[Ifeffit] how to obtain average vacancy files?

Thu May 31 22:56:46 CDT 2012

Dear friends from the Ifeffit list, specially Sebastiano and Bruce,

Thank you very much for your advices. Here I am giving you a better
explanation of the physical problem we are facing, and then I´ll give the
approach we will try to follow now.

We are working on a 50-50 Cr-Fe maghemite, which structure is known by high
resolution XRD and Rietveld method. The synthesis of such a maghemite is
difficult and some magnetic properties are being measured.  We know, also,
that pure Fe maghemite structure at T=10 K has ordered vacancies. We are
trying to verify by XAFS if the local environment of Cr or Fe absorbers
reflects ordered vacancies.

With the aid of crystallographic codes we modeled the various vacancy
configurations, including the one corresponding to the ordered Fe maghemite
structure. By the Rietveld analysis and by the XANES Cr spectrum we know
that Cr occupies only octahedral sites. We have tried to fit with Artemis
the experimental Cr and Fe EXAFS spectra, comparing them with one model of
the ordered vacancies structure. The results are not the best, and then we
“suspect” that the model of ordered vacancies is not the good one.

 As I have written before, our idea was to obtain “average vacancies” FEFF
files and to work with the fully occupied magnetite structure, but with
some kind of “gray” atoms. These gray atoms would be the average of
vacancies and Cr or Fe atoms in octahedral sites. After your comments, it
seems too difficult to obtain the corresponding FEFF functions.

As a way for testing the disordered model, we are now working on the much
easier problem of fitting the magnetite structure to our EXAFS spectra and
to obtain the corresponding interatomic distances and amplitudes. We hope
their interpretation will be also easier than before.

Thanks a lot again for your attention. Best regards

Maria Elena

2012/5/31 <elena.montero at cimav.edu.mx>

> Dear friends:
> Thank you very much for your kind answers. Believe me, now I am in a
> hurry, and I hope this evening I'll answer with more details.
> Regards
> Maria Elena
> Enviado desde mi oficina móvil BlackBerry® de Telcel
>
> -----Original Message-----
> From: Bruce Ravel <bravel at bnl.gov>
> Sender: ifeffit-bounces at millenia.cars.aps.anl.gov
> Date: Thu, 31 May 2012 09:33:29
> To: XAFS Analysis using Ifeffit<ifeffit at millenia.cars.aps.anl.gov>
> Reply-To: XAFS Analysis using Ifeffit <ifeffit at millenia.cars.aps.anl.gov>
> Subject: Re: [Ifeffit] how to obtain average vacancy files?
>
> On Tuesday, May 29, 2012 10:31:43 PM María Elena Montero Cabrera wrote:
> > Dear friends,
> > I am trying to fit with Artemis, not very successfully, a maghemite
> > structure. The main difficulty seems to be the vacancies in the
> structure.
> > My idea, following a recently given Bruce’s advice to a person in this
> > list, is to obtain “average vacancies” FEFF files and to work with the
> > fully occupied magnetite structure. I wanted to obtain each file for a
> > given vacancy configuration in the first coordination sphere and then to
> > multiply the obtained functions by the weight of each configuration and
> > summ them all. I had the hope of doing this by using the feff00nn.dat
> files
> > that I have used when working with SixPack, some years ago.
> Surprisingly, I
> > couldn’t find the Artemis’ corresponding  feff00nn.dat files at any
> folder.
> > What can I do?
>
> There are two questions here.  I'll answer the second one first.
>
> It would be reasonable to say that the whole point of Artemis is to
> free the user from having to manage the feffNNNN.dat files.  They get
> stashed in an out-of-the-way location so that the user can focus on
> creating a fitting model rather than keep track of many (dozens,
> hundreds,...) of little text files.
>
>
> You are not very clear about what, exactly, your confusion is in terms
> of the vacancy thing.  So I can only make a guess.
>
> In general, vacancies are hard to model in Feff.  In any case, it is
> may be imporssible to do a good job of modeling a vacancy with a
> /single/ Feff calculation.  The reason is the same as the reason as
> for dopants -- Feff requires that every point in the cluster be
> occupied by only one kind of thing -- i.e. an atom or nothing.  In
> general, you cannot know how to distribute the vacancy in the small
> cluster represented in the feff.inp file.
>
> The word "vacancy" makes much more sense in a crystallographic context
> where the structure is averaged over 10^23 atoms.  In an EXAFS
> context, you have to figure out how to model both of the main effects
> of missing atom:
>
>  * Reduction in coordination
>
>  * Change in the distances to other neighboring atoms due to
>   relaxation around the missing atom.
>
> I think that, if you spend some time reading papers by the folks who
> answer questions around here, you will find that the prefered approach
> to solving a problem involving vacancies is to run Feff on the
> "native" structure, then to parameterize the fit in such a way that
> models the coordinaton reduction and the changes in bond lengths.
>
> That is, I see dealing with a problem like yours as a modeling problem
> in Artemis, not as a problem requiring a clever use of Feff.
>
> For starters, read some of Scott Calvin's papers on mixed ferrites to
> understand how he approached the problem of mixed occupancy.
>
> B
>
>
>
> --
>
>  Bruce Ravel  ------------------------------------ bravel at bnl.gov
>
>  National Institute of Standards and Technology
>  Synchrotron Methods Group at NSLS --- Beamlines U7A, X24A, X23A2
>  Building 535A
>  Upton NY, 11973
>
>  Homepage:    http://xafs.org/BruceRavel
>  Software:    https://github.com/bruceravel
>
> _______________________________________________
> Ifeffit mailing list
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>

-- 
María Elena

Dra. María Elena Montero Cabrera
Centro de Investigación en Materiales Avanzados (CIMAV)
Miguel de Cervantes 120, Compl. Ind. Chihuahua
Chihuahua CP 31109, Chih. México
Tel (614) 4391123
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