Polymorphism Polytypism

Polymorphism/polytypism Transformations/transitions and/or order/disorder phenomena... 

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. 

Polymorphic forms with structures having different stacking sequences of like layers are called polytypes. 

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... 

Because of their structural relationship, wollastonite and pturawolla-stonite are better described as polytypes rather than as polymorphs. Other polytypes are possible (Deer, Howie, and Zussraan 1963). For example, a new polytype would be formed if regular stacking faults occurred in every third triclinic unit cell, producing the packing sequence... 

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