Solid solubility and transport properties of Ce_1−xNd_xO_2−δ nanocrystalline solid solutions by a sol-gel route

Ce_1−xNd_xO_2−δ (x = 0.05 to 0.55) nanocrystalline solid solutions were prepared by a sol-gel route. X-ray diffraction analysis showed that Ce_1−xNd_xO_2−δ crystallized in a cubic fluorite structure. The lattice parameter for the solid solutions increased linearly with the dopant content. The solid solubility of Nd³⁺ in ceria lattice was determined to be about x = 0.40 in terms of the nearly constant lattice parameters at a dopant level larger than x = 0.40. First-order Raman spectra for Ce_1−xNd_xO_2−δ at a lower dopant content exhibited one band associated with the F_2g mode. At higher dopant contents, F_2g mode became broadened and asymmetric, and a new broad band appeared at the higher frequency side of the F_2g mode. This new band was assigned to the oxygen vacancies. The electron paramagnetic resonance spectrum for x = 0.05 showed the presence of O₂⁻ adsorbed on sample surface at g = 2.02 and 2.00 and of Ce³⁺ with a lower symmetry at g = 1.97 and 1.94. Further increasing dopant content led to the disappearance of the signals for O₂⁻. Impedance spectra showed the bulk and grain boundary conduction in the solid solutions. The bulk conduction exhibited a conductivity maximum and an activation energy minimum with increasing dopant content. Ce_0.80Nd_0.20O_2−δ was determined to give promising conduction properties such as a relatively high conductivity of σ_{700 °C} = 2.44 × 10⁻² S cm⁻¹ and moderate activation energy of E_a = 0.802 eV. The variations of conductivity and activation energy were explained in terms of relative content of oxygen vacancies V_ö and defect associations {Ce_Ce ′V_ö}/{Nd_Ce ′V_ö}.