Also refer to Spherical Harmonics for Preferred Orientation correction and Bragg-Bretano geometry
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:
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. and "A new method for measuring the degree of preferred orientation in bulk textured YBa2Cu3Ox" Th. Leventouri, Physica C 277, 82-86 (1997).
"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."
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)
To get spherical harmonics to be activated do via EXPEDT do: Y L O O H
Then to setup the refinement flags do V N N N Y
(For image-plates only) set sample angle using A:
Omega = -90, Phi = 0, Chi = 0
(For image-plates) set the harmonic order via O to 8
Now exit EXPEDT.
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.