Re: question

[Scott Calvin <scalvin@anvil.nrl.navy.mil>]

Title: Re: question

Hi Stephanie,

Different researchers use different strategies. I work with non-stoichiometric compounds routinely, and this is my strategy:

1) Run ATOMS with a related stoichiometric compound (e.g. if you're doing Zn0.9 Fe2.1 O4 do ZnFe2O4 ).

2) Change some of the atoms in FEFF.INP to the other elements occupying the sites. (In the above example, some of the Zn sites are occupied by iron, so I'd change some Zn atoms to iron atoms.) Make sure to change the potential number in FEFF.INP (each entry for an atom contains a label, which is for your information, and a potential number, which it actually uses). You may also have to add the element to the potentials list at the top.

3) When running FEFFIT (if that's what you're using), define a SOF (guessing or setting, depending on how confident you are of the stoichiometry). Then scale the S02 for that path appropriately. For example, suppose FEFF.INP has 4 Zn atoms at the same distance from the absorber, and you changed one to Fe. The S02 for the Zn path should now be S02*SOF*4.0/3.0, while the Fe path should now be SO2*(1.0-SOF)*4.0/1.0, where SOF is the site occupancy factor for Zn, and sites not occupied by Zn are occupied by Fe.

Things can get somewhat complicated, particularly if the absorbing atom is in more than one site, but that's the basic idea.

--Scott Calvin

Naval Research Lab

> Hello,
> I would like
> to ask you a question about ATOMS and FEFF.
> I want to
> calculate an exafs spectrum of a non-stoichiometric coumpound,
> and
> my problem is
> that ATOMS doesn't take into account occupation ratio (fractional
> occupancies) on the cristallographic positions of
> atoms.
> So, I would
> like to know the best way to proceed ?
> Moreover,
> these positions are occupied by two different atoms
> ...
> I tried to
> modify FEFF.INP file, but It can't reproduce the real
> structure.
> Thank you for
> your attention.
> Sincerely
> yours.
>
> Stephanie
> BELIN

         

> //////////////////////////////////////////////////////////////////////////////////////////////////////////
>

        Dr. Stéphanie BELIN

>                  LURE - CNRS
>                  Bat 209D, Centre Universitaire
> Paris Sud, BP 34
>                  91898 Orsay
> Cedex

       

        Tel   :  01 64 46 82 50 / 88 48

        Fax  :  01 64 46 80 07

        email:  Stephanie.Belin@lure.u-psud.fr

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