Re: [sdpd] Fullprof: preferred orientation

[Armel Le Bail <armel...@fluo.univ-lemans.fr>]

>I wish to know how to make Rietveld Refinement of heavy preferred
>orientation with FullProf.

The following mini-tutorial is based on the 21/01/2000 manual
describing the 20/07/2000 wfp2k Fullprof version. Actually, it
is said that Fullprof can do much more than is described in the
manual, especially concerning preferred orientation, but ordinary
users (like me) have to wait for an available description of these
new possibilities.

1 - See how small can become the Rp and Rwp values 
  (conventional Rietveld values) by applying the "Le Bail method".
   After all, maybe you cannot obtain low R values because of
   other reasons than preferred orientation.
2 - From a Rietveld structure refinement, examine the list of
  intensities and locate the preferred orientation : a series of
   harmonic reflections with Iobs systematically quite larger than
   Icalc.   If you cannot locate such a clear preferred orientation, 
   you may try some possibilities like 001, 100, 010, 110, etc. Enter
   the preferred orientation at line 11-2 (Pr1, Pr2, Pr3).
3 - Choose one of the two possible functions available,
       preferably the March-Dollase one (Nor =1, Line 2).
      It is said that Fulprof has other undocumented possibilities...
4 - Put G1 = 1 at line 11-8-1 if you have selected Nor=1
      (in that case, G1=1 means no preferred orientation)
5 - Add a CG1 codeword at line 11-8-2 for refining that G1
      parameter.
6 - Refine
7 - If G1 becomes frankly less than 1., and the R values decrease
     a lot, you may have found the preferred orientation.  In case
     your structure is hexagonal/trigonal with needle shape, then
     several preferred orientations could occur with hk0 indices
     (in case of flat plate sample measurement), you may select
      001 as Pr1,Pr2,Pr3 and obtain G1 > 1 with R factors decrease.
8 - Be aware that your structure parameters may be affected
       even if your R factors have become reasonably low after
       that preferred orientation correction : try to record a new 
       pattern without preferred orientation is much better...

Info and documentation about those undocumented Fullprof
possibilities are welcome.

Armel Le Bail
http://www.cristal.org/


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