Dear Maurits,

My dissertation looked at the possibility that the kind of change you describe is accompanied by a discontinuity in the third cumulant parameter for (in this case) the nearest-neighbor bond. My evidence was not sufficiently convincing to allow for publication elsewhere, but it was suggestive.

Of course, the third cumulant tends to increase with temperature anyway. One way around this is to measure just above and below the putative transition; a jump in third cumulant (rather than a modest change) would be in accord with what I described in my dissertation. Another way around this is to use one of the theoretical models for how third cumulant should change with temperature in such a system, and look to see if the change is significantly bigger than that.

Changes in the third cumulant correlate in fitting with changes in lattice parameter, so your strategy of using diffraction at the measured temperatures to fix the inter-atomic distances is a good idea. Also, interatomic distances and the third cumulant are weighted differently by k in the EXAFS equation, so if you can collect and analyze data over a wide k-range, you've got a better chance of teasing apart the two parameters.

--Scott Calvin
Sarah Lawrence College

On Dec 9, 2013, at 10:02 AM, Maurits Boeije <M.F.J.Boeije@tudelft.nl> wrote:

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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