Topography and goniometry of very perfect crystals

In order to characterise very perfect crystals such as Si which do not show any visible defects in conventional X-ray diffraction topography, the following topics are reviewed. (1) Intensity distribution of Pendellösung fringes in section topographs. (2) The tails of rocking curves. (3) Borrmann anomalous transmission. (4) Decoration of the defects. (5) Diffuse scattering. The experiments reviewed here indicate that the currently available good crystals are not regarded as ideally perfect crystals. The deviations from ideal perfection in (1), (2) and (3) are due to weak deformation of the crystal matrix surrounding the invisible defects whereas (5) gives us direct information about the atomic structure of the defects.