Dear Naveed, The macros for modeling preferred orientation are an integral part of the Topas software and don't need to be obtained from an external source. Details are available in the technical reference manual.
Preferred orientation results in the central assumption that the measured intensity is proportional to the reflection structure being violated, and it is therefore not surprising that structure solution (or refinement) is more difficult when carried out using incorrect structure factors as input. However, this does not make structure solution impossible - a nice examples is the structure solution of laboratory XRD data for a sample of LiFePO4 which has substantial preferred orientation since it was prepared from crushed single crystals - the data from my lab and the excellent accompanying tutorial written by Arnt Kern are available at: https://sites.google.com/a/stonybrook.edu/mmrrsa_2015/day-2-exercises/exercise2-2 if you would like a controlled example to practice on. As many people will tell you, it is much more effective to eliminate preferred orientation (though improved sample preparation) than to try to model it. A very nice tutorial lecture on sample prep from Pam Whitfield is also available at the same MMRRSA site, linked on the "Final Schedule" tab. The Stephens macro is for anisotropic strain broadening, and deals with variations in peak breadth. However, the integrated intensity of peaks will not change as a result of this type of broadening. As such, this phenomenon is completely different from preferred orientation (which is generally modeled as influencing peak intensities but not peak shapes). -Peter Dr. Peter Khalifah, [email protected]<mailto:[email protected]> Associate Professor Chemist Dept. of Chemistry Dept. of Chemistry Stony Brook University Brookhaven National Laboratory Stony Brook, NY 11794-3400 Upton, NY 11973-5000 Office: 447 Grad. Chemistry Office: Bldg 555, Rm 340 Phone: (631)632-7796 Phone: (631)344-7689 Fax: (631)632-7960 Fax: (631)344-5815 From: [email protected] [mailto:[email protected]] On Behalf Of NAVEED ZAFAR Sent: Tuesday, June 23, 2015 3:54 PM To: [email protected] Subject: TOPAS Macro Language (peak shape broadening macros) Dear Rietveld Experts and Colleagues, For most of the rietveld refinements (both qualitative and quantitative_Launch .INP mode) of XRD powder data, I often use Topas Academic Macro Library, well manged and maintained by John Evans and colleagues. https://community.dur.ac.uk/john.evans/topas_academic/topas_macros.htm But currently for samples, that encounter preferred orientation (e.g. nanotubes etc), the strong preferred orientation somehow influence the intensity distribution within a profile and hamper structural solution efforts using rietveld refinement of powder data. Are there any macros (written by experts) that can assist in such a case. I already tried the " Stephens peak shape broadening macro" but found no change in Rwp value. Also, If there is complete data bank of such macro..s. Please share. Accept my apologies for being ignorant, so far. Best regards, Naveed _________________ Dr. Naveed Zafar Ali https://scholar.google.com/citations?sortby=pubdate&hl=en&user=V4Hsus8AAAAJ&view_op=list_works "Being ignorant is not so much a shame, as being unwilling to learn." .....― Benjamin Franklin<http://www.goodreads.com/author/show/289513.Benjamin_Franklin>
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