Anomalous diffraction with soft X-ray synchrotron radiation: DANES from pentakismethylammonium undecachlorodibismuthate at the K absorption edge of chlorine

Soft X-ray diffraction from synchrotron radiation gives access to the use o f MAD (multi-wavelength anomalous diffraction) and DAFS/DANES (diffraction anomalous fine structure/diffraction near edge structure) methods with rela tively light elements down to Z=14 (silicon), including elements such as su lfur and phosphorus essential in life. It also exploits the very strong dis persion of heavy elements (like uranium) both in resonant magnetic scatteri ng and in protein crystallography, A brief review of the technical progress will be given. Recent measurements on the fine structure of anomalous diff raction from a ferroelectric salt. (NH3CH3)(5)Bi2Cl11 at wavelengths near t he K-X-ray absorption edge of chlorine have shown that diffraction experime nts with 4.4 Angstrom photons are becoming feasible. The dispersion of 60 r eflections (36 unique reflections) has been measured at 30 energies between 2809 and 2838 eV in steps of 1 eV. The theoretical calculation of the disp ersion X-ray diffraction from the chlorobismuthate anion (Bi2Cl11)(5-) sugg ests a strong anisotropic of anomalous dispersion which is confirmed by our data. The data were collected at room temperature. well below the phase tr ansition at 307 K where the salt looses its ferroelectric property. Future experiments aim at the observation of the dispersion of X-ray diffraction a t various temperatures near the temperature of phase transition, where the site-selectivity of DAFS is well suited to follow the role of the central C l-atom bridging the two Bi atoms. Our present experiments provide an insigh t into the present technical state of art of soft X-ray diffraction at the beam line ID1 at the European Synchrotron Radiation Facility in 1999 and wa ys for the improvement of this unique instrument. (C) 2001 Elsevier Science B.V. All rights reserved.