A New Superstructure in Gamma-Ferric Oxide

Abstract

SINCE 1935 it has been generally accepted that gamma-ferric oxide has the structure of magnetite (spinel structure) with cation vacancies¹. The vacancies were assumed to be randomly distributed on all octahedral iron sites of magnetite. Magnetite contains 8 molecules of Fe₃O₄ in the face-centred cubic unit cell and has the space group Fd3m. Gamma-ferric oxide contains 10$\frac{2}{3}$ molecules of Fe₂O₃ in the unit cell. Due to the occurrence of a number of extra reflexions in the X-ray diagram of gamma-ferric oxide also occurring in the diagram of ordered lithium ferrite Fe₈[Li₄Fe₁₂]O₃₂, and to the fact that the material he used contained some water, P. B. Braun² of this Laboratory suggested that in these gamma-ferric oxide specimens the same superstructure occurs as in ordered lithium ferrite. The ideal composition for ordering would be Fe₈[H₄Fe₁₂]O₃₂. For compositions between Fe₈[H₄Fe₁₂]O₃₂ and Fe₈[(Fe${\text{Fe}}_{\text{1}\frac{\text{1}}{\text{3}}}$□$2\frac{2}{3}$)Fe₁₂]O₃₂ the same superstructure reflexions were observed. (The square brackets [ ] denote the cations on octahedral sites, the ordinary brackets ( ) those on the Li-sites in ordered LiFe₅O₈.)