Melt crystallization phase diagrams

Figure 1. A racemic conglomerate (a) the principle of Preferential Crystallization, (b) a packing mode of enantiomers in the crystal with Z = 2, and (c) a representative binary melting-point phase diagram.

Conglomerates and racemates can be easily distinguished from each other on the basis of their melting point phase diagrams [25]. A conglomerate system will exhibit a single eutectic minimum in its phase diagram at the mole fraction of 0.5, since at this enantiomeric ratio the system will consist of an equimolar mixture of two crystalline enantiomers that melts as if it were a pure system. Qn the other hand, a racemate system will exhibit two eutectics on either side of the mole fraction of 0.5 since both enantiomers will be foimd in the unit cells of the crystallized solid. 

Thus, melting points (even in the absence of a full melting point phase diagram) and X-ray powder diffraction patterns are useful criteria for assessing the results of such crystallizations if suitable single crystals are formed, the more elaborate (but far more informative in terms of structural detail) method of single-crystal structure determination provides more definitive confirmation and structural descriptions of phase identities. 

The molecular chirality associated with an optically active molecule is ordinarily manifested in the crystallography of the compound [1], Since the historical development of optical activity was greatly aided by systematic studies of the habits of enantiomorphic crystals, the concepts of molecular dissymmetry, crystallography, and chirality are inexorably linked. As with any other solid, these materials can be characterized on the basis of their crystal structures and through an understanding of their melting point phase diagrams. 

Since conglomerate systems consist of totally independently formed enantiomer crystals and are therefore mere physical mixtures of the enantiomer components, these constitute a binary system. Such binary mixtures are easily described by the phase rule and can be profitably characterized by their melting point phase diagrams [10]. Since the components of a conglomerate racemate will melt indepen-... 

At high pressures, solid II can be converted (slowly) to solid III. Solid III has a body-centered cubic crystal structure. Line bd is the equilibrium line between solid II and solid III, while line be is the melting line for solid III.P A triple point is present between solid II, solid III, and liquid at point b. Two other triple points are present in this system, but they are at too low a pressure to show on the phase diagram. One involves solid II, liquid, and vapor while the other has solid I, solid II, and vapor in equilibrium. 

Unlike low molar mass liquid crystals, these materials do not undergo a nematic-isotropic transition. Instead, they adopt liquid crystal behaviour throughout the region of the phase diagram for which they are in the melt. Above a particular temperature, rather than adopting an isotropic liquid structure, they decompose. 

Binary phase diagrams indicate that the rare-earth dodecaborides do not melt congruently . Owing to the difficulty in preparation of single-phase and single-crystal dodecaborides, little information is available on their physical properties. 

The Ca-Cu system has been reexamined using thermal analysis and x-ray diffraction methods an independent study of the CaCuj-Cu section has also been completed. The resultant phase diagram, although similar to that in ref. 3 at the Cu-rich end, differs markedly for Ca-rich alloys. Supporting evidence for the modifications has been obtained from the Ca-Mg-Cu ternaiy system. Three intermediate compounds are formed in the system CaCuj (950 C) melts congruently, whereas CajCu (488 C) and CaCu (567°C) are formed in peritectic reactions. Single-crystal x-ray diffraction studies verify the stoichiometry of CajCu and examine the polymorphism of CaCu. ... 

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