Abstract
X-ray topography is a powerful non-destructive method for the direct observation of defects in nearly perfect crystals. Since organic molecular crystals usually contain ‘light’ atoms only, they exhibit low X-ray absorption and scattering power. Thus ‘thick’ crystal plates (up to 3 mm for CuKα radiation) can be studied by transmission topography, and defects appear by broad kinematical contrast. The image widths of dislocation lines, however, are large, usually > 20 µm, leading to limited spatial resolution and to the requirement of high-quality crystals.The transmission techniques of projection topography and section topography are explained and illustrated by selected topographs of perfect and imperfect organic crystals. Typical defects observed in nearly perfect organic crystals are inclusions, growth striations, defective growth-sector boundaries, grown-in dislocations and glide dislocations. The determination of Burgers vectors of dislocations and the characterization of planar defects (growth-sector boundaries) by section topography are discussed and demonstrated. Topographs of typical representatives of organic molecular crystals, such as benzil, benzophenone, salol and 2,3-dimethylnaphthalene crystals grown from solution, from undercooled melt, by the Bridgman method or by the Czochralski technique, are presented.
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