Dear kasthuri
Please read the following and recent discussion.
Good luck
Jaap
On Thu, 14 Jan 1999, Jean-Marc Joubert wrote:
>
>
> >> 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
> CNRS - UPR 209
> 2-8 rue Henri DUNANT,
> 94320 THIAIS CEDEX - FRANCE
>
> Phone: 33 (0)1 49 78 12 11
> Fax: 33 (0)1 49 78 12 03
>
> Email: [EMAIL PROTECTED]
>
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