Hi Markus,

it all depends which use you plan to do most; someone can disagree but as far 
as I know parallel beam instruments, with sample in capillary are good for 
Rietveld refinements and structure solution, thanks to reliability of 
intensities. Disadvantages can be the longer data collection because less 
sample is used (a rotating anode generator can help, I guess), and absorption 
problems in case of heavy absorbing samples (the geometry is in transmission).

I'd say that Bragg-Brentano geometry with flat sample plate (reflectance 
geometry) is well suited for quick analysis, as a larger sample can be used and 
sample prep can be quicker on a flat plate rather than in a capillary, but can 
give rise to several issues due to less reliable peak intensities (illumination 
of sample area not constant and depending by diffraction angle, preferrered 
orientation, absorption, surface roughness, vertical sample displacement which 
then requires internal standard use for correct lattice parameter 
determination,...); all these effects can give rise for example to problem in 
refining thermal factors.  

Experts in the list can give more precise opinions, I hope it helps as a first 
answer,

All the best,

marco



-----Original Message-----
From: Markus Valkeapää [mailto:markus.valkea...@tkk.fi] 
Sent: 23 January 2009 10:08
To: rietveld_l@ill.fr
Subject: Bragg-Brentano vs. parallel beam

Dear All,

Due to a laboratory diffractometer purchase I'm making a pros and cons 
list for two configurations:
1) Theta-2theta Bragg-Brentano geometry & Johansson primary beam 
monochromator.
2) Omega-2theta parallel beam geometry with primary beam 2-bounce flat 
crystal monochromator.
Point of view being in powders and Rietveld analysis.

I have papers "N.A. Raftery & R. Vogel, J. Appl. Cryst. 37 (2004) 357 
(DOI: 10.1107/S0021889804003097)" and  "M.S. Haluska, S. Speakman and 
S.T. Misture, ICDD 2003 Advances in X-ray Analysis, Vol 46 p. 192" where 
similar comparisons are made, main difference being that in both works 
primary beam monochromators were not used.

Before reading the papers I was under impression that Bragg-Brentano 
geometry is definetely the best choice for indexing, structure solution 
and Rietveld analysis. But now I'm not so sure anymore. According to 
these papers the differences between the results from parallel beam and 
Bragg-Brentano data are very small.

Any comments on these two different geometries here on Rietveld list? Or 
if you could recommned some relevant references?

Best Regards,
   Markus Valkeapää
--
  Dr. Markus Valkeapaa
  Department of Chemistry, Helsinki University of Technology
  P.O. Box 6100, FI-02150 TKK

  Office: Kemistintie 1, room B206
  tel: +358 50 511 3073, +358 9 451 2596, +358 44 290 2515
  fax: +358 9 462 373
  email: markus.valkea...@tkk.fi
-- 



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