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

Contributions to the Investigation of Inorganic Non-stoichiometric Compounds. XX. Metastable Oxidation of a Series of Solid Solutions — an Access to W-rich Block Structures in the System Nb₂O₅/WO₃ The characteristical region of existence of block structures with building elements that are limited in size in two directions ends in the system Nb₂O₅/WO₃, as was shown by previous investigations, under conditions of equilibrium at a maximum value of 2.654 O/ΣM. For the occurring phases with the ratios Nb₂O₅: WO₃ = 6:1, = 7:3, = 8:5 and = 9:8 as well we now were successful in substituting W for Nb. The original block structure and the corresponding ratio O/ΣM were preserved. The “9:8”-phase W_4/4[Nb₁₈W₇O₆₉], for example, forms solid solutions W_4/4[Nb₁₁W₁₄O₆₉] leaving the size of the building elements ([5 times; 5] blocks) unchanged. Hereby the ratio W/Nb is drastically enhanced from 0.444 to 1.364. By metastable oxidation of these solid solutions at temperatures of about 500°C, for instance in air, one comes back to the system Nb₂O₅/WO₃. In this way the region of existence of block structures could be expanded far beyond the limit at 2.654 O/ΣM to higher W/Nb values.