On the structural complexity of tetragonal tungsten bronze type niobium tungsten oxides
Michael Wörle, Frank Krumeich
Abstract
Abstract 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 Nb 8 W 9 O 47 and Nb 7 W 10 O 47.5 using the structural models of Nb 6.7 W 10.3 O 47 and Nb 8 W 9 O 47 both obtained from X‐ray crystal structure analysis. HRTEM investigations confirm that Nb 8 W 9 O 47 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 Nb 7 W 10 O 47.5 with small domains of Nb 4 W 7 O 31 appearing besides large ones of Nb 8 W 9 O 47 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 “Nb 18 W 16 O 93 ”, a composition recently postulated for battery materials.