Structure and morphology of niobium suboxides

Abstract

The first stages of oxidation of niobium (NbOx) have been studied by electon microscopy. Two phases, called NbOxI and II were detected. A study of NbOz will be published elsewhere. Niobium has been oxidized by a wet argon flow on thin foils or powders, heated at temperatures from 350 up to 400°C. NbOxI, which has the composition Nb8O to Nb6.5O, is tetragonal: a = b = 3.38 Å and c = 3.27 Å. Superstructure spots appear in electron diffraction which are not simple fractions of the basic reciprocal lattice. This superstructure is explained by a particular occupation of interstitial sites of niobium. The NbOxII phase, corresponding to the composition Nb6O, is tetragonal too, with a = b = 3.34 Å and c = 3.278 Å. It is characterized by a superstructure a′ = 4 d110Nb, b′ = 4 d110Nb, c′ = 3 d001Nb (the small deformations being neglected). The structure of NbOxII has been determined.