[Ifeffit] determining S02 from ZnO vs. Zn foil

Jeremy Thorbahn thorbahn at gmail.com
Tue Oct 9 17:48:30 CDT 2012


Thanks very much for your insight. I am glad to hear this is a known
phenomenon, and your reasoning about attenuation makes a lot of sense. My
samples were glasses crushed into powders, so I will proceed cautiously
with the ZnO result.


On Tue, Oct 9, 2012 at 6:09 PM, Scott Calvin <scalvin at sarahlawrence.edu>wrote:

> Hi Jeremy,
> What you describe matches very well with my experience.
> It is difficult to prepare powder transmission samples well enough so that
> there is no attenuation of the EXAFS due to particle size. Not impossible,
> but difficult. Much of the published work I've seen on first-row transition
> metal K edges settles for a little bit of attenuation; if they're fitting
> S02 and are primarily interested in phase variables such as bond length,
> there's little harm done. (Of course, you're looking for an amplitude
> variable, i.e., coordination number!)
> This means powder standards often show a somewhat lower fitted value for
> S02 than foil standards.
> As to which you should use to constrain your samples, it depends on the
> form of the samples. You say they're "glass," and are implying that they
> are measured in transmission. If "glass" means literally a piece of
> macroscopic glass, and the ZnO is distributed homogeneously through it,
> then you should use the foil S02. If it's a glass microscopically, but are
> measuring it as a powder, the ZnO S02 is better. Either way, I'd take into
> account the additional uncertainty in S02 when reporting the uncertainties
> in your fitted coordination numbers.
> --Scott Calvin
> Sarah Lawrence College
> On Oct 9, 2012, at 3:21 PM, Jeremy Thorbahn wrote:
> >
> > [note: when putting together this post I discovered a possible bug
> regarding opening my project files on different PCs. I will submit that
> issue separately]
> >
> >
> > Hi all:
> >
> > I am writing to probe the community's knowledge regarding my current
> problem in measuring S02.
> > I have data on a series of glass samples containing ZnO. I also measured
> a ZnO crystalline powder, and a Zn foil was measured between detectors 2
> and 3 (transmission mode) simultaneously with every run.
> >
> > My goal is to determine coordination numbers around Zn in the glass
> samples, which requires an accurate measurement of S02 for zinc.
> >
> > My main question is if there is any reason why the Zn foil would provide
> a more accurate result than the ZnO powder, or vice versa. I see no reason
> why it should be different, especially since this method depends entirely
> on the 'chemical transferability' of S02 in the first place. Nevertheless,
> I obtain different results from each, with S02 ~ 0.75 from the ZnO and
> ~0.88 from the foil.
> >
> > Statistically, the Zn foil dataset is "better" since it is a merge of
> all of my scans while there are only two scans of ZnO.
> > However, part of me wants to trust the ZnO data more, for somewhat vague
> reasons such as that it is a "more similar" coordination environment to
> what I would expect in my samples, and that it was measured between the
> same two detectors, etc.
> >
> > If anyone has any experience that would indicate why one should be
> better than the other, I would love to hear about it.
> > I will attach Artemis project files for each if you'd like to take a
> look at my fits. It is entirely possible that the discrepancy is due to an
> error in data processing or analysis on my part; I am by no means an
> expert. I hope the project files will not clog up anyone's mailbox, at a
> couple of megabytes each.
> >
> >
> > Thanks,
> > Jeremy
> >
> >
> >
> > <Zn_foil.fpj><ZnO.fpj>_______________________________________________
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Jeremy Thorbahn
thorbahn at gmail.com
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