Stability and evolution of the crystal structure of TbBaCo2O6-δ during thermal oxygen release/uptake

Abstract

A-site ordered double perovskites with the general formula LnBaCo2O6- (where Ln is a lanthanide element) present electrical and electro-catalytic properties that makes them attractive as possible ceramic electrode-materials for solid oxide cells or alkaline electrolyzers. The properties are highly influenced by the anion vacancies 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 LnBaCo2O6- oxides at operating conditions in different applications. TbBaCo2O6- are representative oxides of these layered perovskites systems. The present article reports a study of TbBaCo2O6- by electron diffraction, high resolution electron microscopy and powder neutron diffraction experiments at different tem-peratures. Synthesis of TbBaCo2O6- in air and slowly cooled to room temperature (RT) at 5 ºC h-1, leads to samples formed by distinct phases with different oxygen contents and crystal structure. The 122 and 112 phases (with ap × 2ap × 2ap and ap × ap × 2ap unit cells respectively; ap being the lattice parameter of the simple cubic perovskite structure) are predominant in quasi-equilibrium prepared samples (cooled at RT at 1 ºC h-1) or prepared in Ar-flow and quenched to RT. The evolution of the crystal structure of TbBaCo2O6- 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 TbO1- planes. Anion vacan-cies are not detected in oxygen crystallographic position different that those located within the TbO1- planes even at the highest temperatures, supporting the 2D character of the high anion conduction of the LnBaCo2O6- oxides.