High-resolution dark-field electron microscopy. II. Short-range order in crystals

The simple theory for the intensity distributions in high-resolution dark-field electron microscope images of thin specimens, derived from the phase-object approximation, is applied to discussion of images obtained from the diffuse scattering in electron diffraction patterns from crystals having short-range ordering of atoms on the lattice sites. It is shown that interpretation of the intensity distribution in terms of images of 'microdomains' is not usually justified. It is necessary to take into account the statistics of superposition of defects, possible dynamical diffraction effects and the fact that both positive and negative deviations from the average value of projected potential may give intensity maxima. Examples used to illustrate these points include a hypothetical case of a disordered CuAu crystal and some experimental observations of disordered LiFeO₂ showing a resolution of details on an atomic scale.