CCP14
Methods, Problems and Solutions
GSAS (General Structure Analysis System) Rietveld powder diffraction and Single Crystal software
GSAS - How to apply Occupancy constraints with more than two variables in GSAS.
The CCP14 Homepage is at http://www.ccp14.ac.uk
[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).]
How to apply Occupancy constraints with more than two variables in GSAS.
Via the
Rietveld Mailing List
From: Jean-Marc Joubert
Subject: Re: How to apply constraint with more than two variables in GSAS?
Date: Thu, 14 Jan 1999 07:14:01 -0500
>> Could any of you point me to the good document about the method of applying
>> constraints in GSAS?
>> I am trying to figure out some complex cationic intermixing in a layered
>> cuprate superlatttice compound.
>> But I couldn't find a good documentation in the manual. Specially trouble
>> with a constraint with >2 variables.
>> For example, how do you make a constraint for something like x+2y+2z=1.
>> Thanks for your help.
>In a few words -- you probably can't do that with GSAS. This is not true
>of all Rietveld codes, but in GSAS a variable parameter can only appear
>in one constraint equation. One option that might help would be to put a
>soft constraint on the total composition.
>
Yes you can. You have to use a trick which is detailed in (J.-M. Joubert et
al., J. Appl. Crystalogr. 31 (1998) 327-332).
If you have three atoms on the same site and want to refine occupancy
factors (theoretically it is only possible if you have two data sets, for
example X-ray and neutrons), you have to introduce a fourth fictive atom :
You have, for example :
Atom 1, Ni, occupancy x
Atom 2, Mn, occupancy y
Atom 3, Co, occupancy z
and wants to constrain x+2y+2z=1
You introduce :
Atom 4, Ni, occupancy u
and change occupancy of atom 1 to v in such a way that u+v=x
Set the constraints on the parameters shifts :
du=-2dy
dv=-2dz
You will maintain by this way the condition u+v+2y+2z=x+2y+2z=1
Regards.
Jean-Marc Joubert
---------------------------------------------------------------
Jean-Marc Joubert
Laboratoire de Chimie Metallurgique des Terres Rares
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