Microstructural aspects of the ionic transport properties of strontium-substituted lanthanum cobaltites

The ionic conductivity and microstructure of selected compositions of the solid solution La_1-xSr_xCoO_3-δ (LSC) were examined with respect to possible vacancy ordering phenomena. Homogeneous samples of LSC were prepared by the glycine nitrate process. The ionic conductivity was obtained as a function of the oxygen partial pressure (-3.5 ≤ log[p(O₂)/atm] ≤ 0.5) using a recently developed galvanostatic polarization technique. At 825°C the p(O₂)-dependence of the ionic conductivity of La_1-xSr_xCoO_3-δ (x = 0.4 and 0.6) shows a distinct maximum. Although this behavior has yet to be explained unambiguously it is indicative of decreasing mobility of ionic charge carriers, e.g. due to cooperative vacancy ordering. From the temperature dependence of the ionic conductivity of La_1-xSr_xCoO_3-δ (x = 0.6) activation energies at constant nonstoichiometry (0.20 ≤ δ ≤ 0.28) were obtained. As vacancy association and microstructure are presumed to play a significant role we combined the results of ionic conductivity measurements and electron microscopical investigations. HRTEM images revealed a superstructure within microdomains of about 100 nm in size.