Studies on Niobium Oxides and Polymorphism of Niobium Pentoxide

The oxidation of pure niobium powder in air has been investigated. It is observed that after the initial dissolution of a small percentage of oxygen in the niobium lattice, oxidation commences with the formation of lower-oxides of niobium. The existence of a sub-oxide ascribed as NbOx is detected below 350°C having a structure closely resembling that of the pure metal. The other known lower oxides of niobium, it is thought, exist only as relatively thin layers and therefore cannot be detected by X-ray. At about 400°C, the sub-oxide is found to decompose and further oxidation above this temperature leads to the formation of the pentoxide, Nb2O5. Two allotropic modifications have been observed in Nb2O5 namely, a low-temperature -form which transforms irreversibly to the high-temperature β-form at about 850°C. β-Nb2O5, once formed, becomes stable throughout and hence is the equilibrium phase at all temperatures above room temperature.