[Ifeffit] Itinerant vs localized XAFS

Maurits Boeije M.F.J.Boeije at tudelft.nl
Mon Dec 9 09:02:38 CST 2013

Dear XAFS specialists,

We've performed an experiment at the ESRF in Grenoble to find experimental evidence of an electronic change in our system. We suspect, from ab initio calculations, that there is a change in how electrons are allocated in our Mn/Fe system. Calculations show that some electrons change from being itinerant at high temperatures, to being localized at low temperatures.
We performed an XAFS experiment and hope to see a difference between the low and high temperatures measurements. There is always difference because the cell parameters change as function of temperature, and I used your Artemis program to investigate the influence of the cell parameters on the EXAFS signal. I can get a decent fit using the crystallographic data we have, but I'm not sure if I'm not throwing away any evidence of our electronic change. I can interpret a fitted interatomic distance of 2.1 A (compared to 2.0 A we got from diffraction) as being the mean interatomic distance in our sample, but I could also interpret it as a 0.1 A difference because of our suspected electronic change.
The challenge now lies in seeing an electronic difference despite the crystallographic change. What we would ideally do, is to keep the (known) atomic positions and cell parameters fixed, fit the EXAFS parameters and see if we can predict either the high or low temperature spectrum. Do you think this is possible? Are there any parameters which are calculated by IFEFFIT who are not precise enough for such a conclusion? It could be that the effect we are looking for is so small that they are nullified by the assumptions in the EXAFS equation.

I would appreciate any insight you could have in this particular problem.

With kind regards,

Drs. MFJ Boeije
Fundamental Aspects of Materials and Energy

TU Delft
T +31 (0)15 27 83793

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