Oxygen nonstoichiometry and ionic transport properties of La 0.4Sr 0.6CoO 3- δ

Abstract

Homogeneous samples of La 0.4Sr 0.6CoO 3- δ were obtained by the glycine nitrate process. The oxygen nonstoichiometry was determined from oxygen exchange measurements as a function of oxygen partial pressure (10 −4 bar< P O 2 <10 −2 bar) and temperature (300°C< T<900°C). The chemical diffusion coefficient D̃ was obtained from oxygen exchange measurements applying a stepwise variation of the oxygen partial pressure of the ambient atmosphere of a disk-shaped sample. The amount of oxygen absorbed or desorbed by the perovskite was analyzed as a function of time. Chemical diffusion data were evaluated using simplified and exact fitting procedures taking into account the surface exchange coefficient. Alternatively, galvanostatic polarization measurements were performed in a P O 2 -range between 10 −4 and 10 −2 bar to yield D̃ and the ionic conductivity σ i from the long time solution of the diffusion equation. Values for D̃ from polarization measurements at T=775°C and from oxygen exchange measurements at T=725°C are in good agreement with each other. D̃ and σ i increase with increasing P O 2 (10 −4 to 10 −2 bar). The ionic conductivity shows a maximum at 3- δ≈2.82 and decreases with decreasing oxygen content indicating the possible formation of vacancy ordered structures.