Electronic conduction and stability of solid electrolytes based on lanthanum gallates

The electronic conductivity of some Sr- and Mg-doped lanthanum gallates was measured as a function of the oxygen activity in the pO2 range of about 101–10−17 bar using the Hebb–Wagner polarization technique. The investigated compositions were La0.8Sr0.2Ga1−yMgyO3−δ with y=0.15, 0.2, (La0.8Sr0.2)0.95Ga0.8Mg0.2O3−δ (LSGM19/20), and the Nd-doped (La0.9Nd0.1)0.8Sr0.2Ga0.8Mg0.2O3−δ (LNSGM). Sintered dense pellets of the samples were contacted with glass encapsulated ion-blocking Pt or Au microcontacts in pure N2 atmosphere and a Cu2O/CuO reference electrode. The temperatures ranged between 600 and 750 °C. For pO2>10−8 bar, all samples showed p-type conductivity with the expected dependence σh∝pO21/4 and a weak temperature influence. For pO2<10−12 bar, n-type conduction was found which, however, did not give a corresponding σe∝pO2−1/4 dependence. Instead, the electronic conductivity approached a limiting value below about pO2=10−15 bar. This result and the observation of an additional irreversible current drift after cathodic polarization for long times suggested a degradation of the materials in the low pO2 range. This was most probably caused by the loss of Ga2O and/or by the formation of PtGax-alloys. Both effects can explain the observed limitation of the electron concentration in the n-type range.