Investigation of Factors That Affect the Oxidation State of Ce in the Garnet-Type Structure

Oxide materials that adopt the garnet-type structure (X₃A₂B₃O₁₂) have received attention for a wide variety of applications, one of which is as potential wasteforms for the sequestration of radioactive actinide elements. The actinides are able to be accommodated in the eight-coordinate X site of the garnet structure. This study focuses on the investigation of Ce substitution into the X site as a surrogate for Pu because of their similar chemical properties. This is accomplished through analysis of the Y_{3- z}Ce _zAlFe₄O₁₂ (0.05 ⩽ z ⩽ 0.20) materials. The effects of the Ce concentration, oxidation state of the Ce in the starting materials, annealing environment, and cooling rate on the local structure and Ce and Fe oxidation states were investigated through analysis of the powder X-ray diffraction patterns and Ce L₃-edge and Fe K-edge X-ray absorption spectroscopy (XANES) spectra. Analysis of Ce L₃-edge XANES spectra indicated that Ce was present as both 3+ and 4+ oxidation states, the ratios of which depended on the synthetic conditions. The largest concentration of Ce⁴⁺ was observed when the materials were postannealed at 800 °C following synthesis of the materials at 1400 °C. Variations in the Ce oxidation state are the result of the temperature-dependent Ce³⁺/Ce⁴⁺ redox couple, with Ce⁴⁺ being favored at lower temperatures. Analysis of the Fe K-edge spectra indicated that Fe was only present in the 3+ oxidation state and the Fe coordination number increased with increasing concentration of Ce⁴⁺, which is necessary to charge balance the system. The materials in this study can be described as an oxygen-intercalated garnet-type structure with the formula Y_{3- z}Ce _zAlFe₄O_12+δ.