Phase transformations at high temperatures in polytypic crystals of cadmium iodide

Solid state phase transformations of various kinds have been observed to occur in polytypic crystals of cadmium iodide grown from solution, upon heating them to temperatures well below their melting points. These include the transformation of lower polytypes into higher polytypes and vice versa. All polytypes have been found to transform eventually into the common CdI ₂ type 4H. Characteristic changes in "streaking" and "arcing" on the oscillation photographs have been observed. The results are explicable in terms of the movement of edge dislocations, either already existing inside the crystal or subsequently generated upon heating, and cast light on the kinetics of stacking faults and the role played by them in the formation of polytypes.