Synthesis and Structure of a New Layered Niobium Blue Bronze - Rb2lanb2o7

The reduced layered niobate Rb2LaNb2O7 was synthesized via the reaction of RbLaNb2O7 with rubidium vapor at temperatures between 200 and 250 degrees C. The structures of both materials have been determined by Rietveld analysis of time-of-flight powder neutron diffraction data. The 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 reduced phase Rb2LaNb2O7 (II) also adopts an orthorhombic structure (space group 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 pseudotetragonal orthorhombic structures based on the Ruddlesden-Popper structure type. In I, double perovskite layers are separated by a single rubidium ion and adjacent blocks adopt an eclipsed conformation. The niobium adopts a strongly distorted octahedral coordination and is displaced from the center of the octahedron. In II, the layers are separated by a pair of rubidium ions and adjacent blocks have the staggered conformation usually associated with the Ruddlesden-Popper structure. Considerable expansion within the layers is observed as a result of partial reduction of Nb-V to Nb-IV, accompanied by a reduction in the distortion 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.