[Ifeffit] Phase transformation and space group

[Bruce Ravel]

On Wednesday 28 May 2008 18:39:40 益豪 wrote:
> We have complex perovskites, the series of La(2-x)/3 Nax Mg0.5 W0.5 O3
> where x=0,0.1,0.2,0.3,0.4,0.5. I would like to fit the EXAFS data, and
> according as ICSD to build the atom list by using TkAtoms. However these
> samples might have phase transformation with the increase in x-value.
> First, I use the same structure which the x=0 atom coordinates from ICSD to
> fit the data of x=0-0.2, and use the x=0.5 atom coordinates to fit the data
> of x=0.3-0.5. But the trend of the distance between the central atom(W) and
> the six neighboring oxygen atoms with Na concentration is very strange! The
> distance of O-W with x=0-0.2 is increasing, x=0.2-0.3 is decreasing, and
> x=0.3-0.5 is increasing. I conjecture that the result is so strange because
> we use the different space group. I would like to ask how to choose the
> atom coordinates x=0 and x=0.5 from ICSD to build the theoretical model of
> x=0.1-0.4.

Hi!

Your question is not completely clear.  As I understand it, when x=0
or x=0.5 you are able to use the crystal data from the ICSD as the
basis of your analysis of the W edge data and obtain a good result in
your exafs analysis.  However, for the intermediate values of x, you
find peculiar values for W-O distances.

I would expect that, in the locations occupied by Na, there is
significant local disortion since Na is (I presume) smaller than La.
Consequently, the oxygen atoms might relax around the Na.  In that
case, you might see some significant distortions to the oxygen
octahedra which will show up in your exafs data as additional disorder
in the W-O distribution.

It is common to presume that by "phase transition" you mean
crystallographic phase transition.  Have you done diffraction on the
materials with intermediate values of x?  Do you observe those
materials to be in a different phase from the end members?  If so, and
if no no one has ever published crystallography on those phases,
perhaps you need to refine that diffraction data yourself.  And if you
do not observe a crystallographic phase change, then probably you are
seeing the effect of local disorder.  To interpret your exafs data you
will need to somehow model that disorder.

I hasten to point out that everything I have said is speculation.  I
have never looked at materials quite like yours and I have no personal
knowledge about your system.

You should read the papers on mixed ferrites by Scott Calvin or my
paper of BaTaO2N to see how similar problems have been addressed by
the so-called experts on this list.

Hope that helps,
B


-- 
 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/