Fe~(4+) formation in brownmillerite CaAl_(0.5)Fe_(0.5)O_(2.5+delta)

摘要

A Mossbauer spectroscopy study of brownmillerite Ca_2FeAlO_(5+delta), in combination with the structure refinement from X-ray powder diffraction data, showed that hyperstoichiometric oxygen incorporation into the lattice is accompanied with Fe~(4+) formation in the perovskite4ike layers, which are built of oxygen-metal octahedra and preferably occupied by iron cations. This behavior differs from that of another layered ferrite, Sr_4Fe_6O_(13 + delta) where the point defects formed due to oxidation are primarily accumulated in non-perovskite layers. The incorporation of even minor amounts of extra oxygen in Ca_2FeAlO_(5 + delta) leads to decreasing water absorption by the lattice, probably due to blocking of oxygen vacancies located in the tetrahedral layers of brownmillerite phase.