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 Å), has been found. The oxidation products of Nb₇W₁₀O₄₇ and Nb₄W₁₃O₄₇ frequently comprise less-ordered arrangements of filled tunnels, causing diffuse scattering. In Nb₇W₁₀O_47.5 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* 0.33a* indicates the predominance of 4:9-type domains, whereas r* 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.