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 . Four pentagonal tunnels are occupied by metal-oxygen
strings (black filled circles). A TTB subcell is colored yellow.
be regarded as the end member of a solid solution series Nb8-nW9+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 (TOX<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.
image of Nb4W13O47 along .
The insets show the structural model (framework of octahedra with occupied pentagonal columns marked by a filled circle) and a simulation
image of a microdomain of Nb6W8O39,
a novel TTB superstructure found in the oxidation product of
Nb7W10O47. The interpretation
(right side) 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 on the left side).
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 . Unit cells are framed; some W-rich positions marked
and Disorder in Niobium Tungsten Oxides of the Tetragonal
F. Krumeich, Acta Crystallogr. B 1998, 54,
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