On 07/04/2014 03:51 AM, ZHAN Fei wrote:
In the picture you recommended,it says "The variations in functional form allow Z to be determined (±5or so) from analysis of the EXAFS".But I don't find any publication use it.
Hi Zhanfei, The figure I referred you to was a plot of the scattering amplitude (the bottom panel had the phase shift) for three different scattering elements. I don't know what's plotted in the figure you attached because there are no labels on the axes. In any case, the scattering amplitude is the F(k) term in the EXAFS equation. As such, it's relationship to chi(R) or chi(q) is subtle. And, of course, in real data, all the different scattering paths interfere with one another. As for the Z+/-5 rule, I don't know who first stated that. Perhaps Teo and Lee...? In any case, it is easy to test. Measure, say, a NiO standard. Try replacing O in the feff.inp by N or F and do the analysis. Try again with C or Ne. Try again with B or Na. You will find that the fit using F is basically indistinguishable of the fit with O. Ne will be a bit worse, but not much. Na a bit worse, but not much. Eventually, you will get far enough away from O that you can clearly see the difference in the fit. Some years ago, I tried to work on a FeGa alloy. Although 5 apart, I could not distinguish Fe scatterers from Ga scatterers well enough to say anything about how the dopant was distributed in the lattice.
I simply try an example.I choose the second shell of MoS2 which is the Mo-Mo shell in R-space in Athena,then do the back FT to get amplitude of the second coordination shell,then I compare to amp described in paper http://pubs.acs.org/doi/abs/10.1021/ja00505a003. The peak is at the almost same wave number when q-space use k2 weight(when use higher kweight,q space peak shifts to high wavenumber ),see pic attached. I only get a peak without the vally described in paper when use kweight,but can see a vally without k weight,maybe stress the contribution in high k make the vally obscure.Can this method works in element determination correct to +-5 Z number?
And is there any method to determine whether a coordination peak in R space have one element contribution or more? For example ([Ifeffit] path contribution to fit in low R-space position, but the fit bond length is much longer than that ):a cluster we expected it has both Ni-O Ni-S(normally Ni-S peak is in high position),and when spectrum has a peak between regular Ni-O and Ni-S and a shouler near Ni-O,can I ensure the expection of both Ni-O Ni-S？OR just maybe because of the multiplicity of EXAFS
You seem to be looking for a magic wand that you can wave at your data and have a clear answer pop out. EXAFS analysis, sadly, isn't like that. The information content of the data is quite limited, the data range is usually quite limited, structural and chemical disorder make the EXAFS signal hard to interpret. Scott's discussion that you referred to in a previous email is just a tool to help disentangle all these problems ... it's not a solution. EXAFS analysis does not "solve structures". There is no mathematical operation that can somehow "invert" EXAFS data. The best we can do is test models against real data and do a statistical analysis of the results. In the end, the best we can ever say is whether a fitting model is *consistent* with data. Or perhaps, whether one model is *more consistent* with data than another. In your case, if I understand your description, you need to test models with O scatterers, with S scatterers, and with different mixtures. Hopefully, some of these models will be more successful than others. And hopefully, the successful models will be consistent with other data you have about your samples. O and S are an interesting case. If you have Feff compute Ni-O and Ni-S scatterers at the same distance, then plot chi(k), you sill see that they oscillate out of phase over much of the k-range. This is both great and problematic. It is great because it means that the contrast between the two is about as big as it can be. Ni-O with only a small amount of S will be measurable because the presence of the S serves to significantly reduce the amplitude of chi in the data. However, if you have similar amounts of O and S, you may be in the situation where the overall chi(k) has a very small amplitude due to two things mostly cancelling each other out. As a result, parameters such as sigma^2 for the different scatterers will be highly correlated. So, the "too-long;didn't-read" version of this is that you have to simply try all the different, reasonable fitting models and decide what works best. HTH, B -- Bruce Ravel ------------------------------------ bravel@bnl.gov National Institute of Standards and Technology Synchrotron Science Group at NSLS --- Beamlines U7A, X24A, X23A2 Building 535A Upton NY, 11973 Homepage: http://xafs.org/BruceRavel Software: https://github.com/bruceravel