[Ifeffit] amplitude parameter S02 larger than 1

Anatoly I Frenkel afrenke2 at yu.edu
Fri Mar 20 11:57:02 CDT 2015

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.


From: ifeffit-bounces at millenia.cars.aps.anl.gov [ifeffit-bounces at millenia.cars.aps.anl.gov] on behalf of huyanyun at physics.utoronto.ca [huyanyun at 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.

Quoting Chris Patridge <patridge at buffalo.edu>:

> 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 at 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.html
>> 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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