The article reviews the electron microscope studies of incommensurate phases in dielectrics. General structural properties of the incommensurate phase such as the formation of discommensuration (DC) array near the Lock-in transition point are described. Superspacegroup is used for analyzing the continuously modulated structure, being applied to the analyses of the incommensurate structures of Sr2Nb2O7 and Bi2Sr2CaCu2O8. A DC array corresponds to an array of domain walls in the commensurate phase, which forms a characteristic domain structure, such as a so-called ‘stripple’. The domain configuration around the stripple in the IC phase of Rb2 ZnCl4 is identified from the diffraction contrasts of DCs in the electron microscope images.
The interrelation between the symmetries of CBED patterns and symmetry symbols for incommensurately-modulated structures given by de Wolff, Jannsen and Janner are clarified. 1) Point groups; The symmetry symbol 1 shows no symmetry in CBED patterns. The symbols other than 1 show the symmetries of the corresponding symbols for the average structure. 2) Space groups; The four dimensional glide planes and screw axes give dynamical extinction in the kinematically forbidden reflections due to the symmetry elements. Dynamical extinction rules in CBED patterns were tabulated for one example of the four dimensional space groups.
It has recently been shown that X-ray moire fringes are not exactly the projection of the intensity distribution of the wave field on the exit surface of the crystal, but do oscillate along the beam path behind the crystal. Such nonprojectiveness of moire fringes is inexplicable by the conventional understanding of moiré fringes, and therefore is of interest from a fundamental viewpoint as well as practical one. In this paper the phenomena of the nonprojectiveness are described on the basis of the latest experimental data obtained by synchrotron radiation.