ABSTRACT
In metallic systems, the periodic occurrence of faults to create super-lattice structures is attributed to first-order energy differences. The very large periodicities observed in one-dimensionally disordered structures by lattice imaging in the electron microscope totally defy all theories and explanations. Stacking faults can also occur in crystals with an ordered arrangement. The existence of close-packed structures with a finite periodicity in the c-direction must be attributed to the influence of the more distant neighbours on the energy of a layer if they represent thermodynamically stable phases. Structurally the phenomenon of polytypism represents a special kind of one-dimensional polymorphism but physically it differs from ordinary polymorphism in several respects. Any material whose structure consists of identical layers of structure that can be stacked on top of each other in more than one way so as to achieve the same nearest neighbour relationships for the constituent atoms represents a potential candidate for the occurrence of polytypism.
