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
To Matt: 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