Re: GSAS: Microstrain parameters in rhombohedral SGs
Dear Mr. Werner, this sounds for me like that there would be a problem in GSAS. If you look to the formulas at page 155 of the GSAS manual, in all trigonal/rhombohedral Laue groups there is no separate S004 for rhombohedral settings (in fact S400 = S004). But for the hexagonal settings (not explicitly mentioned in the manual), S310 does not occur as refinable parameter, because S310 (associated with h^3k) is symmetry related to S400. Thus h^3k appears with S400. Only S301 is refinable for Trigonal -3 and -3m (but not for 6/m and 6/mmm, where it is 0). At the moment it appears to me that there may be something wrong in the program. The formulas in the manual appear in principle o.k to me (I went through these formulas by myself some years ago), but the parameters which GSAS/EXPGUI seems to use seem to contradict themselves. This also is in agreement with the Stephens (1999) paper. But please tell me if I got something wrong Best regards Andreas Leineweber Franz Werner wrote: Dear Mr Leineweber actually I am wondering about your parameters. May it be that S310 is instead S301?? If the former is the case, you use the hexagonal setting, which I would also expect if you use also the hexagonal setting for the crystal structure. Thanks for your comment. I just checked the parameter names on EXPGUI's Profile tab again: S400, S004, S202 and S310 (SG R-3, hexagonal setting). Regards Franz Werner
Quantitative analysis
Dear all, I have this question, I have been refined one mixture (of well-know percentage composition) of CaCO3, CaF2, SiO2 and Al2O3, with preferential orientation in CaCO3 (104) and CaF2 (111) with good results. When I add to the mixture FeCO3 and I refine with preferential orientation (104), it happens that when I don´t apply the preferential orientation in all this phases, I have correct values of percentage composition, but when I apply the preferential orientation the refinement is good but with incorrect values of percentage composition. This experiment has been taken in Bragg Brentano geometry. How I should refine this mixture? Thanks a lot by your help. Mario Macías Universidad Industrial de Santander Colombia
RE: Quantitative analysis
Dear Mario, If the quantity of FeCO3 appears to be smaller than the actual value, then it could be due to absorption. If you are using a copper source (CuKalpha) then that excites iron fluorescence, which is radiated equally in all directions... this means that the beam does not penetrate the FeCO3 particles entirely, and results in incomplete diffraction from these particles. There are programs that correct for this problem, but as far as I know there is no freeware to deal with this effect. - Kurt * Kurt Leinenweber Dept. of Chemistry and Biochemistry Arizona State University Tempe, AZ 85287-1604 phone (480)-965-8853 fax (480)-965-2747 *** -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Sent: Monday, October 27, 2008 8:48 AM To: Rietveld_l@ill.fr Subject: Quantitative analysis Dear all, I have this question, I have been refined one mixture (of well-know percentage composition) of CaCO3, CaF2, SiO2 and Al2O3, with preferential orientation in CaCO3 (104) and CaF2 (111) with good results. When I add to the mixture FeCO3 and I refine with preferential orientation (104), it happens that when I don´t apply the preferential orientation in all this phases, I have correct values of percentage composition, but when I apply the preferential orientation the refinement is good but with incorrect values of percentage composition. This experiment has been taken in Bragg Brentano geometry. How I should refine this mixture? Thanks a lot by your help. Mario Macías Universidad Industrial de Santander Colombia
RE: Quantitative analysis
Dear Mario, My experience of quant Rietveld is much the same as yours - poor quant result if using any sort of PO function. Advise as others have here - reduce PO. Capillary geometry may help in many instances but in this case could then have problems with absorption seeing as you will most likely be having to use Co radiation to avoid fluorescence from the iron. Good luck, Martin _ Win an Xbox 360 or £200 Top Shop Vouchers http://clk.atdmt.com/GBL/go/115454062/direct/01/
