Certain aspects of phase transformations in cadmium iodide polytypic crystals have been studied for the first time. It has been shown that the transformation of an ordered polytypic structure into another ordered structure is possible. This fact has been elucidated from the transformation 14H ~ 4H and 18H --+ 4H. Electron-microscope evidence for the occurence of such transformations has been obtained. The phase transformations have also been viewed in the light of the structural geometrical schemes and of the stacking-fault energies of initial and transformed structures. Polytypic transformation from a rhombohedral to a hexagonal type has been observed, 42R--+ 12H. Transformations involving polytypes having similar stacking-fault configurations in their layer sequences have also been studied. The observed transformations of this type are 28H --+ 4H and 36H -+ 4H.

Transformation characteristics of two cadmium iodide polytypes, 18H 6 and 20H9, have been studied. Like other polytypes, these are not based on the most commonly found basic 4H structure; instead they are based on the low-periodicity structure 6H. On vacuum annealing, they transform according to the schemes 18H 6 ~ 4H and 20H 9 ~ 4H. An analysis of these transformations reveals that the growth and transformation processes need not necessarily be complementary. In order to determine a feasible transformation mechanism, the atomic structures (layer sequences) of the polytypes have been evaluated. These correspond to (2211)21122 for 18H6 and (2211)2(11),, for 20H 9. The schematic transformation mechanism has been elucidated.