Phase Diagrams and the Coexistence of Solids

Probably the most widely employed technique now used in phase studies is powder X-ray diffraction. The X-ray powder pattern of a compound can be used as a fingerprint, and data for many compounds are available. This can be illustrated with reference to the sodium fluoride (NaF)-zinc fluoride (ZnF2) system. Suppose that pure NaF is mixed with a few percent of pure ZnF2 and the mixture heated at about 600°C until reaction is complete. An X-ray powder photograph will show the presence of two compounds (or phases), NaF, which will be the major component, and a small amount of a new compound (point A, Fig. 4.1a). A repetition of the experiment, with gradually increasing amounts of ZnF2 will yield a similar result, but the amount of the new phase will increase relative to the amount of NaF until 

Defects in Solids, by Richard J. D. Tilley Copyright 2008 John Wiley Sons, Inc. 

The results of such experiments, carried out over a range of temperatures, are summarized on a phase diagram (Fig. 4.1 b), which can be used to predict the outcome of any preparation in the NaF-ZnF2 system. In such diagrams, each phase is drawn as a line, as in the example of NaZnF3 above, because they show no composition range. Such compounds are referred to as stoichiometric compounds and, sometimes, with the appearance of the phase diagram in mind, line phases. 

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