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Methods, Problems and Solutions

GSAS (General Structure Analysis System) Rietveld powder diffraction and Single Crystal software

Applying Spherical Harmonics Preferred Orientation Correction using GSAS and Brian Toby's EXPGUI for Tcl/Tk Software

The CCP14 Homepage is at http://www.ccp14.ac.uk

[Back to Problems and Solutions] | [Back to GSAS Hints/Resources] | [Back to Applying Spherical Harmonics using EXPEDT]
[Back to Spherical Harmonics for Preferred Orientation correction and Bragg-Bretano geometry]

[The reference to use for GSAS in any resulting publications is: A.C. Larson and R.B. Von Dreele, "General Structure Analysis System (GSAS)", Los Alamos National Laboratory Report LAUR 86-748 (1994).]

[The reference to cite in any resulting publications for using EXPGUI is: B. H. Toby, EXPGUI, a graphical user interface for GSAS, J. Appl. Cryst. (2001). 34, 210-213]

Also refer to Spherical Harmonics for Preferred Orientation correction and Bragg-Bretano geometry

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


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."

Wavelength 0.4447

Spacegroup: P 31 2 1
Lattice params: 4.145 4.145 9.496 90 90 120
Hg: 0.72 0 1/3
S:  0.48 0 5/6

Peak shape: 
GU: 3000 GV: 0 GW: 4.0
LX: 3.0 LY: 0

Only refine GU and LX and if GU goes to zero only refine GU and set LX to
3.0.

To get to spher. harm. correction from main menu:

L O O H

To set order: O n where n is even.

To refine:

V N N N Y

For image-plates only set sample angle using A:

Omega = -90,  Phi = 0, Chi = 0


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.

Rietveld Fit in Liveplot Before performing Spherical Harmonics corrections


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

Enabling Spherical Harmonics via EXPGUI


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.

Rietveld Fit in Liveplot after running Genles with Spherical Harmonics enabled


[Back to Problems and Solutions] | [Back to GSAS Hints/Resources] | [Back to Applying Spherical Harmonics using EXPEDT]

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