Reduction and polymorphic transformation of B-Nb2O5

Abstract

The modification B-Nb₂O₅, theoretically the stable polymorph below ca. 940 K, undergoes no transformation to the high-temperature stable H-Nb₂O₅ under oxidising conditions, below ca. 1 220 K. Reducing conditions facilitate the transformation. At or below 1 020 K, B-Nb₂O₅ is reduced directly to NbO₂, without traversing any intermediate oxides; reaction is topologically controlled by relations between the structures. Under buffered oxygen fugacities, reduction and transformation take place simultaneously at and above ca. 1 100 K. Electron microscopy shows that the product is a non-equilibrium intergrowth of H-Nb₂O₅ with structural elements of the lower, mixed-valence, block-structure oxides. A new regular intergrowth structure, Nb₄₃O₁₀₇, has been detected as an extensive ordered domain and this, with an associated extended defect, suggests a mechanism for the formation of Wadsley defects and disordered intergrowths in the reduction of H-Nb₂O₅ at below 1 300 K. The transformation B-Nb₂O₅→H-Nb₂O₅ is discussed in terms of alternative reaction paths. Formation of block structures is nucleation-controlled, and appears to be catalysed by platinum.