Mode of layer displacements in MX2-type crystals

Using van der Waals potential, the relative strengths of interlayer and intralayer bonds have been estimated in the halides of cadmium and lead. The results have been compared with the recent Raman studies of rigid layer modes in MX₂-type layer compounds and X-ray diffraction studies of fused cadmium iodide. It is found that the interlayer bond is relatively much weaker than the intralayer bond. Accordingly, it is concluded that in the polytypic MX₂-type compounds CdI₂, CdBr₂ and PbI₂, only X-X glide takes place in the polytypic transformations observed during crystal growth and during high temperature treatment of the polytypes. The nature of the X-X glide has been examined in detail.