[Ifeffit] Pb compound determination fitting issue

Wed Feb 25 04:45:15 CST 2015

Dear Pieter,

You wouldn't start with Pb exafs if you had the choice. Pb has a lot of structural disorder in its compounds and is very thermally wobbly leading to poor signal to noise data. I think your dataset is only good to 7 -8 inverse A. That said I think you can learn something about the sample.
Was the sample cold? eg in a cryostat when measured this is about the only way to get OK Pb data from anything but a solid state system with ordered atoms.
In fit 1 (see the history) in the attached project file I use the first shell of massicot and this doesn't give a good fit but nothing ridiculous , in fit 3 I sued the cerrusite 1st shell and e0 goes along way and the first shell distance contracts a lot. This suggests you are likely  have O in the first shell and by playing around more, e.g.
Shortening the data range in Athena to improve the data reduction you should be able to get a better fit than with just one button press, however the useable data range is small so it will be not great
In accuracy terms.
Also you might try comparing the XANES to literature data though the core hole size in Pb L3 washes out xanes a bit so its not the best.

Regards
Fred

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Message: 1
Date: Wed, 25 Feb 2015 11:20:17 +0100
From: Pieter Tack <Pieter.Tack at ugent.be>
To: ifeffit at millenia.cars.aps.anl.gov
Subject: [Ifeffit]  Pb compound determination fitting issue
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Hi everyone,

I recently acquired an EXAFS data set (albeit somewhat noisy) on the
Pb-L3 edge. I have normalized it using Athena, and tried to fit the first shell using Artemis. I attached the Athena project file as well as the Artemis project file of what I obtained thus far.

We have reason to believe the unknown compound is PbO or PbS (or a mixture). However, when I look at the R space curve, the first coordination shell is at rather low path distance (1.7 A), whereas one would expect larger bond distances for PbO and PbS. I found that a similar bond distance as to what the experimental spectrum shows can be found in PbCO3, but a fit using the mineral data of PbCO3 also doesn't do the trick.

In any case, no matter what I try to fit in Artemis, nothing seems decent. Is the EXAFS fitting simply impossible due to the noisy data, did I extract my Chi(k) wrong or am I encountering some other issue?

I realize it's probably not much to go on, but I'm rather new to EXAFS analysis so any help/advice is appreciated.
Thanks a lot in advance!

Best regards,
Pieter Tack

--
X-Ray Micro-Spectroscopy and Imaging Group Department of Analytical Chemistry Ghent University Krijgslaan 281 S12
B-9000 Ghent
Belgium
Phone: +32 (0)9 264 4723
Fax: +32 (0)9 264 49 60

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