Crystal structures of three polytypes of lead iodide: correlation between phenomena of arcing and polytypism

An X-ray study of single crystals of lead iodide grown by the gel method has revealed that the arcing phenomenon occurs rarely in this compound. The polytypism is also not observed frequently. The arcing phenomenon originates from arrangements of partial or unit edge-dislocations created during crystal growth. On the basis of Jagodzinski's disorder theory the polytypism can also be explained in terms of partial edge dislocations. Thus, the two phenomena appear to be interrelated. The complete crystal structures of three PbI₂ polytypes, 10H, 14H and 20H, have been computed. These are found to be (11)₃22, (11)₅22 and (11)₇2112, respectively, in Zhdanov symbols, with space group P3m1. Their formation can be explained in terms of partial edge dislocations created at regular intervals, providing further support to the possibility of the polytypism and arcing phenomena being mutually related.