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GSAS/EXPGUI Alumina tutorial (part 7)
Fitting the Unit Cell

In the previous tutorial, we saw that the unit cell was not doing a very good job of indexing the diffraction peaks. Since a large number of peaks did have computed reflections at least partly overlapping the observed peaks, we are close enough to fit the unit cell parameters using a refinement. If that were not true -- the tick marks did not fall in the range of the peaks, refinement would not be possible.

One the Phase panel, turn on the flag to refine the unit cell parameters (upper right), as shown below.

tutorial screen dump

Also, increase the number of cycles to 6.

tutorial screen dump

Run GENLES (as before). Note there are now 9 parameters (scale, 6 background, + 2 cell) and that the fit significantly improves, as seen below.

tutorial screen dump
(As before, press Enter & load the modified experiment file into EXPGUI.)

The peak positions are fit much better than before, as seen in the LIVEPLOT output below.

tutorial screen dump

Since the peak positions have changed significantly, we now need to rerun POWPREF and use the new peak positions to decide which data points need to be indexed with each reflection.

tutorial screen dump

It is not strictly necessary to run GENLES again, but note that CHI-squared improves from 70 to 60 just from the better indexing.

tutorial screen dump

Previous: Plotting the Initial Fit
Next step: Fit the Diffractometer Zero Correction

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Last modified 18-July-2003 by website owner: NCNR (attn: Craig Brown)
$Revision: 1.1 $ $Date: 2002/07/18 20:45:10 $