Re: Gobbel Mirror

[Andreas Leineweber]

Dear Mr Lancry, I have not worked yet with Göbel (Goebel) mirrors by myself yet, but I have had a couple of discussions on that topic with others. One problem with the Fullprof Rietveld refinement is that the radiation in Bragg-reflected at the Göbel mirror. The alpha1 and alpha2 components are reflected by a slightly different angle, that have also slightly different incident angles with respect to the sample. Therefore, the alpha1-alpha2 separation delta2theta is to simply proportional to d(lam)/lam*tan(theta) but differs by a constant angle, which is the difference between the alpha1 and alpha2 beams leaving the Göbel mirror. Of course it has to be considered, how the two diffraction processes are related to each other ("addition" or "substraction") This has been treated by Toraya, J. Appl. Cryst. (2000). 33, 1317-1323 . The same also holds for usual primary beam monochromators, which are however in powder diffraction most frequently used in order to eliminate alpha2. However, I tried it once with a misadjusted Guinier camera, and it is as it should be. However, this should not affect integral intensities, but only the profiles. The usual Rietveld programs do not consider this, an this may a reason that profile description problems may occur. At least Topas (Bruker) can do it. The problems with wrong integral intensities may have to do with preferred orientation (layer structure)/graininess and the usual parallel character of the radiation leaving the Göbel mirror in contrast to usual Bragg Bentano focussing geometry. With reflection geometry something like MoS2 is probably very difficult to handle. There are however, some procedure to overcome that, e.g. mixing the sampe with some other materials. But I have no experimence there. Of course also polarisation is different, but that should not give a dramatic effect. Best regards Andreas Leineweber eli lancry wrote: Dear Rietvelders, I am working with few molybdenum chalcogenides (hexagonal) and I have noticed that when I measure it (micronic powder in a flat sample holder) in Bruker D8 (Bragg Brentano) with Gobbel mirror, the major peak at around 13 2Th is twice intense of doing the same measurement in Bruker D8 without Gobbel mirror while the rest of the pattern is the same. the refinement (fullprof) is much better for the sample measured with the mirror. my questions is: 1. what could be the reasons for this difference? (absorption??) 2. how can I overcome this problem? (sample preparation, refinement)   Thanks in advance for your help   Eli Lancry Chemistry department Bar-Ilan University, Ramat Gan ISRAEL Phone: 972-(0)3-5317665/8169 Mobile: 972-(0)54-2212717 Fax: 972-(0)3-5351250 E-mail: [EMAIL PROTECTED]                 -- Dr. Andreas Leineweber Max-Planck-Institut fuer Metallforschung Heisenbergstrasse 3 70569 Stuttgart Germany Telephone: +49 (0)711 689 3365 Telefax: +49 (0)711 689 3312 [EMAIL PROTECTED] http://www.mf.mpg.de/de/abteilungen/mittemeijer/english/index_english.htm