Stability and Evolution of the Crystal Structure of TbBaCo₂O_6-δ During Thermal Oxygen Release/Uptake

A-site ordered double perovskites with the general formula LnBaCo₂O_6-δ (where Ln is a lanthanide element) present electrical and electrocatalytic properties that make them attractive as possible ceramic electrode materials for solid oxide cells or alkaline electrolyzers. The properties are highly influenced by the anion vacancy concentration, which is strongly related to the Co-oxidation state, and their location in the structure. Awareness of the stable phases is essential to synthesize, evaluate, and optimize the properties of LnBaCo₂O_6-δ oxides at operating conditions in different applications. TbBaCo₂O_6-δ are representative oxides of these layered perovskite systems. The present article reports a study of TbBaCo₂O_6-δ by electron diffraction, high-resolution electron microscopy, and powder neutron diffraction experiments at different temperatures. The synthesis of TbBaCo₂O_6-δ in air and slow cooling to room temperature (RT) at 5 °C h^-1 leads to samples formed by distinct phases with different oxygen contents and crystal structures. The 122 and 112 phases (with a_p × 2a_p × 2a_p and a_p × a_p × 2a_p unit cells, respectively, with a_p being the lattice parameter of the simple cubic perovskite structure) are predominant in quasi-equilibrium prepared samples (cooled at RT at 1 °C h^-1) or prepared in Ar flow and quenched to RT. The evolution of the crystal structure of TbBaCo₂O_6-δ during thermal oxygen release/uptaking consists of modulation from the 122 phase to the 112 phase (or vice versa during uptaking) by creation/occupation of anion vacancies within the TbO_1-δ planes. Anion vacancies are not detected in the oxygen crystallographic position different from those located within the TbO_1-δ planes even at the highest temperatures, supporting the 2D character of the high anion conduction of the LnBaCo₂O_6-δ oxides.