Hi Abdul: I agree with Pierre. Your model is fundamentally incorrect. It sounds like you are trying to fit an octahedral environment. If there are multiple distances in this octahedron, it is possible that letting all of these distances vary freely will send you off in an incorrect minimum. When I work with these kinds of systems, I always start with a single path and set N=6 in that path. I can bring the other paths in later. With this, I limit the fitting range to just the first shell (usually 1A to about 2A). There should be 4 parameters available and you can let the all vary. If the sigma squared goes negative and the amplitude becomes too small, then you need to constrain the model a bit more. I usually fix sigma squared to 0.003 and then see where the amplitude reduction factor goes. Once I am happy with the amplitude reduction factor, and I am sure that the deltaR is not too big, then I can start amp and deltaR close to their fitted values and then let sigma squared vary. This should get you to a reasonable fit. If amp still coes small and sigma squared is stable and bigger than 0.001 then you might have a problem with self-absorption in your data (if it is fluorescence). If amp is good and sigma squared turns out to be relatively large (say bigger that 0.003) then there is a good chance that you have good enough data to resolve more than one path in that first shell. Under these conditions, there are several things you can do but it is complicated. Post again if this turns out to be the case. Finally, the answer to your second question is that you do NOT need to fit longer paths if all you want to know is the first shell information. Carlo On Mon, 28 May 2018, Abdul Ahad wrote:

Hi all,

I have couple of questions. 1. I found in many tutorials that the value of amp should lie between 0.7 and 1. But when I refined it goes to 0.15, what's wrong with it.

2. The value of sigma2. Some time at high k, value, it turns to negative but small. What I suppose to do?

3. If I need to know the behavior of octahedra distortion with temperature, and it is lies within the first shell. So what is the need of higher shell fitting.???

Thanks in advance

-- Carlo U. Segre -- Duchossois Leadership Professor of Physics Interim Chair, Department of Chemistry Director, Center for Synchrotron Radiation Research and Instrumentation Illinois Institute of Technology Voice: 312.567.3498 Fax: 312.567.3494 segre@iit.edu http://phys.iit.edu/~segre segre@debian.org