Electronic Defect Formation in Fe-Doped BaZrO₃Studied by X-Ray Absorption Spectroscopy

The electronic structure of Fe-doped BaZrO 3 (BaZr 1– x Fe x O 3−δ ) 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 Fe 3+ and 3d 5 L ( L indicates a ligand-hole), respectively. In O K -edge XAS spectra, 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 O 2– (O – ) bridging FeO 6 and FeO 6 and the one bridging FeO 6 and ZrO 6 . 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 BaZr 1– x Fe x O 3−δ is governed by the valence state of the oxide ion between O 2– and O – with the fixed valence state of the transition metal ion of Fe 3+, and no signature of the formation of Fe 4+ is observed.