Synthesis and characterization of Gd3+ and Nd3+ co-doped ceria by using citric acid–nitrate combustion method

A series of Ce0.8Gd0.2−xNdxO2−δ (x = 0–0.20) compositions have been synthesized by citric acid–nitrate combustion method. XRD measurements indicate that all the obtained materials crystallized in cubic fluorite-type structure. Lattice parameters were calculated by Rietveld method and the parameter a values in Ce0.8Gd0.2−xNdxO2−δ system obey Vegard's law, a (Å) = 5.4224 + 0.1208x. The obtained powders have good sinterability and the relative density could reach above 95% after being sintered at 1400 °C. Impedance spectroscopy measurements indicated that the conductivity of Ce0.8Gd0.2−xNdxO2−δ first increased and then decreased with Nd dopant content x. The maximum conductivity, σ700 °C = 6.26 × 10−2 S/cm, was found in Ce0.8Gd0.12Nd0.08O1.9 when sintered at 1300 °C. The corresponding activation energies of conduction had a minimum value Ea = 0.676 eV. The results tested experimentally the validity of the effective atomic number concept of recent density functional theory, which had suggested that co-dopant with effective atomic number between 61 (Pm) and 62 (Sm) was the ideal dopant exhibiting high ionic conductivity and low activation energy.