Mechanism of phase transformation in polytypes.

Abstract

Several materials with a close-packed structure, like ZnS and SiC, are known to undergo solid-state transformation from one polytype structure to another by the insertion of stacking faults. During the course of the phase transformation the stacking faults occur preferentially at such layer spacings as to statistically create the new structure, which is usually faulted. In such cases the classical theories of X-ray scattering from randomly faulted close-packed structures break down and a probability distribution of faults has to be assumed to compute the diffraction effects. This probability distribution depends on the mechanism of the transformation in the material being studied. The nature of the stacking faults involved in the transformation and their distribution can be determined by arresting the transformation at an intermediate stage and studying the disordered partially transformed crystal by XRD techniques. A comparison of the diffraction effects recorded on a single-crystal diffractometer with those computed theoretically for different models of the transformation enable both the type of faults involved and the probability distribution of their occurrence during the transformation to be determined. The application of this method to investigate the mechanism of transformations in ZnS, SiC, ZnxCd1-xS and ZnxMn1-xS is reviewed and the results obtained are discussed.-J.M.H.