ANISOTROPY OF ANOMALOUS SCATTERING IN X-RAY-DIFFRACTION - FORBIDDEN AXIAL REFLECTIONS IN TETRAGONAL SPACE-GROUPS

In continuation of an earlier study on monoclinic and orthorhombic spa ce groups, the effects of Anisotropic Anomalous Scattering (AAS) on th e systematically extinct ('forbidden') axial reflections of all tetrag onal space groups are investigated under the limiting conditions of pu rely sigma-polarized radiation and one generally positioned 'edge atom ' in the asymmetric unit. The compilation of the results, Polarized St ructure Factors and ensuing characteristic intensity functions, I(h ov er arrow pointing right, PSI), offers a concise survey over the differ ent cases of relevant symmetry, and contributes to the understanding o f FRED (Forbidden Reflection near Edge Diffraction). One interesting a pplication of FRED is partial structure determination, i. e. the deriv ation of possible 'edge atom' coordinates, from the intensity variatio ns (h over arrow pointing right, PSI) of a small number of 'forbidden' axial reflections. The method is invariant against the normally unkno wn AAS so that nothing but the occurrence of AAS and a few intensity m easurements at selected azimuthal settings are required. Characteristi c cases for both h00 and 00l reflections are discussed.