On May 6, 2012 11:01 PM, "JeongEunSuk" <eunsuk1986@hotmail.com> wrote:
>
> Hello
> I measured temperature-dependent EXAFS at Pt L3 edge with Pt nanoparticles in room and high temperature(400 C). I have some questions about thermal vibration in EXAFS fit. I read that third and fourth culmulants related with phase and amplitude in anharmonic term, respectively. especially, I concerned third culmulant to relate with phase.
> As you know, the phase also relates with bonding length. So that, the bonding length between Pt-Pt pair considerably correlated with third culmulant. So I can't decide exact bonding length and third culmulant because their correlation. I think that the relation of both bonding length and third culmulant is similar to that of number and debye-waller factor.
> Is it right to find  bonding length and third culmulant like finding number and debye-waller factor using k-weight?

Yes, c3 and R are correlated.  And, as it turns out, in almost exactly the same way that N and sigma2 are.   That doesn't mean they can't be determined accurately, though.   It will tend to make for larger uncertainties, but this is taken into account in ifeffit (at least to first order, ie assuming that the errors are normally distributed and a map of chi-square would be ellipsoidal).

Many people vary the k-weighting to try to "break" these correlations.   I think it doesn't really reduce the correlation that much (certainly not below 50%), but it can't hurt.

You might also consider asserting some mathematical relationship for  R and c3 with temperature, and fit the parameters of those relationships. 
FWIW, my experience is that c3 is rarely significant (ie different from 0), until you get to very high temperature. For 400C for Pt, I'd expect that it would start to be noticeable. 

--Matt