Thermal, mechanical and phase stability of LaCoO₃ in reducing and oxidizing environments

Thermal, mechanical, and phase stability of LaCoO₃ perovskite in air and 4% H₂/96% Ar reducing atmosphere have been studied by thermal mechanical analysis (TMA), high temperature microhardness, and high temperature/room temperature X-ray diffraction. The thermal behavior of LaCoO₃ in air exhibits a non-linear expansion in the 100-400 °C temperature range. A significant increase of coefficient of thermal expansion (CTE) measured in air both during heating and cooling experiments occurs in the 200-250 °C temperature range, corresponding to a known spin state transition. LaCoO₃ is found to be highly unstable in a reducing atmosphere. In case where LaCoO₃ was present as a powder, where surface reduction mechanism would prevail, the reduction starts as earlier as 375 °C with a formation of the metallic Co and La₂O₃ at 600 °C. In the bulk form, LaCoO₃ undergoes a series of expansion and contractions due to phase transformations beginning around 500 °C with very intensive chemical/phase changes at 800 °C and above. These expansions and contractions are directly related to the formation of La₃Co₃O₈, La₂CoO₄, La₄Co₃O₁₀, La₂O₃, CoO, and other Co compounds in the reducing atmosphere. Although LaCoO₃ is a good ionic and electronic conductor and catalyst, its high thermal expansion as well as structural, mechanical, and phase instability in reducing environments present a serious restriction for its application in solid oxide fuel cells, sensors or gas separation membranes.