"Theoretically" U,V,W should not be refined at all as they describe instrumental broadening, and this is exactly what I do (usually). After diffractometer alignment, a good quality pattern in whole possible angular range has to be collected from "Rietveld standard" which currently is LaB6. From these data it is possible to calculated UVW assuming high quality (e.g. step size 0.01 deg. 2theta or less, 5-10 sec/point, etc.). These UVW could be used w/o refinement in any other experiment obtained at the same diffractometer settings and alignment. Thus separate instrumental files have to be created for each diffractometer configuration used (e.g. different slits).
Peter Zavalij Dr. Peter Y. Zavalij University Crystallographer Institute for Materials Research and Chemistry Department Binghamton University, SUNY, Vestal Pkwy, East Binghamton, NY 13902-6000, USA Tel: (607)777-4298 Fax: (607)777-4623 E-mail:[EMAIL PROTECTED] http://materials.binghamton.edu/zavalij -----Original Message----- From: Maxim V. Lobanov [mailto:[EMAIL PROTECTED] Sent: Thursday, March 18, 2004 10:50 AM To: [EMAIL PROTECTED] > Your sample will probably broaden the lines (LX, LY, etc.) so much that any attempt to vary the Gaussian >coefficients will yield nonsense. Just some remark (of course, I am not a great specialist): At least to my experience, there is always some Gaussian broadening from the sample as well, and (again it is my humble opinion only) at least U is better to allow to be refined. For example, Rietan manual states the problem in the following way: { U, V, and W tend to be highly correlated, with a result that various combinations of quite different values can lead to essentially the same variance, sigma^2. These three parameters, therefore, do not converge in a stable manner when refined simultaneously (Prince, 1993). In particular, refining P in addition to U, V, and W almost certainly affords a singular (non-positive definite) coefficient matrix. Of the four profile-shape parameters in Eq. (5), V and W depend not on specimens but only on instruments (Young & Desai, 1989). Then, these two instrumental parameters may well be fixed at values obtained by the Rietveld refinement of a well-crystallized sample where profile broadening is negligible, i.e., P = 0 . } Sincerely, Maxim. __________________________________ Maxim V. Lobanov Department of Chemistry Rutgers University 610 Taylor Rd Piscataway, NJ 08854 Phone: (732) 445-3811
