Hexagonal Perovskite-Type Phases in the BaO-Rich Part of the BaO–WO3–Nb2O5System

Abstract

BaO-rich phases with the nominal compositions Ba6NbxW3−xO15−x/2 (x=1, 2), 9L-Ba9Nb2W4O26, 12-layered (12L)-Ba4Nb2WO12, and 27L-Ba9Nb6WO27 were synthesized as polycrystalline powders using solid-state techniques and characterized by X-ray powder diffractometry and transmission electron microscopy. Of these powders only the 5L2-Ba6NbxW3−xO15−x/2 (x=2) and 9L-Ba9Nb2W4O26 were obtained as single-phase materials. The formation of Ba6NbxW3−xO15−x/2 (x=1, 2) started with the occurrence of binary compounds. Thus, 5L1-Ba6NbxW3−xO15−x/2 (x=1) appeared over binary Ba4Nb2O9 in contrast to 5L2-Ba6NbxW3−xO15−x/2 (x=2), which formed through a reaction between Ba5Nb4O15 and other barium–tungsten oxides. 5L2-Ba6NbxW3−xO15−x/2 (x=2) appeared in two different polymorphs structurally related to the hexagonal Ba6Ta2WO14 and the orthorhombic β-Ba4Nb2O9. The structural refinement of the high-temperature 5L2-Ba6NbxW3−xO15−x/2 (x=2) was carried out using the space group P3m1. The fitted parameters of the hexagonal unit cell corresponded to aH=6.030(1) A and cH=12.44(1) A. Additional reflections of variable intensity and strong, diffuse scattering in the electron diffraction patterns indicated a strong structural disorder. The formation of 9L-Ba9Nb2W4O26, 12L-Ba4Nb2WO12, and 27L-Ba9Nb6WO27 started with the reaction between the binary compounds and continued over a series of intermediate hexagonal perovskite-type phases lying on the Ba3W2O9–Ba5Nb4O15 tie line.