a basic (?) EXAFS question
Hello, I have a question about fitting an EXAFS dataset. The compound is FeAs2, crystalline, and will be used as a model for further study. I was able to fit the first three shells very well. However, when I try to fit the fourth shell, I either get amplitude that is too high (> 2, with large errors), or 0.0. There are several single-scattering and several multiple-scattering paths in that region. Sometimes, only excluding one of the MS paths leads immediately from amplitude of >2 to 0.0. I'd appreciate any advise about what is the meaning of these changes and the inability to fix the spectrum. Attached are few pictures from Artemis. Thank you, Juro Majzlan ---------------------------------- Juraj Majzlan Institute of Mineralogy and Petrology Albert-Ludwig-University of Freiburg Albertstrasse 23b Freiburg, D-79104, Germany telephone +49-761-203-6416 fax +49-761-203-6407 http://www.minpet.uni-freiburg.de
Hi Juro, I think you'd have to give us more information about how you did the fits. A few obvious questions are: what R range did you use for the fit? what was floated and what was fixed in the fit? how far out does the data go in k-space? I'm afraid that I don't know the crystallographic details of loellingite, but the EXAFS for many crystal structures get very complex by the fourth shell. This is especially true in slightly disordered structures where multiple-scattering and single paths nearly overlap. Part of the reason for this is that multiple-scattering paths can "spread out" in R-space, making it difficult to know where to stop including paths. --Matt On Sun, 17 Apr 2005, Juraj Majzlan wrote:
Hello,
I have a question about fitting an EXAFS dataset. The compound is FeAs2, crystalline, and will be used as a model for further study. I was able to fit the first three shells very well. However, when I try to fit the fourth shell, I either get amplitude that is too high (> 2, with large errors), or 0.0. There are several single-scattering and several multiple-scattering paths in that region. Sometimes, only excluding one of the MS paths leads immediately from amplitude of >2 to 0.0.
I'd appreciate any advise about what is the meaning of these changes and the inability to fix the spectrum.
Attached are few pictures from Artemis.
Thank you,
Juro Majzlan
---------------------------------- Juraj Majzlan Institute of Mineralogy and Petrology Albert-Ludwig-University of Freiburg Albertstrasse 23b Freiburg, D-79104, Germany telephone +49-761-203-6416 fax +49-761-203-6407 http://www.minpet.uni-freiburg.de
Matt,
thanks a lot for the few simple questions. One of them -
the R range - seems to be the problem, since the range
wasn't big enough. Oh well ... The structure of loellingite
has very well defined shells, up to the fifth one.
Thanks again,
Juro
On Sun, 17 Apr 2005 14:52:18 -0500 (CDT)
Matt Newville
Hi Juro,
I think you'd have to give us more information about how you did the fits. A few obvious questions are: what R range did you use for the fit? what was floated and what was fixed in the fit? how far out does the data go in k-space?
I'm afraid that I don't know the crystallographic details of loellingite, but the EXAFS for many crystal structures get very complex by the fourth shell. This is especially true in slightly disordered structures where multiple-scattering and single paths nearly overlap. Part of the reason for this is that multiple-scattering paths can "spread out" in R-space, making it difficult to know where to stop including paths.
--Matt
On Sun, 17 Apr 2005, Juraj Majzlan wrote:
Hello,
I have a question about fitting an EXAFS dataset. The compound is FeAs2, crystalline, and will be used as a model for further study. I was able to fit the first three shells very well. However, when I try to fit the fourth shell, I either get amplitude that is too high (> 2, with large errors), or 0.0. There are several single-scattering and several multiple-scattering paths in that region. Sometimes, only excluding one of the MS paths leads immediately from amplitude of >2 to 0.0.
I'd appreciate any advise about what is the meaning of these changes and the inability to fix the spectrum.
Attached are few pictures from Artemis.
Thank you,
Juro Majzlan
---------------------------------- Juraj Majzlan Institute of Mineralogy and Petrology Albert-Ludwig-University of Freiburg Albertstrasse 23b Freiburg, D-79104, Germany telephone +49-761-203-6416 fax +49-761-203-6407 http://www.minpet.uni-freiburg.de
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---------------------------------- Juraj Majzlan Institute of Mineralogy and Petrology Albert-Ludwig-University of Freiburg Albertstrasse 23b Freiburg, D-79104, Germany telephone +49-761-203-6416 fax +49-761-203-6407 http://www.minpet.uni-freiburg.de
Hi Juro, Just a note of caution--if I've followed this discussion, you're fitting the amplitude for each shell separately. While there are sometimes good reasons to do this (it's the way Anatoly Frenkel determines crystallite size and morphology, for example), if you think you have fairly ordered, fairly large (> 10 nm, say) crystallites, then you need a physical justification. Although the match between fit and data will of course usually look marginally worse by using the same amplitude for every shell, it often has the side benefit of reducing uncertainties in the guessed parameters. In terms of fitting strategy, when I was an EXAFS neophyte I would start my fits by having lots and lots of parameters and then would gradually reduce the number. Since then I've switched around to what most people recommend, which is starting with very few parameters (many recommend starting with none!), and then adding guessed parameters as necessary. The latter has the benefit of forcing you to think about what's physically reasonable, reducing the incidence of bizarre "false minima," giving you a sense of what's going on early in the process, and being easier to defend in print. --Scott Calvin Sarah Lawrence College
Matt,
thanks a lot for the few simple questions. One of them - the R range - seems to be the problem, since the range wasn't big enough. Oh well ... The structure of loellingite has very well defined shells, up to the fifth one.
Thanks again,
Juro
On Sun, 17 Apr 2005, Juraj Majzlan wrote:
I have a question about fitting an EXAFS dataset. The compound is FeAs2, crystalline, and will be used as a model for further study. I was able to fit the first three shells very well. However, when I try to fit the fourth shell, I either get amplitude that is too high (> 2, with large errors), or 0.0. There are several single-scattering and several multiple-scattering paths in that region. Sometimes, only excluding one of the MS paths leads immediately from amplitude of >2 to 0.0.
participants (3)
-
Juraj Majzlan -
Matt Newville -
Scott Calvin