ANISOTROPY OF ANOMALOUS SCATTERING: NEW EXPERIMENTAL EVIDENCE. A. Kirfel, Mineralogical Institute, University of Würzburg, and T. Lippmann, HASYLAB/DESY, Hamburg, FRG

Anisotropy of Anomalous Scattering (AAS) is caused by resonant X-ray scattering occurring in the vicinity of an absorption edge and giving rise to X-ray optical effects in both transmission and diffraction. The description requires a generalized scattering model which predicts the intensity variation and polarization properties of the scattered radiation upon rotation [[psi]] of the crystal about the scattering vector [1]. As a special consequence, the excitement of 'forbidden' reflections systematically extinct due to screw axes and/or glide planes is of particular interest, because the observed signal allows direct experimental proof of the model, independent of size and complexity of the non-resonant scattering structure. This selectivity of scattering provides the opportunity of partial structure analysis, site discrimination in structures possessing non-equivalent cation sites, and the distinction of resonance type. New experimental evidence of the model validity is obtained by synchrotron radiation experiments. For Cuprite, Cu₂O, (Pn3m), reflection 300 , by i) rotation of the polarization direction of the incident radiation about the beam axis, i.e. rotation of the scattering plane, and ii) direct polarization analysis by secondary reflection using Ge analyzer crystals. For orthorhombic Sodium Nitroprusside, Na₂[Fe(CN)₅(NO)] 2H₂O (Pnnm), the intensity variations, I([[psi]]), of several 'forbidden' axial reflections are used for an ab initio structure determination. Finally, the intensity patterns of AAS induced 'forbidden' reflections in cubic Magnetite, Fe₃0₄ (Fd3m), clearly reveal that the observed signals are solely due to a mixed dipole-quadrupole transition in Fe³⁺ occupying the tetrahedral position (43m), whereas the octahedral site (3m) fails to contribute significantly.

[1] KIRFEL, A., PETCOV, A. & EICHHORN, K. (1991). Acta Cryst. A47, 180-195.