Bronze-type Niobium Tungsten Oxide

Nb₈W₉O₄₇ can be regarded as the end member of a solid solution series Nb_8-nW_9+nO₄₇ (n=0-5) in which Nb⁵⁺ is formally substituted by Nb⁴⁺ (or W⁴⁺) and W⁶⁺. The amount of reduced cations in these phases can be oxidized, and that provides a novel access to the fully oxidized system Nb₂O₅/WO₃. Remarkably, the oxidation on air at varying temperatures leads to different products. Of particular interest is the oxidation at low temperatures (T_OX<1000°C) that results in novel metastable structures, among them a TTB superstructure observed here for the first time (see figure below). Different variants of diffuse scattering that appear in the electron diffraction patterns are associated with little ordered occupations of the pentagonal tunnels. For the characterisation of the oxidation products, high-resolution transmission electron microscopy (HRTEM) is an indispensable tool: in projection along the short axis, the structure can be determined directly since the positions of the cations can be recognized as dark dots.

Recently, scanning transmission electron microscopy with a high-angle annular dark field detector (HAADF-STEM) has been established as a good alternative since not only the metal atoms are imaged in projection but the intensity reveals also some information about the occupation (Z contrast imaging). In addition, annular bright field (ABF) STEM images reveal the oxygen positions as tiny black dots besides the metal ones (large black dots).