[Ifeffit] Ratio of peak amplitudes in R-space

[Scott Calvin]

Hi Jatin,

It's really best to model such a system using Ifeffit or similar software. There's a lot that goes into peak height. For one thing, I'm assuming you're looking at the peak height in the magnitude of the Fourier transform. But the Fourier transform of the individual scattering paths is complex-valued, not positive-definite. Thus, two paths contributing at some value of R can sometimes result in less amplitude than either path alone. Depending on data ranges and windowing, there's likely to be some contribution from the path that you're thinking of as corresponding to the first peak at the R of the second, and vice-versa, and that contribution is not additive when viewed in the magnitude.

In addition, for the peak at higher-R there may be a significant contribution from multiple scattering.

It should also be noted that a single peak may be due to unresolved splitting. For example, a nearest-neighbor octahedron might be distorted, with different axial and radial distances. In a case like that, a change in the amount of distortion can have a dramatic effect on the height of the peak.

Having said that, you can make qualitative comparisons in a series like the one you describe. If you have a suspicion of what might be happening (change in coordination, change in disorder, change in splitting, etc.), then you can see it progress by just inspecting the graphs in the way you suggest, and could probably identify, e.g., a phase change. But for quantitative determinations, you really need to model the system.

--Scott Calvin
Sarah Lawrence College

On Sep 5, 2012, at 8:01 AM, Rana, Jatinkumar Kantilal wrote:

Dear Ifeffit community,

I was wondering If I can make a qualitative comparison about the amplitude ratio of first two shells for various states of same sample ?

For example, I have a sample which is measured at various states, say A, B, C, D and E during any physical or chemical process.

I get systematic change in the ratio of first two peaks at each state. This could be interpreted either as change in disorder or as change in number of atoms associated with first two shells as a result of any physical or chemical change material has undergone.

As we know that amplitude of peak decreases with increase in distance from central absorber.

Can I compare the amplitude ratio of first two peaks for various states of same sample ??

Many thanks in advance !!!

With best regards,
Jatin