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A co-doping strategy is used to enhance the ionic conductivity of doped ceria. Recent density functional theory has suggested that Pm with atomic number 61, is the ideal dopant exhibiting low activation energy for oxygen diffusion, and consequently high ionic conductivity. Taking the atomic number of Pm³⁺ into account, Sm³⁺ and Nd³⁺ are selected as co-dopants for ceria to test this hypothesis. Different compositions of Sm_x/2Nd_x/2Ce_1−xO_2−δ(x=0.01–0.20) are synthesized using conventional solid state route. The lattice parameters are estimated after the detailed analysis of X-ray diffraction data using least-squares extrapolation technique. The lattice parameter in Sm_x/2Nd_x/2Ce_1−xO_2−δ system obeys Vegard's law, even at higher dopant concentration, indicating that short-range oxygen vacancy ordering is comparatively lower than in other systems. The ionic conductivity of the samples is measured in the …