The structural determination of the CeO2/Gd2O3 mixed system is a non-trivial problem because of the close resemblance between the ionic sizes of Ce4+ and Gd3+ and between the crystal structures of CeO2 and Gd2O3. (Ce1−xGdx)O2−x/2 powder samples with x ranging between 0 and 1 have been synthesized by coprecipitation of mixed oxalates and subsequent thermal decomposition in air at 1200 °C followed by slow cooling. Synchrotron powder X-ray diffraction data were collected and refined by the Rietveld method. Lattice parameters do not follow Vegard's law and no peak splitting has been observed for any composition, meaning that no biphasic regions exist over the whole compositional range. The same hybrid structural model – a proper mixture of the structures of the two pure oxides – was used for the refinements, allowing to account for the data observed.
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