Cation Deficient Layered Ruddlesden−Popper-Related Oxysulfides La₂LnMS₂O₅ (Ln = La, Y; M = Nb, Ta)

The structures of the new oxysulfide Ruddlesden−Popper phases La₂LnMS₂O₅ (Ln = La, Y; M = Nb, Ta) are reported together with an iodide-containing variant:  La_3 - xNb_1 + xS₂O₅I_2x (0 ≤ x < 0.11). Structures were refined against powder-neutron or single-crystal X-ray diffraction data. All of these compounds exhibit an intergrowth structure with NaCl-type slabs [La₂S₂] alternating regularly with perovskite-type oxide slabs [LnMO₅] or [La_1 - xNb_1 + xO₅I_2x]. In the oxide slabs, the trivalent and pentavalent cations are disordered on the long-length scale probed by diffraction methods, but bond length considerations suggest that they must be ordered at least on the length scale of the unit cell. The [LnMO₅] block of the iodide-free compounds derive from the ideal [Ti₂O₅] blocks found in Ln₂Ti₂S₂O₅ (Ln = Nd−Er; Y) by the formal substitution of two Ti⁴⁺ ions with one Ln³⁺ and one M⁵⁺ion. The unusual partial insertion of iodide in the perovskite voids of the [LaNbO₅] block in La₃NbS₂O₅ was found to be coupled to a La/Nb substitution, maintaining the charge balance within the [La_1 - xNb_1 + xO₅I_2x]^2- block. The Nb⁵⁺ ions were found to be too resistant to reduction to undergo the intercalation of alkali metals observed in the Ln₂Ti₂S₂O₅ series.