Origin of polytype structures in CdI₂: application of faulted matrix model revisited

Abstract

Cadmium iodide is known to crystallize in a large variety of crystallographically different modifications, called polytypes. The origin of polytype structures in CdI₂ has been attributed to spiral growth round screw dislocations of integral as well as non-integral Burgers vectors created in a perfect or faulted basic matrix. Several workers have theoretically deduced the polytype structures that can result by the spiral growth mechanism. In all these deductions, it has been assumed that polytype structures having similar looking Zhdanov symbols, like the rhomohedral structures 31 and 13, are crystallographically equivalent. Recently, it has been shown by Jain and Trigunayat [Acta Cryst. A33 (1977) 257] that for MX₂ type structures, two sets of Zhdanov symbols are crystallographically equivalent if one is obtainable from the other either by an "evenshif" of the starting point or by literally reversing the sequence plus rewriting it after an "odd shift" of the starting point. This point was overlooked in the earlier deductions and this can drastically affect the theoretically predicted series of polytype structures. We have therefore rederived the theoretically expected polytype structures that can result by the spiral growth mechanism taking into account the suggestions of Jain and Trigunayat. The most probable series of polytype structures that can result from 4H and 2H basic matrices of CdI₂ have been predicted on the basis of stacking fault energy calculations. It is shown that the origin of almost all the known polytype structures of CdI₂ can be understood in terms of the screw dislocation mechanism.