Formation of Intergrowth Phases

One of the central themes in chemistry is to understand how atoms and molecules react together to form the products of any chemical reaction. This is true of all reactions of solids, and one of the most fascinating aspects of the phases described in the previous Sections is how they form and how they react chemically. This is especially so of the polytypes, where mechanisms for the formation of these complex structures are being sought, and the occurrence of such materials remains an enigma yet to be solved. 

One of the major problems in attempting to account for the existence of these complex structures is a lack of experimental data. It is clear from the examples quoted that the true complexity of these systems can only be revealed by using high-resolution electron microscopy. Such instruments have only been available for some 10—15 years, and although the technique is now becoming a more widespread tool of the crystal chemist, the initial emphasis, almost inevitably, had been to characterize the structures found and to carry out at least partial phase analysis in a minera-logical or ceramic sense. The time devoted to a study of the formation and reactivity of these phases has, to date, been small and the number of recent results which can be presented here are few. 

The first piece of information needed in such cases is to determine whether the intergrowth phase is formed from a solid-solid reaction, or whether the crystals grow from a liquid phase or from the vapour phase. Strangely enough this is not known, or 

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