Bronze-type Niobium Tungsten Oxide

In the pseudo-binary system Nb₂O₅/WO₃, the phase Nb₈W₉O₄₇ crystallizes in a threefold superstructure of the tetragonal tungsten bronze (TTB) type caused by the systematic occupation of 1/3 of the pentagonal channels with metal-oxygen strings (see figure).

Threefold TTB superstructure of Nb₈W₉O₄₇ along [010]. Four pentagonal tunnels are occupied by metal-oxygen strings (black filled circles). A TTB subcell is colored yellow.

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.

HRTEM image of Nb₄W₁₃O₄₇ along [001]. The insets show the structural model (framework of octahedra with occupied pentagonal columns marked by a filled circle) and a simulation (EMS program).

HRTEM image of a microdomain of Nb₆W₈O₃₉, a novel TTB superstructure found in the oxidation product of Nb₇W₁₀O₄₇. The interpretation (right) shows the distribution of filled pentagonal tunnels (white dots). Two different orientations of the unit cell (marked as frames) occur. Besides this twinning, disorder of the tunnel occupation causes diffuse scattering in the electron diffraction pattern (inset in the left image).

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).

HAADF-STEM (left) and ABF-STEM images of Nb₈W₉O₄₇ along [001]. Unit cells are framed. On the left, some bright dots that indicate W-rich positions are marked by arrows. On the right, a pentagonal of MO₆ octahedra is outlineded taking the faint dots corresponding to O positions into account.

Selected Publications

Order and Disorder in Niobium Tungsten Oxides of the Tetragonal Tungsten Bronze Type
F. Krumeich, Acta Crystallogr. B 1998, 54, 240-249 DOI

Intergrowth of Niobium Tungsten Oxides of the Tetragonal Tungsten Bronze Type
F. Krumeich, Z. Naturforsch. B 2020, 75, 913-919 DOI

On the Structural Complexity of Tetragonal Tungsten Bronze Type Niobium Tungsten Oxides
M. Wörle and F. Krumeich, Z. allg. anorg. Chem 2021, 647, 98-106 DOI

The Complex Crystal Chemistry of Niobium Tungsten Oxides (Review)
F. Krumeich, Chem. Mater. 2022, 34, 911-934 DOI

ETH Zürich | ETH chemistry department | ETH inorganic chemistry