Anisotropy of anomalous dispersion in X-ray diffraction

Abstract

180MARTENSITIC TRANSFORMATIONS IN ZnS CRYSTALS mation and against a random distribution of stacking faults in the polytypes is overwhelming.The observation (see Fig. 1) that the asymptotic value of NR/NL is two indicates that the transformation mechanism does contribute to the randomization of layer positions so that for structures created by a large number of transformations the type of the layer in the M + 1 ,position does indeed become randomly related to the first one. This however does not imply that the stacking faults are randomly distributed.References MARDIX, S. (1986a). Phys. Rev. B, 33, 8677-8684. MARDIX, S. (1986b). BulL Mineral. 109, 131-142. PAULING, L. (1945 AbstractPolarization-dependent resonant Bragg diffraction in crystals is investigated both theoretically and experimentally. In order to describe the effects of anisotropic anomalous dispersion on intensity and polarization of kinematically diffracted X-radiation, a general scattering model is developed on the basis of site-symmetry-compatible second-rank scatteringfactor tensors for the absorbing atoms. For conventional four-circle single-crystal diffractometry it is shown that intensity and polarization of the diffracted beam can be predicted as functions of both crystal orientation and polarization of the incident radiation. In principle, anisotropy of anomalous dispersion may affect any reflection. In particular, it can give rise to the observation of intensities for reflections being systematically extinct by space-group symmetry. Both effects are discussed. Experimental proof of the model's validity was obtained by synchrotron-radiation X-ray diffraction measurements of mainly 'forbidden' reflections in cubic cuprite, Cu20 and tetragonal rutile type TiO2 and MnF2. The experiments were carried out at the respective K-absorption edges of the cations using different instruments at HASYLAB/DESY during dedicated mode of DORIS II (3.78 GeV). Significant anisotropy of anomalous dispersion due to excitation of K electrons into p states was observed in each case, allowing studies of the dependence of 'forbidden' reflection intensities on both radiation energy and rotation (

Polarization-dependence of anomalous scattering in brominated DNA and RNA molecules, and importance of crystal orientation in single- and multiple-wavelength anomalous diffraction phasing

“…Such anisotropy of anomalous scattering is, however, only significant in the near-edge region, as has been experimentally demonstrated in numerous investigations for salts and small-molecule compounds (Templeton & Templeton, 1982, 1995Kirfel et al, 1991;Dmitrienko, 1983). A comprehensive review of these studies has been presented by Dmitrienko et al (2005).…”

Single-Crystal X-Ray Diffractometry Using Synchrotron Radiation

“…The reason for this is that crystallographically equivalent atoms do not have the same scattering power any more exactly (both magnitude and phase may differ). Experimental evidence for the occurence of space-group extinct reflections due to AAD has been reported by Templeton and Templeton [27,[30][31] for cubic ΝaBrO3 , by Petcov, Kirfel and Fischer [28] for LiNbO3 , and by Kirfel, Petcov and Eichhorn [29] for Cu2O, TiO2 and MnF2.…”

“…The result is X-ray dichroism and double refraction. Such anisotropies have been studied for several compounds like vanadyl bisacetyl acetonate, ubidium uranyl acetate and sodium bromate [25][26][27]; LiNbO3 [28]; Cu2O, TiO2 , MnF2 [29], and more.…”

Resonant X-ray scattering in the presence of several anisotropic factors

“…This approach is widely used, e.g. by Blume (1994), Brouder (1990), Kirfel et al (1991), Kissel et al (1995) and Templeton (1998). The resonant part of the scattering factor F res H , describing the resonant X-ray scattering into the re¯ection with H hkl is equal to…”