Zur Präparation und Struktur gemischtvalenter Niob‐Wolframoxide der Zusammensetzung (Nb,W)₁₇O₄₇

Abstract

Durch formale Substitution von je zwei Nb5+ durch Nb4+ (bzw. W4+) und W6+ sind – ausgehend von Nb8W9O47 – gemischtvalente Niob‐Wolframoxide unterschiedlicher Nb/W‐Verhältnisse denkbar, von denen Zusammensetzungen Nb8‐nW9+nO47 mit n = 1 bis 5 durch isothermes Erhitzen (1 250°C) der entsprechenden Gemenge von NbO2 (bzw. WO2) mit Nb2O5 und WO3 phasenrein dargestellt werden konnten. Diese Produkte kristallisieren alle in der Struktur des Nb8W9O47 und sind somit als Mischphase (0 ≤ n ≤ 5) aufzufassen, wie die röntgenographische (Pulvermethode) und durchstrahlungs‐elektronenmikroskopische Untersuchung belegt. Präparate mit niedrigerem Nb‐Gehalt (n > 5) erhielten wir dagegen nicht mehr phasenrein.

In-situ Oxidation Products of Nb4W13O47: A High-Resolution Transmission Electron Microscopy Study

“…This destruction indicates that there are considerable energies involved in this reaction since the TTB framework is a very stable structure and has been largely preserved in the course of the in-situ oxidation of Nb 7 W 10 O 47 [12] [25]. While the reorganization of the TTB structure leads to the generation of domains of WO 3 , the other phase expected from the phase dia- (1250°C, 4 d) a mixture of the binary oxides in the corresponding molar ratio (NbO 2 : WO 3 = 4 : 13), as described elsewhere [4].…”

“…The oxidation of Nb 4 This separation indeed occurs during an oxidation in air at a temperature T OX = 1200°C whereas a variety of apparently non-equilibrium products were observed at lower temperatures T OX [6,7]. The in-situ oxidation of Nb 4 W 13 O 47 generated different structures.…”

“…Nb 4 W 13 O 47 (orthorhombic, a = 1.225, b = 3.659, c = 0.3897 nm [4]) crystallizes isostructural with Nb 8 W 9 O 47 ( fig. 1(a) [8]) in a threefold superstructure of the TTB type.…”

“…Rows of modified TTB units (figs. 4,6) form the boundary between WO 3 domains with different orientations (twinning plane {210} ReO 3 ). In these boundaries, TTB units are modified by substituting a triangle and a pentagon of octahedra by two squares of octahedra.…”

“…This method was recently used for the characterization of the structural changes caused by the oxidation (in air) of niobium tungsten oxides belonging to the solid solution series Nb 8±n W 9+n O 47 (0 < n £ 5) [4±7]. These phases are formally obtained by a successive substitution of n(Nb 5+ ) by n(Nb 4+ + W 6+ ) or n(W 4+ + W 6+ ) [4], starting with the fully oxidized phase Nb 8 W 9 O 47 [8]. The oxidation of these oxides, which contain different amounts of cations with lower valency, gives an access to the pseudobinary system Nb 2 O 5 /WO 3 .…”