[Ifeffit] how to best implement fitting u (oxygen parameter) in an spinel

[Ravel, Bruce]

The assumption we make when doing EXAFS analysis with Feff and any 
Feff-using tool (Artemis, for example, but there are others) is that a 
small change in local configuration affects the argument of the sin() in 
the EXAFS equation significantly, but the scattering function negligibly.

To say that another way, a small change in length between absorber and 
scatterer is clearly visible in sin(2kr).  However, if you were to 
recalculate the scattering function upon making that small change in 
configuration, you would find that the scattering function changes very 
little.

Thus, to a good approximation, we can compute the scattering functions 
using a starting configuration, then model changes in configuration as 
deltaR terms in the analysis.

In Artemis, this means writing interesting math expressions in the 
deltaR fields for the paths that you include in the fit.  Those math 
expressions should encapsulate the way that a change in your u parameter 
changes the absorber-scatterer distances.

This is discussed in the Artemis user manual and in LOTS of EXAFS 
analysis papers in the literature that do this.  Look up papers by names 
that you recognize from this mailing list.

HTH,
B


On 04/01/2019 01:46 PM, Juan de la figuera wrote:
> Dear all,
> 
> I have a naive question. I have some mixed spinels, of which I want to 
> fit among other parameters the oxygen parameter u which indicates the 
> distortions around the oxygen anions, using only the single scattering 
> paths.
> 
> What would be the simplest way of fitting that? In the tests I did 
> before using Artemis, I would start with the structure (usually with a 
> cif file), using atoms/FEFF to calculate all the paths. But changing u 
> changes the initial structure.
> 
> One way I can think of is to apply a correction, obtained by hand, to 
> deltaR on each O-M path. Of course, this is not a general solution but 
> only good for a few paths. Am I missing something?
> 
> Thanks in advance,
> Juan
> 
> 
> 
> ----------------------------------------------------------------------------
> Juan de la Figuera
> Instituto de Quimica Fisica "Rocasolano"
> c/ serrano, 119
> Madrid, Spain 28006
> Phone: +34 91 745 9517
> http://surfmoss.iqfr.csic.es/ 
> <http://surfmoss.iqfr.csic.es/surflab/people/juan-de-la-figuera>
> http://scholar.google.com/citations?user=GOiP624AAAAJ
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-- 
  Bruce Ravel  ------------------------------------ bravel at bnl.gov

  National Institute of Standards and Technology
  Synchrotron Science Group at NSLS-II
  Lead Beamline Scientist, 6BM (BMM)
  Building 743, Room 114
  Upton NY, 11973

  Homepage:    http://bruceravel.github.io/home/
  Beamline:    https://www.bnl.gov/ps/beamlines/beamline.php?r=6-BM
  Software:    https://github.com/bruceravel
  Demeter:     http://bruceravel.github.io/demeter/