Re: [Ifeffit] Artemis

Juan Antonio Macia Agullo juan.macia at ua.es
Fri Nov 10 08:10:20 CST 2006



Thanks a lot Scott, I will do it. Sorry, but I do not understand what you mean 
with the sentence "in a physically reasonable way: delr might need to be 
scaled by reff". Could I use the same delR for all single scattering paths 
(even distant paths)? In principle, I am not going to consider MS paths.

I hope fits increasing R-range will improve very much fits in k-space because 
now they are not very good. 

Best regards,
JA




---- Mensaje Original ----

> Hi Juan,
> 
> The same S02 should be OK; that's generally thought to be chemically 
> transferable.
> 
> Unless I'm misunderstanding something about your system, there is no 
> physical basis to justify the same delr. Think about it: why would 
> both phases show the same difference in bond length between the model 
> and the actual structure?
> 
> Using the same sigma2 is also problematic; it seems unlikely that the 
> disorder and vibrational spectrum is the same in the two cases.
> 
> deltaE0, to first approximation, may be the same. In both phases the 
> valence of the absorbing atom is the same (i.e. 0), so assuming the 
> same E0 is not unreasonable.
> 
> To my mind, therefore, you need at least 6 parameters (plus whatever 
> else you're using; e.g. the fraction of each phase present).
> 
> So I'll use this opportunity to beat one of my favorite drums: why 
> not try a multishell fit? It sounds to me like you have phases that 
> are likely to be fairly crystalline. If your data is decent, you may 
> be able to extend your fit out to several coordination shells without 
> adding many new free parameters. (You have to do this in a physically 
> reasonable way: delr might need to be scaled by reff, etc.) This 
> might be a net gain in degrees of freedom, and also might reduce 
> uncertainties in some of the fitted variables by reducing correlations.
> 
> --Scott Calvin
> Sarah Lawrence College
> 
> At 03:25 AM 11/9/2006, you wrote:
> 
> 
> >Hi people!!
> >
> >I was fitting with Artemis bimetallic catalysts with two different 
> >structures:
> >platinum structure plus an intermetallic compound structure. As I only use a
> >small range of R space (1st shell) for fitting, I have a small number of
> >independents points and I used an appropriate number of variables. The
> >question is: I used the same values of SO2, delR, sigma^2 and 
> >deltaE0 for both
> >structures, is that ok? or maybe I have to increase until 8 the number of
> >variables?
> 
> 
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