Interface, bond redistribution steps

Vacancies may participate in the nucleation step in thermal decomposition. The aggregation of vacancies precedes the initiation of structural reorganization. Removal of any lattice constituent, by decomposition or migration, is an important contributory factor in interface reactions. The availability of vacancies, perhaps in combination with other imperfections, provides the space which may be required during bond redistribution steps. 

The application of absolute reaction rate theory to a chemical change at an interface is only useful if the calculations refer to an identified, or at least reliably inferred, model of the controlling bond redistribution step. This is a problem, because it is particularly difficult to characterize the structures of the immediate precursors to reaction in many solid state rate processes of interest. The activated species are inaccessible to direct characterization because they are usually located between reactant and product phases. The total amount of reacting material present within this layer, often of molecular dimensions, is small and irreversible chemical and textural changes may accompany opening of such specialized structures for examination or analysis. Moreover, the presence of metallic and/or opaque, ill-crystalUzed product phases may prevent or impede the experimental recognition of participating intermediates or essential textural features. 

The inaccessibility of reaction interfaces to investigation means that indirect methods must be used to explore the chemical reactions that occur within these active zones. The determination of the sequence of bond redistribution steps that results in the transformation of a crystalline reactant into a (usually different but often crystalline) product phase is the fimdamental objective of mechanistic studies [74]. All intermediates and the factors that determine the rate and energetics of the transformation (reactivity) must be identified. 

Decomposition is suggested [63] to involve the formation of Mn " ions at the salt-oxide interface and subsequent bond redistributions to involve electron transfer steps, resembling those in solution, thereby regenerating active Mn " ions at the interface. In the absence of oxygen, the carbon monoxide evolved reduces the oxide product to a finely divided and very reactive form of MnO. Decomposition is more rapid when oxygen gas is present, due to increased availability of the intermediate Mn at the nucleus surface and this species is abundant in the residual oxide phase. 

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