Hi Fuxiang,
How much background do you have in EXAFS fitting? Have you taken a
course?
Do you have access to a good EXAFS primer book (e.g. Grant Bunker's
or Scott Calvin's)?
1) Which is better, to refine only the first shell or more
shells.
The results seems to be quite sensitive on the selection
of kmin/kmax and Rmin/Rmax, I cannot judge when can get reliable
results (base on chi?)
In general, I would take a kmin that is above the edge and away
from any white line because background
removal in those areas (white line, or on the rising edge) is
suspect. I would take a kmax that avoids data that cannot be
distinguished from noise. I picked Rmax on the high side to
isolate the 1st peak in the FT. I picked Rmin based on the
magnitude of the 1st Feff shell and a need to get enough R-range
to do a 6
parameter fit. It was a nervous choice since too low an Rmin could
bring in unwanted artifacts. What it
comes down to is: do the choices allow a sufficient amount of data
for use in fitting; and are
they defensible in review.
What I considered good fits were based in part on looking for
where chi^2 is a minimum and
in part from inspection of the results - were the results
consistent with a basic understanding
of how atom-atom distances should manifest. You applied this as
well when you initially wrote
and noted your fit was giving Pd-Pd distances shorter than Pd-Ni,
and you questioned that.
As for fitting higher shells, what is the problem you are trying
to answer? Is the Pd distribution
statistical or showing evidence of clustering? The higher shells
are weak and have multiple scattering
as well as back-scattering paths. The error in just the first
shell Pd content is large, owing to the
strong correlation with the mean-square-relative-displacement. I
would expect this to also be
problematic in higher shells - more so. So, can you build a
defensible argument based on a first
peak analysis? Is taking more data an option (going to higher k
with less noise)? This was just
the Pd data. How consistent is the Ni data fit?
2) You used Rmax=2.56, while Rpd is 2.68, not sure
if there's any contradict
The sine term in the EXAFS equation is sin(2kR+delta). What is
delta?
Did you measure reference foils for Ni and Pd? Where does the
first peak manifest
in the |FT(k^n * chi(k))| - at what R-value? By how much is the
peak shifted from the
crystallographic distance for the different standards?
3) For the Nni and Npd, you change them manually and check
the chi values during fitting, is that right?
Correct. Try this yourself. I suggest you tabulate and plot them
so you can see
the different subsidiary minima and maxima and how the fit results
vary between them. Perhaps use
a broad range to get a sense of what a global minimum might be.
Please note: The questions I have posed are intended to stimulate
thinking and need not warrant
a response.
Another curious note: If you look at the chi(k)'s for the 1st
shell Feff paths, they are almost
180 degrees out of phase at higher k...a suppressing effect that
could make it difficult to
study higher concentration mixtures.
cheers,
-R.
On 2/21/2017 6:22 AM, Fuxiang Zhang
wrote:
Hi, Robert,
Thank you very much for the help. Based on the nominal
composition, there should have more than one Pd atoms in the
1st shell and both the R values you got are reasonable.
I still have some questions about the fitting
1) Which is better, to refine only the first shell or more
shells.
The results seems to be quite sensitive on the
selection of kmin/kmax and Rmin/Rmax, I cannot judge when can
get reliable results (base on chi?)
2) You used Rmax=2.56, while Rpd is 2.68, not sure
if there's any contradict
3) For the Nni and Npd, you change them manually and check
the chi values during fitting, is that right?
Thanks again
Fuxiang
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