Cerium effect on the phase structure, phase stability and redox properties of Ce-doped strontium ferrates

Nanostructured perovskite-type Sr1−aCeaFeO3−x, (0⩽a<0.15) powders have been prepared by citrate–nitrate smoldering auto-combustion. Their phase structure and stability, surface and morphological properties, reduction behavior and interaction with oxygen have been investigated by X-ray Powder Diffraction combined with Rietveld Analysis, 57Fe Mössbauer and X-ray Photoelectron Spectroscopies, N2-adsorption method, Temperature Programmed Reduction and Oxidation experiments. Our results reveal that citrate–nitrate auto-combustion method is effective in obtaining single phase Sr1−aCeaFeO3−x. The Sr1−aCeaFeO3−x structure is cubic only for a⩾0.06, while for a<0.06 remains tetragonal. Moreover, for a⩾0.06 after semi-reductive treatment under inert gas, an expanded cubic phase is obtained instead of the brownmillerite-type structure, which is known to have ordered vacancies. Stabilization of octahedral Fe3+ by cerium doping appears to be the main factor in determining the structural properties of Sr1−aCeaFeO3−x. The highest oxygen consumption for Ce-doped SrFeO3 occurs for a=0.06. Preliminary impedance measurements show that Sr0.94Ce0.06FeO3−x has the lowest area-specific resistance.

Fig. 2a. The Ce-doped SrFeO3 structure changes with cerium content: undoped SrFeO3 is tetragonal (I/4mmmI/4mmm), 2 mol%-doped SrFeO3 is still tetragonal with higher symmetry (P/4mmmP/4mmm), whereas from 6 to 15 mol% the structure becomes pure cubic. The cubic (or pseudo-cubic) reticular constant increases with cerium content.Figure optionsDownload as PowerPoint slide