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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