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# Polytype

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 $\infty$ layers.