Electron Paramagnetic Resonance in Combination with the Thermal Analysis, X-ray Diffraction, and Raman Spectroscopy to Follow the Structural Properties of ZrxCe1-xO₂ Solid Systems and Precursors

The Zr x Ce 1 - x (OH) 4 solids were prepared by simple precipitation of hydroxides from known solutions of ZrOCl 2 ·8H 2 O and Ce(NO 3 ) 3 ·6H 2 O in aqueous ammonium hydroxide. For x ≤ 0.5, the presence of a CeO 2 phase with a mixed Zr−Ce hydroxide has been evidenced by thermal analysis and Raman and EPR spectroscopies. For this latter technique, the EPR signal of Ce 3+ ions was used as a probe to determine the presence of the CeO 2 phase. For x > 0.5, a pure Zr x Ce 1 - x (OH) 4 was found without the formation of the ZrO 2 phase. After calcination under flow air at 700 °C of hydroxide samples, Zr x Ce 1 - x O 2 solid solutions were formed. For x ≤ 0.5, the solids possess a cubic phase slightly modified from that corresponding to the pure CeO 2 phase structure, since the Zr 4+ ions replace in the first step the Ce 4+ ions located on the surface rather than in the bulk of CeO 2 . For x > 0.5, the presence of Zr 3+ ions corresponding to d 1 ions in an octahedral environment with strong tetragonal distortion was evidenced. This ion was used as a probe to determine the phase nature of the Zr x Ce 1 - x O 2 solid solution. This phase was attributed to a tetragonal one except for the pure ZrO 2 solid, where a mixture of monoclinic and tetragonal phases was evidenced. As in the case of the XRD technique, EPR has been shown to efficiently evaluate the percentage of tetragonal or monoclinic phases in the ZrO 2 solid.