Dear all,

when I try to solve my question{[Ifeffit] path contribution to fit in low R-space position, but the fit bond length is much longer than that},I find a method from Prof.Calvin to distinguish whether coordination shell is composed by light or heavy element: 

http://millenia.cars.aps.anl.gov/pipermail/ifeffit/2007-December/007983.html

A clue can perhaps be obtained by noting the relative height of the 
peak near 2.3 angstroms compared with the large peak you've fit. As k-
weight is raised from  0 to 1 to 2 to 3, the peak at 2.3 angstroms 
does not grow relative to the first peak. That suggests the scattering 
may be from another low-Z element like oxygen.

But I still don't know ,why it works.

Thanks for your patience,

Sincerely, 

zhanfei