Mixed valence cerium substitution in Gd_2-xCe_xTi₂O_7+δ pyrochlores

Highlights: • Variation of the Ce oxidation state in Gd_2-xCe_xTi₂O_7+δ was investigated. • XPS and XANES confirmed the presence of Ce³⁺ and Ce⁴⁺. • Increase in Ce oxidation state is charge balanced by O intercalation. - Abstract: Pyrochlore-type oxides having the formula Gd_1.6Ce_0.4Ti₂O_7+δ have been synthesized by ceramic and co-precipitation methods and annealed at 1300 °C under different environments. Rietveld refinement of powder X-ray diffraction (XRD) patterns indicated that the products adopt the pyrochlore-type oxide structure with Ce ions substituting for Gd ions. Analysis of Ce 3d X-ray photoelectron spectroscopy (XPS) spectra confirmed the presence of both Ce³⁺ and Ce⁴⁺, which was corroborated by analysis of Ce L₃-edge X-ray absorption near edge spectroscopy (XANES) spectra. Incorporation of Ce⁴⁺ imparts disorder on the Ti octahedra as observed by analysis of Ti K-edge XANES spectra. Due to the presence of Ce⁴⁺ in the A-site of the pyrochlore-type structure, oxygen is likely intercalated into the vacant anion 8b site to charge balance the system. This intercalation results in an increase of the average Ti coordination number from 6 to between 6 and 7, which leads to the formula unit being better represented as Gd_1.6Ce_0.4Ti₂O_7+δ.