Ordering Principles in Niobium Tungsten Oxides of the Tetragonal Tungsten Bronze Type

In the system Nb 2 O 5 /WO 3 , phases with structures related to the tetragonal tungsten bronze (TTB) type occur at three well‐defined compositions: Nb 2 WO 8 (two modifications), Nb 8 W 9 O 47 , and Nb 4 W 7 O 31 . Samples with a deviating ratio O/ΣM (M = Nb,W) form intricate intergrowths of Nb 8 W 9 O 47 with the other phases and with less‐ordered regions. Depending on the state of order, X‐ray powder diffraction patterns show the reflections of the threefold TTB superstructure of Nb 8 W 9 O 47 or only those of the basic TTB structure. In contrast, the evaluation of high‐resolution electron microscopy images (here: HAADF‐STEM) reveals the real structure. While in O‐rich samples, segregations of ReO 3 ‐type structure appear, the variation of composition in the Nb‐rich range is realized by a flexible but never completely random occupation of pentagonal tunnels in a perfectly ordered TTB substructure. The long searched‐for phase Nb 12 W 11 O 63 has not been obtained yet though a structure with this composition occurs in nanodomains.