Development of Rare Earth Metal Iron Oxides as Mixed Ionic and Electronic Conductors

Abstract

In recent years there has been an increasing interest in finding a new and less energy demanding alternative to cryogenic distillation in the production of pure oxygen gas. The use of oxygen permeable membranes has been considered a preferable alternative, and extensive studies into candidate materials for use in these membranes, performed. Problems concerning cationic demixing and breakdown of the material structure has been reported for many of these materials were oxygen is transported through oxygen vacancies usually requiring an operating temperature above 800°C. Lately, an increasing interest in materials that can transport oxygen through oxygen interstitials instead of vacancies has emerged, since oxygen interstitials can be transported at temperatures below 600°C where cationic diffusion is usually prevented. A study of the oxygen storage ability of rare earth ferrites has recently been reported, where an oxygen storage ability of an x=0.5, and a good cycling ability has been found for LuFe2O4. A problem with the use of rare earth ferrites for this purpose is that the powder preparation route involves metallic iron as one of the precursors, requiring the use of a glove box in the preparation of the material. In this study, phase pure powder of another rare earth ferrite, YbFe2O4 has been performed from a conventional solid state synthesis route. Strong indications of the possibility of obtaining the material from a sol-gel synthesis route is also reported. Heat treatment of the material in oxygen atmosphere resulting in an oxygen rich phase, YbFe2O4.5, was quantified from in situ TGA measurements, while an ex. situ heat treatment performed under oxidizing and reducing conditions strongly indicates a good cycling ability of the material. The increase in the oxygen content during heat treatment in oxygen atmosphere took place below 500°C, and is therefore expected to originate from oxygen interstitials being implemented in the structure. A significant change in structure is also observed after heat treatment in oxygen atmosphere, emphasizing this theory. Indications of a p-type conductivity from a van der Pauw measurement is also reported. The new and more convenient powder preparation route, significantly increases the potential of rare earth ferrites for use in oxygen permeable membranes, and a further investigation into their reported multiferroic properties. A replacement of Lu with Yb in the material significantly decreases the material cost, and therefore the use of these materials in up-scale production.