Syntheses, structure, and a Mössbauer and magnetic study of Ba ₄Fe₂I₅S₄

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. Ba₄Fe ₂I₅S₄ 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 by _∞¹[Fe ₂S₄] chains. Each Fe atom, which is located on a site with 4 symmetry, is tetrahedrally coordinated to four S atoms. The FeS₄ tetrahedra edge-share to form linear _∞¹[Fe ₂S₄] chains 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 derealization and an average Fe oxidation state of +2.5. The molar magnetic susceptibility of Ba₄-Fe₂I ₅S₄, 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.