X-ray standing waves in noncentrosymmetric crystals and the phase problem in crystallography

The effect of the variation of f' and f'' on the phase of the X-ray standing wave field is discussed with the example of a semiforbidden GaAs(200) reflection and incident X-ray energies around Ga and As K edges. In noncentrosymmetric crystals the phases of the structure factors contribute to the phase of the standing wave field relative to lattice atoms in the unit cell. Generation of standing waves with (006) and (00 anti 6) reflections in a (00 anti 1)-cut LiNbO_3 crystal and detecting Nb fluorescence yield demonstrates how the phases of the structure factors F(006) and F(00 anti 6) with respect to the origin at an Nb atom can be determined. The phase determination does not require the incident energy to be tuned very close to an absorption edge. (orig.)Also published in: Materlik, G.; Sparks, C.J.; Fischer, D. (eds.): Resonant anomalous X-ray scattering. Theory and applications. Elsevier Science B.V., 1994, p. 119-143Available from TIB Hannover: RN 5581(94-02) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman