Synchrotron X-ray Absorption Spectroscopy Study of Local Structure in Al-Doped BiFeO₃ Powders

The Al-doped BiFeO₃, i.e., BFA_xO powder samples with x = 0, 0.025, 0.05, and 0.1, were prepared via the hydrothermal route. The effects of Al substitution on the structural, electrical, and optical properties of BFA_xO samples were investigated. It is found that the substitution of Al ions at B-site of BiFeO₃ did not cause structural change and it still retains the rhombohedral perovskite structure with R3c symmetry, which was confirmed by the X-ray diffraction (XRD) and Raman measurements. X-ray absorption fine structure (XAFS) above the Fe K-edge and Bi L₃-edge in BFA_xO powders was also measured and analyzed. Fe ions exhibit mixed valence states (Fe²⁺/Fe³⁺) while Bi ions keep the + 3 valence state in all the samples. Fe K-edge XAFS also indicated that there was a competition between hybridization of Fe 3d and Al 3d with O 2p orbitals and occurrence of the more 4p orbitals with Al doping. The Bi L₃-edge XAFS revealed that transition from 2p_3/2 to 6d state increased, so did the energy of 6d state. Besides, Al ion doping affected both the nearest-neighbor and next-nearest coordination shells of Fe atom and nearest-neighbor shells of Bi atom. Ultraviolet-visible (UV-Vis) spectroscopy results show the BFA_xO prepared by hydrothermal method could be an appropriate visible-light photocatalytic material.