The importance attributed to modulated structures in recent years is reflected by a rapidly increasing number of publications. Modulated structures can be considered as crystals of a basic structure with a periodic distortion. The distortion can be either displacive (variable atomic positions) and/or occupational (variable composition) in nature. In either case this variability reveals itself by the existence of so-called satellite reflections, often weaker than the main reflections of the underlying basic structure. The introduction of area detectors has enabled structural scientists to recognize these characteristic diffraction patterns more easily and with JANA2000 a software tool is available that allows to handle the resulting intensity data according to the specific requirements of superspace refinement. KNbOB2O5, a recent problem from our Application Lab, provides an excellent example for an understandable modulated structure, which, at the same time, fully demonstrates how to proceed with indexing, data processing, structure solution, and refinement. The underlying concepts of the superspace approach will be presented and discussed step by step via our sample structure. We believe that this type of project-based learning is a valuable addition to the traditional textbook method and will gain more significance in the future as the subjects of crystallographic teaching become more and more complex. Acta Cryst. (2002). A58 (Supplement), C187 Keywords: TEACHING METHODS MODULATED STRUCTURES SUPERSTRUCTURE DESIGN OF MODULAR CRYSTALLOGRAPHIC LECTURES WITHXML BASED TECHNOLOGIES G. Schuck W. Steurer ETH Zurich Laboratorium fur Kristallographie Sonneggstr. 5 ZURICH CH-8092 SWITZERLAND Web-based courses in crystallography have become quite common. Since these applications are mostly HTML-or PDF based, we find often that content, structure and presentation are lumped together. HTML-or PDF presentations can be produced with standard scientific programs like Latex or Microsoft Word but sharing, reuse and modification of these data is impeded. That means HTML-and PDF based courses are somewhat inflexible and also devicedependent. Most of the web-based crystallographic courses have an auditorium with a very different educational background. Crystallographic lectures may be of interest for earth scientists, materials scientists, chemists, physicists or biologists. Consequently, we need different lectures for each target group and a high degree of flexibility and modularity in the teaching environment. In order to achieve such a flexibility, course modules may be prepared in the Extensible Markup Language (XML) [1], maintained in a repository and organized way using metadata based on standards like ARIADNE [2]. Markup descriptions are usually device-independent. The device-dependent information is stored in a separate description, called style sheet. The word 'style' refers to the mapping between the markup tags and the semantics. There are several proposed standards for describing style information like XSL (Extensible S...
The reflection overlap problem is the primary limitation in structure determination from powder diffraction data, and over the last decade, a procedure that addresses this problem experimentally by exploiting preferred orientation has been developed [1]. The intensity variation in the X-ray diffraction pattern obtained from a textured sample as a function of sample orientation (tilt χ and rotation ϕ) can be applied to resolve the relative intensities of reflections that overlap in a conventional powder diffraction pattern. The availability of a new version of the 1-dimensional Si-microstrip detector (Mythen II) on the Materials Science Beamline at SLS [2] and new features in the computer program Maud [3] have allowed both the experimental and the data analysis procedures to be redesigned. By applying these modifications, it was expected that a significant improvement in the quality of the extracted reflection intensities could be achieved.To test this modified method, two samples of a zirconium phosphate framework material (ZrPOF-pyr) with a known crystal structure [4] were measured: (1) a textured sample using the revised experimental setup and data collection strategy, and (2) an untextured sample in a 0.5mm capillary. Both datasets were collected at the SLS using the Mythen II detector. For the analysis, reflections up to a minimum dspacing of 1.0 Å were considered, and a reflection separation factor of 0.5*FWHM was used to define the overlap groups.Comparison of the structure factors extracted from conventionally measured powder diffraction data with those derived from a textured sample indeed shows the latter to be significantly better. The agreement between the amplitudes obtained with a LeBail extraction using Maud for the untextured data and those calculated from the refined crystal structure [4] can be expressed as an R value of 0.453. For overlapping reflections (80.2% of the reflections) an R value of 0.547 was obtained. Amplitudes derived in a joint extraction using 32 powder patterns collected on a textured sample yielded R values of 0.380 and 0.408 for all and overlapping reflections, respectively. This improvement was obtained even though the peaks in these patterns are broader (presumably a result of multiple measurements of this particular sample) and have 91.9% overlap.The quality of the extraction could be further improved by combining the amplitudes derived from the textured sample with those derived from the sharper untextured one. This combination results in R values of 0.326 for all and 0.385 for overlapping reflections. This obvious improvement in the estimation of the structure factor amplitudes should allow some crystal structures that were previously inaccessible to be solved. If the remaining difficulties with the estimation of the background and the description of the peak shapes can also be solved, the method would become even more powerful.[1] Ch. Baerlocher et al
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