[Ifeffit] experimental standard

Matthew Marcus mamarcus at lbl.gov
Tue Nov 18 17:50:44 CST 2014

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.

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-bounces at millenia.cars.aps.anl.gov [ifeffit-bounces at millenia.cars.aps.anl.gov] on behalf of Matthew Marcus [mamarcus at 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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