[Ifeffit] a question

Bruce Ravel bravel at bnl.gov
Tue Aug 25 12:20:50 CDT 2009

On Monday 24 August 2009 04:21:01 pm Dr Somaditya Sen wrote:
> I am having problems in comparing the real bond length as obtained from
> EXAFS data and Reitveld analysis of XRD data of te same sample. The bond
> lengths as observed from the XAFS data seems to be much smaller that that
> from XRD. Is there some multiplication factor known in literature to
> address this issue?

As Jason said, you seem to be neglecting the scattering phase shift in
the EXAFS equation, which has the effect of shifting a peak in the
Fourier transform by about 0.3 - 0.5 angstroms to a lower value.  This
is taken into account correctly by the data analysis software such
that results reported from a fit represent a measurement of an actual
interatomic difference.

There is a broader question, though.  Should you expect Reitveld and
XAS to give the same answer for your sample?  Reitveld measures
long-range correlations while XAS is an inherently local probe.  If
there is some kind of local disorder in your material, then it may not
be reasonable to expect the two to agree.  Indeed, the manner in which
they disagree is often the very purpose of doing the XAS experiment.

Your question is pretty fundamental.  There is a lot of material
available at http://xafs.org to help you get started understanding
your XAS measurements.  I think you would benefit by poking around



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

 National Institute of Standards and Technology
 Synchrotron Methods Group at NSLS --- Beamlines U7A, X24A, X23A2
 Building 535A
 Upton NY, 11973

 My homepage:    http://xafs.org/BruceRavel
 EXAFS software: http://cars9.uchicago.edu/~ravel/software/exafs/

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