FURTHER STUDIES OF POLYTYPISM IN LEAD IODIDE

An x-ray study of over 600 crystals of PbI2 has revealed some important facts for the formulation of a satisfactory epitaxial theory of polytypism. Some new polytypes are described, the largest being 48R with c=167.5 Å. The distribution of rhombohedral polytypes found can be described by a Poisson distribution, ps=e-mm8/s!, with m=12. Evidence for a mechanism whereby an nH polytype together with a Shockley partial dislocation a/3[01̄10] can result in a 3nR polytype is presented. The general applicability of this nH-3nR mechanism is discussed. Miscellaneous results obtained from an investigation of syntactic coalescence are reported and shown to be in harmony with the epitaxial theory. © 1968 The American Institute of Physics.