Transmission electron microscopic study of the defect structure in Sr 4Fe6O12+δ compounds with variable oxygen content

Sr4Fe6O12+δ (δ = 0.6, 0.8) compounds were prepared by a solid-state reaction in evacuated sealed silica tubes. The structure of these materials was studied by X-ray powder diffraction, electron diffraction, and high-resolution electron microscopy. The materials are characterized as a complicated superstructure appearing at three different levels:  (a) modulations with q = αa* due to changing δ, so that δ = 2α; (b) cooperative atomic displacements in the Fe2O2+δ layers resulting in the formation of distorted tetragonal pyramids and distorted trigonal bipyramids around the Fe cations; and (c) different stackings of neighboring Fe2O2+δ layers. Symmetry changes occurring at each of these three levels are discussed. A comparison is made between the Sr4Fe6O12+δ structures in bulk and in thin film.