Abstract
A nonstoichiometric block structure oxide, Nb22O54 (NbO2.455), with monoclinic symmetry was characterized following in situ oxidation in a controlled environment, high-resolution electron microscope. This instrument is based on a JEOL 4000EX electron microscope equipped with a gas reaction cell. The oxidation reaction was carried out by introducing ca. 20 mb of oxygen gas to the specimen region and heating the specimen by means of a focussed electron beam. The resulting structures obtained after the oxidation reaction were not totally homogeneous. (1) Mainly microdomains of Nb10O25 were found, which consists of [3×3] blocks of octahedra linked through tetrahedrally coordinated sites—such a structure is isostructural with PNb9O25. Nb10O25 seems to be a metastable phase in the Nb–O system, and was found here for the first time as extended domains. (2) A highly disordered structure was observed in some areas of the crystal after oxidation, with a corresponding electron diffraction pattern similar to an “X” phase interpreted as two-dimensionally disordered H-Nb2O5. Very clear lamellar defects were also found after oxidation.
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