It could be Ba K-edge, but you're right that trying to do EXAFS at the Ti edge
is probably a no-hoper. You get to k8.5 before you hit the Ba L3-edge,
which would just about be enough to get an average Ti-O distance and maybe
something about a second shell but not much more.
Anyway, more speculation about intentions is unwarranted until Yun Xu tells us
what he (? - I can't tell with Chinese names) actually has in mind and what his
system really is.
mam
On 11/18/2014 3:35 PM, Anatoly I Frenkel wrote:
I think that they study Ga K-edge because Ba L3 and Ti K overlap, hence, those
are very difficult to refine in such a complex system.
I also think that if your system is a solid solution (it was not obvious but it
is probably what you imply) then the environment of Ga will have a mixture of
Ti and Ga somewhere in the r-space and you will have to use a realistic model
structure to learn how to do this type of an analysis, not a less sophisticated
experimental standard. The latter may likely not even exist, and if it did, it
will not be less sophisticated because it will also have a mixture of Ga-Ti and
Ga-Ga as your unknown material, hence, the same problem.
I would just make sure it is a solid solution and try to make use of XRD data.
They should tell you if it is an isostructural material to one of the pure
components, or a new structure.
If a new structure, and if you can't do Rietveld refinement, then - theory may
be the best next step...
If isostructural to something more simple, use that known structure to
construct FEFF model.
Anatoly
________________________________________
From: ifeffit-boun...@millenia.cars.aps.anl.gov
[ifeffit-boun...@millenia.cars.aps.anl.gov] on behalf of Matthew Marcus
[mamar...@lbl.gov]
Sent: Tuesday, November 18, 2014 6:09 PM
To: XAFS Analysis using Ifeffit
Subject: Re: [Ifeffit] experimental standard
You didn't say what you're trying to find out or even which edge you're doing
this at. That being the case, it's a little hard to know
what the best strategy is. A few things come to mind:
1. If you're looking for the environment of the Ga, then obviously, BaO
and Ti2O3 aren't terribly useful. You'd want something
with Ga in an environment similar to that of the unknown. Is there such a
thing as BaGa8Ox with the same structure as BaTi8O16, which
I assume is known? Another possibility is to use as a reference something in
which instead of Ga as the central atom, you have something
nearby in the periodic table, but in a similar environment. Ga2O3 would only
be useful if all you're after is Ga-O and maybe Ga-Ga distances.
Check for other, known oxides in the Ba-Ti-Ga-O system or at least Ba-Ga-O or
Ti-Ga-O.
2. If you're after what the Ga does to the environment of the major
elements, then perhaps you could use BaTi8O16 (if that exists)or
barium titanate. These would give you the Ba-O, Ti-O, and (Ba,Ti)-(Ba,Ti)
paths. My intuitive guess is that if x is small, then you'd be looking
for small changes, so the best standard of all is the x=0 material, if it
exists. Fit that, then add in (Ba,Ti)-Ga paths as appropriate and let
the other paths change and see what happens for x >0.
3. Is the doped material available as a single crystal? Has the structure
been solved? Perhaps that information could be used to constrain
the structure.
4. The stoichiometry you give, Ba(Ti,Ga)8O16, requires the (Ti,Ga) to be
in a mixed-valent state (assuming Ba=2, average = 30/8 = 15/4 = 3.75).
That being the case, I'm guessing that the Ga substitutes for the Ti3+, either
shifting the average valence so that O isn't 16 anymore or just
replacing the Ti3+ until you've replaced it all. Either way, Ti XANES would be
informative. There are many papers about the interpretation
of Ti K-edge XANES.
5. If my guess about Ga substituting only for Ti3+ is correct, *and* you
quoted the correct stoichiometry, then,
a) Something should happen for x>2 because you can't get an
average valence of 3.75 anymore.
b) If the Ti3+ and Ti4+ sites are different enough, say
octahedral+tetrahedral, then the Ga EXAFS should look like
that of the Ti3+ only.
c) Ga3+ is slightly smaller than Ti3+, so you might see a small
contraction around the Ga relative to the Ti.
Good luck.
mam
On 11/18/2014 2:32 PM, Yun Xu wrote:
Hi all
I have a complex oxide BaTi8-xGaxO16. What standards I should run?
Ga is the dopant. I guess I should run the sample without dopant BaTi8O16 as
standard, or the simple oxide as BaO, TiO2 Ga2O3? or is the standard necessary?
Also, I have colleage working on DFT, I assume the theoretical standard is the
result from the DFT calculation, and by using the coordinates of the atoms, I
can run FEFF and fit the data.
Thanks
Yun
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