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[sdpd] Rotating anode X-ray tube and Rietveld refinement?

Dear all,

I wonder if there is a significant difference between a rotating anode
source and conventional x-ray tube from the point of view of Rietveld
refinement? I'm having difficulties on refinements with data from
rotating anode instrument, for both intensity and the profile. Since
preferred orientation was expected due to the samples, my first
thought was that somehow I just describe pref. orientation wrong (I
give the direction as 001, since crystallines in question are
platelets with c-axis perpendicular to the plane of the platelet).
Maybe this is wrong but a test with Al2O3 xrpd data also gives bad
results. From FullProf summary file :  

=> Conventional Rietveld R-factors for Pattern:  1
=> Rp: 8.57     Rwp: 10.7     Rexp:    2.94 Chi2:  13.3  

and a warning about the high Chi2: "...residuals are dominated by
systematic errors in profile."

I can see from the plot that the calculated profile is not coping well
with the observed one on the higher 2theta side of the Bragg peak,
close to the background. Observed profile is wider (it is not a
question of peak cut-off value). For above mentioned R-values the
profile function used was "Pseudo-Voigt function convoluted with axial
divergence asymmetry function" Finger, Cox & Jephcoat, J. Appl. Cryst.
27, 892, 1994 (nr.12 in FullProf). Radiation is Cu K-alpha&beta from a
fixed tube position and data range is from 3 to 100 deg. 2theta, with
0.02deg step size. I have tried refinements with and without
additional asymmetry parametes, U,W,V refinement for alfa2, as well
with other profile functions. R-values and Chi2 do change but only a
little. 

Any comments or suggestions for diagnostics? 

Best Regards to everyone,
  Markus 

--------
Markus Valkeapaa, markus...@msl.titech.ac.jp
Yamauchi-Karppinen Laboratory
Materials and Structures Laboratory
Tokyo Tech, Japan






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