Interface structures, topotactic

Examples of the diversity of possible interface structures are as follows. Topotactic interfaces. Primary valence forces may link closely juxtaposed, or perhaps coherent, reactant and product phases so that the crystalline product retains the orientation and some structural features of the reactant [29,30]. The interface, with thickness of molecular dimensions, is defined by the discontinuity of structure and bonding within which reactivity is locally enhanced by the strain field. Product catalysis. If the residual solid is a catalyst for breakdown of a reactant constituent, decomposition may occur within chemisorbed material at the product... 

The structures of metal oxides and mixed oxides are often relatively simple, so that many features of reaction, such as the properties and dispositions of extended imperfections (Section 9.4.), can be characterized more easily than for more complex sohds. The ability of these compounds to deviate from stoichiometry does, however, increase interpretational difficulties. Topotactic behaviour, arising from structures based on simple ions, is important in formulating mechanisms [87], The surface chemistry and interface reactions of oxides are also of importance in heterogeneous catalysis and metal oxidations. 

Probably the most powerful tool for the exploration of phase relationships at interfaces is the use of synchrotron radiation to determine structures. This can be undertaken for a sequence of small volumes across the reaction zone [6]. The potential of the method is considerable, but, as yet, the number of applications has been small because there are few of these expensive facilities available. X-ray diffraction measurements are, however, widely used to confirm or identify the structure of both reactants and products, and can be extended to detect any topotactic relationship between them. Assumptions do have to be made concerning the absence of structural changes on cooling from the reaetion temperature to the temperature of strueture determination, if these temperatures are different. 

Figure 2.8. The mechanism of magnetite reduction (a) describes the reducibility of a magnetite brick under standard conditions as a function of its porosity. In the range of poor reducibility the reaction is topotactic and leads to a core and shell structure visible in an optical microscope and shown schematically in (b). Section (c) describes the situation at the wustite-magnetite interface as can be seen in an electron microscope.

---