On giant screw dislocations in ZnS polytype crystals

Vapour-grown platelet crystals of ZnS containing long-period polytypes have been found by x-ray topography to contain a single screw dislocation with a very large Burgers vector parallel to the c axis. Individual polytype regions can have highly perfect lattices; the few dislocations observed in them in addition to the giant screw have Burgers vectors in the basal plane. A divergent-beam section topograph arrangement has been used to measure giant Burgers vectors to an accuracy of about 30%. Within this uncertainty, the Burgers vector magnitudes match the polytype repeat period in the five crystals upon which measurements have been made, the repeat periods ranging from 50 Å to 106 Å. The x-ray topographic studies provide good evidence in support of the periodic slip process for polytype formation.