Extension of the A nB nO3 n+2 Slicing/Oxidizing Process from 2D Layered to 1D Columnar Perovskites: The La7W7-xM xO30 Structural Family with Di-, Tri-, and Tetravalent Transition Metal M Substitutes to Tungsten.

Abstract

The A7B7O30 structural type is a columnar-perovskite-type arrangement originally evidenced in La7Mo7O30 [Goutenoire et al. J. Solid State Chem. 1999, 142, 228], an intermediate reduction product of fast oxide-ion conductor La2Mo2O9. Subsequently, this new structural family has been extended to La7W6+4M5+3O30 (M = Nb, Ta) [Goutenoire et al. J. Solid State Chem. 2005, 178, 2811]. Here we show that it is possible to further extend the solid solution through partial substitution of hexavalent tungsten by di-, tri-, or tetravalent transition metals. The general formula is La3+7W6+7- xM m+ xO30 with x = 3/(6 - m), or m = 6 - 3/ x. Single phase samples with M m+ = Zn2+, Fe3+, and Ti4+ have been prepared, and their crystal structure refined using X-ray powder diffraction. In the structure, the transition metal with lower valence is preferentially located in the octahedral site shared by consecutive trans-connected perovskite cages, whose diagonal runs along the threefold axis of the structure. Close similarities are noticed with the cationic repartition and distortion of coordination polyhedra in compounds belonging to the A nB nO3 n+2 series of layered perovskites. A rationale is proposed, which includes the A7B7O30 structural type as a subsequent 1D step to the 3D → 2D A nB nO3 n+2 scheme of perovskite framework slicing by progressive oxygen insertion through decreasing n.