Homogeneous solid state transformations in niobium oxides

Abstract

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, Nb ₂₂ O ₅₄ , Nb ₂₅ O ₆₂ and Nb ₅₃ O ₁₃₂ , 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 -Nb ₂ O ₅ , 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, Nb ₂ O ₅ . In oxidized Nb ₂₂ O ₅₄ , 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 Nb ₂ O ₅ , Nb ₁₀ O ₂₅ , while blocks (5 x 3) octahedra in cross section are formed by disproportionation and built into microdomains of H -Nb ₂ O ₅ . 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 crystal b -axis and relatively slowly from block to block along rows and up files of blocks. Extended defects and compromise structures are formed where effects spreading from different initiating centres impinge; these faults serve as sinks for eliminating cation vacancies. The final result, stable up to 950 °C, is a domain structure, with nearly continuous walls of Nb ₁₀ O ₂₅ enclosing slabs of H -Nb ₂ O ₅ structure, and intersected by nearly continuous walls of fault structure. In oxidized Nb ₂₅ O ₆₂ , the first stage detected is the formation of triangular nuclei; one side of the triangle is formed of H -Nb ₂ O ₅ structure, the other side of Nb ₁₀ O ₂₅ structure, with an abrupt boundary to unchanged Nb ₂₅ O ₆₂ in each case. As these nuclei expand, by propagation of atom row displacements along rows and files of blocks, they merge to create relatively extensive domains of H -Nb ₂ O ₅ structure, separated by narrow walls of Nb ₁₀ O ₂₅ and linked by faults. Transformation of oxidized Nb ₅₃ O ₁₃₂ is similar. Long annealing of all the oxides at 900 °G gives products with essentially similar domain structures, but differing in the relative proportions of H -Nb ₂ O ₅ and Nb ₁₀ O ₂₅ domains. They are stable up to 950 °C; at higher temperatures, transformation into H -Nb ₂ O ₅ appears to be a catastrophic, and probably a heterogeneous process, for after heating at 950-1100 °C, any one crystal is either fully transformed into recrystallized H -Nb ₂ O ₅ or retains its domain structure with no significant change. There is some evidence that the domain structure is mobile at high temperatures.