[Ifeffit] questions: EXAFS

Robert Gordon ragordon at alumni.sfu.ca
Tue Feb 19 20:15:36 CST 2019

Hi Vineetha,

I advised during the course that students should use the ifeffit mailing 
list for questions post-school.
An advantage of doing so is that one could receive assistance from any 
member of the community
and not be at the mercy of the time constraints of any particular 
researcher. Another advantage is that
someone more familiar with the type of problem you are studying might be 
able to offer more insight.
I would like again to encourage you to make use of the mailing list. 
Other researchers may also see
the problem with which you are dealing and it could help them too.

Some general advice:

Mixed phase samples can be problematic. If you know which two phases are 
present, you can simulate them
separately and examine the local environments. If the near-neighbour 
environments are very similar,
then some fitting can be done with reasonable results. You still need to 
keep in mind the resolution
criteria for being able to resolve closely spaced paths. Looking at your 
O-scattering paths, not having seen
your k-space data and range, I suspect you will not be able to resolve 
the splitting in the first shell,
and so, would treat it as a single path (similar argument for Hf paths 
in 2nd shell).

The starting model serves to give you amplitudes and phases for fitting. 
In that regard, whether you use monoclinic
or orthorhombic starting model should not matter. Once fit, you would 
examine how well the fit result
compares to either model. I would hope you also measured standard(s) of 
known composition and structure.
Fitting such would help guide the fitting of your unknown.

Model calculations can be done for your two suspected phases, including 
temperature effects
with the DEBYE card in the FEFF input. A quick google search yielded 
this paper:
which gives a calculated Debye temp of 484K for monoclinic HfO2. You can 
check the
literature further for other research on this material which may give an 
experimental value.
With the model chi(k)'s, you can do linear combinations (manually) and 
see if you find some ratio
that approaches agreement with your data. I don't see how to get the 
model chi(k) out of Artemis though.
I even tried running the feff6 executable manually (with associated 
dll's) but it did not generate the chi.dat
file I was expecting. I would suggest, if you wish to generate models 
and compare in this fashion, that you
invest in a newer version of FEFF that does allow you to generate this 
output (i.e. get FEFF9).

Other thoughts:

XANES? Did you measure it for the unknown and standards? If so, are the 
two phases distinguishable?

Background removal? Can you improve it to reduce the amplitude below 1 

Always show the original data (merged ok), not simply the transforms 
when asking for help.
That helps one assess the problem better.


On 2019-02-19 9:43 a.m., Mukundan, Vineetha wrote:
> Hi Dr. Gordon,
> I am Vineetha and had met you at the EXAFS short course @ BNL in 
> November’18. I am trying to analyze a couple of my data on Hafnia and 
> am stuck. So I am seeking help with the following questions. Apology 
> in advance if it will some of your time.
> 1.I generated the scattering paths. See attached. It is safe to 
> discard the multiple scattering paths here. So I generated the plots 
> with Hf-O single scattering paths and Hf-Hf scattering paths. Clearly 
> it shows the first shell is mostly one or all of O1.1, O2.1 and O2.3 
> paths. And the second shell is mostly influenced by Hf1.2, Hf1.3 and 
> Hf1.5. Am I correct in making this assumptions so far? IS there any 
> other way to figure out which paths are important or not important, 
> other than plotting them together with the data.
> 2.While fitting, I am really unsure how to set the parameters for each 
> path in my example. I am not sure how to set up the parameter when we 
> have multiple paths contributing to say first shell or second shell. 
> Also, I know using the Nyquist equation it is possible to figure out 
> the number of independent variables. I read up the ARTEMIS pages on 
> FeS2 example described but there the structure is cubic but my sample 
> is monoclinic. Here I am trying to fit only the monoclinic paths with 
> my data.
> 3.Also, I know my sample has monoclinic and orthorhombic structure. So 
> how does one go about generating the simulated EXAFS for a particular 
> structure on FEFF? say in my case the monoclinic and orthorhombic 
> ones. I have the .cif files for these 2. For figuring out the % amount 
> of monoclinic and orthorhombic phases in the sample, I am assuming I 
> will have to do a linear combination analysis. Am I correct?
> Any help in this regard will be really useful.
> Thank you,
> Vineetha
> *Vineetha Mukundan, PhD*
> Post-doc, Prof. Diebold’s Group
> Colleges of Nanoscale Science and Engineering
> SUNY Polytechnic Institute
> 257, Fuller Road
> Albany, NY 12203 USA
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