Analysis of Metal Site Preference and Electronic Structure of Brownmillerite-Phase Oxides (A2B′ x B2−x O5; A = Ca, Sr; B′/B = Al, Mn, Fe, Co) by X-ray Absorption Near-Edge Spectroscopy

X-ray absorption near-edge spectroscopic (XANES) measurements of the Fe and Co K-edges for a series of Brownmillerite-type transition-metal oxides (A2B′ x B2−x O5; A = Ca, Sr; B′/B = Al, Mn, Fe, Co) have been made. In these compounds, the metal atoms are present in both octahedral and tetrahedral coordination environments. The spectra were interpreted with the aid of electronic structure calculations. Significant changes in intensity and energy were observed in the Fe K-edge spectra as the average Fe3+ coordination number (CN) was changed from 5.5 (Ca2AlFeO5) to 4.2 (Ca2FeMnO5). In Ca2Co x Fe2−x O5, the Fe K- and Co K-edge XANES spectra indicated that Fe has a slight octahedral site preference in these series. This confirmed that size effects play a significant role in controlling the site preference of metal atoms in these compounds. The change in the average Fe CN with Co substitution was examined by analysis of a 3D plot, which compared all of the changes observed in the spectra. This type of analysis gives an accurate depiction of the change in Fe CN as compared to standard materials. The Fe K-edge peak intensities in Ca2−y Sr y Fe2O5 decreased with higher values of y, a result of decreased Fe 4p(3d)−O 2p overlap with increasing Sr concentration.