Hexagonal Perovskite-Type Phases in the BaO-Rich Part of the BaO-WO_3-Nb_2O_5 System

摘要

BaO-rich phases with the nominal compositions Ba_6Nb_xW_(3-x) O_(15-x/2)(x=1, 2), 9L-Ba_9Nb_2W_4O_(26), 12-layered (12L)-Ba_4Nb_2WO_(12), and 27L-Ba_9Nb_6WO_(27) 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-Ba_6Nb_xW_(3-x)O_(15-x/2)(x = 2) and 9L-Ba_9Nb_2W_4O_(26)were obtained as single-phase materials. The formation of Ba_6Nb_xW_(3-x)O_(15-x/2) (x = 1, 2) started with the occurrence of binary compounds. Thus, 5L1-Ba_6Nb_xW_(3-x)O_(15-x/2)(x = 1) appeared over binary Ba_4Nb_2O_9 in contrast to 5L2-Ba_6Nb_xW_(3-x)O_(15-x/2) (x = 2), which formed through a reaction between Ba_5Nb_4O_(15) and other barium-tungsten oxides. 5L2-Ba_6Nb_xW_(3-x)O_(15-x/2) (x = 2) appeared in two different poly-morphs structurally related to the hexagonal Ba_6Ta_2WO_(14) and the orthorhombic β-Ba_4Nb_2O_9. The structural refinement of the high-temperature 5L2-Ba_6Nb_xW_(3-x)O_(15-x/2)(x = 2) was carried out using the space group P3m1. The fitted parameters of the hexagonal unit cell corresponded to a_H = 6.030(1) A and c_H = 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-Ba_9Nb_2W_4O_(26), 12L-Ba_4Nb_2WO_(12), and 27L-Ba_9Nb_6WO_(27) started with the reaction between the binary compounds and continued over a series of intermediate hexagonal perovskite-type phases lying on the Ba_3W_2O_9-Ba_5Nb_4O_(15) tie line.