[Ifeffit] Help with Artemis fit

[Han Sen Soo]

Hello,
I'm a post-doc at LBL and I recently attended the XAFS Summer School at IIT. I just started trying to fit some data on titanium sites embedded in amorphous silica and I was hoping to get some advice on how to improve my fit.

My first objective is to get an idea of the coordination number of Ti, and I'm trying to do this in Artemis by using the parameter N as my S0^2 and letting the amplitude of S0^2 vary. I'm not sure if this is appropriate?

A problem I am encountering is that the physically reasonable fits look visually unacceptable. I am just trying to fit the first Ti-O shell initially and plan to work on the 2nd shell subsequently. The R space fit for the first shell has 2 peaks that are about 0.3 Å apart at 1.2 and 1.5 Å (uncorrected). With only 1 shell of O atoms at a distance I found for similar materials, a reasonable fit was found (including background fitting) but the amplitude of the fit at the 1.5 Å peak is too high while the amplitude at the 1.2 Å peak is too low. I naively tried to improve the fit by adding a 2nd shell at a shorter distance and tried different combinations of distances 0.02 Å apart, but I consistently find that the sigma^2 values for one or both shells become negative. I also realized that the resolution of the data may not be sufficient to distinguish shells that are less than 0.2 Å apart, which is what I am expecting based on data on similar materials. I have attached a pdf image of one of the bad fits.

My question then is, should I not try to improve the fit and accept the 1 shell fit, or am I mis-interpreting the fit and are there ways I can introduce the 2nd shell without having an unphysical model (the main problem being the negative sigma^2)?

Thanks for reading and any suggestions!
han sen
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