Real space crystallography and defects in molecular crystals

Abstract

An image plane technique of crystallographic analysis (real space crystallography) is shown to be particularly useful in studying defects in thin molecular crystals. Contrast in electron images from line defects is enhanced at bend contours and details of the interaction between the strain fields of dislocations and the bend contours give an indication of the Burgers vector. The shift in bend contours at coherent planar boundaries separating two orientationally related structures (as occur in martensitic transformations) is found to be directly related to a variation in deviation (excitation) parameter as a result of a change in the position of a particular reciprocal lattice point perpendicular to the electron beam. Observations are consistent with a previously suggested model for such a transformation in 1∶8 dichloro-10-Me anthracene. In addition, dark-field images taken around a bend contour pole in anthracene provide evidence for the existence of forbidden reflections resulting from double diffraction in the space group P2_1/a .