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: La3 - xNb1 + xS2O5I2x (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 [La1 - xNb1 + xO5I2x]. 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 [La1 - xNb1 + xO5I2x]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.