Oxygen nonstoichiometry in Sr4Fe6O13-δ:: The derivatives [Sr8Fe12O26]•[Sr2Fe3O6]n

Different samples of the ferrite "Sr4Fe6O13", synthesized in air, nitrogen, and argon, have been studied by high-resolution electron microscopy. It is shown that this oxide is oxygen-deficient, i.e., Sr4Fe6O13-delta, and that its structure is modulated. The electron diffraction study shows that the modulation vector is p $(a) over bar $* + q $(c) over bar $*, with p = (1 - delta)/2 varying in the range [0.4-0.5] and q close to 1. It is shown that the component of the modulation vector along (c) over right arrow induces complex modulated structures. It is also demonstrated that oxygen nonstoichiometry corresponds to the generation of microphases [Sr8Fe12O26] [Sr2Fe3O6](n), whose first integral member n = 1, which corresponds to p = 0.4, is isolated here for the first time by synthesis under argon. Structural models involving intergrowths of single perovskite [Sr2Fe2O6](infinity) layers with complex [Sr2Fe4O7-delta)](infinity) layers involving two kinds of pyramidal ribbons, FeO5 trigonal bi-pyramids and FeO5 distorted tetragonal pyramids, explain the oxygen deficiency.