Effects of bond character on the electronic structure of brownmillerite-phase oxides, Ca[subscript 2] [prime]B[subscript x]Fe[subscript 2-x]O[subscript 5] (B = Al, Ga): an X-ray absorption and electron energy loss spectroscopic study

Brownmillerite-phase transition-metal oxides, Ca{sub 2}B'{sub x}Fe{sub 2-x}O{sub 5} (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 Ca{sub 2}Ga{sub x}Fe{sub 2-x}O{sub 5}, 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 Ca{sub 2}Al{sub x}Fe{sub 2-x}O{sub 5} 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.