Hi Jhonny,

This article on AgBr(x)Cl(1-x) explains how to do this type of analysis, and if there are still questions, look for other articles (in Phys. Rev. B and Phys. Rev. Lett.) on mixed ionic salts, by the same authors:

https://pubweb.bnl.gov/~frenkel/BUCKLING-PHB/buckledPHB.pdf

It is a model independent way to find local composition of Br and Cl around Ag. You are better off doing that (limit it to the 1st shell analysis) rather than using particular configurations you are proposing, and then see if you get the same composition (x) for Ag-Br as you obtain for composition y corresponding to the Br-Br coordination number found from Br K-edge. That will mean that your alloy is random. Otherwise, there may be either short range order or segregation that you will be able to identify by comparing x and y in each individual case. Rely on XRD results also, to tell whether you really have a solid solution or not.

If you only measured Ag edge, you still can make some conclusions but you should rely on the bulk composition to compare the "x" and "y".

Anatoly

From: ifeffit-bounces@millenia.cars.aps.anl.gov [ifeffit-bounces@millenia.cars.aps.anl.gov] on behalf of johnny huertas [johnnyhf@hotmail.com]
Sent: Saturday, July 14, 2012 1:51 PM
To: Lista fe IFEFFIT
Subject: Re: [Ifeffit] Ifeffit Digest, Vol 113, Issue 11

Hi Matt:
Thanks for your response. First of all I am beginner in this issue. Because of my little experience I don't know exactly how to specify my problem but I think you write an interesting phrase in your response: "are you asking if you can distinguish random substitution from ordered substitution? I'm not sure why the Ag-Br and Ag-Cl parameters would be the same".
I don't know how to create a robust fitting model to distinguish random substitution from ordered substitution of the example found in Bruce site about AgBr(1-x)Clx. What are the parameters that I must consider to fit the spectra and to verify if exist random or ordered substitution? I am attaching a document very explicit about this. Thanks again.

Jhonny Huertas Flores
Rio de Janeiro-Brasil

> From: ifeffit-request@millenia.cars.aps.anl.gov
> Subject: Ifeffit Digest, Vol 113, Issue 11
> To: ifeffit@millenia.cars.aps.anl.gov
> Date: Sat, 14 Jul 2012 12:00:01 -0500
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> Message: 1
> Date: Fri, 13 Jul 2012 22:21:50 +0000
> From: johnny huertas <johnnyhf@hotmail.com>
> To: <ifeffit@millenia.cars.aps.anl.gov>
> Subject: [Ifeffit] problems
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> Content-Type: text/plain; charset="iso-8859-1"
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>
> Members of list:
> I have some problems to resolve a question. I have been working a lot in fit the AgBr(1-x)Clx example of FEFFIT site. My fits were good in my opinion, but I don't know how to identify the Br and Cl atomic distribution around Ag (random or specific?). I didn't find any example about this in the site. Conversely I have made some attempts to identify such distribution. I have made two fits preparing for each one feff file, a feff file puting Br and Cl atoms in face and other in medirian position around Ag knowing the CN=6 of this central atom and fiting only the first shell. Unfortunatelly I don't have found satisfactory results because the paremeter are equal in both cases. I would like you help showing a similar example. Thanks.
> Jhonny Huertas Flores
> Rio de Janeiro-Brasil
>
>
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> Message: 2
> Date: Fri, 13 Jul 2012 22:20:41 -0500
> From: Matt Newville <newville@cars.uchicago.edu>
> To: XAFS Analysis using Ifeffit <ifeffit@millenia.cars.aps.anl.gov>
> Subject: Re: [Ifeffit] problems
> Message-ID:
> <CA+7ESbruuJaW0KVyuE=fUP5Mzz_W63yEqc-v3OAE_KhQj6hN-A@mail.gmail.com>
> Content-Type: text/plain; charset=ISO-8859-1
>
> Hi,
>
> On Fri, Jul 13, 2012 at 5:21 PM, johnny huertas <johnnyhf@hotmail.com> wrote:
> > Members of list:
> > I have some problems to resolve a question. I have been working a lot in fit
> > the AgBr(1-x)Clx example of FEFFIT site. My fits were good in my opinion,
> > but I don't know how to identify the Br and Cl atomic distribution around Ag
> > (random or specific?). I didn't find any example about this in the site.
> > Conversely I have made some attempts to identify such distribution. I have
> > made two fits preparing for each one feff file, a feff file puting Br and Cl
> > atoms in face and other in medirian position around Ag knowing the CN=6 of
> > this central atom and fiting only the first shell. Unfortunatelly I don't
> > have found satisfactory results because the paremeter are equal in both
> > cases. I would like you help showing a similar example. Thanks.
>
> As is often requested on this list, it would be most helpful if you
> asked specific questions, and possibly post a project of what you've
> done and a clear explanation of what you are trying to do. Open ended
> questions are just too difficult to address. Are you asking how to
> include both Ag-Br and Ag-Cl in a fit, or asking if you can
> distinguish random substitution from ordered substitution? I'm not
> sure why the Ag-Br and Ag-Cl parameters would be the same, but it's
> too hard to speculate without knowing what you actually did.
>
> --Matt
>
>
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