The oxidation of Nb 12 O 29 : electron microscopy of a solid-state reaction

Abstract

The two forms-monoclinic and orthorhombic - of the mixed valence Nb (IV, V) oxide Nb 12 O 29 oxidize at relatively low temperatures to two new modifications of Nb 2 O 5 . These, like the starting materials, are of the ‘block’ structure type, derived from the DO 9 structure by crystallo­graphic shear. Electron microscopy and electron diffraction show that the first distinct stage after complete oxidation is the formation of a super­-lattice structure derived from the structure of the original Nb 12 O 29 ; this undergoes internal reorganization leading, in the case of the monoclinic oxide, to the formation of a new modification of niobium pentoxide, Nb 10 O 25 , with a (3 x 3) 1 block structure. By using high resolution lattice imaging methods, the course of the initial oxidation step, the formation of the superlattice and its subsequent rearrangement have been traced in considerable detail. This low temperature oxidation process, which takes place with measurable speed at temperatures as low as 110 °C, clearly turns upon the diffusion of oxygen through the open channels of the DO 9 structure, and permits of a homogeneous solid state reaction involving only single unit jumps of atoms from original lattice sites into interstitial positions. At 440 °C and above, a second oxidation mechanism becomes competitive, involving reaction and rearrangement at the surface of the crystals. A reaction front then passes through the crystal by the co­-operative diffusion processes that are involved in the migration of crystal­lographic shear planes.