Polycrystalline samples of (Eu1−xYbx)2O3 (x = 0.0, 0.1, 0.2, 0.5, 0.8, 0.9 and 1.0) were synthesized by a sol–gel process. X‐ray diffraction data were collected and the crystal structures were refined by the Rietveld method. All samples are found to have the same crystal system and formed solid solutions over the whole range of x. The lattice parameters are found to vary linearly with the composition x. The cationic distribution over the two non‐equivalent sites 8b and 24d of the space group is found to be random in the range 0.0 < x≤ 0.5 and preferential in the range 0.5 < x≤ 1.0. Replacing Eu3+ and Yb3+ by each other introduces slight changes in the atomic coordinates. Crystallite size and microstrain analysis are performed on single and multiple orders for each sample using profile fitting and the Warren–Averbach method. The obtained values of microstrain are correlated with the distribution of the rare earth (RE) ions over the two cationic sites of the structure. The average crystallite size ranges from 35 to 96 nm and the mean‐square strain from 0.052 to 0.225 × 10−2.

Nanosized polycrystalline samples of (Er 1Àu Gd u ) 2 O 3 (0 u 1.0) were synthesized by a sol±gel technique. X-ray diffraction data were collected and the crystal structures were re®ned by the Rietveld method. All samples are found to have the same crystal system and formed solid solutions over the whole range of u. The Er 3+ and Gd 3+ ions were randomly distributed over two cationic sites, 8b and 24d, in the space group Ia " 3 (206) in all re®ned structures. The lattice parameter was found to vary non-linearly with the composition (u). The average microstrain and average crystallite size have been calculated from the Williamson±Hall plots for each sample. The average size ranges from 50 to 70 nm, and the microstrain from 0.4 to 1.7%.

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