[Ifeffit] Itinerant vs localized XAFS

[Matt Newville]

On Mon, Dec 9, 2013 at 9:02 AM, Maurits Boeije <M.F.J.Boeije at 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.

A difference between 2.0 Ang to 2.1 Ang is pretty big.  How can you
get a decent fit to the crystallographic data and have a distance
shift of 0.1Ang?  Do you mean you started with the crystallographic
structure, refine the distances, and find they moved 0.1Ang?    As
Chris suggests, such a shift could imply a change in valence that
could affect the XANES.

I'm not sure I understand what you are looking to do.  "Keep the known
atomic positions" and "fit the EXAFS parameters"  may not be
consistent.  Do you mean *not* refine the inter-atomic distances, but
other parameters (sigma2, third cumulant, as Scott suggests)?

Without knowing more about your system, I'd recommend care when
comparing diffraction results and XAFS.  Diffraction does not directly
measure interatomic distances, and  XAFS does not care about
crystallographic sites.   For example, simple refinements of
diffraction data may put an atom at a high symmetry site with a large
u^2 when the atom randomly occupies many possible sites around that
high symmetry site.  This would look very different to XAFS.

--Matt