Reduction in a niobium tungsten bronze, 2Nb₂O₅.7WO₃, studied by 1 MV HRTEM

1 MV high-resolution transmission electron microscopy (HRTEM) reveals that the reduction in a niobium tungsten bronze, 2Nb₂O₅.7WO₃, is initiated by the release of a small amount of oxygen from the -M-O-M-O- strings in the pentagonal tunnels. Its content can be measured from the weight change. At the second stage of reduction the ordered structure of 2Nb₂O₅.7WO₃ is destroyed, i.e. the ReO₃-type structure at the center of the unit cell disappears. As a result an apparently disordered matrix appears, in which isolated unit cells of 2Nb₂O₅.7WO₃ and 4Nb₂O₅.9WO₃ are irregularly formed. At the third stage the microdomains of 4Nb₂O₅.9WO₃ appear and grow. The presence of oxygen vacancies in the strings at each stage is revealed by the technique utilizing the knock-on of atoms by the impact of electrons. The number of knocked-on atoms is analyzed with the help of computer simulation on image contrast. Circular diffuse scattering is excited from the disordered matrix and its origin is related to the clusters composed of nine tunnel sites.