Ordering of Interstitial Impurities in Niobium

The ordering phenomenon in niobium is studied by electron microscopy and by electron diffraction. The phenomenon is found in parts of niobium foils subjected to an annealing treatment in vacuum. Domains are formed which, on the basis of the present model, are due to the ordering of interstitial oxygen atoms. The insertion of the interstitials on a superlattice in the nobium matrix causes a deformation of the latter in a 〈110〉 direction. The domain structure results from the fact that several orientations are possible for the superlattice. Further evidence for this ordering is deduced from the presence of superlattice dislocations and of supplementary spots in the diffraction patterns. The superlattice is deduced from the diffraction data and a composition Nb₂O is attributed to this metallic oxide phase. The domains are revealed by fringe contrast at the boundaries and by the change in background intensity. Both contrast effects are associated with the slight deformation due to the ordering. A discussion is included of a second type of fringe pattern which is often observed in annealed niobium. Patterns of this type have previously been attributed to the presence of stacking faults; it is shown here that they are due to plate‐shaped precipitates of some oxide phase in the niobium matrix.