Running Rietveld/FullProf calculations using the "Parameter Turn-On" dialog

The so-called "Parameter Turn-On" dialog is used to setup and run a Rietveld refinement calculation. It can be opened either by selecting the "Rietveld (FullProf)" command in the "Tools" menu, by pressing the corresponding "FP" button in the center of the toolbar, by pressing “Ctrl + Shift + R” on your keyboard, or by pressing the "Run new calculation" on the "Rietveld"-tab on the right-hand side of the Match! window.

Prerequisites

Before you use the Parameter Turn-On dialog, you first have to perform the normal phase identification procedure, i.e. import (raw/profile!) diffraction data, find the peaks, run a search-match and select one or more “matching phases”.

Once you have run one of the commands mentioned above to open the Parameter Turn-On dialog, Match! checks if any of the entries you have selected as “matching phases” does not contain a full crystal structure data set (e.g. if the atomic coordinates are missing). In this case you will be prompted to import suitable crystal structure data from cif-files.
The background is that it is not possible to run Rietveld refinements without complete structural models. Once complete crystal structure data sets are available for all matching phases, the “Parameter Turn-On” dialog will be displayed.

Please note:

Concept/layout of the Parameter Turn-On dialog

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".

We will now discuss the four approaches of using the “Parameter Turn-On” dialog in more detail:

Automatic Rietveld Refinement

The easiest way to setup a Rietveld refinement calculation is to activate the “Automatic Rietveld refinement” option at the top of the “Parameter Turn-On”-dialog. If this option is active, all remaining options are disabled. Afterwards, you can run the automatic refinement calculation by pressing the “Run FullProf” button on the lower left-hand side. The calculation(s) will be run automatically; at the end, the result will be displayed on the “Rietveld”-tab on the upper right-hand side of the Match! window.

For the automatic Rietveld refinement it is possible to apply user-defined refinement sequences other than the factory one. In order to do so, please press the Configure... button at the top of the "Parameter Turn-On" dialog. This will open the Define Parameter Sequence for Automatic Rietveld Refinement dialog in which you can select which parameters shall be refined in which order (cycle) during the automatic Rietveld refinement procedure. The user-defined refinement sequences can be stored in this dialog and recalled later on in the Parameter Turn-On dialog using the Preset combo box at the top.

By default, either the "Standard" (factory) parameter sequence set or the one that has been selected as "default" in the Define Parameter Sequence for Automatic Rietveld Refinement dialog is run during automatic refinement. If you would like to run a different user-defined parameter set, please select it from the Preset combo box.

Beginner

The next step for learning how to do Rietveld refinement is to deactivate the “Automatic Rietveld refinement” option, and to use the options (checkboxes) on the “Beginner” tab. On this tab, a selection of Rietveld parameters is available, along with a suggested order in which they shall be turned on in successive calculations.

For example, in the first calculation only the scale factors and specimen displacement parameters are recommended for refinement. If this first refinement calculation is successful, the unit cell and first background parameters can be activated in addition to the first parameters, and another refinement calculation is performed. If this is successful, the W and atomic coordinate parameters are added etc.

In fact, this is just the order in which the calculations are carried out during the automatic refinement. However, you now have more control and can stop at certain points or repeat steps if you like. This order is taken from the excellent book "The Rietveld Method" edited by R. A. Young (1993).

Advanced

Once you are familiar with the principal procedure and main parameters, you are ready for more detailed options and parameters. Hence, we have introduced the “Advanced” tab, on which a reasonable amount of parameters is grouped according to their effect: “Peak positions”, “Peak intensities” and “Profile shape”.

You can e.g. address specific problems e.g. concerning the positions of the peaks by marking one or the other parameter on the “Peak positions” tab for refinement.

Another feature on the “Advanced” tab (that is also available on the “Expert” tab) is the possibility to define constraints for a certain parameter. In order to do so please press the “...” button to the right of the corresponding parameter (e.g. “Shift on 2theta-axis”). A new dialog window will open (Fig. 18) in which you cannot only modify the parameter value or mark the parameter for being optimized, but also define contraints by entering the limiting numerical values in the “Minimum value” and/or “Maximum value” fields.

At this stage you should also note the table of “Active parameters” on the right-hand side of the Parameter Turn-On dialog: Here you can get an overview over the parameters that are currently marked for being varied in the next refinement calculation. Please note that the parameter names given in the left-most column are the original full parameter identifiers used in FullProf. If you double-click on a certain line of the table, the parameter modification dialog just mentioned will be opened, so that you can modify parameter settings directly without having to search for the corresponding “...” button or tab.

Expert

This is the most comprehensive but also the most demanding option for setting up the Rietveld refinement parameters. All FullProf parameters that are currently supported by Match! are available here. You can adjust global settings like “Zero” or “Lambda” on the “Global” tab, as well as select or modify parameters for individual phases (e.g. unit cell parameters or atomic coordinates). For each phase of the Match list, a separate tab is available. All parameters belonging to a certain phase are distributed to subtabs like “Quant.”, “Profile” or “Crystal structure”.

Pattern calculation

If you do not mark any of the “vary” checkboxes in the "Parameter Turn-On" dialog (i.e. if the table of “Active parameters” on the right-hand side is empty), you can instruct FullProf to calculate the powder diffraction pattern (based on the current settings and parameter values) simply by pressing the “Run FullProf” button. When the calculation has finished, you can e.g. export the calculated pattern using the menu command File/Export/Profile data (Rietveld).

Running the calculation

When you have selected all parameters you would like to refine, the Rietveld refinement calculation is started by pressing the Run FullProf button at the bottom left of the Parameter Turn-On dialog. When the calculation has finished, 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.

Buttons in “Parameter Turn-On” dialog

There are a couple of buttons located at the bottom of the “Parameter Turn-On”-dialog: