Contrasting Magnetic Structures in the Quaternary Sulfides Ba₂FeMS₅ (M = Sb, Bi)

Ba₂FeSbS₅ and Ba₂FeBiS₅ are two isostructural quaternary sulfides that crystallize in the Pnma space group with four formula units per unit cell. Ba₂FeSbS₅ has lattice parameters a = 12.08609(3) Å, b = 8.83426(2) Å, and c = 8.89114(2) Å, and Ba₂FeBiS₅ has a = 12.09610(3) Å, b = 8.89281(2) Å, and c = 8.82437(2) Å at room temperature. They comprise infinite [FeMS₅]^4- (M = Sb, Bi) chains, where Fe³⁺ is present in FeS₄ tetrahedra and M³⁺ ions reside in edge-sharing MS₆ distorted octahedra, each of which shares an edge with an FeS₄ tetrahedron. Powder neutron diffraction measurements confirm the presence of long-range antiferromagnetic order of the Fe³⁺ moments in both materials, where Fe-S···S-Fe super-superexchange interactions which act along the direction of the [FeMS₅]^4- (M = Sb, Bi) chains are the driving force for this antiferromagnetic order. The magnetic Bragg reflections reside on a k-vector with k = (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 b-axis in Ba₂FeSbS₅, whereas in Ba₂FeBiS₅ they lie along the longer crystallographic a-axis, reflecting the weak directional preference of the Fe³⁺ moments. Furthermore, an additional incommensurately modulated ordering of the Fe³⁺ moments is suggested for Ba₂FeSbS₅ (but not Ba₂FeBiS₅) by the appearance of small additional magnetic Bragg peaks in the neutron diffraction data, which may be a consequence of greater magnetic frustration in Ba₂FeSbS₅.