Effects of bond character on the electronic structure of brownmillerite-phase oxides, Ca₂B′_xFe_2−xO₅ (B′ = Al, Ga): an X-ray absorption and electron energy loss spectroscopic study

Brownmillerite-phase transition-metal oxides, Ca₂B′_xFe_2−xO₅ (B′ = Al, Ga), have been examined by use of X-ray absorption near-edge spectroscopy (XANES) and electron energy loss spectroscopy (EELS). These studies were performed to examine how the electronic structure was affected as a more (Ga) or less (Al) electronegative metal was substituted for Fe. The oxygen deficient perovskite-like structure is built up of alternating layers having either octahedrally or tetrahedrally coordinated metal atoms. Analysis of the Fe L-, Ga K-, and Al L-edge XANES and EELS spectra confirmed that the group III metals substitute primarily into the tetrahedral site, regardless of the size of the atoms. Through examination of O K-edge spectra, compared to LMTO calculated crystal orbital Hamilton population plots, the change in O–metal bond character with substitution was investigated. It was found that in Ca₂Ga_xFe_2−xO₅, the peaks in the O K-edge spectra resulting from an excitation of O 1s electrons into hybridized, unoccupied, O 2p–Ga 4p/4s antibonding states decreased in energy and increased in intensity with greater values of x. This is a result of the formation of more covalent O–Ga bonds. No shifts in energy were observed in O K-edge spectra from Ca₂Al_xFe_2−xO₅ as the more ionic O–Al states are overlapped by stronger O–Ca antibonding states. This study shows that the electronic structure of these materials is tunable through selective substitution of metals into the tetrahedral site.