[Ifeffit] amplitude parameter S02 larger than 1

huyanyun at physics.utoronto.ca huyanyun at physics.utoronto.ca
Fri Mar 20 12:44:05 CDT 2015


Hi Josh and Anatoly,

Thank you for bringing up this point.

I agree that P would be 100% correlated with S02 if we assume both  
sites have the same first-shell distance. In my fit, the correlation  
between the percentage parameter P and S02 is -0.73.

Do you have any good idea on how to break this kind of correlation, or  
alternative way to find the 'true S02' and 'true P'?

Best,
Yanyun


Quoting Joshua Kas <joshua.j.kas at gmail.com>:

> Anatoly makes a very good point, and his example can also be used to show
> that there could be very large correlation between s02 and your percentage
> parameter. For example, if you assume that the first shell distances are
> the same for the two sites, and take Anatoly's example of 4 and 6 nearest
> neighbors, then your parameter P is 100 percent correlated with s02, i.e.,
> all it does is change the coordination smoothly from 4 to 6. In a real
> system this probably won't happen, but the correlation could still be
> large. Have you looked at it?
>
> Josh
>
> On Fri, Mar 20, 2015 at 10:00 AM, <ifeffit-request at millenia.cars.aps.anl.gov
>> wrote:
>
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>> Today's Topics:
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>>    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
>> >>
>> >>
>> >>
>> >>
>> >>
>> >>
>> >>
>> >>
>> >>
>> >>
>> >>
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