Beiträge zur Untersuchung anorganischer nichtstöchiometrischer Verbindungen. XX. Metastabile Oxydation von Mischkristallreihen — ein Zugang zu W‐reichen Blockstrukturen im System Nb₂O₅/WO₃

Abstract

Das charakteristische Existenzgebiet der Blockstrukturen mit zweidimensional begrenzten Bauelementen endet im System Nb2O5/WO3, wie bereits frühere Untersuchungen gezeigt haben, unter Gleichgewichtsbedingungen bei einem Maximalwert von 2,654 O/O/ΣM (M  Nb, W). Es gelang nun, in den mit den Verhältnissen Nb2O5: WO3 = 6:1, 7:3, 8:5 sowie 9:8 auftretenden Phasen im weiten Umfang Nb durch W zu substituieren, wobei ihre ursprüngliche Blockstruktur und das jeweilige Verhältnis O/ΣM erhalten blieben. So bildet z. B. die «9:8»‐Phase W4/4[Nb18W7O69] bei unveränderter Größe der Bauelemente ([5 × 5]‐Blöcke) einen Mischkristall W4/4[Nb11W14O69]; das Verhältnis W/Nb wird damit drastisch von 0,444 auf 1,364 erhöht. Durch metastabile Oxydation der so erhaltenen Mischkristalle bei Temperaturen um 500°C, z. B. an der Luft, gelangt man in das System Nb2O5/WO3 zurück. Das Existenzgebiet der Blockstrukturen konnte in dieser Weise bis weit über die Grenze bei 2,654 O/ΣM hinaus zu höheren W/Nb‐Werten erweitert werden.

The Complex Crystal Chemistry of Niobium Tungsten Oxides

“…An example is Nb 12 O 29 (Nb 2 4+ Nb 10 5+ O 29 ), 92,93 in which the two Nb 4+ can be substituted by Ti 4+ , resulting in Ti 2 Nb 10 O 29 . 94 Similar substitution experiments are possible for Nb−W oxides: in many cases, Nb 5+ can be substituted by equal amounts of M 4+ and W 6+ with a preservation of the ratio O/ΣM and the corresponding 95 For instance, the Nb in Nb 18 W 8 O 69 could partly be substituted by W according to 2Nb 5+ = Nb 4+ + W 6+ . In the obtained solid solution series Nb 18−n W 8+n O 69 , the ratio W/Nb was increased from 0.444 to 1.364 while the ratio O/ΣM was kept constant and the structure consisting of [5 × 5] blocks preserved.…”

“…In the obtained solid solution series Nb 18−n W 8+n O 69 , the ratio W/Nb was increased from 0.444 to 1.364 while the ratio O/ΣM was kept constant and the structure consisting of [5 × 5] blocks preserved. 95 In all cases, the samples were phase-pure and highly crystalline according to XRD powder patterns. The molar volume slightly decreased with increasing W content.…”

“…The thermodynamically stable phases were obtained at oxidation temperatures above 800 °C, whereas at lower temperatures (here: T OX = 500 °C) metastable products arose. 95 If the amount of substituted metal was low, the XRD powder pattern still showed the characteristic reflections of the starting phase, while for high amounts, broad reflections and diffuse intensity were observed, which nonetheless provided evidence of the preservation of block structures despite the presence of disorder. By such metastable oxidation reactions, the existence range of block structures could be enlarged compared to the conventional synthesis from the binary oxides.…”

“…The molar volume slightly decreased with increasing W content. 95 The range of these solid solution series in the direction toward higher amounts of reduced cations could be extended by chemical transport experiments with HgCl 2 as the transport agent performed either in a temperature gradient or isothermally (dashed blue lines in Figure 11). 18 Block phases with other cations crystallizing isostructural to Nb−W oxides are rare and as yet known with molybdenum only.…”

“…In 1983, solid solution of the four basic block structures Nb 12 WO 33 , Nb 14 W 3 O 44 , Nb 16 W 5 O 55 , and Nb 18 W 8 O 69 were prepared by heating mixtures of NbO 2 , Nb 2 O 5 , and WO 3 in appropriate ratios at above 1300 °C and subsequent quenching to room temperature . For instance, the Nb in Nb 18 W 8 O 69 could partly be substituted by W according to 2Nb 5+ = Nb 4+ + W 6+ .…”