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The crystallographic properties of the Ce_{1&#8722;<i>x</i>}(Nd_0.63Dy_0.37)<i>_x</i>O_{2&#8722;<i>x</i>/2} system (0 &#8804; <i>x</i> &#8804; 0.6) were studied by means of synchrotron powder X-ray diffraction and compared to the ones of Sm-doped ceria. The aim of this work was to investigate the effect of substituting Sm³⁺ by a mixture of a smaller and a larger ion that ensures a more pronounced Ce⁴⁺/dopant size mismatch while having the same average ionic size as Sm³⁺. Two main findings came to light: (a) the compositional region of the CeO₂-based solid solution widens up to <i>x</i> ranging between 0.4 and 0.5, and (b) the cell parameter is larger than the one of Sm-doped ceria at each composition. Both effects are expected to play a significant role on the ionic conductivity of the material. The results are discussed in terms of disorder and cation-vacancy association.