Material incongruently melting composition

There is some question as to whether material of the composition InClz is a compound melting sharply at 235°, an incongruently melting compound, or indeed a true compound at all. ... 

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). 

Both the low (B) and high (o) temperature polymorphs of NiS dissolve excess sulfur to form solid solutions. The B phase shows a limited range of homogeneity but the a phase shows a broad limit extending to approximately NiS (8). The o phase is normally designated o-Ni to emphasize this feature. However, the sulfur-poor limit of this phase has the stoichiometric composition to at least 873 K at 1070 K the deviation from stoichiometry is less than 0.05 weight percent nickel (8). Above 1079 K the deviation of the sulfur-poor limit from stoichiometry increases more rapidly and a material of stoichiometry NiS will exist as an equilibrium mixture of liquid NiyS. a hd solid a-Nij S (8). Thus, NiS melts incongruently. 

The material to be crystallized melts incongruently, and cannot be grown from a melt of the same composition. 

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