Solid state reactions in the system Nb2O5·WO3: An electron microscopic study

Lattice imaging electron microscopy has been used to study the mechanism of solid state reactions of the type: A_s → B_s + C_s, in which the product B is able to intergrow coherently with the starting material A, but the product C cannot do so. C must be formed by a fully reconstructive, heterogeneous process; formation of B is only partially reconstructive, and essentially homogeneous. Reactions were the reversible phase reactions in the system Nb₂O₅WO₃: disproportionation of the (5 × 4)₁ block structures of 8Nb₂O_5·5WO₃, to form (4 × 4)₁ blocks of 7Nb₂O_5·3WO₃ as coherent product, and that of 9Nb₂O_5·8WO₃ (with (5 × 5)₁ blocks), forming (5 × 4)₁ blocks of 8Nb₂O_5·5WO₃ as coherent product. The coherent product structure is formed in isolated rows of blocks, or small packets of such rows, running across each crystal. The reaction does not work in progressively from some surface initiating step, with an interface between unchanged and converted material, but represents a block-by-block conversion, linearly propagated. Nb₂O₅ and WO₃ must be abstracted, in appropriate stoichiometric ratio, from each block but must ultimately reach and react at the surface, to form the incoherent product (a pentagonal tunnel network structure, in both cases). Some homogeneous transport process involving lattice diffusion must be invoked. Domains of highly anomalous structure, regarded as relicts of transient conditions, are occasionally observed. From reactions at relatively low temperatures, these have structures that can be regarded as partially ordered nonstoichiometric solid solutions; after prolonged heating, and at higher temperatures they form well ordered strips of metastable block structures. Both types represent strong, spontaneous fluctuations of composition, which impose a corresponding structure locally. These fluctuations may be associated with the transport of WO₃ and Nb₂O₅ away from the locus of reaction. Evidence about the mechanism of the reactions, the role of dislocations and the nature of cooperative processes is considered.