Bruker AXS cordially invites you to the following workshop which will be held to accompany the XVIIIth IUCr Congress in Glasgow: "30 Years of Rietveld Analysis: The Next Generation" Wednesday 4th August 1999 This workshop gives an overview of the latest developments in the field of Rietveld analysis. The techniques presented will dramatically extend the possibilities of all profile analysis methods, starting from single line analysis and ending up with ab-initio structure solution from powder data. The workshop will emphasize the following topics: 1. A new fundamental parameters approach for describing X-ray line profile shapes 2. Rietveld refinement without the need of a parameters turn-on sequence 3. Ab-initio structure solution from powder data as part of the Rietveld refinement process In the abstract below a more detailed description of the workshop contents is provided. The procedures presented lay the foundation for the next generation of Rietveld analysis. Organizers: Dr. A. Kern, Dr. A. Coelho & Dr. M. Winter, Bruker AXS For further information please email to [EMAIL PROTECTED] or refer to http://www.bruker-axs.com/Events/glaspg1.htm Date: August 4th, 1999 10 am - 4 pm Workshop site: Lecture Theatre G29 The Gilbert Scott Building University of Glasgow Workshop fee: 30 GBP (standard rate) / 20 GBP (student rate) Fee includes workshop materials and luncheon For information about registration and the preliminary schedule please refer to http://www.bruker-axs.com/Events/glaspg1.htm ----------------------------------------- ABSTRACT: X-ray diffraction line profile fitting approaches, from single line fitting up to whole powder pattern fitting, with or without reference to a structural model, have been widely recognised to be the most important evaluation methods for material characterisation. In particular, the Rietveld structure refinement method has turned out to be uniquely valuable for (micro)structural analysis of nearly all classes of crystalline materials. Nevertheless, in spite of many exciting developments in the past 10 years, all recent profile-fitting approaches suffer severely from a number of disadvantages. Most of these fall into one of the following categories: * poor modelling of all of the various contributions to the powder diffraction pattern. This is particularily true for the main objectives: Bragg peaks * poor numerical stability as well as extended evaluation times due to poor calculation speed and unavoidable parameter turn-on sequences. THE NEW FUNDAMENTAL PARAMETERS APPROACH It is particularly the poor modelling of X-ray line profile shapes that limits the capabilities of current profile fit approaches. This, in turn, precludes the elucidation of most important, though subtle (micro)structural details. Both is of major interest, as it is well known latest since the pioneering work of Klug & Alexander in 1954, that profile shapes are a convolution of (i) the emission profile, (ii) an instrument component and (iii) specimen aberrations. Nevertheless, until now, profile fit routines have simply applied empirical models to describe X-ray line profiles. These do not distinguish between the different contributions and they are therefore quite inadequate in modelling the peak shapes actually observed in X-ray diffraction data. The fundamental parameters approach (FPA) presented in this workshop uses a convolution based method to synthesise X-ray line profiles. Instrumental and specimen aberrations are convoluted with the emission profile to form the final line profile. Peak position, shape and asymmetry are described by the instrument and sample contributions, resulting in accurate estimates of Bragg angle and profile shape. Sample related effects, such as specimen absorption, crystallite size and strain broadening, are entered as refinable values. The physical parameters of the diffractometer, such as the receiving slit length, horizontal divergence, and the primary and secondary soller slit angles, are measurable quantities and are not usually refined but can be if required. This theoretical modelling of line profiles provides information on diffractometer misalignment and/or geometric irregularities. Main features of this FPA are: * The instrument and sample contributions are calculated from first principles. This eliminates the need to determine the "instrumental profile function" by measuring "strain free" standard materials with "infinite crystallite size". With fundamental parameters a "standard-free" real structure analysis can be performed. * The physical parameters of the diffractometer, such as the receiving slit length, horizontal divergence, and the primary and secondary Soller slit angles, are measurable quantities that can be refined. This is necessary as in practice one would not expect to obtain refined values matching the actual diffractometer values exactly; there are too many second order effects in diffractometer profiles and some instrumental effects cannot be described accurately enough to make this feasible. * In addition, any user supplied function can be easily incorporated into the convolution process. The benefits that follow from being able to synthesize, and fit, X-ray line profiles accurately in terms of fundamental parameters are manifold and clearly show up from the accurate description of line profile shapes across the complete 2q range for a wide range of instrumental and sample conditions: * In contrast to conventional profile fitting methods (i.e. using empirical profile functions), with FPA the refined numerical parameters have physical significance. The accuracy of all refined parameters can be easily assessed. * The accurate description of the emission profile and instrument aberrations, together with the accurate description of the specimen aberrations due to the geometry of the experimental conditions opens the path for investigating structurally based specimen aberrations. * It is possible to identify whether or not a diffractometer is operating at its optimum resolution and determine unequivocally whether or not an observed profile indicates specimen broadening. * Synthesized profiles rather than profiles from reference materials can be used as the basis for deconvoluting instrumental effects from observed profiles. This allows real structure analysis to be carried out at much lower levels of X-ray line broadening, possibly up to apparent crystallite s
