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 image whose α and γ values are, respectively, equal to 1/3 and 1. The ferrite is described as a three-fold superstructure with image, image and image (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.