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In continuation of an earlier study on monoclinic and orthorhombic space groups, the effects of Anisotropic Anomalous Scattering (AAS) on the systematically extinct (‘forbidden’) axial reflections of all tetragonal space groups are investigated under the limiting conditions of purely  σ -polarized radiation and one generally positioned ‘edge atom’ in the asymmetric unit. The compilation of the results, Polarized Structure Factors and ensuing characteristic intensity functions,  I ([unk], Ψ ), offers a concise survey over the different cases of relevant symmetry, and contributes to the understanding of FRED (Forbidden Reflection near Edge Diffraction). One interesting application of FRED is partial structure determination, i.e. the derivation of possible ‘edge atom’ coordinates, from the intensity variations  I ([unk], Ψ ), of a small number of ‘forbidden’ axial reflections. The method is invariant against the normally unknown AAS so that nothing but the occurrence of AAS and a few intensity measurements at selected azimuthal settings are required. Characteristic cases for both  h 00 and 00 l  reflections are discussed.

Anisotropy of anomalous scattering in X-ray diffraction. ‘Forbidden’ axial reflections in tetragonal space groups