Order and Disorder in Niobium Tungsten Oxides of the Tetragonal Tungsten Bronze Type

Two superstructures of the TTB type (tetragonal tungsten bronze) exist in the system Nb2O5/WO3. They are caused by an ordered arrangement of pentagonal tunnels filled with metal–oxygen strings. Besides the well known 4:9 phase (Nb8W9O47), which has a tripled b axis, evidence for a new orthorhombic structure, Nb6W8O39 (a = b = 27.3 A), has been found. The oxidation products of Nb7W10O47 and Nb4W13O47 frequently comprise less-ordered arrangements of filled tunnels, causing diffuse scattering. In Nb7W10O47.5 the electron diffraction patterns (along [001]) of many crystal fragments show circular diffuse scattering, which appears around the main reflections of the TTB substructure with two distinct radii (r*). r* \simeq 0.33a* indicates the predominance of 4:9-type domains, whereas r* \simeq 0.41a* corresponds to Nb6W8O39. In Nb4W13O49 the diffuse scattering pattern is cross-shaped; this is due to the presence of long slabs of diamond-linked pentagonal columns. Structure models for these particular states of order have been derived from high-resolution transmission electron microscopy images. Some general principles for the formation of the different arrangements are deduced. Apparently, two distinct types of link between the filled pentagonal tunnels are important features which lead to the stability of these structures.