Helical dislocations in crystals of lead iodide

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Abstract
The appearance of 'zig-zag' dislocation lines during the decomposition of crystals of lead iodide in the electron microscope can be explained by the climb of existing dislocations into a flattened helical form. The simple zig-zags are often distorted into cusped and looped forms and it is shown how these distortions may be accounted for by the elastic interaction between neighbouring segments of the helices. Such internal re-arrangement within helices is possible only as a result of the marked anisotropy of climb and glide processes in these particular crystals. The climb of arrays of dislocations leads to interactions between neighbouring helices. Some examples of such interaction are described and it is shown how these may lead to the formation of isolated loops of dislocation and double helices. An examination of the nature of loops formed in this way suggests an interesting possibility for the creation of new regenerative sources of glissile dislocations.