Scott,
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.
Jeremy
On Tue, Oct 9, 2012 at 6:09 PM, Scott Calvin
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
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-- ------------------------- Jeremy Thorbahn thorbahn@gmail.com