What is the correct procedure for refining U,V,W? It is my understanding that those parameters are a function of instrument geometry. Does one use a standard material to determine U,V,W and then fix their values for the instrument you're using?....or do the values of U,V,W change depending on the sample being examined? If so, why do the values change?
The GSAS manual covers the latter part of you questions pretty well, though perhaps indirectly. In theory, U and W should be instrumental constants that will not change with sample, while V can have both an instrumental and a residual stress component. However, this assumes that one also refines a crystallite size parameter, P when needed, which many people (myself included) do not. In that case, U, V & W will all change to compensate for crystallite broadening.
I do recommend using a standard with good sharp peaks (SRM LaB6 is the ideal, though there are likely to be many other oxides handy that work reasonably.) If you can't get a good fit to your standard, then you do not want to advance to an unknown until you understand the problems with your instrument/technique.
Where possible, I try to start a refinement with values that are close to correct for U, V & W (+ X & Y where significant) and put off refining them until late in the refinement, when they tend to be pretty stabile. Initially, I usually refine U, V & W together and then refine X and then Y solo and then finally in combinations until everything is refined together. Look for parameters that are refining to zero and turn them off, since GSAS does not deal with that very well. Also look at the widths vs 2theta (widplt) to see if the functions are reasonable.
The routine in CMPR for fitting U, V & W values to a set of peak widths has been useful for me where I don't have good calibration information for an instrument.
FullProf might be a bit more stable, but I think the process there is about the same. (BTW, if anyone works out how to convert GSAS profile terms to ones used in FullProf, I'd be interested to get those relationships into CMPR; I am not sure if the scaling is only between centidegrees**2 and degrees**2.)
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