The structure and stoichiometry of anodic films on V, Nb, Ta, Mo and W

Abstract

Anodic films on Nb were found to have the expected amorphous structure for all formation voltages starting at 6 volts. Films on V, Ta, Mo, and W, on the other hand, were amorphous only when thick, with films below 9 to 30 nm (6 to 10 volts) showing a polycrystalline pattern by reflection electron diffraction. Although the patterns of the thin films were difficult to identify because of their poor quality, the following assignments are proposed: not V₂O₅, TaO _z plus δ — Ta — O, MoO₂ · H₂O, WO₂. To get information on the stoichiometry of thick anodic films on V, Mo, and W, specimens were crystallized by heating them at 300 to 500° C in air, but it was shown that the resulting observation of V₂O₅, MoO₃, and WO₃ has no relevance, as oxidation is both thermodynamically and kinetically possible. Vacuum crystallization was shown to be quite unsatisfactory with V and Mo, though is possibly acceptable with W; the information obtained from a comparison of film weights with metal-removal weights, namely that the stoichiometries were close to V₂O₅, MoO₃, and WO₃, is more relevant. The stoichiometries TaO _z , MoO₂ · H₂O, and WO₂ in the thin films are thermodynamically understandable. A greater problem arises in explaining the state of crystallinity of these as well as other anodic films. A parallel is proposed between anodic and ion-bombarded oxides, for in both cases the state of crystallinity tends to be the same and, furthermore, correlates with the ratio (crystallization temperature)/(melting temperature). A model based on crystallization due to energy deposition is discussed.