Structures des Oxydes de Niobium

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Abstract
The formation of the niobium oxides by heating thin films and the powder of Nb in various conditions has been followed by electron and X-ray diffractions. Numerous forms of Nb oxide are identified: Nb-(O) (solid solution), NbO x (tetragonal), Nb-O δ-form (hexagonal), γ-Nb2O5 (monoclinic) and α-Nb2O5 (monoclinic) are formed by heating in air at either atmospheric or reduced pressure; while, NbO z (tetragonal), NbO (cubic) and NbO2 (tetragonal) are formed only in air of reduced pressure. The transformation relations between these oxides are made clear. At first, the oxidation of Nb occurs through penetration of oxygen atoms into the metallic lattice of Nb. Then the structures of the oxides transform successively by rearrangement of niobium and the oxygen atoms. The transformation δ→γ can take place through a relatively small change in the atomic distribution, while the transformation γ→α occurs through a remarkable rearrangement of both kinds of atoms, passing two distinct intermediate stages. The δ-form, which has been considered hitherto to be a poorly crystallized state of the γ-Nb2O5, may be an oxide inferior to NbO2. The tetragonal form NbO2 has a super lattice derived from the rutile structure.

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