Polytypism and Polymorphism

The bilayer arrangement of fatty acid chains is the most common packing structure for natural fats, including milk fat. However, with quench cooling to 8°C, Lopez et al. (2001) found evidence of the coexistence of both the bilayer and trilayer lamellar structures in milk fat. 

The (3 polymorph is the most thermodynamically stable. It has the highest melting point and is therefore the least soluble in a melt at a given temperature below its melting point. Despite this, nucleation for the a-polymorph is favored. Although the a-crystal is less stable, it has a lower crystal-melt interfacial tension and lower heat of crystallization than the (3 - and (3-polymorphs (Timms, 1995). Nucleation in milk fat typically 

Although the a-polymorph is meta-stable, it can have a relatively long lifetime in milk fat at low temperatures compared to other fats (Walstra et al., 1994). The a-crystal may be stabilized by the formation of compound crystals in milk fat (Walstra et al., 1999). A consequence of both polymorphism and mixed crystal formation in milk fat is that the material is rarely at equilibrium (Walstra et al., 1994). 

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Polytypes and polytypoids can be regarded as a special form of polymorphism and are treated in more detail in Ref. 10. 

These arranged layers are called "polytypes" and are prevalent where simple compounds such as SiC and SiN are involved. In many cases, the properties of such compounds depend, to a large extent, upon the specific stacked layers obtained during formation. For SiC, we can also have "polytypes" where two stacking sequences like 4H - 6H can combine to form a unit. Another "polymorph" is 4H -15R. This phenomenon has been thoroughly studied and polymorphs of 87R and 270R have been reported. 

C AHjg forms also two polymorphs Oj and a, the latter one is more stable and shows less disordered structure [83]. They both are the polytypes and reveal the hexagonal unit cell—a, a=511 pm, c=6408 pm, CI2 a=511 pm, c=2137 pm. 

Silicon carbide exists in several modifications being polymorphic and polytypical and crystallizing in a diamond lattice, like silicon [13]. 

Alpha SiC is the high temperature form of SiC. Unlike pSiC which is a single compound, aSiC has a large number of polytypes, approximately 250 having been identified so far.OiHiz] These polytypes have either a rhombohedral or a hexagonal structure. Polytypes, unlike polymorphs, are the same thermodynamic phase, are formed under the same conditions of temperature and pressure, and have similar properties and structure. Their close-packed layers ( 0001 for hexagonal) are identical but have a... 

An element or compound is polytypic if it exists in two or more layerlike crystal structures that differ in their layer-stacking sequences. Polytypes are nearly identical in structure and composition and polytypism is a special case of polymorphism the two-dimensional translations within the layers are essentially preserved, whereas the lattice spacing normal to the layers varies between polytypes and is indicative of the stacking period. There is increasing evidence that some polytypic structures are characterized either by small deviations from stoichiometry or by small amounts of impurities. " ... 

Polytypism is polymorphism in a narrow and restricted sense and is probably the more precise term to apply to the layer silicates. But polymorphism is not incorrect and has been much more widely used in the older literature it will be retained in this chapter. 

Polymorphism in nonmetals has also received a great deal of study and is particularly clearly discussed in a book by two Indian physicists (Verma and Krishna 1966) which also links to the phenomenon of polytypism, discussed in Section 3.2.3.4. 

Silicon carbide exhibits a two-dimensional polymorphism called polytypism. All polytypes have a hexagonal frame of SiC bilayers. The hexagonal frame should be viewed as sheets of spheres of the same radius and the radii touching, as illustrated in Figure 1.5. The sheets are the same for all lattice planes. However, the relative position of the plane directly above or below are shifted somewhat to fit in the valleys of the adjacent sheet in a close-packed arrangement. Hence, there are two inequivalent positions for the adjacent sheets. 

A. R. Verma and P. Krishna, Polymorphism and Polytypism in Crystals, New York, John Wiley Sons, 1966... 

A subclass of polymorphism known as polytypism is found for one-dimensional close-packed and layered structures such as SiC, Cdl2, GaSe, micas and clay... 

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