polytype

A polytype is a crystal form of a compound that shares the same chemical composition and the same local atomic arrangement but differs in the long-range stacking sequence of its atomic layers along a principal axis. These variations arise from different sequences in how stacked layers are arranged, typically in layered or tetrahedrally coordinated materials. The term is widely used in crystallography and materials science to describe polytypism, where a single compound can crystallize in multiple structural forms without changing its stoichiometry.

In close-packed structures, layers can be stacked in different ways. The most common motifs are cubic close packing (ABC stacking) and hexagonal close packing (AB stacking). A polytype is defined by its repeating stacking sequence; for example, 3C-SiC has an ABCABC... sequence, 2H-SiC has ABAB..., 4H-SiC has ABACABAC..., and 6H-SiC has ABCACB ABCACB... The same chemical formula can thus produce several polytypes with distinct space groups and properties. Hexagonal boron nitride also exhibits polytypism, with 2H and 3R forms.

The existence of polytypes affects physical properties such as band gap, dielectric behavior, and radiation hardness, making polytypism important for electronic and optoelectronic applications. Polytypes can form during crystal growth due to fluctuations in temperature, pressure, or impurities, and can sometimes transform to other polytypes under heat or stress. Detection typically relies on X-ray diffraction and electron microscopy to identify characteristic stacking sequences.