Electronic Defect Formation in Fe-Doped BaZrO3 Studied by X-Ray Absorption Spectroscopy

The electronic structure of Fe-doped BaZrO3 (BaZr1-xFexO3-delta) has been studied by X-ray absorption spectroscopy (XAS) to understand electronic defect formation reactions. The analysis of Fe L-edge XAS spectra suggests that the valence state and ground state of Fe under an oxidizing atmosphere are Fe3+ and 3d(5)L (L indicates a ligand-hole), respectively. In O K-edge XAS specta, the formation of two pre-edge features by Fe doping is observed. One of these features is related to the unoccupied O2p-Fe3d hybridizing state, whereas the other is assigned to an acceptor level composed of an unoccupied nonbonding O2p state. The acceptor level splits into two fine structures of localized holes on O2- (O-) bridging FeO6 and FeO6 and the one bridging FeO6 and ZrO6. The intensity of the acceptor level is very sensitive to both Fe concentration and the partial pressure of oxygen gas for heat treatment, both of which modulate the acceptor concentration through defect chemical reaction. The present results reveal a surprising conclusion that the oxidation state of BaZr1-xFeO3-delta is governed by the valence state of the oxide ion between O2- and O- with the fixed valence state of the transition metal ion of Fe3+, and no signature of the formation of Fe3+ is observed.