[Ifeffit] amplitude parameter S02 larger than 1

Fri Mar 20 12:13:07 CDT 2015

On Fri, Mar 20, 2015 at 10:00 AM, <ifeffit-request at millenia.cars.aps.anl.gov
> wrote:

> Send Ifeffit mailing list submissions to
>         ifeffit at millenia.cars.aps.anl.gov
>
> To subscribe or unsubscribe via the World Wide Web, visit
>         http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
> or, via email, send a message with subject or body 'help' to
>         ifeffit-request at millenia.cars.aps.anl.gov
>
> You can reach the person managing the list at
>         ifeffit-owner at millenia.cars.aps.anl.gov
>
> When replying, please edit your Subject line so it is more specific
> than "Re: Contents of Ifeffit digest..."
>
>
> Today's Topics:
>
>    1. Re: amplitude parameter S02 larger than 1 (Anatoly I Frenkel)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Fri, 20 Mar 2015 16:57:02 +0000
> From: Anatoly I Frenkel <afrenke2 at yu.edu>
> To: XAFS Analysis using Ifeffit <ifeffit at millenia.cars.aps.anl.gov>
> Subject: Re: [Ifeffit] amplitude parameter S02 larger than 1
> Message-ID:
>         <
> 545F89325F51764C902D37B42C9F4D530148FDA88C at YUWEXCPM11.yuad.uds.yu.edu>
>
> Content-Type: text/plain; charset="us-ascii"
>
> 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 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.
>
> Best,
> Yanyun
> 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
> >>
> >>
> >>
> >>
> >>
> >>
> >>
> >>
> >>
> >>
> >>
> >> _______________________________________________
> >> Ifeffit mailing list
> >> Ifeffit at millenia.cars.aps.anl.gov
> >> http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
> >
> > _______________________________________________
> > Ifeffit mailing list
> > Ifeffit at millenia.cars.aps.anl.gov
> > http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
>
>
>
> _______________________________________________
> Ifeffit mailing list
> Ifeffit at millenia.cars.aps.anl.gov
> http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
>
>
> ------------------------------
>
> _______________________________________________
> Ifeffit mailing list
> Ifeffit at millenia.cars.aps.anl.gov
> http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
>
>
> End of Ifeffit Digest, Vol 145, Issue 31
> ****************************************
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://millenia.cars.aps.anl.gov/pipermail/ifeffit/attachments/20150320/5ef05401/attachment.html>