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 Ce0.8Gd0.2-xPrxO1.9 (with 0.01 less than or equal to x less than or equal to 0.03) was measured as a function of oxygen activity in the range from a(O2) approximate to 10(2) to a(O2) approximate to 10(-15) (a(O2) = 1 corresponding to a gas with oxygen partial pressure p(O2) = 1.013 bar) for temperatures between 600 degrees C and 750 degrees C and compared with results for Pr-free Ce0.8Gd0.2O1.8. The Hebb-Wagner polarization technique was used with an encapsulated Pt-microcontact in an N-2 atmosphere and a Cu2O/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 degrees C: from an oxygen activity of 5x10(-4) for Ce0.8Gd0.2O1.9 to 5x10(-6) for Ce0.8Gd0.17Pr0.03O1.9). The activity dependence of the electronic conductivity in the n-type and p-type range slightly deviates from the theoretically expected values (sigma(n,p)(proportional to)a(O2)(=1/m) with 4<m less than or equal to 5). The effect of Pr-substitution on the F-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 Pr3+ to Pr4+ under oxidizing conditions. The activation energy as determined from the temperature dependence of the electronic conductivity was E-A,E-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,E-h =1.16 eV for Ce0.8Gd0.2O1.9 to E-A,E-h = 0.7 eV for Ce0.8Gd0.17Pr0.03O1.9. Measurement of the total impedance yielded an increase in the overall oxygen ion conductivity with Pr addition due to a lower grain boundary resistivity. (C) 1999 Published by Elsevier Science B.V. All rights reserved.