Polytype formation in nonmetallic substances

A systematic account is given of the known data on polytypism in nonmetallic substances with various structures. The existing hypotheses concerning the nature of polytypism are discussed. It is shown that polytype formation in nonmetallic crystals is a solid-state relaxation rearrangement accompanied by the formation of planar defects of various types, inducing the breakdown in the periodicity of the packing of atomic layers. Such defects and hence structural elements of polytypes may be structural (growths, deformation) and chemical packing defects, microtwins, antiphase boundaries, layers of vacancies, as well as layers of other phases coupled coherently with the basic structure. For diamond-like phases (SiC, AlN, BN), intragranular polytype transformations, induced by the migration of boundaries, occur together with intragranular polytype transformations. The bibliography includes 294 references.