SYNTHESIS AND STRUCTURE OF A NEW LAYERED NIOBIUM BLUE BRONZE - RB2LANB2O7

The reduced layered niobate Rb2LaNb2O7 was synthesized via the reactio n of RbLaNb2O7 with rubidium vapor at temperatures between 200 and 250 degrees C. The structures of both materials have been determined by R ietveld analysis of time-of-flight powder neutron diffraction data. Th e cream-colored host compound RbLaNb2O7 (I) crystallizes in the space group Imma with cell constants a 5.4941(4) Angstrom A, b = 21.9901(6) Angstrom A, c = 5.4925(4) Angstrom A, and Z = 4. The blue-black reduce d phase Rb2LaNb2O7 (II) also adopts an orthorhombic structure (space g roup Cmcm) with a = 22.30955(9) Angstrom A, b = 5.69748(10) Angstrom A , c = 5.69365(10) Angstrom A, and Z = 4. Both compounds adopt pseudote tragonal orthorhombic structures based on the Ruddlesden-Popper struct ure type. In I, double perovskite layers are separated by a single rub idium ion and adjacent blocks adopt an eclipsed conformation. The niob ium adopts a strongly distorted octahedral coordination and is displac ed from the center of the octahedron. In II, the layers are separated by a pair of rubidium ions and adjacent blocks have the staggered conf ormation usually associated with the Ruddlesden-Popper structure. Cons iderable expansion within the layers is observed as a result of partia l reduction of Nb-V to Nb-IV, accompanied by a reduction in the distor tion of the NbO6 octahedra. The fractional expansion within the layers arising largely from electronic effects exceeds that perpendicular to the layers arising from steric effects.