Abstract
The Rietveld method is the most reliable and powerful tool for refining crystal structure when powder diffraction data are available. It requires that the structure model to be adjusted is as close as possible to the true structure. The Rietveld method usually represents the final step of the powder solution process, in particular when a new structure is going to be determined and published. EXPO is a software able to execute all the steps of the solution process in a mostly automatic way, by starting from the chemical formula and the experimental diffraction pattern, passing through computational methods for locating the structure model and optimizing it, and ending to the Rietveld refinement. In this contribution, we present the most recent solution strategies in EXPO, both in reciprocal and direct space, aiming at obtaining models suitable to be refined by the Rietveld method. Examples of Rietveld refinements are described, whose results are related to different solution procedures and types of compounds (organic and inorganic).
Highlights
The Rietveld method [1], born from the simple and brilliant idea of refining crystal structure together with parameters describing the diffraction profile employing directly the profile intensities, has been one of the most innovative and still widely applied methods for studying materials from powder diffraction data
We briefly describe the basic principles of direct methods used by EXPO for obtaining the structure model and its main strategies for optimizing the model up to Rietveld method expectations
It requires that the structure model to be adjusted is physically and chemically reliable diffraction data
Summary
The Rietveld method [1], born from the simple and brilliant idea of refining crystal structure together with parameters describing the diffraction profile employing directly the profile intensities, has been one of the most innovative and still widely applied methods for studying materials from powder diffraction data. In the case of powder diffraction peak overlap, background not always simple and described, preferred orientation and limited experimental resolution are concurrent problems that compromise, sometimes considerably, the process of extracting the structure factor moduli from the experimental powder diffraction pattern [5], and lower the reliability of the moduli estimates In this scenario, using the experimental structure factor moduli as observations in the least-squares minimization for refining structural parameters can be unsuccessful, or at least it needs a powerful weighting scheme of reflections for compensating for the low accuracy of the extracted moduli [6]. Due to the dependence on numerous variables, the minimization process can fall into false minima or diverge Such a risk can be reduced and possibly avoided if the quality of the experimental diffraction pattern is good, peak and background are described by suitable functions and, especially, the structure model makes physical and chemical sense. We describe briefly the EXPO strategies and show examples of Rietveld refinement by EXPO
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