Crystal structure of Cs₂GdNb₆Cl₁₅O₃ in the structural evolution of niobium oxychlorides with octahedral Nb₆-cluster units

Cs2GdNb6Cl15O3, dicaesium gadolinium hexaniobium pentadecachloride trioxide, was synthesized by solid-state reactions starting from a stoichiometric mixture of CsCl, Gd2O3, Nb, NbCl5, and Nb2O5. The crystal structure is based on octahedral Nb-6 cluster units (point group symmetry 3.2) with composition [(Nb(6)Cli(9)Oi(3))Cla(6)](5-) where i and a denote inner and outer ligands. Cs2GdNb6Cl15O3 exhibits 14 valence electrons per cluster unit. The cluster units are linked to each other by Cs-I and Gd-III atoms, whereby Cs-I (site symmetry 3..) is 12-coordinated by six Cl-i and six Cl-a ligands belonging to six neighboring cluster units and GdIII (site symmetry 3.2) is 9-coordinated by three O-i and six Cl-i ligands belonging to three adjacent cluster units. The arrangement of cluster units corresponds to a stacking of ... AA'A... layers along [001]. Cs2GdNb6Cl15O3 is isotypic with Cs2UNb6Cl15O3.