On the structural complexity of tetragonal tungsten bronze type niobium tungsten oxides

In this comparative study, two samples with a slightly different oxygen : metal ratio have been characterized by various electron microscopy methods and by single-crystal and powder X-ray diffraction (XRD). A reasonably good fit to the experimental powder XRD patterns could be achieved by Rietveld refinement for both samples Nb8W9O47 and Nb7W10O47.5 using the structural models of Nb6.7W10.3O47 and Nb8W9O47 both obtained from X-ray crystal structure analysis. HRTEM investigations confirm that Nb8W9O47 crystallizes as a pure phase in the well-known threefold superstructure of the tetragonal tungsten bronze type (TTB). In contrast, a more complex real structure was found for the sample Nb7W10O47.5 with small domains of Nb4W7O31 appearing besides large ones of Nb8W9O47 and in addition extended TTB-based areas with short-range order only. The diffuse scattering caused by this disorder is easily detected in the electron diffraction pattern but does not affect the Rietveld refinement of the powder XRD patterns. However, on the basis of powder diffraction experiments, it is clearly possible to distinguish between the above compounds and the hypothetical, but non-existing compound Nb18W16O93, a composition recently postulated for battery materials.

Automated summary

This study characterized two samples with slightly different oxygen : metal ratios using various electron microscopy methods and X-ray diffraction. The Rietveld refinement showed good fit for both samples, revealing structural differences between Nb8W9O47 and Nb7W10O47.5. The complex real structure of Nb7W10O47.5 was distinct from the pure phase crystal structure of Nb8W9O47.