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 A and c = 3.27 A. 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 A and c = 3.278 A. 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. Les auteurs ont etudie les premiers etats rencontree dans l'oxydation du niobium, et particulierement deux phases appelees NbOzI et II. L'etude de NbOz fera l'objet d'une publication prochaine. L'oxydation est obtenue en faisant passer un courant d'argon humide sur des feuilles minces ou des poudres portees a une temperature comprise entre 350 et 400°C. NbOxI une composition variant de Nb8O a Nb6,5O; le reseau de niobium est quadratique, a = b = 3,38 A, c = 3,27 A. Il est caracterise en diffraction electronique par la presence de reflexions satellites autour de chaque reflexion principale. Elles sont attribuees a une occupation particuliere de sites interstitiels du reseau de niobium par l'oxygene. La phase NbOxII est observee pour une composition Nb6O; le reseau de niobium est quadratique: a = b = 3,34 A, c = 3,278 A. Elle est caracterisee en diffraction electronique par une surstructure quadratique a′ = 4 d110Nb, b′ = 4 d110Nb, c′ = 3 d001Nb. La structure de NbOxII a ete determinee.