Polytypism of Cadmium Iodide and its Relationship to Screw Dislocations

Abstract

This paper shows how the screw dislocation theory of crystal growth supplies a satisfactory mechanism for generating the known structural polytypes of cadmium iodide. It was found that all the structures of the “22…11” series could be generated by spiral growth from single nonintegral screw dislocations in an “ideal” 4H structure. All of the other known structures not belonging to this series could be accounted for in a similar manner by assuming the presence of two or more cooperating dislocations and/or an alternate arrangement of the initial ions as they take their places at the bottom of a growing step. Structural series like “22…1111”, “22…33” and “22…2123” are to be expected as a result of these considerations. The proposed causes of dislocations are outlined and then it is shown how they are related to the former explanations of polytypism. Other polytypic substances such as silicon carbide and graphite are considered in the light of these findings. It is pointed out that polytypism is not to be expected in certain substances because of the nature of their “ideal” structures.