Abstract
The effect of the A-site cation ordering on the chemical stability, oxygen stoichiometry and electrical conductivity in layered LaBaCo₂O(5+δ) double perovskite was studied as a function of temperature and partial pressure of oxygen. Tetragonal A-site cation ordered layered LaBaCo₂O(5+δ) double perovskite was obtained by annealing cubic A-site cation disordered La(0.5)Ba(0.5)CoO(3-δ) perovskite at 1100 °C in N₂. High temperature X-ray diffraction between room temperature (RT) and 800 °C revealed that LaBaCo₂O(5+δ) remains tetragonal during heating in oxidizing atmosphere, but goes through two phase transitions in N₂ and between 450 °C and 675 °C from tetragonal P4/mmm to orthorhombic Pmmm and back to P4/mmm due to oxygen vacancy ordering followed by disordering of the oxygen vacancies. An anisotropic chemical and thermal expansion of LaBaCo₂O(5+δ) was demonstrated. La(0.5)Ba(0.5)CoO(3-δ) remained cubic at the studied temperature irrespective of partial pressure of oxygen. LaBaCo₂O(5+δ) is metastable with respect to La(0.5)Ba(0.5)CoO(3-δ) at oxidizing conditions inferred from the thermal evolution of the oxygen deficiency and oxidation state of Co in the two materials. The oxidation state of Co is higher in La(0.5)Ba(0.5)CoO(3-δ) resulting in a higher electrical conductivity relative to LaBaCo₂O(5+δ). The conductivity in both materials was reduced with decreasing partial pressure of oxygen pointing to a p-type semiconducting behavior.