[Ifeffit] Ifeffit Digest, Vol 104, Issue 9

[Soham Mukherjee]

For the first shell, I used cubic phase as my starting model and that gave a
pretty good fit. However, when I take wurtzite phase as my starting model
and vary two bond lengths (both Zn-S, with the axial bond longer by only
0.004 Ang) independently, my delr and ss go crazy. I think that as the first
two S subshells are so close to each other, my data can't identify them
(delr and ss) independently. So I defined their delr and ss to be same and
the
results match pretty well with the cubic phase. The r-factor and reduced-chi
square with the two models are very similar.Thanks a lot !

On Sun, Oct 9, 2011 at 10:30 PM,
<ifeffit-request at millenia.cars.aps.anl.gov>wrote:

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> Today's Topics:
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>   1. lifetime broadening of pre-edge peaks? (Matthew Marcus)
>   2. Re: Zinc Sulfide fitting (JeongEunSuk)
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> ----------------------------------------------------------------------
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> Message: 1
> Date: Sat, 08 Oct 2011 14:36:56 -0700
> From: Matthew Marcus <mamarcus at lbl.gov>
> To: XAFS Analysis using Ifeffit <ifeffit at millenia.cars.aps.anl.gov>
> Subject: [Ifeffit] lifetime broadening of pre-edge peaks?
> Message-ID: <4E90C278.3060602 at lbl.gov>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>
> Is the core-hole lifetime broadening of pre-edge XANES peaks the same as at
> the main edge?  I'm looking into issues of de-noising of XANES epsctra, so
> need to figure out how narrow real features can be.
>        mam
>
>
> ------------------------------
>
> Message: 2
> Date: Sun, 9 Oct 2011 07:06:52 +0900
> From: JeongEunSuk <eunsuk1986 at hotmail.com>
> To: ifeffit <ifeffit at millenia.cars.aps.anl.gov>
> Subject: Re: [Ifeffit] Zinc Sulfide fitting
> Message-ID: <BLU151-W2672826EC60532B7643F18DAFF0 at phx.gbl>
> Content-Type: text/plain; charset="ks_c_5601-1987"
>
>
> hello
>
> Two phase can be mixed in ZnS. You need to check r-factor and reduced-chi
> squrare with two model.
> If you want to fit only first shell,
> I suggest.
> first use cubic zinc-blende model in FEFF, and check goodness of fit.
> second, fix the bond length of 3 bonding length and change the bond length
> of a rest.
> when you use my suggestion, the variables in Artemis is delrS1(changable),
> sigmaS1(changable), others(fix)
>
>
>
>
> Date: Thu, 6 Oct 2011 13:32:24 +0530
> From: soham.chem at gmail.com
> To: ifeffit at millenia.cars.aps.anl.gov
> Subject: [Ifeffit] Zinc Sulfide fitting
>
> Dear Ifeffit Community,
>
> Zinc Sulfide exists largely in two phases, either the cubic zinc-blende
> (F-43m) phase, or the hexagonal wurtzite (P63cm) phase.
>
> The details of the NN distances are as follows :-
> 1.  For cubic phase, the NN distances (Zinc to Sulfur bond lengths) are
> 2.3028 Angstroms each, 4 such bonds exist in a tetrahedral arrangement;
> 2.  For hexagonal phase, there are 2 different NN distances (Zinc to Sulfur
> bond lengths) 3 bonds having the length 2.3421 Angstroms each, and 1
> axially
> elongated bond of length 2.3465 Angstroms. So they differ only by  0.0044
> Angstroms !
>
> I am performing a first shell fit to Zinc Sulfide which exists in the
> wurtzite phase (hexagonal_P63cm). However, I could fit it quite well taking
> the cubic phase (Zinc blende_F-43m) as my starting model. The only thing is
> that since bond distances are higher in a hexagonal phase than the cubic
> phase, I end up with a considerable delr as I underestimated my starting
> bond length value. Taking a wurtzite model involves defining two different
> bond lengths (which differ only by 0.0044 Angstroms !), paths 1 and 2 in
> the
> FEFF, whose sigma^2 values are quite similar. This gives unusually large
> delr values and high sigma^2 for the longer axial bond, which looks
> suspicious !
>
> Do I really need to define the two bond distances separately of so close
> values? Or taking a cubic phase will be sufficient? Please find the
> attached
> project file (artemis.apj) where I have tried out some simulations with
> both
> cubic and hexagonal models.
>
> Here are a few details you asked for :-
>
> The sample is a thin layer, it's pretty homogeneous and had no pinholes
> during measurements. The fit details are :-
> k_min = 2, k_max = 14
> R_min = 1, R_max = 3
> The potential is self consistent in these calculations
> Fitting was performed in the R-space
>
> Best regards,
> Soham.
>
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