Also refer to Spherical Harmonics for Preferred Orientation correction and Bragg-Bretano geometry
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The following is performed using Brian Toby's EXPGUI which can be a very user friendly way to control GSAS refinements. It is written in Tcl/Tk so works in Windows and UNIX, is freely available, and the source can be freely modified and added to.
EXPGUI Homepage with download and installation information on EXPGUI for Windows and UNIX
Download FTP Site and Mirrors
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Spherical Harmonics can be very effective for performing preferred orientation
correction. In the following HgS example using image plate data provided by Scott Belmonte,
we will perform spherical harmonics preferred orientation correction to obtain a better
fit to the data. This assumes you understand a bit about the geometry implications about
Spherical Harmonics which in GSAS is rigourously implemented in a solid manner.
Information from Scott Belmonte (including EXPEDT commands which are superfluous for this demonstration as we are using the EXPGUI interface): For a scientific explanation of what is going on and the where's and why's of using Spherical Harmonics in GSAS, refer to Von Dreele, R. B. (1997). Quantitative texture analysis by Rietveld refinement. J. Appl. Cryst. 30, 517-525. "The number you should watch in the Texture Index in the PO menu. 1 means no texture, 3 is strong texture, higher than that and you should be aware that you may be overfitting."
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Run EXPGUI and load up the EXP file. Following is the state of the refinement obtaining the best fit possible without using spherical harmonics preferred orientation correction. (if using the example files, don't forget to run Powpref to start with before going to run genles). Following is the output from the EXPGUI Liveplot program.
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In EXPGUI, click on the SH Pref Orient Spherical Harmonics tab. Click
on Refine ODF Coefficients and Confirm Sample symmetry is Cylindrical
(is high pressure image plate data)
Check that Omega = -90, Phi = 0, Chi = 0
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Now run Genles and then look at the plot. You may have to play with a few things such as the Spherical Harmonics order; and keep in mind geometry and spacegroup considerations when setting up for this type of correction.
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