X-ray diffraction analysis of the modulated structure of Bi2Sr2CaFe2O y

The modulated structure of the Bi₂Sr₂CaFe₂O _y compound isomorphous with the Bi₂Sr₂CaCu₂O _y phase was refined by the method of the full profile analysis on the basis of the powder X-ray diffraction data. The analysis was carried out in the superspace group ofN:Abmm: 1 – 11 using the parameters of the unit cell of the basic structurea = 5.466(3),b = 5.446(5) andc = 31.25(9) Å and the modulation vectorq = 0.220(5)b* +c*. The model of excess oxygen interstitials into the BiO plane was used by applying a saw-tooth-type function of the form:U = 2U ₀ (X ₄ -X ₄ ⁰ /)] 1]. The results point to a strong displacement of the atoms from their positions in the basic structure. An essential improvement of theR factors was obtained by taking into account the modulation waves of the higher harmonics.