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 of MAD (multi-wavelength anomalous diffraction) and DAFS/DANES (diffraction anomalous fine structure/diffraction near edge structure) methods with relatively light elements down to Z=14 (silicon), including elements such as sulfur and phosphorus essential in life. It also exploits the very strong dispersion of heavy elements (like uranium) both in resonant magnetic scattering and in protein crystallography. A brief review of the technical progress will be given. Recent measurements on the fine structure of anomalous diffraction from a ferroelectric salt, (NH3CH3)5Bi2Cl11 at wavelengths near the K-X-ray absorption edge of chlorine have shown that diffraction experiments with 4.4 ? photons are becoming feasible. The dispersion of 60 reflections (36 unique reflections) has been measured at 30 energies between 2809 and 2838 eV in steps of 1 eV. The theoretical calculation of the dispersion X-ray diffraction from the chlorobismuthate anion (Bi2Cl11)5? suggests a strong anisotropic of anomalous dispersion which is confirmed by our data. The data were collected at room temperature, well below the phase transition at 307 K where the salt looses its ferroelectric property. Future experiments aim at the observation of the dispersion of X-ray diffraction at various temperatures near the temperature of phase transition, where the site-selectivity of DAFS is well suited to follow the role of the central Cl-atom bridging the two Bi atoms. Our present experiments provide an insight 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 ways for the improvement of this unique instrument.