Cation miscibility in CeO2–ZrO2 oxides with fluorite structure. A combined TEM, SAED and XRD Rietveld analysis

Abstract

Structural and microstructural features of Ce_1−xZr_xO₂ (0 < x < 1) samples, prepared by solid state reaction at 1350 and 1500 °C, were analyzed by means of powder X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques. In samples prepared at 1350 °C the solubility was very low, but in samples prepared at 1500 °C, it increased considerably. In all of the analyzed samples, the fluorite structure was detected; however, Rietveld analysis of the XRD patterns showed that for samples with a high cerium content the solid solution adopts cubic symmetry (S.G. Fmm) and suffers a structural transformation as the zirconium content increases, firstly to tetragonal (S.G. P4₂/nmc), and later to monoclinic (S.G. P2₁/c). The relationship between the Ce/Zr ratio and the microstructural properties has been investigated by means of selected area electron diffraction (SAED) and high resolution electron microscopy (HREM), combined with EDS microanalysis at the nanoscale level. Although no intracrystalline compositional heterogeneities were detected, samples with intermediate compositions consisted of two phases with compositions close to Ce_0.2Zr_0.8O₂ and Ce_0.7Zr_0.3O₂. Finally, solubility ranges deduced from Rietveld analysis are discussed in terms of phase diagrams previously reported.