Michael,

The values for C3 and C4 should be very small -- they are in Ang^3 and Ang^4, and should be numerically smaller than sigma2 (in Ang^2).   I would expect values of 0.001 for C3 in a highly disordered system.   I'm not sure anyone has actually seen a value for C4 that was not consistent with 0.0 -- and several of us have looked!   So, I'd suggest starting with C4 = 0, at least until the effect of C3 with sigma2 and deltaR has been explored.

Values of 0.5 for either of these is much too big, and could make the fit bad enough to fail.   Guessing a value of 0. or 1.e-5 is probably wise.

--Matt



On Mon, Jun 16, 2014 at 10:15 AM, Michael Kömm <michael.koemm@uni-jena.de> wrote:
Hello,

I'm trying to make an EXAFS analysis on the Structure of Tb in a borogermanate glass System. The reference is an .cif of Terbium-Gallium-Garnet (TGG).

I expect some disorder caused to working in glass. Therefore I would like to use the 3rd and/or the 4th parameter. I tried to set a guess parameter to evaluate those parameters. Could you tell me a reasonable value for these parameters? The 3rd only varies a little bit around its guessed value (+- 0.01) and if I try to do the same with the 4th parameter no fit at all takes place. The errors in the GDS-window are exactly 0.00000 with no fit taking place (enot = 0, amp = 0 (guess amp = 1) delr = 0 and ss = 0.003) and the R-factor of the fit is way over the top (>10^20).

I've chosen a value of 0.5 for the 3rd and the 4th parameter just to see if they're working and I was only doing a quick first shell fit with the surrounding oxygen-shell.

Please can somebody tell me how to deal with this problems.

Thanks a lot for your time and efford.

Sincerly yours,

Michael Kömm


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--Matt Newville <newville at cars.uchicago.edu> 630-252-0431