Abstract text (incl. figure legends and references)
At present, niobium tungsten oxides are intensively studied as potential battery materials as their structural flexibility permits a reversible high-rate Li-ion exchange [1]. However, synthesis and characterization of phases in the pseudo-binary system Nb2O5-WO3 was a major topic in structural research already in the 1960s [2]. Remarkably, the versatile structural chemistry was explored by HRTEM investigations right from the onset of this method in the early 1970s [3].
Several phases with odd but well defined stoichiometry appear in the system Nb2O5-WO3. Their structures are based on frameworks of corner-sharing octahedra MO6 (M = Nb,W) [4]. The structure of Nb8W9O47 is closely related to that of the tetragonal tungsten bronzes (TTB) [5]: a part of the pentagonal tunnels (PTs) is filled by metal-oxygen strings in such a way that a threefold superstructure arises (Figure 1). This structure appears to be thermodynamically favored for the ratio oxygen:metal = 47:17, and any deviation from this ideal composition must somehow be compensated structurally. On the oxygen rich side of Nb8W9O47, two simultaneously occurring effects accommodate an increased O/M ratio (Figure 1) [6]: (i) formation of little ordered variants with less PTs occupied (e. g. Nb6W8O39); (ii) incorporation of single units of the neighboring phase Nb4W7O31, an intergrowth of the TTB and the ReO3 type as firstly determined by HRTEM [3]. The metal-rich side of Nb8W9O47 towards the block phases is scarcely investigated yet. The HAADF-STEM characterization of samples with composition Nb18W16O93 pointed to a denser occupation of the PTs: nano domains of units a yet unknow phase with composition Nb12W11O63 and of Nb8W9O47 appear [7]. The close relation of all these structures to the TTB-type substructure enables their intimate intergrowth as schematically shown in Figure 1 and demonstrated by the HRTEM image of a little ordered area shown in Figure 2.
The results discussed here confirm that the details of such a complex real structure as realized by niobium tungsten oxides can only be figured out by a direct visualization by high-resolution TEM or STEM [8].
Figure 1: Schemes and HAADF-STEM images of Nb-W oxides that are either superstructures of the tetragonal tungsten bronze (TTB) type (blue squares) or a coherent intergrowth of TTB and ReO3 type (orange square).
Figure 2: HRTEM image demonstrating the coherent intergrowth of Nb8W9O47 and Nb4W7O31 domains. The Fourier transform (inset) shows circular diffuse scattering besides the Bragg reflections of both phases.
References: [1] Griffith et al., Nature 559 (2018) 556. [2] Roth, Waring, J. Res. Natl Bur. Stand. 70A (1966) 281. [3] Iijima, Allpress, Acta Crystallogr. A 30 (1974) 22. [4] Krumeich, Chem. Mater. 34 (2022) 911. [5] Sleight, Acta Chem. Scand. 20 (1966) 1102. [6] Krumeich, Acta Crystallogr. B 54 (1998) 240. [7] Krumeich, Crystals 11 (2021) 1514. [8] Electron microscopy was done at ScopeM (ETH Zurich).