polytypism

Polytypism is a special, restricted kind of polymorphism. Ordinary polymorphs are the same atoms arranged in genuinely different structures (diamond versus graphite). Polytypes are far more subtle: the atomic layers are essentially identical and packed the same way within each layer; only the stacking sequence along one direction changes. It is like a deck of identical cards that can be stacked in many repeating patterns.

The textbook examples are silicon carbide and zinc sulfide. Silicon carbide alone has more than 200 known polytypes. They are named by the number of layers in the repeat and the resulting symmetry: 2H is the two-layer hexagonal (wurtzite-like) form, 3C is the three-layer cubic (zinc-blende) form, and 4H, 6H, and 15R are longer repeats. Within any single layer the atoms are indistinguishable between polytypes; what differs is whether the next layer goes to position B or C, repeated over long periods.

Polytypism matters because subtle stacking changes shift real properties. In silicon carbide the electronic band gap depends on the polytype (3C is about 2.4 eV, 6H about 3.0 eV, 4H about 3.3 eV), which is why device engineers care which polytype they grow. It also cleanly illustrates the difference between structure types: polytypes are not really new structures, just new stacking periods of the same building block.