Re: RE: Rietveld: U,V,W
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
Re: Rietveld: U,V,W
Hi, Self Citation: NIST Standard Reference Materials for Characterization of Instrument Performance Industrial Applications of X-ray Diffraction Ed by F. H. Chung D. K. Smith, pp 903-917. A pdf is available; email to request. In this I outline how I use SRM 660 (it was 660 in 2000, then it was 660a until ~July '07, we are working on 660b) for qualifying and characterizing instrument performance with both profile fitting and the Rietveld method, via GSAS. I suggest obtaining a set of TCH parameters with LaB6. The instrument specific ones thereafter remained fixed while the other are used as a floor for refined values. With this approach I was able to obtain stable and meaningful results from laboratory equipment. Regards, Jim At 08:25 PM 11/30/2008, you wrote: To all: 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? Thanks in advance. Frank May From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Sent: Sun 11/30/2008 1:32 PM To: rietveld_l@ill.fr Subject: Re: I am a newcome, how can I begin my rietveld refinement analysis Hi, Li: To me, the most wonderful tool to determine initial peakshape parameters is CMPR. CMPR is especially oriented to GSAS and gives you GU, GV, GW etc. And when you use EXPGUI for GSAS, you can also try Graphs-widplt to see how FWHM develops when parameters are tuned. When you prefer Fullprof, you should take a factor to get U, V, W etc. i am not sure about the facor exactly, maybe GX~100X(X=U, V, W), er..? Anyway, just go ahead and make a try. Faithfully Jun Lu -- Lst. Prof. Lijie Qiao Department of Materials Physics and Chemistry University of Science and Technology Beijing 100083 Beijing P.R. China http://www.instrument.com.cn/ilog/handsomeland/ James P. Cline Ceramics Division National Institute of Standards and Technology 100 Bureau Dr. stop 8520 [ B113 / Bldg 217 ] Gaithersburg, MD 20899-8523USA [EMAIL PROTECTED] (301) 975 5793 FAX (301) 975 5334
Rietveld: U,V,W
To all: 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? Thanks in advance. Frank May From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Sent: Sun 11/30/2008 1:32 PM To: rietveld_l@ill.fr Subject: Re: I am a newcome, how can I begin my rietveld refinement analysis Hi, Li: To me, the most wonderful tool to determine initial peakshape parameters is CMPR. CMPR is especially oriented to GSAS and gives you GU, GV, GW etc. And when you use EXPGUI for GSAS, you can also try Graphs-widplt to see how FWHM develops when parameters are tuned. When you prefer Fullprof, you should take a factor to get U, V, W etc. i am not sure about the facor exactly, maybe GX~100X(X=U, V, W), er..? Anyway, just go ahead and make a try. Faithfully Jun Lu -- Lst. Prof. Lijie Qiao Department of Materials Physics and Chemistry University of Science and Technology Beijing 100083 Beijing P.R. China http://www.instrument.com.cn/ilog/handsomeland/
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
Re: Rietveld: U,V,W
Why my reply went to the author's personal mail box? I am very sorry for that. Now I am hunting for a standard reference material to determine the profile parameters of our instrument X'pert Pro. -- Mingtao Li State Key Laboratory of Multiphase Flow in Power Engineering School of Energy and Power Engineering Xi'an Jiaotong University Xi'an, 710049 P.R.China Tel: +86-29-8266 8296 Fax: +86-29-8266 9033 Email: [EMAIL PROTECTED]
RE: Rietveld: U,V,W
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
RE: Rietveld: U,V,W
Brian wrote: 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. Maxim wrote: 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? Dear Brian and Maxim, In fact, the term responsible for the Gaussian strain component is U, not V. Regards, Leonid *** Leonid A. Solovyov Institute of Chemistry and Chemical Technology 660049, K. Marx 42, Krasnoyarsk, Russia www.icct.ru/eng/content/persons/Sol_LA www.geocities.com/l_solovyov ***
