Hi Deltsidis: Using Artemis, you can compare the calculated Fourier Transforms of the Se-Fe and Se-Li paths to see if the latter will actually be visible. I suspect that it will not if it is an impurity phase with small phase fraction. I would probably start with only Se-Fe and see how well it can fit the data. The eins(T, thetae) is the temperature dependent term. What is missing the static disorder that needs to be added into the sigma^2 to account for the temperature independent residual disorder. Carlo On Thu, May 27, 2021 at 9:43 AM Alexandros Deltsidis < adeltsidis@iesl.forth.gr> wrote:

Dear mailing list,

I am currently analyzing some EXAFS data. I am studying a Lix(C5H5N)FeSe system in a temperature grid that extends from 20 K to 300 K and I have 4 such datasets which correspond to different amount of doping (x). Right now, im focusing on fitting the 1st coordination cell, in Artemis for the Se K-edge. My starting model is the simple P4/nmm FeSe. So, in my system the 1st coordination cell, in the Se K-edge, corresponds to the Se (absorber) - Fe (backscatterer) pair. I have 2 questions: 1) I realize now, that I have a certain impurity in the high doping range on my system, namely Li2Se. I try to include a scattering path from the respective Li2Se crystal model in my fits, since a Se (absorber) - Li (backscatterer) pair is present in the R-range of my fit in the Forward Fourier Transform. My question here is if this makes sense since Li is much smaller scatterer compared to Se. In other words, does it make sense to look for physical parameters (Li-Se bond length and DW factor respectively) of a signal (Se-Li) that is "tucked" in below the main peak coming from the "majority" Se-Fe signal in the FFT? 2)Also, I'm attempting to extract an Einstein temperature for each of those datasets, by utilizing the "eins(T, thetae)" function implemented in Artemis. What is the equation that is parametrized here? Does it include the s0^2 offset term that accounts for the overall configuration disorder in the system? And if that is the case is there same way to separate it from the temperature dependent s^2 term?

Thank you in advance, Deltsidis Alexandros PS:I am attaching a png. file exported from Artemis that is relative to my question 1)

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Hi Alex, I agree with everything Carlo and Matthew said. As you say, Li is very light and so the scattering should be weak compared to Fe. It will also die out much more quickly with "k" than the Fe (or Se) scattering will. So, if you have enough k-range, simply starting your fit at higher k than normal (perhaps 4 or 5 Ang^-1) or increasing the k-weight used in the Fourier transform (perhaps to 3 or 4) would de-emphasize the Se-Li scattering to a level that it was safe(r) to ignore. FWIW, I would imagine that trying to fit coordination number or sigma^2 for Se-Li at percent-level concentrations would not work very well. If you did indeed get values that were clearly telling you that there was definitely Se-Li scattering contributing, I would wonder if there was something else going on (say, from another ligand, some multiple scattering, or some other phase). The formula for the Einstein temperature is a scale factor times "coth(theta/(2t)) / (r_mass * theta)" where t is the temperature, theta the Einstein temperature, and r_mass the reduced mass of the atoms in the path. See https://github.com/xraypy/xraylarch/blob/726136d0184d9a006546002722b7573f6c6... for details. This will not include S0^2 -- they are conceptually totally different. As Carlo said, the sigma^2 in the EXAFS equation does not distinguish between static and thermal disorder. But if you have temperature-dependent data, modeling the sigma2 values as a static offset + a term that depended on temperature with an Einstein model would be a fine way to go. Hope that helps! On Thu, May 27, 2021 at 9:43 AM Alexandros Deltsidis < adeltsidis@iesl.forth.gr> wrote:

Dear mailing list,

I am currently analyzing some EXAFS data. I am studying a Lix(C5H5N)FeSe system in a temperature grid that extends from 20 K to 300 K and I have 4 such datasets which correspond to different amount of doping (x). Right now, im focusing on fitting the 1st coordination cell, in Artemis for the Se K-edge. My starting model is the simple P4/nmm FeSe. So, in my system the 1st coordination cell, in the Se K-edge, corresponds to the Se (absorber) - Fe (backscatterer) pair. I have 2 questions: 1) I realize now, that I have a certain impurity in the high doping range on my system, namely Li2Se. I try to include a scattering path from the respective Li2Se crystal model in my fits, since a Se (absorber) - Li (backscatterer) pair is present in the R-range of my fit in the Forward Fourier Transform. My question here is if this makes sense since Li is much smaller scatterer compared to Se. In other words, does it make sense to look for physical parameters (Li-Se bond length and DW factor respectively) of a signal (Se-Li) that is "tucked" in below the main peak coming from the "majority" Se-Fe signal in the FFT? 2)Also, I'm attempting to extract an Einstein temperature for each of those datasets, by utilizing the "eins(T, thetae)" function implemented in Artemis. What is the equation that is parametrized here? Does it include the s0^2 offset term that accounts for the overall configuration disorder in the system? And if that is the case is there same way to separate it from the temperature dependent s^2 term?

Thank you in advance, Deltsidis Alexandros PS:I am attaching a png. file exported from Artemis that is relative to my question 1)

Ph.D candidate, Institute of Electronic Structure and Laser (IESL), Foundation for Research and Technology - Hellas (FORTH)_______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit Unsubscribe: http://millenia.cars.aps.anl.gov/mailman/options/ifeffit

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