[Ifeffit] "RE: Contents of Ifeffit digest..."

Matt Newville newville at cars.uchicago.edu
Sat Jul 14 19:34:43 CDT 2012

On Sat, Jul 14, 2012 at 4:19 PM, johnny huertas <johnnyhf at hotmail.com> wrote:
> 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.

Actually, I think I still don't fully understand what you are asking.
When you say that the sentences I wrote are interesting, am I supposed
to interpret that to mean that this is what you are asking?  And,
though I see the fits in your attachment, I don't really see a
question that I understand.   Of course, an answer to any question
like "what parameters to use to fit spectra" would depend on what
you're trying to model.    Going out on a limb and guess that you are
asking about random versus ordered substitutions, here's some thoughts
that might help....

First shell EXAFS is not, by itself, sensitive to distinguishing
ordered from random substitutions.  That is, if there are 3 Ag-Cl and
3 Ag-Br bonds, a simple analysis of the first shell EXAFS won't tell
how the Cl and Br are arranged relative to each other, only how the Br
and Cl are arranged relative to the Ag.

If you have a single crystal and measure the polarization dependence
of the EXAFS, you might be able to distinguish the relative bonding in
the different crystallographic directions.  Another approach is to go
past the first shell, and analyze the multiple-scattering paths.
Those can be sensitive to order, as the fraction of linear Ag-Br-Ag-Cl
to linear Ag-Br-Ag-Br would depend on the ordering.

You might also consider measuring the Br and/or Cl edges so that, for
example, you might be able to see the relative fraction of Br-Br and
Br-Cl in the Br second shell.

Hope that helps,


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