Preferred orientation in EXPGUI/GSAS

In GSAS/EXGUI has implemented 2 approaches to deal with preferred orientation

1) March-Dollase correction requires that you state specific hkl values.

2) Spherical harmonics correction (Orientational Distribution Function) is a bit more flexible, and is the one I would suggest starting with.

Slides at the bottom of the page give more info on both approaches. (Note: these are taken from GSAS Parameters & Controls chapter of Brian Toby's excellent series of web-based GSAS/EXGUI tutorials.

http://www.aps.anl.gov/Xray_Science_Division/Powder_Diffraction_Crystallography/

GSAS Parameters & Controls: What to Refine when? (Refinement Recipes)

Also, there are some hints here: http://sdpd.univ-lemans.fr/du-sdpd/nexus/ccp14/web/solution/gsas/spher_h2.htm

For Debye-Scherrer geometry, the spherical harmonics terms of omega, chi & phi should be set to (0,0,0).  I'm told that the default values of omega, chi & phi (0,90,0) are appropriate for Bragg-Brentano diffraction.  Also, according to "experience" (aka Bob VonDreele),  texture is "rare" for Debye-Scherrer data, changing these (0,0,0) terms is really needed.

Start with a low order number for the spherical harmonic correction, and slowly increase.  If preferred orientation is present, this should starting improving the fit.

You can also see p141-144 of the GSAS Manual for a full description of the coordinate  systems and Eulerian rotation angles used for spherical harmonics texture. or for more about  Spherical Harmonics in GSAS, refer to Von Dreele, R. B. (1997). Quantitative texture analysis by Rietveld refinement. J. Appl. Cryst. 30, 517-525.

hope this helps

-Matthew