[Ifeffit] How to distinguish whether the coordination element is heavy or light

[Scott Calvin]

Hi Zhanfei,

A few more words to add to Bruce's answer on the +/- 5 "rule."

It refers to cases where the bond length is treated as a free parameter. There's a gradual change of phase shift due to scattering with Z which looks a bit like a change in bond length. So if you're fitting bond length, the fit can compensate for an incorrect choice of scatterer by also returning an incorrect bond length. If you don't know what the bond length should be and your comparing two fits with scattering atoms of similar Z, then you don't know which reported bond length is incorrect and thus which fit is more valid.

But some people (Joe Woicik comes to mind) have pointed out that sometimes you do know what bond length would correspond to which Z. As an example, you might be trying to determine which of two possible known structures is found in a nanoparticulate sample. Each candidate structure might have bond lengths that are well known. In that case, bond length would not be a free parameter, but would be fixed to the appropriate value for each fit. In a case like that, a change in Z of 1 is often distinguishable; i.e., there is no +/- 5 rule.

--Scott Calvin
Sarah Lawrence College

On Jul 4, 2014, at 9:06 AM, Bruce Ravel <bravel at bnl.gov> wrote:

> On 07/04/2014 03:51 AM, ZHAN Fei wrote:
>> In the picture you recommended,it says "The variations in functional
>> form allow Z to be determined (±5or so) from analysis of the
>> EXAFS".But I don't find any publication use it.
> 
> Hi Zhanfei,
> 
> The figure I referred you to was a plot of the scattering amplitude (the 
> bottom panel had the phase shift) for three different scattering 
> elements.  I don't know what's plotted in the figure you attached 
> because there are no labels on the axes.  In any case, the scattering 
> amplitude is the F(k) term in the EXAFS equation.  As such, it's 
> relationship to chi(R) or chi(q) is subtle.  And, of course, in real 
> data, all the different scattering paths interfere with one another.
> 
> As for the Z+/-5 rule, I don't know who first stated that.  Perhaps Teo 
> and Lee...?
> 
> In any case, it is easy to test.  Measure, say, a NiO standard.  Try 
> replacing O in the feff.inp by N or F and do the analysis.  Try again 
> with C or Ne.  Try again with B or Na.  You will find that the fit using 
> F is basically indistinguishable of the fit with O.  Ne will be a bit 
> worse, but not much.  Na a bit worse, but not much.  Eventually, you 
> will get far enough away from O that you can clearly see the difference 
> in the fit.
> 
> Some years ago, I tried to work on a FeGa alloy.  Although 5 apart, I 
> could not distinguish Fe scatterers from Ga scatterers well enough to 
> say anything about how the dopant was distributed in the lattice.