Homogeneous Solid State Transformations in Niobium Oxides

In homogeneous solid state reactions, transformation takes place within the crystals, and structural continuity is preserved; reactant and product structures are coherently intergrown. If the transformation starts independently at numerous loci, the incompletely reacted state of the crystals is inherently disordered. Crystallographic methods that average the structure over the whole crystal cannot fully characterize the actual, real-space structure, as it evolves during reaction, but that structure can, in favourable cases, be sampled by high resolution electron microscopy. Homogeneous reactions in the niobium oxide ‘block structure’ type crystals lend themselves to this technique. This study relates to the oxidation of the lower niobium oxides, Nb22O54, Nb25O62 and Nb53O132, to niobium pentoxide. In each case, complete oxidation takes place at very low temperatures; the first products are reported to differ little in structure from the parent oxides, and to transform continuously into the stable polymorphic form, H-Nb2O5, over a wide temperature range and via several intermediate structures. By using lattice imaging methods, the structure elements present at each stage in the reactions have been identified, and their rearrangement has been followed. The initial oxidation step is completely non-reconstructive, and produces cation-deficient modifications of the original structures; all subsequent changes take place at constant composition, Nb2O5. In oxidized Nb22O54, a drastic rearrangement sets in at 350-400 °C, involving (a) an exchange of places and an apparent diffusion of the columnar structure elements (or blocks) and (b) a disproportionation that creates blocks of different cross sections. By a suitable mapping method, it is possible to infer exactly where, and how many, niobium atoms have undergone displacement into new sites, and to show that a single mechanism, proposed by Andersson and Wadsley, effects the observed changes of structure. Only a few atoms at a time, at the edge of one block, need to shift. Passing through a highly disordered state, blocks (3x3) octahedra in cross section are progressively segregated into microdomains of a new polymorphic structure of Nb2O5, Nb10O25, while blocks (5 x 3) octahedra in cross section are formed by disproportionation and built into microdomains of H-Nb2O5. These changes are initiated at random centres, 8-10 nm apart, starting at the edge of a single block; the displacements are propagated rapidly up the