Fluorination of Iron Hexagonal Perovskites Promoting Low Temperature Oxygen Mobility

Abstract

New materials with original mechanisms driving low temperature oxygen mobility in solids are needed to reduce the operating temperature of solid-oxide fuel cells. Instead of the standard chemical modification of the cationic array in oxides in order to optimize the concentration of anionic carriers, we show that the partial incorporation of F − anions in fluorinated-hexagonal perovskites (FHP) gives raise to efficient low-temperature O 2− mobility. In the Ba/Sr−Fe−O system, fluorides incorporate preferentially highly stable hexagonal [BaF x O 3-y ] layers, thus creating extended defected structures. Contrarily to metastable fluorinated-cubic perovskites, FHP show thermal stability above 1000 °C without significant fluoride removal. Such striking stability is made possible by the reversible reorganization of the anionic array (O 2−, F −, and vacancies). It leads to an original “breathing effect” within the hexagonal slabs which improves the redox properties of FHP and suggests a reversible incorporation of oxygen above 200 °C. This phenomenon is rapid and nondestructive as observed from the easy intercalation of FHP-single crystals, fully conserving their crystallinity.