Abstract
The crystals of cadmium iodide and lead iodide when subjected to X-ray examination have exhibited the rhombohedral polytypes occurring quite frequently in coexistence with their respective basic hexagonal types 4H and 2H, respectively. This hexagonal - rhombohedral phase transformation taking place during crystal growth can be easily understood in terms of stacking faults occurring periodically either during growth or due to creation and movement of Shockley partials, generated either independently or as a result of dissociation of unit edge dislocations. It has been found that the basic hexagonal types are transformed more frequently into those rhombohedral structures for which (i) the hexagonality is almost the same as that of their parents, (ii) the top and bottom layers of two successive sandwiches, respectively, around the boundary of rotation are not in the same orientation, and (iii) the stacking fault energy is minimum. Non-occurrence of the types 6R and 12R, (13) 3, in CdI 2 and PbI 2 as a result of transformation of their basic types have been explained. The possibility of occurrence of polytypes with Zhdanov number 4 is pointed out.
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