Re: [sdpd] Synchrotron powder diffraction

[Armel Le Bail <alb...@cristal.org>]

Synchrotron is unrivalled for SDPD. This cannot be discussed.

Since it is easy to access, a more interesting question is now, which 
synchrotron powder diffractometer is the best ? 

Or are they all similar ? 

You may try to answer to that question by yourself at least by a 
comparison of the refinement quality that you will obtain from 2
patterns of the same annealed CeO2 sample made for the Size Strain 
Round Robin, one at the NSLS X3B1 beamline, and the other at
the ESRF BM16 beamline :
http://www.boulder.nist.gov/div853/balzar/s-s_rr.htm

Using Fullprof (le Bail method ;-), I obtained in the first case :

  =>  R-factors (not corrected for background) for Pattern:  1
  => Rp: 12.5     Rwp: 15.4     Rexp:    3.72 Chi2:  17.1      L.S. refinement
  => Conventional Rietveld R-factors for Pattern:  1
  => Rp: 14.0     Rwp: 16.5     Rexp:    3.99 Chi2:  17.1    
  => Deviance: 0.194E+06     Dev*  :  17.16    
  => DW-Stat.:    0.6519     DW-exp:     1.9444
  => N-sigma of the GoF:     1209.125

and in the second case :

  =>  R-factors (not corrected for background) for Pattern:  1
  => Rp: 6.66     Rwp: 9.40     Rexp:   13.05 Chi2: 0.518      L.S. refinement
  => Conventional Rietveld R-factors for Pattern:  1
  => Rp: 7.14     Rwp: 9.87     Rexp:   13.71 Chi2: 0.518    
  => Deviance: 0.182E+04     Dev*  : 0.5414    
  => DW-Stat.:    3.2858     DW-exp:     1.8977
  => N-sigma of the GoF:      -19.502

This gives elements of reflexion for deciding 
where to apply for beam time.

Unfortunately, the same data for the increasing
number of synchrotron powder diffractometers
in the world are not available. The situation
is much more clear for conventional powder
diffractometers.

The problem with the NSLS data was clearly due
to high asymmetry and bad peak positions, sometimes 
displaced toward large or small angles ! Probably
better results would be obtained with a special
software rather than by using Fullprof. But a
standard user may prefer to be able to cope with his 
data by using a standard Rietveld software.

Time for an honest comparison of the powder data coming
from all the synchrotron sources ?

Armel

PS - I "realized" (thanks to the local contacts) only 
2 synchrotron experiments in my life, one at Daresbury 
and the other at the ESRF. Not much could be published 
because most of the samples were highly resisting to an 
approach by using conventional powder diffraction,
and they were found to resist to synchrotron as 
well (due to anisotropic line broadening,
uncertainties about indexing results, and so on).


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