Diffraction Anomalous Fine Structure on Forbidden and Superlattice Reflections

The phenomenon of diffraction anomalous fine structure (DAFS) has been examined in relation to structural symmetry and is discussed for PbZrO₃ in the space group Pbam. It is found that DAFS appears on the superlattice reflection, even if the intensity of the reflection is free from the anomalous dispersion of heavy atoms at the relevant absorption edges. For reflections that are forbidden by the glide symmetry, DAFS oscillation can be observed, unaffected by the influence of absorption and of X-ray absorption fine structure oscillation; the amplitude of DAFS oscillation depends on the polarization of incident X-rays. The role of DAFS signals in superstructure investigation is discussed with respect to highly absorbing crystals of finite size.