On May 19, 2009, at 5:16 AM, wahyu bambang wrote:
Is it alright if I substract the background and refine it a little
first using another refinement software before I go through GSAS?
Wahyu,
While I agree with what was said in other messages and prefer to
see people fit background using functions that are refined rather
than do a subtraction, let me address the question above directly.
The goal of a Rietveld analysis is to fit diffraction intensities
using a crystallographic model, which requires that one model a whole
bunch of effects (such as peak shape, background,...) that are
artifacts of how the measurement was done. In the case of background,
the goal is to account for the "extra" non-Bragg intensity that shows
up in the collected data. There is nothing wrong with subtracting out
any smooth arbitrary curve from the data before you fit -- as long as
the uncertainties are computed from the original intensity values,
not from the values after subtraction. Since this subtraction does
not change the Bragg intensities or the weights, the results of the
crystallographic fit are unchanged.
If you do subtract a background, you are right to also do a
refinement of background too. My personal opinion on this (which
differs from what Rietveld-expert Dave Cox has said) is that
regardless of what one does to define background manually, one should
always refine a background contribution, if possible. The reason for
this is that the location of the background curve indirectly
determines the Uiso values for the atoms. If the background is fixed
and not refined, then the uncertainty on the Uiso values is
unrealistically low. For patterns having lots of overlapped peaks at
high angles, this location is no better determined by eye than by
refinement and in some cases there is no unique solution. When this
happens, the inability of the refinement to find a minimum (or a
minimum with reasonable Uiso values), lets you see that the data are
not sufficient to determine Uiso and background -- a very useful
thing to know.
Brian
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Brian H. Toby, Ph.D. office: 630-252-5488
Senior Physicist/Materials Characterization Group Leader
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