[Ifeffit] Bond lengths

[Scott Calvin]

Hi Bindu,

This is an important and subtle concept.

The error bars generated in EXAFS fitting are an attempt to represent  
the uncertainty in the absolute value of each parameter. A large part  
of that uncertainty is often due to correlation with other variables;  
these correlations are listed in the log file.

Now suppose you have a sequence of data in which it is known that only  
one parameter should vary systematically. For example, suppose delE0  
and delr for each temperature is unknown, but it is thought that delE0  
does not change with temperature. (This is only an example! It  
certainly might, particularly if oxidation state changes.) The delr  
will have a large uncertainty, but much of that is due to the  
correlation with E0: they both could be higher or lower together. But  
if it is known that only delr is changing with temperature, then  
changes in delr that are smaller than the error bars may be detected  
and meaningful.

You'll also see people refer to the resolution of EXAFS data. They are  
talking about something like a Rayleigh criterion: how close can two  
bond lengths be and be distinguished as different bonds via EXAFS? But  
again, suppose it is known that there are two closely spaced bonds,  
even though they can't be resolved. If one changes relative to the  
other, it will change the MSRD (I.e. show up as a form of static  
disorder) if they are modeled as a single bond, and also change the  
"bond length" of the combined peak.

So there are plenty of cases where changes smaller than the error bars  
are meaningful. In general, though, they require additional knowledge  
about the system. If you know nothing about the system and see a  
systematic change in the bond length that is much less than the error  
bars, it might actually be a change in a correlated variable. Also,  
for very small changes, watch the third cumulant as a possible  
culprit! The third cumulant is often strongly dependent on  
temperature, and correlates with bond length. If you don't fit the  
third cumulant, you can think you have a bond length change when you  
don't--or even think it's changing in the opposite direction from  
what's happening.

Having said that, the fourth decimal point is very hard to pin down.  
While it's certainly possible to get a systematic effect over a series  
of spectra that shows up in the fourth decimal point, how do you know  
for certain it's not one of the other effects I've described? In other  
words, I'd say that it's possible to see a phase transition like that  
via EXAFS curve fitting, but it's difficult to know exactly what the  
structural change that's accompanying it is.

--Scott Calvin
Sarah Lawrence College

On Jul 25, 2008, at 7:58 AM, Bindu R. wrote:

> Hi,
>
>
>
>
> Could you please clarify me, up to what decimal place in the bond  
> lengths obtained from EXAFS fitting is reliable?
>
>
>
> If there are changes in the bond lengths in the fourth decimal place  
> around the region of phase transition, can one consider this as a  
> genuine one even though the error bars obtained from the EXAFS  
> fitting are more than the variation in the bond lengths?
>
>
>
> Regards
>
> Bindu
>
>
>
> Dr.Bindu R.
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