One possible scenario: If one site has 6 nearest neighbors and the other - 4, and if you choose the site with 4 neighbors to construct FEFF to model your EXAFS data; and if you set your degeneracy equal to 4 and make your amplitude factor as S02*x of one site + S02*(1-x) of another site, then your S02 will come out larger than it should be because it will compensate for the fact that you underestimate the degeneracy of the 6-coordinated site.
Anatoly
________________________________________
From: ifeffit-bounces@millenia.cars.aps.anl.gov [ifeffit-bounces@millenia.cars.aps.anl.gov] on behalf of huyanyun@physics.utoronto.ca [huyanyun@physics.utoronto.ca]
Sent: Friday, March 20, 2015 12:04 PM
To: XAFS Analysis using Ifeffit
Subject: Re: [Ifeffit] amplitude parameter S02 larger than 1
Hi Chris,
Thank you for your suggestion. But I don't have a standard. Also, I
wonder whether a multiple-site situation could be different from the
normal one-site case with respect to S02.
Best,
Yanyun
Quoting Chris Patridge
Hi,
One thing that could be considered is transferring the SO2 factor from a reliable source such as a standard and then use that value in the fit. Chemical transferability of SO2 to similar systems is often acceptable. You could also try constraining the value in the fit as well. SO2 and Debye are also correlated so this may also affect the value.
Hope that helps,
Chris
Sent from my iPhone
On Mar 19, 2015, at 6:32 PM, huyanyun@physics.utoronto.ca wrote:
Hi all,
I know this question has been asked for many times. S02 is expected to be around, but smaller than 1, a fact that has been explained, such as in the following previous emails, in our mailing list.
http://www.mail-archive.com/ifeffit%40millenia.cars.aps.anl.gov/msg02237.htm... http://millenia.cars.aps.anl.gov/pipermail/ifeffit/2003-February/000230.html
However, I am continually get S02 value larger than 1 for a series of similar samples when I fit data in Artemis. I think my fit is very good, because my suspected model(based on other technique) could be verified in XAFS analysis (i.e., defensible in physics), the statistics is good ( R=0.01, reduced chi-square=31.4, fit-range:1.5~6 Angstrom, k-range: 3~14 angstrom-1) and all the parameters such as the bond length, sigma2 are physically reasonable. The only thing makes me uncomfortable is that parameter S02 keeps between 1.45 to 1.55 during the fitting.
In my system, the absorber atom occupies two crystallographic sites. So I built a model with paths generated from two FEFF calculations. For paths generated from the 1st and 2nd FEFF calculation, the amplitude parameters are set to be S02*P% and S02*(1-P%) respectively, where P% is the first site occupancy percentage. Both S02 and P are free parameters during the fit, and P is an important conclusion I want to extract from XAFS fitting.
However, the fit result gives me S02=1.45 ~ 1.55 and P=0.51 ~ 0.56 all the time (i.e., for each path the 'total amplitude' S02*P% or S02*(1-P%) are about 0.7~0.8, smaller than 1). It looks to me that I got a 'perfect' fit but I am not sure if S02 larger than one is defensible. So I have to ask:
1) Is my current fit with S02 larger than one reasonable? If not, what could be suggested to get around it?
2) What's the meaning of S02? It is interpreted in physics that it is a reduced electron excitation parameter, but is it possible that S02 will be affected by any experimental condition?
3) Can anyone share whether you had the multiple site system that gets S02 larger than one?
Looking forward to your help.
Best, Yanyun
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