POLYTYPEFORMATION IN NONMETALLIC SUBSTANC ES

Available data about the polytypism in nonmetallic substances with dif ferent nature of chemical bonding and lattice type are systematized. M ain attention is paid to consideration of the phenomenon in (sic) carb ide and aluminium nitride, including practical aspect. Known conceptio ns of polytype formation (sic) this materials are discussed. It is con cluded that polytypism in nonmetallic crystals is a solid state relaxa tive reconstruction with formation of various planar defects which cau se regular disturbance of periodicity of atomic layers stacking. Struc ture (growth, deformational) and chemical stacking faults, microtwins, antiphase boundaries, layers of vacancies and other coherent with mat rix phases (including phases with different coordination number or sta cking sequence) may be such defects and therefore elements of polytype structure. In diamond-like phases (SiC, AlN, BN) side by side with in tragranular polytypic transformations, intergranular transitions take place, which are induced by a migration of grain boundaries. Such tran sformations are controlled by an action of lattice power field of poly type-substrate, on which mass-transfer occurs from the initial polytyp e.