[Ifeffit] distinct crystal positions for the absorber atom

[Chris Patridge]

Hello everyone,

I am working on fitting the first shell of a double layer vanadium oxide 
material.  The data is V K edge data with 3 scans merged.  There is two 
positions for vanadium in the unit structure with equal occupancy.  I 
started modeling using equal weighting between the 2 feff calculations 
for each unique vanadium.  Setting amp*0.5 seems straight forward.  The 
first shell consists of oxygen coordinated and the next shell is 
dominated by the vanadium of the adjacent edge sharing polyhedra.  Feff0 
appears to give a better fit by itself with lower uncertainties than 
using both model in concert or Feff1 by itself.  Can this problem lie in 
the fact that the bond lengths are quite short (1.6 A) for V-O and 
discerning the background and signal causes an error?   I feel 
reasonably confident in the crystallography.  I pasted the fit results 
below and included bmps and artemis file there.  If there is something 
suspect please let me know or if you have a suggestion.

Thank you all in advance,

Christopher Patridge
PhD Candidate
SUNY University at Buffalo
Dept of Chemistry
315-529-0501

 Independent points          =       7.691406250
Number of variables         =       4.000000000
Chi-square                  =    2503.896487489
Reduced Chi-square          =     678.304233648
R-factor                    =       0.029837426
Measurement uncertainty (k) =       0.000474272
Measurement uncertainty (R) =       0.000888467
Number of data sets         =       1.000000000


Guess parameters +/- uncertainties  (initial guess):
  amp             =     0.8044660   +/-      0.1118000    (1.0000)
  enot            =    -1.9212800   +/-      1.8083430    (0.0000)
  ssO             =     0.0001680   +/-      0.0018760    (0.0030)
  alpha           =     0.0011620   +/-      0.0078840    (0.0000)
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