Dystectic point

This maximum is called a dystectic or indifferent point. The justification for the latter name, introduced by Duhem, will appear in chap. XXIX. 

Attractive interactions between the two co-crystal components in the liquid phase may significantly change the picture in Figure 12.1 in that the eutectic and dystectic temperatures are further decreased and the temperature differences between the eutectics and the melting points of the components will increase. However, the stabilities of a mixture of the individual soUd components and the co-crystal relative to the molten state are affected by these attractive interactions to the same extent, and the relative thermodynamie stability of the co-crystal and the individual components will still be refleeted correctly in the phase diagram. 

The ternary phase diagram will additionally change with temperature. Two examples in the form of a contour map are represented in Figure 12.5 based on the thermodynamic data from Figure 12.1(a) and (b). The solubility lines of the individual components and the co-crystal extend into curved surfaces and the invariant points become eutectic grooves on both sides of the co-crystal phase boundary surface. Relatively symmetric cross sections can be seen at all temperatures for the phase diagram derived from the system with a dystectic. Very asymmetric temperature cross sections are found in the example with a peri-tectic melting of the co-crystal. 

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