[Ifeffit] how to recognize element with a difference of Z<2?

[Bruce Ravel]

Sebastiano,

You are correct that EXAFS is relatively insensitive to atoms that are
nearby on the periodic table.  This is because the backscattering
amplitudes and phase shifts are functions of, in part, Z number.

In the case of stochastic replacement of a host atom by a dopant on
the exact position of the lattice site, it can be difficult or
impossible to distinguish nearby atoms.  However, if the introduction
of the dopant is accompanied by a distortion to the lattice, the
situation might be a lot friendlier.

In that case, you may be able to notice two (or more) distances in the
neighboring shells, one of which can be explained by the host atom and
the other(s) by the dopant.

Even better, you can measure data at the edges of both host and dopant
and corefine the data.  That additional information might be enough to
figure out what's going on.

I also think that your case -- an alloy -- might be a good candidate
for high energy diffraction using PDF analysis methods.  PDF, or PDF
combined with EXAFS, might help for your problem.

Good luck,
B

On Tuesday 15 May 2007, Cammelli Sebastiano wrote:
> Dear
>
> I am Sebastiano and I am working on some metallic alloy composed by
> several diluted elements in a Fe matrix. The main elements are: Cu, Cr <
> 0.5at%; Mn, Ni < 1.5at%, I collected some spectra at different K edge
> (Mn, Ni, Cu). Submitting this alloy by annealing treatment (775K) and
> neutron irradiation, some clusters are expected mainly composed by Cu;
> Ni and Mn play an important role but it is not so clear. Some problems
> rise when I try to analyze these data:
>
> "It is known" that it is not so easy to recognize by EXAFS the
> difference from near elements as Fe and Mn (Z=26,25) and Cu, Ni (29,28)
> because these elements has a similar electronic distribution. Performing
> the analysis by Artemis I found some 'realistic' value of Mn Fe and Ni
> in the first and second shell (Cu K edge), have these values any
> physical meaning since I cannot distinguish element as Fe and Mn?
>
> Do you know any tricks that could help me? Maybe the right answer is
> that EXAFS is not enough to investigate these alloys, but if I cannot
> distinguish the different atoms, which kind of quantitative information
> could I get? The interatomic distances are important but which is the
> meaning of distance between the absorber (Cu) and an unknown atom (Fe or
> Mn, Cr or Mn, Cu or Ni?)?
>
> Thank you help and for your time.
>
> Sebastiano
>
>
>
> Sebastiano.cammelli at psi.ch
>
> Paul Scherrer Institute, Villigen 5232 (CH)


-- 
 Bruce Ravel  ---------------------------------------------- bravel at anl.gov

 Molecular Environmental Science Group, Building 203, Room E-165
 MRCAT, Sector 10, Advanced Photon Source, Building 433, Room B007

 Argonne National Laboratory         phone and voice mail: (1) 630 252 5033
 Argonne IL 60439, USA                                fax: (1) 630 252 9793

 My homepage:    http://cars9.uchicago.edu/~ravel 
 EXAFS software: http://cars9.uchicago.edu/~ravel/software/