You definitely will see the difference (sorry that I am replying instead of Scott who will not mind of course) because from Ni edge the result means an effective mixture of Ni-Ni and Ni-Fe bond lengths, and for edge - the effective mixture of Fe-Ni and Fe-Fe bonds. That's why you get different results. If you could measure separately Fe-Ni and Ni-Fe (you can't ) their lengths should have been the same, of course, but in your case you are not doing that, hence the difference.
It could have been caused by a million other reasons, e.g. the correlation with other parameters or bad data quality or mistakes in the model or how you constructed the fitting procedure, but it is besides the point.
Anatoly
Sent from my iPhone
On Jun 23, 2016, at 5:51 PM, Fuxiang Zhang mailto:zhangfx@umich.edu> wrote:
Hi, Scott,
In fact, I have a series of single crystal Ni-Fe alloys (Fm-3m) with known atomic ratio, and measured both Fe and Ni edges for samples and standards. I just want to see if there's any difference between the local bonding environment of Fe and Ni, such as R, N and sigma2. Now using the suggested method by Matt by changing atoms in FEFF.inp file, I can only get ss, deltr and E0 from fitting (I don't know how to refine N and R). But the fitting with Ni (or Fe) edge can only generate the same R and deltr for Ni-Ni and Ni-Fe pairs. Fitting the edges of Ni and Fe will yield a little difference in the R and deltr. Which results should I believe, or something wrong I am using the program because I cannot generate different R for Ni-Ni and Ni-Fe pairs with the same data set.
Many thanks for your suggestions and helps.
Fuxiang Zhang
On Thu, Jun 23, 2016 at 4:23 PM, Scott Calvin mailto:dr.scott.calvin@gmail.com> wrote:
One additional thought:
Distinguishing Ni-Ni and Ni-Fe by creating two different feff paths and fitting the spectrum as a mixture of the two is very difficult, to put it mildly, as both Anatoly and Matt indicated.
But the bond length determined by your fit may be able to give you a clue, particularly when compared to appropriate standards. Depending on the details of your structure, the average bond length may depend on the fraction of sites occupied by iron to a degree which is measurable by EXAFS. Even if you simply use a nickel-nickel scattering path in feff to model your structure, differences in bond length could be indicative of changes in site occupancy.
Ideally, this could be done by creating a series of standards with known iron-nickel ratios, fitting the bond length for each, and then comparing a fit of a sample with a similar structure but unknown iron-nickel ratio to the results for the standards.
—Scott Calvin
Sarah Lawrence College
On Jun 23, 2016, at 3:41 PM, Fuxiang Zhang mailto:zhangfx@umich.edu> wrote:
Hi, Matt and Anayoly,
Thank you very much for the help.
Fuxiang
On Thu, Jun 23, 2016 at 1:51 PM, Anatoly I Frenkel mailto:afrenke2@yu.edu> wrote:
Whatever you decide to do, remember that Ni Ni and Ni Fe pairs look very similar by EXAFS, so it does not matter how you end up doing it.
You can as well use pure Ni but you may want to consider both fcc and bcc structure for making the model because the alloy may end up to be either that or that, especially if it is a nanoalloy.
Anayoly
Sent from my Verizon Wireless 4G LTE Tablet
-------- Original message --------
From: Fuxiang Zhang mailto:zhangfx@umich.edu>
Date: 6/23/2016 12:51 PM (GMT-05:00)
To: ifeffit@millenia.cars.aps.anl.govmailto:ifeffit@millenia.cars.aps.anl.gov
Subject: [Ifeffit] How to input atoms in solid solution for Artemis
Hi, All,
I am new in using EXAFS techniques. Just wondering how to input atomic structure of NiFe solid solution to Artemis. It has a simple Ni-structure, but both Ni and Fe occupy the same site. If I use partial occupancy, Artemis program does not allow. Anyone can give me an ideal?
Also the coordination number N cannot be refined in this simple system (12), is that right? In fact I just want to get the N through the fitting of EXAFS because of the partial occupancy.
Thank you very much
Fuxiang Zhang
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