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
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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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@millenia.cars.aps.anl.gov [ifeffit-bounces@millenia.cars.aps.anl.gov] on behalf of Matthew Marcus [mamarcus@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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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-bounces@millenia.cars.aps.anl.gov [ifeffit-bounces@millenia.cars.aps.anl.gov] on behalf of Matthew Marcus [mamarcus@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
_______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
_______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit _______________________________________________ Ifeffit mailing list Ifeffit@millenia.cars.aps.anl.gov http://millenia.cars.aps.anl.gov/mailman/listinfo/ifeffit
participants (3)
-
Anatoly I Frenkel -
Matthew Marcus -
Yun Xu