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Methods, Problems and Solutions

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

Discussions on modelling macroscopic strain within GSAS

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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).]

Via Rietveld Mailing List

Subject:   New GSAS kits
From: "Bob Von Dreele" [vondreele@lanl.gov]
To: [Rietveld_l@ill.fr]
Date: Mon, 16 Dec 2002 10:32:19 -0800

Hi everyone,

New Linux & Windows versions of gsaskit have been placed 
on the LANL ftp server (ftp://ftp.lanl.gov/public/gsas). 
Both are identical. See notes below for latest updates 
(there have been a few important ones).

All the best for the Holidays & have a Happy New Year.

Bob Von Dreele

GSAS News.......

Dec. 15, 2002

Still more fixes & an addition...

1. A new constant wavelength X-ray and neutron profile 
function (#5) has been defined. This has the same form as 
#2 except that the "Lij" are now "Dij". These are 
coefficients for a macroscopic strain on the lattice 
parameters for the phase in a given histogram with respect 
to the lattice parameters given for that phase. The strain 
is given by

del-2th = 

This function has it's uses; e.g. for combining data sets 
taken at slightly different temperatures on the same 

Subject:  Re: New GSAS kits
Date: Tue, 17 Dec 2002 12:15:09 +0100
To: rietveld_l@ill.fr
From: Jonathan WRIGHT [wright@esrf.fr]
Reply-To: rietveld_l@ill.fr


This sounds great - just what we are after for using anisotropic thermal 
expansion data! I'm assuming the distance-angle restraints are calculated 
based on the unit cell parameters, so we should try to use a "averaged" 
values for cell par's in multipattern fits? One other question - are these 
parameters forced to retain the symmetry of the lattice? For example, a 
strained sample with a "cubic" lattice might have different values for D11, 
D22 and D33, so that the (100), (010) and (001) peaks no longer exactly 
overlap. Are you calculating the contributions of all of the various peaks 
using a peaklist generated P-1, regardless of the symmetry, or does the 
user need to be consistent with their choice of space group so that the 
program can use a multiplicity factor for symmetry equivalent reflections? 
This could be very useful for refinements of small distortions. Often one 
wants to use a space group which has a higher symmetry than the lattice, 
which can present a challenge for getting the list of atoms and constraints 



Subject:   Re: New GSAS kits
From: "Bob Von Dreele" [vondreele@lanl.gov]
To: rietveld_l@ill.fr
Date: Tue, 17 Dec 2002 08:36:46 -0800

Hi Jon,

Couple of things. This function is only effective if the 
lattice parameter changes are small (at most 0.1% or so) 
so the effect on things like bond length restraints are 
negligible. The assumption is that the effect is like a 
strain so that the atom positions are unaffected. There 
are no symmetry constraints; all the Dij are independent. 
My experience with it for lysozyme (tetragonal) is that 
D11 & D22 follow each other quite well with D33 different 
(& opposite sign with pH changes). I've varied lattice 
parameters & Dij for one out of two histograms - works 

I haven't tried it with data showing small distortions 
(e.g. cubic -> rhombohedral in a perovskite) from symmetry 
but should work very well.


Subject:   Re: New GSAS kits
From: "Bob Von Dreele" [vondreele@lanl.gov]
To: rietveld_l@ill.fr
Date: Tue, 17 Dec 2002 09:08:17 -0800

Hi Jon (again),

Reflecting on your comment & thinking about it a it more - 
the symmetry effect is tricky. A change in symmetry makes 
reflections not equivalent & makes them appear in 
different places in the pattern. Consequently, reducing 
the reflection set symmetry to P-1 and processing over all 
of them is the right way to account for the effect of 
changing symmetry with Dij refinement in function #5. Down 
side is the "expense" of doing all those profile function 
calculations. I'll have to think about this some more. 

However, function #5 will be OK for those cases where the 
symmetry doesn't change.


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