Pb4Sr13Fe24O53: an original terrace structure, related to Sr4−xPbxFe6O13−δ

The Pb for Sr substitution in Sr4Fe6O13±δ evidenced a limited existence range of solid solution Sr4−xPbxFe6O13−δ, with x⩽0.7x⩽0.7. For higher lead content, an original phase has been discovered for the composition Pb4Sr13Fe24O53 with a terrace-like structure. The cell is monoclinic and the parameters refined from the powder X-ray diffraction data are aT=35.039(2) ÅaT=35.039(2) Å, bT=5.5897(2) ÅbT=5.5897(2) Å, cT=25.662(2) ÅcT=25.662(2) Å and β=98°102(2)β=98°102(2), space group Cc or C2/c. The reconstruction of the reciprocal space allowed describing the structure from a monoclinic sub-cell with different sets of modulation vectors. The high resolution electron microscopy showed that the new phase has a terrace-like structure corresponding to the formation of two types of ribbons parallel to the [302]sub direction. One ribbon is made of 2212-type bricks, 3/2a→sub large, and the second is made of 2201-type bricks, 2a→sub large. The new structure is therefore described as the intergrowth of 2201-type slices, m octahedra large, and 2212-type slices, n   octahedra large. It is therefore the m=4m=4 and n=3n=3 member of a large new structural family [Fe2(Sr2)FeO6.5±1/2δ]2201m[Fe2(Sr3−xA2+x)Fe2O9.5±1/2δ]2212n. The magnetic measurements evidence an anti-ferromagnetic-like magnetic transition at T≈730 KT≈730 K showing that the samples, as the Sr4−xPbxFe6O13−δ, ones are magnetic at room temperature.

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