Meeting report

American Chemical Society symposium

A symposium entitled "Advances in Structure Determination by X-ray Diffraction and Related Methods" was organized by A. Clearfield at the Spring American Chemical Soc. meeting. P. Coppens described the use of the extremely high brilliance beam at Advanced Photon Source at Argonne Nat'l Lab. to study photoinduced excited states and transient species. These results contribute to a better understanding of photochemical processes. D. Poojary described the solution of unknown structures with up to 50 non-hydrogen atoms by application of "direct" and/or Patterson methods to powder data. Excellent software now exists for deconvolution of the powder pattern and refinement of the structure by Rietveld techniques. B. Toby (NIST) describing the use of simulated annealing techniques for structure solution. B. Blessing (Hauptman-Woodward Inst) presented a succinct summary of the phase problem and its solution. He traced the history of the Harker-Kasper inequalities, the Sayre equation, Hauptman-Karle direct methods and the Shake-and-Bake Method of macromolecular structure solution. C. Campana described the advantages of using a CCD detector on small or poorly diffracting crystals (twinned, split, multiple and those with a large mosaic spread). T. Koetzle (Brookhaven) and A. Albinati discussed neutron diffraction and incoherent inelastic neutron scattering (IINS) as complementary techniques for studies of order-disorder reactions, ion exchange processes in zeolites, and dehydration reactions. Synchrotron radiation coupled with area detectors on a 1.5 sec time scale and specially designed high temperature cells are used to obtain resolved powder data. A. Schultz (Argonne) discussed neutron diffraction sources, applications to transition metal cluster hydride complexes, studies of Jahn-Teller distortion switching under applied pressure and the location of interlayer cations and water in layered oxide materials. D. Dorset achieved direct determination from electron diffraction intensities of structures of an aluminum-iron alloy, metal layers on silicon and the hydrogen position in brucite. K. Hodgson (Stanford Synchrotron Radiation Lab) used EXAFS and XAS to determine the coordination about metal centers and the oxidation state of the metal distinguishing between Cu(I), Cu(II) and Cu(III) in bridged dicopper oxo or hydroxo complexes. A. Wilkinson described the use of EXAFS and powder diffraction to match phase evolution and site specific inhomogeneities in mixed potassium tantalum niobates. D. Jones and J. Roziere (CNRS, U. Montpellier) described the use of X-ray absorption near edge spectroscopy (XANES) and EXAFS as probes for the local structural and electronic changes which occur for lithium extraction from lithium manganese spinels. B. Bujoli (U. of Nantes) utilized a combination of NMR, EXAFS and IR spectroscopies to study amine intercalates of zinc phenylphosphonate where zinc changes from six coordinate to four coordinate. This finding was corroborated by an independent powder X-ray study (Clearfield, Texas A&M). J. Jorgensen (Argonne) presented a very elegant summary of the oxygen defect chemistry of high-temperature copper-oxide superconductors. B. Von Dreele (Los Alamos) is exploring the limits of powder techniques gathering data on porcine insulin, and myoglobin. T. Egami (U. of Pennsylvania) dealt with atomic pair-density functional analysis of crystalline materials (PDF). The method of PDF analysis has been widely used in the study of liquids and glasses, but it is equally applicable to the study of crystalline materials, for materials with internal disorder this technique has particular advantages.

A. Clearfield