Disordered commensurate structure of the 2212-related phase Fe2(Bi0.69Sr2.31)Fe2O9.5±1/2δ and structural mechanism

The structure of the Fe2(Bi0.69Sr2.31)Fe2O9.5−1/2δ has been carried out by single crystal X-ray diffraction. Its structure can be considered as commensurately modulated with a basic structure of Fmmm   symmetry and with a modulation vector q⇒=αa⇒*+γc⇒* whose α and γ   values are, respectively, equal to 1/3 and 1. The ferrite is described as a three-fold superstructure with a≈3ap2, b≈ap2 and c≈26Å (space group Bbmb). It is described using a disorder model in agreement with diffuse scattering observations. The structure is built up from the intergrowth sequence between [(Bi, Sr)Fe2O3.5±1/2δ]∞ complex layers, related to the rock salt (RS)-type blocks, and double perovskite layers. Large analogies are found between the present structure and the so-called 2212 modulated structures related to the high Tc superconductor copper oxides or to the corresponding Fe or Co oxides; they are discussed here. A statistical disorder characterizes the [Fe2O2.5±1/2δ]∞ double layer. Taking advantage of similar structural arrangements in related oxide compounds, a structural model is proposed. A description of this double layer is then given in terms of sequence of polyhedra (tetragonal pyramids, mono-caped tetrahedra and trigonal bi-pyramids), in comparison with other related modulated structures with different periodicities. The powder sample exhibits a statistical distribution of α around an average value of 0.35; the oxygen non-stoichiometry appears as the driving force for the α value, i.e. for the modulation, commensurate or incommensurate. The Mössbauer study has been performed and interpreted on the basis of the refined structure.

Schematical representation of the structure projected along b. The average periodicity of the Fe and O atoms in the rock salt-type type layers is shown.Figure optionsDownload as PowerPoint slide