Contrasting Magnetic Structures in the Quaternary Sulfides BaFeMS (M = Sb, Bi).

BaFeSbS and BaFeBiS are two isostructural quaternary sulfides that crystallize in the space group with four formula units per unit cell. BaFeSbS has lattice parameters = 12.08609(3) Å, = 8.83426(2) Å, and = 8.89114(2) Å, and BaFeBiS has = 12.09610(3) Å, = 8.89281(2) Å, and = 8.82437(2) Å at room temperature. They comprise infinite [FeMS] (M = Sb, Bi) chains, where Fe is present in FeS tetrahedra and M ions reside in edge-sharing MS distorted octahedra, each of which shares an edge with an FeS tetrahedron. Powder neutron diffraction measurements confirm the presence of long-range antiferromagnetic order of the Fe moments in both materials, where Fe-S···S-Fe super-superexchange interactions which act along the direction of the [FeMS] (M = Sb, Bi) chains are the driving force for this antiferromagnetic order. The magnetic Bragg reflections reside on a -vector with = (1/2 0 1/2), and the relative orientations of the moments are similar in the two cases One significant difference is that the moments are aligned along the crystallographic -axis in BaFeSbS, whereas in BaFeBiS they lie along the longer crystallographic -axis, reflecting the weak directional preference of the Fe moments. Furthermore, an additional incommensurately modulated ordering of the Fe moments is suggested for BaFeSbS (but not BaFeBiS) by the appearance of small additional magnetic Bragg peaks in the neutron diffraction data, which may be a consequence of greater magnetic frustration in BaFeSbS.