Rietveld Refinement using FullProf

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Rietveld refinement is a tool that tries to model a full powder diffraction profile based on crystal structure data, specimen and instrument effects. This is achieved by introducing certain functions that describe typical phenomena in powder diffraction experiments, like 2theta errors or peak broadening, and by fitting the corresponding parameters afterwards. The parameters are varied using a least-squares procedure, in order to minimize the difference between the calculated and the experimental powder diffraction pattern.

The method was first introduced by Hugo Rietveld in 1967 and 1969. Details of the method are available in the literature:

• H.M. Rietveld, Acta Cryst. 22, 151-2 (1967).
• H.M. Rietveld, J. Appl. Cryst. 2, 65-71 (1969).
• R.A. Young (ed.), "The Rietveld Method", International Union of Crystallography, Oxford University Press, New York 1993.

One of the key issues of Rietveld refinement is that the method is rather sensitive to problems and errors in the model, resulting in non-convergence and other issues. Hence, it has become a common agreement for many scientists that a successful phase analysis should be proven by a successful Rietveld refinement converging at low R- and chi²-factors. In addition, running Rietveld refinements on your diffraction pattern will give you an opportunity to think and learn more about your sample, your experiment and your model.

Today a variety of excellent programs for Rietveld refinement are available, so there is little need to invent the wheel again and again. Match! uses the well-known Rietveld program FullProf (J. Rodriguez-Carvajal, Physica B 192, 55 (1993)) to actually perform the calculations. You do not have to interact with FullProf directly though; instead, you can use the Match! user interface to define ("turn-on") the parameters, setup the calculations and evaluate the results.

In order to introduce users that are new to Rietveld refinement to this method, we have implemented four different approaches to define ("turn-on") the options and parameters for the calculation. The method that is easiest to use ("Automatic Rietveld Refinement") is available simply by marking the corresponding checkbox at the top of the Parameter Turn-On dialog, while the three remaining facilities "Beginner", "Advanced" and "Expert" are present as tabs in the main part of the dialog. The parameters on the three tabs are linked to each other, so that you can start turning on parameters e.g. on the "Beginner" tab and finish as an "Expert".

Once you have selected all parameters you would like to refine, the Rietveld refinement calculation is started simply by pressing the "Run FullProf" button at the bottom left of the "Parameter Turn-On" dialog. Match! will automatically create the control and data files required for FullProf, run the calculation, and read the results. The results (e.g. R-factor, chi2, FullProf comment) will be displayed in the "Rietveld" tab to the right of the Match! window. You will also be informed if the calculation was successful (reached convergence) or not.

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