[Ifeffit] EXAFS reference materials
frenkel at bnl.gov
Thu Mar 22 15:03:34 CDT 2007
Thanks, Scott! - Good point.
From: ifeffit-bounces at millenia.cars.aps.anl.gov on behalf of Scott Calvin
Sent: Thu 3/22/2007 3:48 PM
To: XAFS Analysis using Ifeffit
Subject: Re: [Ifeffit] EXAFS reference materials
Following up on Anatoly's excellent post, I would say that I
personally distinguish between a "reference" and a "standard."
When I talk about a reference, I usually mean something I'm measuring
simultaneously with the sample. The primary purpose is energy
alignment, although it also gives a check on beam quality issues
(e.g. harmonics). For this purpose, I generally use whatever is
convenient and well known, such as a metal foil. Sometimes I'll
measure the reference by itself, particularly if it's a beamline or
edge that is new to me.
On the other hand, I think of a standard as a known compound that is
similar to my sample. I often work with nanoparticles, so my standard
might be the corresponding bulk material. Or if I'm working with a
doped compound, the standard might be the undoped version. The
standard is measured under the same conditions as the sample...in
fact, that means that I measure the standard with a reference in
place, if that makes sense. :)
So I use a reference for Anatoly's purpose (1) and a standard for
Anatoly's purposes (2) and (3).
I hope that's helpful.
Sarah Lawrence College
At 03:28 PM 3/22/2007, you wrote:
>I am cc'ing this email to IFEFFIT mailing list. There are several
>reasons (I am listing just a few) you should measure a reference
>sample simultaneously with unknown materials.
>1) to align the unknown sample scans prior to merging. If those data
>are noisy, reference sample can be used for the alignment. In
>addition, if you want to compare samples before and after some
>changes that may affect their energy positions (e.g., oxidation,
>reduction, doping, metal-insulator transition, etc.) you want to
>align all of their references to reliably analyze these changes.
>For this purpose, it does not matter whether you use a metal or a
>metal oxide or any other standard with the same resonant element.
>2) to find S02 value. It is usually the same in most samples which
>have the same resonant element.
>For this purpose, what matters the most is whether you can reliably
>find S02 from your standard (and then impose it on your unknown
>sample). Thus, some exotic reference which may still be good for 1)
>may not be as good for 2) since it may be difficult to refine its
>structure and get precise value for S02. The metals would be the
>best, but if you have other materials with "open lattices", e.g.,
>fcc, rock salt, etc., with well defined first shell that you can fit
>with the miniumum parameters, they will all work fine.
>3) to use as a model for your analysis.
>If you analyze an alloy, as in your case, you want to compare EXAFS
>results in your alloys against those in relevant pure compounds. For
>binary alloys, the best reference will be a pure metal, for ternary
>- an intermetallic compound, etc. An oxide would not be so useful
>for modeling. For that reason, people often measure more than one
>reference, as per 1),2) and 3).
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