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 -- -- Scanned by iCritical.
