Effect of the Synthesis Conditions on the Crystal, Local, and Electronic Structures of Ce2x4+Ce2-2x3+M2O7 + x (M = Zr, Hf)

Resumo

The effect of synthesis conditions (type of cations, annealing temperature, reductive or oxidative atmosphere) on the chemical composition and structure of compounds formed in the MO₂–CeO₂/Ce₂O₃ systems (M = Zr, Hf) was studied by X-ray diffraction, X-ray absorption spectroscopy, Raman spectroscopy, and thermogravimetric analysis. The isothermal annealing of the precursors at temperatures below 1000°C in air gives Ce_0.5M_0.5O₂ powders with fluorite type cubic structure (space group Fm3̅m). High-temperature annealing above 1000°C gives rise to additional tetragonal (space group P4₂/nmc) (for zirconates) or monoclinic (space group P2₁/a) (for hafnates) phases. The annealing in a hydrogen atmosphere affords the compounds Ce_2x⁴⁺Ce_2-2x³⁺M₂O_{7 + x} with an intermediate oxidation state of cerium and the x value depending on both the reduction conditions and the pre-annealing parameters. Vacuum annealing at 1400°C considerably decreases the content of Ce(IV) in the samples and affords a pyrochlore structure (space group Fd3̅m) with predominating Ce(III) cations.