Markus, it is an interesting question given the wide range of choices now available. The three considerations are sample, resolution and intensity.
Marco Sommariva mentioned capillary/transmission geometry - which I had not thought of in the context of the question, but must be considered. If your samples are lowly adsorbing (low mass absorption), then a BB geometry is not optimal (but doable) and a parallel beam or transmission technique may be better. You mentioned Johansson (Ka1) and two bounce parallel beam - so it looks as if you are going high resolution/low intensity. I have had experience with neither, but whoever wants to sell equipment to you should be asked to collect data on a known sample for comparison purposes. If you are going to do indexing/Rietveld you are going to have to model the diffraction peaks - it is paradocical that it is easier to model poor resolution data (but poor resolution data may not give you what you want). So a question is if you can model the data. I have had no luck with parallel beam for large cell, low absorption data - mainly for model reasons. I just can't model the low angle data. One advantage of BB is that it suits position sensitive detectors (as long as the acceptance angle is kept low, 2 degs at 240mm is OK but marginal, 6 deg at 240mm is a nightmare [so I'm told]) which allows faster data collection, that can't done in parallel beam if the post-diffraction optic is a colliminator or mirror. Lots of luck - I'd suggest you have data collected on target equipment for the sample types of interest and see if you can model it. Tony Raftery ________________________________________ From: Markus Valkeapää [markus.valkea...@tkk.fi] Sent: Friday, 23 January 2009 8:08 PM 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 --
