X-ray Diffraction Study of the Structural Change from 2H to 4H in CdI2 Crystals

The structural changes, 2H → faulted 2H → faulted 4H → 4H, in CdI2 crystals, which are caused by successive heat treatments, were studied by X-ray diffraction. The faulted 2H and faulted 4H structures, which give diffraction patterns composed of sharp spots and diffuse streaks, were solved on the assumption that sharp spots arise from the `average structure' and diffuse streaks from a stacking disorder of layers. It was revealed that (1) each of the faulted 2H and faulted 4H structures is maintained unchanged over a certain range of heating temperature and (2) when two kinds of layer, V and V′, are stacked in the faulted structures, a small number of V′ layers are always followed by a large number of V layers.