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Abstract
The oxidation of Nb7W10O47 at relatively low temperature TOX = 500 °C leads to the formation of domains of the Nb8W9O47 structure between which coherently embedded planar defects appear. The basic Nb8W9O47 structure was identified by powder X-ray diffraction (XRD) and subsequent Rietveld refinement. The structure of the planar defects building up the boundaries was determined by scanning transmission electron microscopy (STEM) images recorded with a high-angle annular dark field (HAADF) detector and an annular bright field (ABF) detector. We compare STEM images of the well-known Nb8W9O47 structure with images of the oxidation product of Nb7W10O47 and show that domains of Nb8W9O47 are also present while additional coherently embedded planar defects appear. HAADF-STEM images reveal that these defects often comprise arrays of three hexagons of octahedra that are locally arranged in a one-dimensional periodic way. ABF-STEM images reveal that two pairs of octahedra in one of the hexagons are actually connected by edge-sharing instead of the usual and hitherto assumed corner-sharing.
Highlights
A multitude of phases and structural variants occurs in the pseudobinary system Nb2O5-WO3 [1], which have recently gained broad attention because of their possible applications for Li storage [2,3,4] and as thermoelectric materials [5,6]
Scanning transmission electron microscopy with a high-angle annular dark field detector (HAADF-scanning transmission electron microscopy (STEM)) represents a good alternative as the positions of the metal atoms are recognizable in projection but in addition some information about the occupancy of positions by Nb and W can be retrieved from their brightness [13,14,15]
The oxidation product of Nb7W10O47 obtained at low temperature is still related the threefold tungsten bronzes (TTB) superstructure of the starting phase as evidenced by the presence of the corresponding reflections in the selected area electron diffraction (SAED) pattern as well as in the X-ray powder diffractogram (Fig. 2a)
Summary
A multitude of phases and structural variants occurs in the pseudobinary system Nb2O5-WO3 [1], which have recently gained broad attention because of their possible applications for Li storage [2,3,4] and as thermoelectric materials [5,6]. The connection of octahedra by sharing corners and edges forms various frameworks covering the range of the oxygen:metal ratio O/ΣM between 2.5 and 3.0: block structures (2.5 O/ΣM
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