[Ifeffit] How to distinguish whether the coordination

Matt Newville newville at cars.uchicago.edu
Mon Jul 7 14:04:55 CDT 2014


Hi Michael,

On Mon, Jul 7, 2014 at 10:49 AM, Michael Weir <mgweir at udel.edu> wrote:

> 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".
>

Oh, yeah, that's absolutely correct.  The example was definitely "what Z
sensitivity does XAFS alone have?".   Zn-Rb and Zn-Ga make absolutely no
sense (part of the fun, really!).   Zn-Br would be at a very different
distance (again, the bond-valence idea).


>
>    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?
>

Yes, just having an XRF spectrum for the sample can probably tell you that
many atomic species are not present, which might be obvious but is highly
useful.  For ZnSe, I think we could confidently conclude that the Ge and As
concentrations were something below 10 ppm.

Anyway, yes, definitely use "chemical sense", either qualitative (Zn-Rb
makes no sense)  or quantitative (the As concentration is at least 2 orders
of magnitude lower than Se concerntration).

--Matt
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