Electronic conductivity of Gd-doped ceria with additional Pr-doping

The electronic conductivity of ceria doped with 20− x mole% Gd and x mole% Pr according to the composition Ce 0.8Gd 0.2− x Pr x O 1.9 (with 0.01≤ x≤0.03) was measured as a function of oxygen activity in the range from a O 2 ≈10 2 to a O 2 ≈10 −15 ( a O 2 =1 corresponding to a gas with oxygen partial pressure p O 2 =1.013 bar) for temperatures between 600°C and 750°C and compared with results for Pr-free Ce 0.8Gd 0.2O 1.8. The Hebb–Wagner polarization technique was used with an encapsulated Pt-microcontact in an N 2 atmosphere and a Cu 2O/CuO reference electrode. Addition of 1–3 mol% Pr leads to a slightly decreased electron conductivity in the n-type range, but to a large increase in the p-type range. Moreover, the minimum of the electronic conductivity is shifted to lower oxygen activities by doping with Pr (e.g. at 700°C: from an oxygen activity of 5×10 −4 for Ce 0.8Gd 0.2O 1.9 to 5×10 −6 for Ce 0.8Gd 0.17Pr 0.03O 1.9). The activity dependence of the electronic conductivity in the n-type and p-type range slightly deviates from the theoretically expected values ( σ n, p ∝ a ±1/ m O 2 with 4< m≤5). The effect of Pr-substitution on the p-type conductivity is explained by a model which assumes low lying 4f levels of Pr that cause an excess p-type conductivity due to a beginning valence change from Pr 3+ to Pr 4+ under oxidizing conditions. The activation energy as determined from the temperature dependence of the electronic conductivity was E A,e=2.5 eV for electrons in the n-type range, independent of the Pr concentration. In the p-type range, the activation energy decreases with Pr concentration: e.g. E A,h=1.16 eV for Ce 0.8Gd 0.2O 1.9 to E A,h=0.7 eV for Ce 0.8Gd 0.17Pr 0.03O 1.9. Measurement of the total impedance yielded an increase in the overall oxygen ion conductivity with Pr addition due to a lower grain boundary resistivity.