Effects of bond character on the electronic structure of brownmillerite-phase oxides, Ca2B′xFe2−xO5 (B′ = Al, Ga): an X-ray absorption and electron energy loss spectroscopic study

Andrew P. Grosvenor; Farshid Ramezanipour; Shahab Derakhshan; Christian Maunders; Gianluigi A. Botton; John E. Greedan

doi:10.1039/B914983J

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†

Andrew P. Grosvenor,‡*^ab Farshid Ramezanipour,^ab Shahab Derakhshan,^ab Christian Maunders,^c Gianluigi A. Botton^cd and John E. Greedan^ab

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* Corresponding authors

^a Department of Chemistry, McMaster University, Hamilton, Ontario, Canada
Fax: +1 306 966 4730

^b Brockhouse Institute for Materials Research, McMaster University, Hamilton, Ontario, Canada

^c Canadian Centre for Electron Microscopy, McMaster University, Hamilton, Ontario, Canada

^d Department of Materials Engineering and Science, McMaster University, Hamilton, Ontario, Canada

Abstract

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.

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Article information

DOI: https://doi.org/10.1039/B914983J
Article type: Paper
Submitted: 24 Jul 2009
Accepted: 03 Sep 2009
First published: 27 Oct 2009

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J. Mater. Chem., 2009,19, 9213-9220

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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

A. P. Grosvenor, F. Ramezanipour, S. Derakhshan, C. Maunders, G. A. Botton and J. E. Greedan, J. Mater. Chem., 2009, 19, 9213 DOI: 10.1039/B914983J

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Journal of Materials Chemistry