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Methods, Problems and Solutions

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

GSAS - Spherical Harmonics for Preferred Orientation correction and Bragg-Bretano geometry

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]
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[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).]

Setting up Constraints in GSAS

Via Rietveld Mailing List

Date: Wed, 30 May 2001 19:30:14 -0800
To: rietveld_l@ill.fr
From: Sergey Ushakov [svushakov@ucdavis.edu]
Subject: spherical harmonics in GSAS question

Dear All,

Could somebody give me a hint on what should be omega, phi and chi 
angles ideally for Bragg-Bretano geometry (or tell why they should 
change)?

Thanks in advance,
Sergey

----------------------------------
Sergey Ushakov

post-doc
University of California at Davis
Thermochemistry Facility
Dept. of Chem.Engr.&Mat.Sci.
One Shields Ave., Davis, CA 95616
ph 530/754-5863
fax 530/752-9307


Date: Thu, 31 May 2001 10:42:23 -0400
From: "Brian H. Toby" [Brian.Toby@nist.gov]
Organization: NIST Center for Neutron Research
To: rietveld_l@ill.fr
Subject: Re: spherical harmonics in GSAS question

Under normal circumstances, you can leave the spherical harmonics
orientation angles as zero. Their purpose is for pole figure analysis,
where you are doing multiple measurements, with your sample in different
orientations (or with detectors in various positions relative to your
sample).

Brian


********************************************************************
Brian H. Toby, Ph.D.                    Leader, Crystallography Team
Brian.Toby@NIST.gov      NIST Center for Neutron Research, Stop 8562
voice: 301-975-4297     National Institute of Standards & Technology
FAX: 301-921-9847                        Gaithersburg, MD 20899-8562
                http://www.ncnr.nist.gov/xtal
********************************************************************

Date: Thu, 31 May 2001 10:43:56 -0600
To: rietveld_l@ill.fr
From: vondreele@lanl.gov (Bob Von Dreele)
Subject: Re: spherical harmonics in GSAS question

Dear Sergey (& All),

The default values of omega, chi & phi (0,90,0), which you will find under 
'A' in histogram editing, are appropriate for Bragg-Brentano diffraction 
geometry. For Debye-Scherrer geometry these should be set to (0,0,0). Since 
experience has shown that texture is "rare" for D-S data, changing these is 
rarely needed. The sample rotation angles found in the spherical harmonics 
texture menu are only needed for "full" texture analysis to correct for 
small sample orientation errors. If you "tilt" the sample then the 
appropriate values must be changed to reflect the rotation of the sample. 

See p141-144 of the GSAS Manual for a full description of the coordinate 
systems and Eulerian rotation angles used for spherical harmonics texture.

Bob Von Dreele


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