Material congruently melting composition

Congruent Melting. The melting point of a solid is said to be congruent if the material melts completely at a fixed temperature and without change in composition, (cf. incongruent melting). 

In all of the above discussion we have been concerned with systems which crystallize without change of composition, i.e., pure materials or congruently melting compounds. In the more general case, with composition as an additional variable, the techniques of controlled nucleation and crystal growth have been fruitfully employed in recent years. Applied in various contexts they form the basis of several important commercial processes... 

The skutterudites do not melt congruently and involve pnicogens (P, As, Sb) that generally have high vapor pressures at the formation temperatures of the compounds. The high melting temperatures of Fe, Ru and Os coupled with the reactivity of the lanthanide metals with convenient crucible materials (e.g., Si02) makes the synthesis of many of these compounds difficult. As a result, variations in the reported properties of a particular lanthanide skutterudite compound can often be traced to differences in sample composition and quality. 

In the above two cases the solidus and liquidus curves meet tangentially to an isothermal line at a congruent point the solution freezes at this temperature without any change in composition. Au-Ni alloys exhibit the behavior depicted in Fig. 3.16.2. The melting point of the intermediate composition is below that of either pure material. Inasmuch as > 0 for this case, the solid solution for T < Tc is less stable than a mixture of phases this is indicated by the dotted curve at the bottom of the diagram. 

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