To all: OBSERVATION: Most of the reported X-ray Rietveld analyses I've seen include refined values for U, V, & W which are dependent on the particular sample of interest. As you say below regarding U, V, & W: "They can be a bit hard to determine unless a very high quality pattern is used for the calibration." Therefore, I am puzzled why it is current practice to allow U, V & W to vary during Rietveld refinement. If U, V, & W are really INSTRUMENT parameters, shouldn't they be determined independently using "a very high quality [specimen to determine the] pattern [which] is used for the calibration" -- and then FIXED FOR FUTURE ANALYSES? Can anyone explain why U, V, & W are refined? Frank May Research Investigator Department of Chemistry and Biochemistry University of Missouri - St. Louis One University Boulevard St. Louis, Missouri 63121-4499 314-516-5098 [EMAIL PROTECTED]
________________________________ From: Von Dreele, Robert B. [mailto:[EMAIL PROTECTED] Sent: Mon 6/25/2007 12:27 PM To: [email protected] Subject: RE: Caglioti U V W parameters Dear Stefan (& all, I suppose), >From the original formulation by Caglioti, et al. U>0, V<0 & W>0 for a "nonfocusing" neutron CW instrument and describes a parabolic curve with the minimum at roughly the 2-theta angle that matches the monochromator "take-off" (really 2-theta) angle. For a Bragg-Brentano powder diffractometer the curve is usually quite flat yielding very much smaller FWHM especially for the low angle portion rising only at high angles. I suppose the minimum is about the 2-theta for the analyser crystal. Thus, U, V & W are usually quite small but the relationship of U>0, W>0 & V<0 still holds. They can be a bit hard to determine unless a very high quality pattern is used for the calibration. The expression using U, V & W yields (FWHM)^2 so it REALLY can't ever have a negative result as Yaroslav noted in his message. Rietveld refinement codes will protect against the possibility of a negative square root in various ways (if not they can crash on a "negative sqrt" error). Bob Von Dreele R.B. Von Dreele IPNS Division Argonne National Laboratory Argonne, IL 60439-4814 -----Original Message----- From: Stefan Berger [mailto:[EMAIL PROTECTED] Sent: Monday, June 25, 2007 11:18 AM To: [email protected] Subject: Caglioti U V W parameters Dear All, i am a little bit confused about the magnitudes of the U V W profile parameters of the caglioti function for instrumental broadening. Is it true, that U and W are always positiv (larger 0) and V is always smaller 0 (negativ)? Thanks for your hints and explanations. Best Regards, Stefan
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