[Ifeffit] amplitude parameter S02 larger than 1

[Bruce Ravel]

On 03/23/2015 04:10 PM, huyanyun at physics.utoronto.ca wrote:
>
> This is to follow up the *test* experiment you suggested. Attached are
> three Artemis files.
>
> I chose the Fe (im-3m) structure to do the test. The normal crystal
> structure has its first-shell distance at 2.48 angstrom. A large
> structure was created so that the first-shell distance reaches 3.11
> angstrom.
>
> Both normal and large structure are calculated in JFEFF in the same
> manner (i.e., same path number, same calculation procedure, same
> sigma2=0.0045 for all paths) to generate the calculated chi data.
>
> File #1 attached was exactly following the procedure you mentioned. The
> quick first shell fits very well, four parameters except S02 give normal
> results. The amplitude parameter S02 is very large (2.2+/-0.14).
>
> I extended the test in #2 and #3 for comparison. In #2, calculated data
> based on large structure is presented to fit to the same large
> structure. As expected, we get reasonable fit looking with other
> parameters normal. However, amplitude S02 fits to 1.80+/-0.03. In #3,
> both data and structure are normal. In this case, it is not surprising
> to get good fit and all parameters including S02 turn out to be close to
> their true values.
>
> I think it is clear that first-shell distance larger than 3 angstrom
> does has effect in making amplitude artificialy large.

Yanyun,

Very interesting!  It's rather difficult to understand how to relate
what you have found to your actual sample.  That is certainly true in
a quantitative sense.  I don't think this tells you how to interpret
the actual value that you are finding for S02.  But I think it does
give a hint for why you are getting such a large value in your case.

I will be very interested to see how you address this when you publish
your results.

For everyone else, I would caution against reading too much into what
Yanyun has found.  (Even more so than I would caution her!)  She is
looking at a really unusual situation.  Her materials really do have
extraordinarily long near neighbor distances.  In fact, last fall when
she showed me her data, I was quite shocked that such a thing exists.
In all my years doing XAS and being a beamline scientist, I had never
seen such a long near-neighbor distance.  The FT of the data was
really quite remarkable!

Trying to understand a large S02 is a common question on this mailing
list and elsewhere.  However, I don't think that Yanyun's experience
gives much insight to most such problems.  Her situation is quite
unusual.  If you are seeing an oddly large S02, that is something you
need to figure out about your sample or your data.  In most cases, you
cannot explain it away by asserting that Feff made a mistake with the
muffin tin radii.

As an example, I rather doubt that Yanyun's experience can shed much
light on the questions that Jatin Rana was asking over the weekend.

Anyway, wow!  What an interesting day on the mailing list!

B


-- 
  Bruce Ravel  ------------------------------------ bravel at bnl.gov

  National Institute of Standards and Technology
  Synchrotron Science Group at NSLS-II
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