[Ifeffit] Using Larch to analyze uranium compounds

[John J Rehr]

Hi Juejing,

  It also wasn't clear to me which edge you were measuring, but U XAFS are
interesting. The large size of Uranium atoms typically improves convergence
of XAFSs calculations, so FEFF calculations often work well for L-edge XAFS
spectra with 17 keV x-rays. On the other hand, K-edge XAFS of U compounds
is often  not  useful due to the large core-hole broadening  of U which
limits the XAFS  to short near neighbor distances, and  the large > 100 keV
K-edge energy  is not available at many synchrotron sources. I'll be
interested to hear what source you used to measure the XAFS.  Thanks.

  With best wishes,
  John

On Sat, May 21, 2022 at 10:13 AM Matt Newville <newville at cars.uchicago.edu>
wrote:

> Hi Juejing,
>
> Sorry for the trouble. It's hard to guess from the screenshots alone - if
> only we had a Project file you could send (soon, soon).
>
> One thing I see is it looks like the Feff calculation you're using was
> done at the U K-edge.  Is that correct?  Is the data U K-edge too?
>
> I don't know if that would completely explain the difference, and Mike's
> suggestion of looking at the coordination number is a good idea.
>
> XAS Viewer writes a "feffit script" in your user folder
> (C:\Users\jiuju\larch\xas_viewer\feffit_script.lar).  Compared to the GUI
> form, that's a little bit cryptic and formal, but it is also what the
> program is really doing.  If the suggestions above don't help clarify the
> problem, can you post that?
>
>
>
> On Fri, May 20, 2022 at 7:35 PM <juejing.liu at gmail.com> wrote:
>
>> Hi,
>>
>>
>>
>> I am recently trying to use Larch to fit XAS spectra of uranium compounds
>> (e.g., UO2, using an existing CIF file). I did not achieve a good fit and
>> the amplitude of the model is way lower than the data. Then I tried to plot
>> the uranium oxygen 1st path with all 8 atoms a sigma2=0.001 by using the
>> plot model function. The resulting scattering peak is still far lower than
>> it should be (images below). Did I do something strange with the program?
>>
>>
>>
>> The Larch version I used was 0.9.59. I can send the cif file and the UO2
>> data I used if necessary.
>>
>>
>>
>> Best,
>>
>>
>>
>> Juejing
>>
>>
>>
>>
>>
>>
>>
>>
>>
>> *Juejing Liu*
>>
>> Guo Group
>>
>> Materials Science and Engineering program
>>
>> Alexandra Navrotsky Institute for Experimental Thermodynamics
>>
>> Washington State University
>> Pullman, WA  99164-4630
>> Office: Fulmer Hall, Room 630
>> Phone: 208-206-7304
>>
>>
>>
>>
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>
>
>
> --Matt
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