[Ifeffit] How to distinguish whether the coordination

[Michael Weir]

Hi Matt and Zhan,
   I think this is the point where adding another approach would be very
useful. In my limited experience, XAFS was much more powerful when combined
with other techniques to eliminate potential models. We also needed to use
our "chemical sense".
   Is there a reason you would suspect Se, Ge, Ga, or any of the other
elements in your material? Could you use your beamline to look for these
elements by a quick scan of one of their stronger transitions? If there is
a full coordination shell, detecting the other element this way is
feasible. However, as someone else mentioned recently, dopants would be
much more difficult to detect.
 Is there another analytical technique you could use to narrow down the
possible elements in your system?
I hope that answering those questions helps you answer your question about
the system.
Also, thank you Bruce, Matt, Scott and others for the very useful
discussions on this list.
   Michael Weir





Hi Zhan,


On Mon, Jul 7, 2014 at 3:40 AM, ZHAN Fei <zhanfei at ihep.ac.cn> wrote:

> Hi Matti?
>  Thanks for your attention and patience.
>  The back fourier transform of the specific peak (use window)of Chi(R)
> gives the amplitude,donated by amp_bft?As in previous mails,I ask whether
> the compare the amp_bft and the amplitude of specific Z number element can
> determine the Z.
>

Sorry, I'm not sure I understand this.  I didn't understand the figure you
attached in your earlier mail.   Generally speaking, I find
back-transformed data to be not very useful -- making any sense of these
requires very well isolated shells of atoms.   The amplitude of the
back-transformed chi(k)  (is that what you mean by amp_bft??) has many
contributions, and is not simply f(k).  It will have Z dependence, but it
will have other dependencies too.


> And thanks for telling me the useful trick using the total phase shift of
> the specific element.
>
>
> The discrepancy between R and Rphcor is below,the Zn and Br is close to
> Se(the best fit)'s 0.013,
>

Yes, Ge actually gives the closest match, and Se the second closest match,
and I left Ga out of the test.  I would probably say that anything closer
that 0.015 Ang (and, really, maybe 0.02 Ang) is pretty darn close.   So the
phase-correction approach appears to be (in this case) not as sensitive to
Z as the reduced chi-square, but does provides a check on
self-consistency.    The fits with Zn and Rb are noticeably worse than the
fit with Se... hence Z +/- 3 or perhaps 5 seems like a reasonable rule of
thumb, and sometimes one might be able to do better.


> should the enot also be important criterion in this trick?and dose the
> plus or minus of enot  indicate the lighter or higher element relative to
> the specific coordination shell?
>

Well, an E0 shift > 10 eV probably indicating that something is off ....
but that could be just the selection of E0 for the experimental data.   I
wouldn't put much meaning of the absolute value of E0 though for any single
fit.

  "a bond valence sum can be an independent check on the consistency of N,
> R, and valence",can your give the ref. paper of this method? Thanks
>
>
http://en.wikipedia.org/wiki/Bond_valence_method   The idea is that N, R,
and valence are not independent.

--Matt
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