Oxygen thermodynamics and ion conductivity in the solid solution La _1-xSr_xCoO_3-δ at large strontium content

The equilibrium oxygen content was measured in the model system and important oxygen permeable material La_1-xSr_xCoO _3-δ, where x = 0.6, in the temperature range 650-900 °C and oxygen partial pressure range between 10^-5 and 1 atm. The data were utilized to obtain changes in the partial entropy and enthalpy of oxygen in the solid as a function of the oxygen content. It is shown that the initially cubic perovskite undergoes to a phase transition to a tetragonal structure at δ > 0.3. The oxygen permeation of La_1-xSr_xCoO _3-δ at 700-900 °C is found to be controlled by bulk solid state processes. The activation energy equals about 0.8 eV at high oxygen pressure and small oxygen nonstoichiometry. Increasing oxygen deficiency results in a rapid increase in the activation energy. In combination with thermodynamic data, these changes can be explained as resulting from the intrinsic, spatial inhomogeneouty in oxygen vacancy distribution which varies both with temperature and oxygen nonstoichiometry. It is shown that, when the oxygen deficiency increases at constant temperature, the oxygen vacancies form locally ordered microdomains (clusters), which eventually results in a transition of the cubic perovskite structure to the tetragonal structure. The oxygen ion conductivity depends strongly on the development of the ordering.