From Nb12O29 to Nb22O54 in a Controlled Environment High-Resolution Microscope

Abstract

A nonstoichiometric Nb12O29(NbO2.417) block structure oxide with monoclinic symmetry was slightly oxidized in a controlled environmental high-resolution electron microscope. This instrument is based on a JEOL 4000 EX electron microscope equipped with a unique gas reaction cell in which the oxide could be oxidized by introducing O2 at pressures up to 30 mbar, while promoting local reaction by electron beam heating. The oxidation process was seen to start in the regions where some stacking faults perpendicular to the c axis appear (in which the structure has local orthorhombic symmetry). The initial movement of atoms, involving a formation of lamellar defects, could be traced with great certainty. Such defects provide empty rectangular tunnels, similar to those found in the NaNb13O33 structure, which is a (4×3) block structure with Na cations situat ed in the rectangular tunnels. This stage of the oxidation reaction implies a minimal rearrang ement, usually involving a simple unit jump (the primitive diffusion step) by atoms in certain sites. Then, the cooperative unit jumps along rows of atoms propagate the transformation through the crystal. A further stage in the oxidation gives rise to the Nb22O54(NbO2.455) structure in some areas of the crystal, which could be identified in the high-resolution transmission electron microscopy image as well as in the electron diffraction patterns.