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ContentsEvidence of stacl ' 6.1 One-dimensional disorder ; phenomena of "streaking" and "arcing" 6.2 Formation of arcs 6.3 Arcing and polytypism; role of stacking faults Effect of temperature and pressure on transformation of polytypes 7.1 Effect of temperature 7.2 Effect of pressure A critical evaluation of the various theories 8.1 Frank's screw dislocation theory 8.2 Jagodzinski's disorder theory 8.3 Theories of Schneer and Peibst 8.4 Vand and Hanoka's epitaxial theory 8.5 M a r k and Steinberger's theory 8.6 Knippenberg's hypothesis of electron energy correlation faults in polytypic crystals Concluding Remarks !ferences Evidence of Stacking Faults in Polytypic Crystals One-dimensional disorder; phenomena of %treaking" and "arcing"It is well-known that close-packed crystals, such as h.c.p. and f.c.c. crystals, nerally contain many types of faulfs in the stacking of their close-packed omic layers. These "mistakes" or "stacking faults" have been observed in th naturally occurring and artificially grown crystals. They constitute one the principal defects in certain structures, e.g. in crystals of A1IBV1 compounds 341. The faults consist of displacements of the layers from their correct 'sitions in the structure. The direction of the displacements being parallel the layers, their effect is t o destroy the regularity of the structure in the rection perpendicular t o the layers, thus introducing "one-dimensional gorder" in it, which is easily recognised in single-crystal X-ray diffraction l) Part I see phys. stat. sol. (a) 4, 9 (1971). --. -2) Sections 1 to 5 see Part I.

A combined optical and X-ray diffraction study has been undertaken of the phenomenon of polytypism in cadmium iodide crystals, which grow by a dislocation mechanism. 88 CdI2 crystals have been investigated to study Frank's ideas of a 'buckle followed by slip' mechanism in the creation of dislocations and subsequently of polytypes. A correlation between Che spiral step height and X-ray unit cell is found for a few 2H and 4H structure types. However, for polytypes with larger unit cells no such correlation was observed. This is at variance with previous observations and with Frank's theory and cannot yet be fully explained. Some X-ray diffraction photographs show 'diffuse background' and 'smearing of diffraction spots', with spots elongated along streaks, similar to Jagodzinski's observations on silicon carbide crystals. 13 (possibly 18) new polytypes of CdI 2 are described. In addition, several crystals resembling 2H and 4H types, but having either intensity differences or conspicuous absences of certain reflections, have been found.