Red-ox behaviour in the La0.6Sr0.4CoO3±δ-CeO2 system

Abstract

The compositions in the (100 − x)La_0.6Sr_0.4CoO_3±δ-xCeO₂ (LSCC) system with 5 ≤ x ≤ 76 are two-phase at room temperature. They consist of the modified perovskite with rhombohedral symmetry (Rc) and modified ceria with fluorite structure (Fmm). The cross-dissolution of La, Sr, Co and Ce cations between the initial La_0.6Sr_0.4CoO_3±δ (LSC) and CeO₂ takes place and results in the modification of the initial phases. This is particularly important for the modified ceria. The lattice parameter of the modified ceria increases due to the dissolution of La and Sr cations with larger ionic radii, thereby changing noticeably the oxygen sublattice in the fluorite structure. Above 300 °C LSCCx composites are three-phase due to the reversible change in the symmetry from rhombohedral (Rc) to cubic (Pmm) within the perovskite phase. Red-ox behaviour of the LSCC composites has been explored under air and argon atmospheres in terms of evolution of the chemical composition at the grain's surface and phase interfaces, formation of oxygen vacancies and thermochemistry of this process. Reversible red-ox behaviour was observed in LSCCx with x = 8–37 most probably due to an observed high surface concentration of Co cations, that can be easily involved in the reduction/re-oxidation cycle. The increase in the surface concentration of Ce⁴⁺ cations together with the decrease in surface concentration of Co cations seems to result in the differences in the reduction and oxidation behaviour under air in LSCCx with x = 57–76. Formation of oxygen vacancies in LSC, LSCC02 and LSCCx with x = 5–76 in air was not accompanied by any distinct thermal events. This process becomes more endothermic with further increase in oxygen nonstoichiometry (δ) above certain values: δ > 0.08 in LSC, δ > 0.13 in LSCC02, and LSCC with x = 5–76. The LSCCx with x = 5–37 and with x = 57–76 show slightly different reduction behaviour under a(o₂) = 7.4 × 10⁻⁵. In the composites with a relatively low CeO₂ content, the extent of the reduction is proportional to the Co content in a composition, whereas the reduction of the LSCCx with x = 57–76 was more significant than expected. The changes in the enthalpy of oxygen vacancy formation and the kinetics of reduction have been discussed.