| 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 NbO x 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, Nb20 5• Two allotropic modifications
have been observed in Nb20 5, name~v, a low-temperature o{-jorm which transforms
irreversibly to the high-temperature {3-form at about 850°C. f3-Nb 20 5, once
formed, becomes stable throughout and hence is the equilibrium phase at all temperatures
above room temperature. |
