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Syntheses, Structure, and a Mössbauer and Magnetic Study of Ba4Fe2I5S4

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posted on 07.01.2008, 00:00 by Danielle L. Gray, Gary J. Long, Fernande Grandjean, Raphaël P. Hermann, James A. Ibers
The compound Ba4Fe2I5S4 has been prepared at 1223−1123 K by the “U-assisted” reaction of FeS, BaS, S, and U with BaI2 as a flux. A more rational synthesis was also found; however, the presence of U appears to be essential for the formation of single crystals suitable for X-ray diffraction studies. Ba4Fe2I5S4 crystallizes in a new structure type with two formula units in space group I4/m of the tetragonal system. The structure consists of a Ba−I network penetrated bychains. Each Fe atom, which is located on a site with 4̄ symmetry, is tetrahedrally coordinated to four S atoms. The FeS4 tetrahedra edge-share to form linearchains in the [001] direction. The Fe−Fe interatomic distance in these chains is 2.5630(4) Å, only about 3% longer than the shortest Fe−Fe distance in α-Fe metal. Charge balance dictates that the average formal oxidation state of Fe in these chains is +2.5. The Mössbauer spectra obtained at 85 and 270 K comprise a single quadrupole doublet that has hyperfine parameters consistent with an average Fe oxidation state of +2.5. The Mössbauer spectrum obtained at 4.2 K consists of a single magnetic sextet with a small hyperfine field of −15.5 T. This spectrum is also consistent with rapid electron delocalization and an average Fe oxidation state of +2.5. The molar magnetic susceptibility of Ba4Fe2I5S4, obtained between 3.4 and 300 K, qualitatively indicates the presence of weak pseudo-one-dimensional ferromagnetic exchange within a linear chain above 100 K and weak three-dimensional ordering between the chains at lower temperatures.