RE: [sdpd] Synchrotron powder diffraction

["Yokochi, Alexandre" <alex.yokochi...@orst.edu>]

Hi all,

	My main reason to interject a contribution to this discussion is a
bewildered amazement with the refinement results that Armel just reported.
Is the first data set really that bad?  Do you think it would be repeatably
bad?  And as for the second, are you bothered by the Chisquared of 0.5 and
the DW of about 3.5?  What's the deal there?  (inquiring minds just want to
know)

	And, for the record (since we're all speaking of records), I also
agree that synchrotron X-ray source is extremely valuable for powder
diffraction in general.  I am also one of those that believes the effort
required to get some synchrotron beamtime is usually too large given the
advantages I get over my lab equipment - but then again, maybe I am just
fortunate in the equipment my colleagues have amassed for our use.  I also
tend to think getting neutron data time is a lot of work, but then, my
colleagues have not been so kind as to get us a local neutron source (we do,
however, have a teaching reactor on campus, and I have fantasized about
things on more than one occasion).

	Finally, as nice as fire and forget sounds, I tend to think better
data comes from my fret and worry system, though a lot slower.  And yes,
fret and worry tends not to be suitable for researchers of all temperaments,
and especially not if the researchers count the minutes as precious data
that could have been collected.  Maybe this is the real crux of the issues
being debated here.

	AlexY

Dr. Alexandre F. T. Yokochi
Assistant Professor (Senior Research)
Director, X-ray Crystallographic Facilities
Department of Chemistry
Oregon State University
Corvallis, OR 97331-4003

Ph#  (541) 737-6724       Email: alex.yokochi...@orst.edu
Fax# (541) 737-2062       Web Page: www.chem.orst.edu/yokochi/


 -----Original Message-----
From: 	Armel Le Bail [mailto:alb...@cristal.org] 
Sent:	Sunday, September 09, 2001 7:32 AM
To:	sdpd...@yahoogroups.com
Subject:	Re: [sdpd] Synchrotron powder diffraction

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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