As Maxim says, strain-related parameters are U and Y. By the same token, size-related parameters are P and X (both go with 1/cos(Theta). The reason is that this Rietveld model (the so-called TCH) assumes a Voigt function for both size and strain profile and therefore has to have both Lorentzian and Gaussian components.
When refining the parameters, one has to keep in mind that instrumental broadening in general depends on the same parameters. Therefore, first refine all profile parameters on the diffraction pattern of the standard sample (as Brian said, LaB6 or similar) and then refine ONLY four parameters (U,P,X,Y) for the sample of interest, as far as instrumental conditions stay the same. More details and how to calculate size and strain from U,P,X, and Y were published in JAC 37 (2004) 911-924. The reprint and original data from seven different instruments (including synchrotron and neutron) can be found at http://mysite.du.edu/~balzar/s-s_rr.htm. Davor BalzarUniversity of Denverwww.du.eduwww.du.edu/~balzar ----- Original Message ----- From: "Maxim V. Lobanov" <[EMAIL PROTECTED]> Date: Sunday, November 30, 2008 10:37 pm Subject: RE: Rietveld: U,V,W To: rietveld_l@ill.fr > Dear colleagues, > > > > one question on that: > > > > >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. > > as far as I understand, the strain broadening term should have FWMM~theta > dependence, i.e. Lorentzian Y in "standard" (GSAS) notation. > > For the Gaussian part you have > > FWHM^2=U*tan^2(th)+V*tan(th)+W+P/cos^2(th) > > and the term that varies roughly as ~theta is U, not V. > > Therefore, based on that and probably erroneously, I used to refine U > parameter rather than V (+ P, X, Y, of course). > > > > I am wondering if somebody could make this more clear? > > > > >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.) > > So far I noticed that Fullprof uses interchanged (v.r.t. GSAS) XY, > i.e. X > corresponds to strain term there. There might be also other > differences, of > course... > > > > > > Sincerely, > > Maxim. > > > > From: Brian H. Toby [mailto:[EMAIL PROTECTED] > Sent: Monday, December 01, 2008 5:21 AM > To: May, Frank > Cc: rietveld_l@ill.fr > Subject: Re: Rietveld: U,V,W > > > > 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.) > > > > Brian > > > > ******************************************************************** > > > Brian H. Toby, Ph.D. office: 630-252-5488 > > Materials Characterization Group Leader, Advanced Photon Source > > 9700 S. Cass Ave, Bldg. 433/D003 work cell: 630-327-8426 > > > Argonne National Laboratory secretary (Marija): 630-252-5453 > > Argonne, IL 60439-4856 e-mail: brian dot toby at anl dot gov > > > ******************************************************************** > > > > > > > >
