[Ifeffit] Re[2]: a question about background refinement

[Stanislav Stoupin]

To Matt:

>Normally, you'd want to keep
>Rbkg/Rmin below the 'first shell peak', which is sometimes difficult
>to define for the asymmetric peak common with short metal-oxygen
>distances, but if the peak is around 1.3Ang, the Rbkg/Rmin of 1.5Ang
>seems like it might be too high.  Can you get decent results with
>a lower Rbkg/Rmin?
No, I can not. 1.5A is around the minimum value in Chi(R) magnitude
where the first peak can be separated from the others. However, the peak
is a result of superposition of background, oxygen path and also
the alloy structure.
As for the correlation between Ru-O variables and the background
parameters, they are somewhat correlated I think
If I do not play with the rmin value and keep it equal to 1.0A these
correlations are:

   delr_ru_o and bkg01_05   -->  0.5440
   delr_ru_o and bkg01_04   --> -0.3878
   dwf_pt_pt and bkg02_11   -->  0.3551
         x_o and bkg01_01   --> -0.2856
    delr_ru_o and bkg01_03   -->  0.2760
  delr_ru_ru and bkg01_05   -->  0.2709
     dwf_ru_o and bkg01_01   --> -0.2614
         x_o and bkg01_02   -->  0.2600
    dwf_ru_o and bkg01_02   -->  0.2595
  delr_ru_pt and bkg02_11   -->  0.2562

  x_o means the amplitude of Ru-O path
  e0 for Ru-O path is constrained to be the same as for all paths with
  Ru central atom




  To Jeremy:

>You mention that the amplitude remains consistent at about 0.3.
>However, what is happening to R and E0?  Are these values also
>reasonable?  I have found that, on occasion, a negative DWF is telling
>me that oxygen may not be the correct nearest neighbor.  In these cases,
>R and E0 are always anomalous for the metal-oxygen path.

I have two e0 parameters in the model that correspond to ru and pt
data sets (they are e0_ru and e0_pt)
the same e0_ru I use for Ru-O path
Typical values that they take are about:
e0_pt           =     7.2318013   +/-      1.0171952    (0.0000)
e0_ru           =    -9.1526876   +/-      0.7730603    (0.0000)

and this is consistent through all data that I have and also their
values almost do not depend on whether I use rmin=1.0A or 1.5A

the same I can say about Ru-O distance. Typical value is:
delr_ru_o       =    -0.0899771   +/-      0.0406058    (0.0000)


To Bruce:

>I have a question for you.  Are you trying to determine the oxide
>content?  That is, do you want to know what fraction of the Ru atoms
>are in an oxide environment?  Or are you trying to account for the
>oxide so that you better measure the metallic component?

My advisor (Carlo Segre) has already answered this question
I just want to add that the observed amplitude of Ru-O path (0.3)is
approximately in agreement with the number of atoms on the
nanoparticle surface. This may indicate that all Ru atoms are on the
surface and all they have oxygen bonds.


Stanislav