Polytype
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Polytypes are variations of the same chemical compound that are identical in 2 dimension and differ in the third. Thus, they can be viewed as layers stacked in a certain sequence. The atoms of those layers can be placed on one out of three "locations", A, B or C to achieve closest packing. To ensure the close-packing of the structure, when a layer is placed on one of the three locations, the two neighboring layers must not be on the same location. The stacking sequence of those layers defines the structure. The unit cell is the shortest periodically repeated sequence of the stacking sequence. The shortest unit cell being AB, and the longest having 
layers. The most investigated compunds which show the phenomenon of polytypism are silicon carbide (SiC) and zinc sulfide (ZnS). The different polytypes of the same compound show slight variations in some of their physical properties (optical, semi-conducting,...), which would make it interesting to be able to grow one certain polytype of a crystal. This would allow very fine tuning of the physical properties. Still, it is not possible to influence the formation of specific polytypes, because the details of the process of formation remains unknown. As a matter of fact, different polytypes form under the exact same circumstances and oftentimes one crystal may contain several polytypes. The identification of polytypes is performed by the analysis of the Bragg diffracton pattern of x-ray photographs of the crystal.
