Topotactic relationships

X-Ray and electron diffraction measurements have been most usually used to characterize the phases present in any reactant mixture, and provide a means of identification of solid reactants, intermediates and products. In addition to such qualitative analyses, the method can also be used quantitatively, with suitable systems, to determine the amounts of particular solids present [111], changes in lattice parameters during reaction, topotactical relationships between reactants and products, the presence of finely divided or strained material, crystallographic transformations, etc. 

Direct kinetic measurements from the changes in diffracted beam intensities with time during heating of the reactant are illustrated in the work of Haber et al. [255]. Gam [126] has reviewed the apparatus used to obtain X-ray diffraction measurements in thermal analysis. Wiedemann [256] has designed equipment capable of giving simultaneous thermo-gravimetric and X-ray data under high vacuum. X-Ray diffraction studies enable the presence, or absence, of topotactic relationships between reactant and product to be detected [92,102,257—260], Results are sometimes considered with reference to the pseudomorphic shape of residual crystallites. 

It is appropriate to emphasize again that mechanisms formulated on the basis of kinetic observations should, whenever possible, be supported by independent evidence, including, for example, (where appropriate) X-ray diffraction data (to recognize phases present and any topotactic relationships [1257]), reactivity studies of any possible (or postulated) intermediates, conductivity measurements (to determine the nature and mobilities of surface species and defects which may participate in reaction), influence on reaction rate of gaseous additives including products which may be adsorbed on active surfaces, microscopic examination (directions of interface advance, particle cracking, etc.), surface area determinations and any other relevant measurements. 

X-ray diffraction measurements enable the presence or absence of epitactic and topotactic relationships between reactant and solid product to be established [7]. Electron diffraction may be useful when only small particles are available, and is especially valuable for the examination of selected small areas during electron microscopic examination of the sample. The detailed surface structure, including the presence of chemisorbed species, may also be revealed. 

Kim et al. [44] reported results from a similar study of the decompositions of Mg(OH)2 and MgCOj in the TEM. They were able to determine in detail the product/reactant orientation relationships, by careful preparation of specimens, using ion-beam milling where necessary. They detected no intermediate phase in either decomposition. The decomposition of Mg(OH)2 produces MgO with a single orientation relationship to the substrate, while the decomposition of MgCOj yields MgO in two principal orientations. These topotactic relationships are explained in terms of the orientations of oxygen octahedra in the reactants and product. 

For any kinetic study, the chemical change should be clearly specified, though this is not always achieved [25], and the identities of all reactants, intermediates and products must be established using suitable analytical techniques. Unlike homogeneous reactions, the ciystallographic structures, topotactic relationships and states of sub-division of all participating solids should also be described. 

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. 

Chemically the interlayers in the modular phases described previously are relatively reactive. This has been exploited to fabricate large numbers of phases that are not available via the normal solid-state chemistry preparative method of high-temperature synthesis from mixed oxides or oxide precursors. The technique generally employs low temperatures, typically room temperature to 300°C, and long reactions times, of the order of 1 week. This procedure, termed chimie douce , is often translated as soft chemistry but is better called mild chemistry or gentle chemistry . Under such conditions, much of the stmcture remains intact, and the product phases have a strong topotactic relationship to the starting stmctures. For the modular perovskite phases, in... 

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