[RMCProfile-users] Spikes in PDF when using distance window and Bragg

[Shiliang Zhou]

Hi all,

I have been getting spikes in my PDF model when I try to fit the PDF and
Bragg datasets at the same time. The system is Fe2Mo3O12, and I have them
in the order of [Fe Mo O]. The spikes usually happen at the distance window
limits that I set. When I set up a distance window by including the
following block in the .dat file,

DISTANCE_WINDOW ::
 > MNDIST :: 0 0 1.84 0 1.64 0
 > MXDIST :: 0 0 2.14 0 1.94 0

I got a spike at r=0 even though the starting point for the PDF to fit is
at 1.61 A. (Pic #1)

When I set up the distance window by having a separate .dw file with the
following cotent,

0 0 1.84 0 1.64 0
0 0 2.14 0 1.94 0

it split the Fe-O and Mo-O peaks into basically two sets of spikes at 1.84,
2.14, 1.64 and 1.94. (Pic #2)

But if I don't include the Bragg datasets, it doesn't have these issues.
(Pic #3) The Bragg fit was pretty good to start with (Pic#4). Do you have
any idea what the driving force of the spikes may be?

I do find the following in the v6 EXAFS manual, which says "DISTANCE
WINDOW" and "MINIMUM DISTANCES" constraints frequently cause unphysical
spikes in partial PDFS at the distance limits set by these constraints....

If that's just a program bug and it coincidentally happens to me when I try
to include the Bragg datasets, how exactly should I set up the restraints
on the peak tails? It says to fit a fourth-order polynomial to the tail of
interest. But how do you define the tail, do I just choose the data range
from the onset to the highest point of the peak?

I tried including the following block in the .dat file, but the same spikes
as in Pic #2 showed up.

LEFT_TAILS ::
  >START_FINISH :: 1.62 1.76 (start and end of the tail)
  >PARTIAL :: 5 (partial number 5, which is Mo-O)
  >COEFFICIENTS :: -156.617 3826.67 -25411.7 55786.8 (coefficients I
obtained by fitting the PDF between 1.62 to 1.76 using Origin)
  >WEIGHT :: 0.01 (How much weight should I generally put into this?)

[image: Fe_bragg_0_2.JPG]
[image: Fe_bragg_0_dw_file.JPG]

   [image: Li_nobragg_0.JPG]
[image: bragg_fit.JPG]
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