This example uses example Cimetidine data from the EXPO software
A method to get Le Bail extracted HKL intensities is to use a variety of
Rietveld packages. The advantage of this is you can have great control
over the Le Bail fit - as well as model in secondary phases of a known
structure. In this example, we will use the GUI LHPM-Rietica for Windows
to do a Le Bail fit, output the extracted intensities in either Shelx or
EXPO format, and generate an EXPO (*.EXP) control file.
For a full list of available programs that will do Le Bail fitting refer: Programs that will perform Le Bail extraction
Setting up the Rietica-LHPM input fileAssuming you have the data file already, start Rietica and select File, New Input.
Select under "Phases" that you wish to do an extraction and unless you already have the synchrotron diffractometer defined as an "instrument", stick to the defaults and we will change these in the input file later.
Select Model, General and setup the general information as shown in the following screen dump. Number of cycles, automatic Marquadt damping, data file format, we wish to obtain a EXTRA/Sirpow97 HKL file, and that we are using Synchrotron data.
This would be a good time to save and name the file. Select File, Save and browse to the directory that holds the Powder Diffraction data for cimetidine that you are using.
Select Model, Phases and input the basic information. That we want to do a Le Bail Extraction, Spacegroup and cell constants. the cell constants and two-theta offset should be from your unit-cell refinement so the Rietveld program should be right on the spot for starting the extraction. Le Bail fitting with an inaccurate cell can put you in a world of pain as the Le Bail fitting via the Rietveld software goes beserk.
Select Model, Histograms and input the basic information. Setting the start, step and stop to zero tells LHPM to use the full pattern for the extraction. A Chebyshev function should be OK for fitting the background. Set the background function to refine.
Select Model, Sample and input the sample information. FJC Pseudo-Voight can be a flexible function to use that should work. As this is synchrotron data, set a thin width. Normally it would be good to fit a standard such as NIST/NBS Silicon or NIST/NBS Cerium Oxide to get good starting width and shape values. But if we don't have this information, starting off with a constant width should be OK.
Now we are ready to start the Le Bail fitting. The only parameters
we are fitting at the moment are the background parameters. Select
Rietveld, Refine, select the data file, select 10 cycles.
Now start the refinement by selecting Start then Step. Don't be intimidated if the fit is very bad to start with. This is due to the initial eqipartition of starting intensities. Bad initial fits can also be due to problems width and shape. This will improve as we refine these options.
(be careful to watch for weirdess in the refinement. You will notice that in the following Rietveld plot, not all the HKLs are matching up with observered lines. This is because an error was made in the selection of the spacegroup (P 21/N instead of P 21/A). These errors are easy to make so always keep an eye out for errors of this sort. The screen dumps after this were made with the correct spacegroup.
Now go into a series of release parameter, refine set of refinement cycles.
General (depending on the range of data and peak assymetry) releasing,
i) zero offset, ii) unit cell, iii) W-width iv) gamma1 shape,
v) V-width vi) gamma2 shape, vii) U-width, viii) gamma3 shape.
(if necessary, the asymmetry (S/L and D/L) parameters can be released.) Depending
on data quality, the parameters can be released in bunches or one at a time. You will
only find out the hard way. Don't release the scale factor parameter as this is
meaningless with Le Bail fitting.
Following is the fit after unitcell, 2-theta offset, V, W and Gamma 1 shape have been released.
Completed fit, all width and shape parameters, and one assymetry parameter.
Completed Le Bail fitThe *.FOU file is ready for use in EXPO or processing by Overlap.
Using File, Export, you can create a starting Sirpow/EXPO file ready for structure solution. To use the following EXPO file created by LHPM-Rietica, all you have to do is name the FOU file, and put in the cell contents.