Influence of synthesis conditions on the crystal structure of the powder formed in the "hfO₂ - CeO₂/Ce₂O₃" system

Influence of synthesis conditions (type of atmosphere: reduction or oxidation, annealing temperature) on the chemical composition and structure of the compounds formed in the "HfO₂ - CeO₂/Ce₂O₃" system has been investigated by X-ray absorption fine structure spectroscopy combined with Raman spectroscopy, X-ray diffraction and thermogravimetric analysis. It was revealed that isothermal annealing of precursor at temperatures less than 1000°C in air leads to formation of Ce_0.5Hf_0.5O₂ powders with cubic fluorite-type structure (space group Fm-3m). Further increase of annealing temperatures above 1000°C causes decomposition of formed crystal structure into two phases: cubic and monoclinic. Annealing in reduction hydrogen atmosphere causes formation of Ce⁴⁺ _2xCe³⁺ _2-2xHf₂O_7+x compounds with intermediate oxidation state of cerium, where value of x depends on the reducing conditions and treatment parameters. Annealing in vacuum at 1400°C strongly reduces the content of Ce⁴⁺ in a powder samples and leads to formation of pyrochlore structure (space group Fd-3m) with predominant +3 oxidation state of cerium.