Melting congruent state

Figure 8.22 shows an Mn0-Al203 phase diagram. The equimolar composition melts congruently, i.e., there is a direct transformation between the solid and liquid states without a change in the number of phases (in this case X = 1). Ba0-Ti02 is another example of a two-component... 

Consider a hypothetical pair of substances denoted by A and B. Substance A melts at 650°C and substance B melts at 575°C. The two substances form two compounds in the solid state, A2B and AB2, but mix in all proportions in the liquid phase. A2B melts congruently at 550°C. AB2 melts incongmently to give pure A and a solution with... 

In the Au-Bi system the compound Au2Bi is stable in a restricted range of temperature only it is formed by a peritectic reaction (371°C) and, at a lower temperature (116°C), it is decomposed according to the eutectoidal reaction Au2Bi — (Au) + (Bi). In the Zn-Te system, finally, we have the congruently melting compound ZnTe. In this system a miscibility gap in the liquid state may also be noticed. 

Figure 2.19. Examples of systems in which intermediate phases corresponding to small composition ranges are formed. These are SnTe (congruent melting), HfRu (congruent melting), ZrV2 (peritectic formation) and TaV2 (formed through a solid-state reaction).

Figure 3.3. Schematic representation of the diagrams for the alkali metals with a selected number of elements of the p-block. In each box the solid intermediate phases are represented in the positions approximately corresponding to their compositions (long bars congruent melting phases short bars non-congruent phases). In the top part of each box every mark corresponds to a characteristic composition of the liquid phase for which the formation of an associate ( liquid compound ) may be suggested, for instance by the presence of an extremum in the trend of some property of the liquid phase. The symbol 2 L shown for certain ranges of compositions in a few diagrams indicates the presence of a miscibility gap in the liquid state, that is two liquid phases.

Figure 14.29 shows the (solid + liquid) phase diagram for (benzene + hexafluoro-benzene). A congruently melting solid molecular addition compound with the formula QFU-CeFe ) is evident in this system.26 The rounded top of the freezing curve (solid line) for the addition compound results from almost complete dissociation of the addition compound in the liquid mixture. In other words, benzene and hexafluorobenzene act as independent molecular species in the liquid state and combine together as the addition compound only in the solid state. 

A phase diagram investigation (494) of the l-pentene-AgBF4 system has shown the 2 1 and 3 1 complexes with congruent melting points at 43.3 and —30.3°C, respectively. In addition, evidence is provided for the existence of a 1 1 complex stable only in the solid state at temperatures below about — 10°C. It appears to be formed by a solid state reaction between the 2 1 complex and AgBF4. 

From the complex physico-chemical analysis of the system LiF-KF-K2NbF7, it follows that the behavior and properties of the melts are affected by the formation of the congruently melting compound KsNbFg. Regarding the presence of the anion [NbFg] in the molten state the system behavior is not far from the ideal. 

C. Equilibrium between Solid and Liquid Phases only. I. The Components are Completely Miscible in the Liquid State. a) The pure components only occur as solid phases. Polymorphism of components. Determination of the equilibrium curve. Example, b) Compounds are formed with a congruent meltings point. The indifferent point. Determination of the composition of a compound by thermal analysis. Examples, (c) Compounds are formed with an %ncongruent melting-point. Determination of the composition of the coinpound by thermal analysis. Example. (d) Solid solutions or " mixed crystals are formed, i) The two components can form an unbroken series of solid solutions. Examples. Melting-point curve. Example. Fractional crystallisation of solid solutions, h) The two components do not form a continuous series of solid solutions. Examples. Changes in solid solutions with the temperature. II. The Components are not Completely Miscible in the Liquid State. Suspended transformation. 

---