A new light on the iron coordination in Sr4Fe6O13±δ: Super space formalism and structural mechanism

The incommensurate modulated structure of Sr4Fe6O13±δ refined in super space formalism, from single crystal X-ray diffraction data. The analysis of this structure shows that the geometry of the iron polyhedra, their respective positions and arrangement are fundamentally different from those previously described for this oxide. Three types of iron environment, namely trigonal bipyramids, tetragonal pyramids and monocaped tetrahedra are observed in the double iron oxygen layers. The peculiar role of a structural unit built up from double trigonal bipyramids is outlined and the notion of disorder is detailed. The structural mechanism of this modulation character of the double iron layer is analyzed, compared to other related phases built of one single perovskite layer inter-grown with a complex slice made of one modulated double layer sandwiched between two [AO]∞ layers.