RESUMEN
A-site ordered double perovskites with the general formula LnBaCo2O6-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 LnBaCo2O6-δ oxides at operating conditions in different applications. TbBaCo2O6-δ are representative oxides of these layered perovskite systems. The present article reports a study of TbBaCo2O6-δ by electron diffraction, high-resolution electron microscopy, and powder neutron diffraction experiments at different temperatures. The synthesis of TbBaCo2O6-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 ap × 2ap × 2ap and ap × ap × 2ap unit cells, respectively, with ap 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 TbBaCo2O6-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 TbO1-δ planes. Anion vacancies are not detected in the oxygen crystallographic position different from those located within the TbO1-δ planes even at the highest temperatures, supporting the 2D character of the high anion conduction of the LnBaCo2O6-δ oxides.
RESUMEN
Layered perovskites of the Gd0.8-xBa0.8Ca0.4+xFe2O5+δ system show oxygen reduction reaction (ORR) activity. The layered crystal structure of these oxides is established by the interplay of the Gd3+, Ba2+, and Ca2+ locations with the ordering of the coordination polyhedra of the Fe3+ cations. Substitution of Gd3+ by Ca2+ increases the oxygen deficiency that is accommodated by the formation of layers of FeO5-squared pyramids intercalated with A-O layers containing mainly Gd3+. The presence of FeO5-squared pyramids in the crystal structure promotes the oxygen diffusion and then the ORR activity. Therefore, GdBa2Ca2Fe5O13 is the oxide of the system which presents lower area specific resistance (ASR) values when it is applied as an electrode in symmetrical cells using Ce0.9Gd0.1O2-δ as an electrolyte.
