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46868-SE
Diffuse Scattering for the Masses: the Characterization of Local Structural Correlations in Molecular, Macromolecular, and Inorganic Crystals. American Crystallographic Association Transactions Symposium, July 2007, Salt Lake City, UT
The symposium topic reflects the now-common observation of diffuse features in area-detector images from virtually all material classes, and the increasing awareness that local structural features strongly influence many of the useful properties of real crystals. The well-attended full-day symposium, comprised of 12 speakers and 2 poster presentations, touched on a range of current scientific, instrumental and computational developments, while also devoting significant attention to introductory concepts, practical data collection strategies and methods of qualitative interpretation. The papers presented at the symposium will be published online via the ACA website by our Transactions editor, Jim Britten (McMaster University).
Two foreign keynote speakers were supported by the SE grant at the level of $1,200 USD each: Richard Welberry (Research School of Chemistry, Australian National University) and Friedrich Frey (Dept. of Geological and Environmental Science, Ludwig-Maximilians-University of Munich). Their participation greatly enhanced the stature and the content of the symposium.
Richard Welberry opened the morning session with a brief history of diffuse scattering methods, and proceeded to describe the use of highly-versatile Monte Carlo simulation methods to analyze and interpret diffuse scattering patterns. By strategically exploring a small number of interaction parameters (e.g. intermolecular spring constants, intramolecular torsion constants, local displacement-displacement correlation coefficients, etc.) one can identify atomistic disorder models that reproduce experimental diffuse scattering features. Richard then demonstrated the use of these state-of-the-art tools to uncover framework faults in zeolite catalysts and nanopolar domains in piezoelectric relaxor materials.
Friedrich Frey opened the afternoon session by introducing the foundational concepts of diffuse scattering for newcomers to the field, and illustrated them with practical examples from solid electrolytes and other complex oxides. These concepts included the description and experimental identification of the average structure, size and strain effects, anisotropic displacement parameters, static displacements, substitutional/chemical short-range order, and domain disorder, as well as descriptions of some common interpretive errors.
Branton Campbell (Brigham Young University) reviewed the challenges and prospects of fitting local-structure models against 3D volumes of diffuse scattering data. Jim Britten (McMaster University) illustrated the application of modern graphic-visualization routines to interactively explore large volumes of diffuse-scattering data obtained from a standard crystallographic data collections. George Phillips (University of Wisconsin-Madison) described the use of normal-mode-analyses based on Gaussian Network Models to discover the elusive details of protein dynamics. This approach was further illuminated during the poster session by his student, Demian Riccardi. Lars Meinhold (Caltech) presented the joint use of diffuse scattering experiments and molecular dynamics simulations to determine the phonon vibration spectrum and the conformation energy landscape of macromolecules. Andrew Beasley (Australian National University) demonstrated the application of Welberry's Monte Carlo intermolecular force method to obtain the structure of a strongly-disordered polymorphic organic system. Abbas Ourmazd (University of Wisconsin-Milwaukee) proposed a novel solution to the problem of determining molecular orientations at X-ray free-electron laser sources based on the statistical correlation analysis of coherent diffuse scattering patterns. Stephan Rosenkranz (Argonne National Laboratory) presented plans for CORELLI, a dedicated single-crystal time-of-flight neutron diffuse-scattering instrument with elastic-energy discrimination to be built at the new Spallation Neutron Source. Simon Billinge (Michigan State University) updated us on the state-of-the-art in powder pair-distribution-function analysis: dedicated neutron instruments, rapid x-ray data-collection strategies, open-source analysis software, and a variety of local-structure models. Ross Angel (Virginia Tech) showed that a simple qualitative interpretation of diffuse scattering data will often yield a wealth of local-structure information. The diffuse scattering from octakis-octasilicate, and the unusual columnar faulting implicated, was described in detail by Carla Slebodnick (Virginia Tech) during the poster session. John Konnert (Naval Research Laboratory) described the analysis of fully 3D x-ray diffuse scattering data to obtain the structure of a liquid-crystal elastomer comprised of two cross-linked molecular components.
The symposium presentations demonstrated that a variety of specialized tools are already available for use with data from standard diffractometers and detectors. A few key components (e.g. 3D visualization) are likely to become integrated with standard systems soon. Standard GUI-driven tools for generating local-structure models and simulating single-crystal diffuse-scattering patterns are currently feasible and could be available within a few years if the crystallographic community maintains the current level of interest. The symposium and subsequent interactions amongst its participants helped to bring a number of pressing needs into focus and fostered new collaborations to address those needs.
