Examination of the site preference of metals in NiAl2−xGaxO4 spinel-type oxides by X-ray absorption near-edge spectroscopy

Abstract

Materials adopting the spinel-type structure have received considerable attention owing to the compositional diversity and the large number of potential applications for these materials. Although many studies of ternary spinel-type oxides have been completed, few studies have investigated quaternary materials. The NiAl2−xGaxO4 spinel-type system was investigated in this study by X-ray absorption near-edge spectroscopy (XANES) and powder X-ray diffraction to study how the metal-site preference changed depending on composition. The Al L2,3-edge XANES spectra showed that Al occupied the tetrahedral and octahedral sites when x in the chemical formula was low, and preferentially occupied the octahedral site as x increased. The Ga K-edge XANES spectra confirmed that Ga3+ has a strong preference for residing in the tetrahedral site and that this ion only partially occupied the octahedral sites when the concentration of Ga3+ in the system was sufficiently large. The Ni K-edge XANES spectra showed that Ni2+ has a strong preference for residing in the octahedral site, and that more Ni2+ was present in the tetrahedral site in NiAl2O4 vs. NiGa2O4. Rietveld refinement of powder X-ray diffraction patterns from the ternary end members were in general agreement with these results. This study has demonstrated the utility of using XANES to investigate the site preference of Al and Ga through the examination of Al L2,3- and Ga K-edge spectra, respectively.