type Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF), was studied by thermogravimetric analysis, coulometric titration and room-temperature neutron diffraction, with the aim to identify the possible cause of such a discrepancy. Comparative analysis of the obtained results and available literature data indicates that the Rietveld refinement of neutron diffraction data yields strongly overestimated δ values, by 0.20–0.31 oxygen atoms per formula unit, compared to other methods. The uncertainty of oxygen content in BSCF from the neutron diffraction data is discussed in terms of strong diffuse scattering leading to undervalued oxygen occupancy in a standard Rietveld refinement.