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Methods, Problems and Solutions - and Tutorials

GSAS (General Structure Analysis System) Rietveld powder diffraction and Single Crystal software

Doing Le Bail (whole pattern structureless) fitting in GSAS

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[The reference to use for GSAS in any resulting publications is: A.C. Larson and R.B. Von Dreele, "General Structure Analysis System (GSAS)", Los Alamos National Laboratory Report LAUR 86-748 (1994).]

This example cubic Y2O3 data, I A -3 (206) Spacegroup and cell length of ~10.604

Tricks and Hints

Make sure you have an appropriate GSAS Parameter file and put this and the raw data file into a directory.

(One trick that may be necessary on other samples is to put in the wrong unit-cell to allow a nasty background to lock in, then put in the correct cell, run Powpref and start the Le Bail again.)

One reason why people can fail doing Le Bail with GSAS is they keep running Powpref before Genles Least Squares; resetting all the extracted intensities. Do not keep re-running Powpref so that the previously fitted intensities are recycled for the next round of least squares in GSAS.

Summary of the following is run GSAS as you normally would, insert a dummy atom to allow Genles to run and the Le Bail controls are in the Least Squares control part of EXPEDT.

Brian Toby's EXPGUI for Windows and UNIX can make it very easy setting the various parameters if you are new to GSAS.


Some advice from Brian Toby at NIST:

". . . . trick is to converge the
intensity extraction by running lots of 0 cycle GENLES runs until the
factors are staying fixed and then start refining. This needs to be
repeated every time POWPREF is run."


Also, the latest October 2000 GSAS allows you to set a Le Bail damping parameters. On the Expedt menu:

  • L - Least squares refinement set up
  • L - Edit least squares controls
  • E - Change powder data Fobs extraction flags
  • D - Change damping for leBail extraction


To access the Marquardt damping option in GSAS, via EXPEDT:

  • L - Least squares refinement set up
  • L - Edit least squares controls
  • D d - Marquardt factor


Run GSAS and setup a new EXP file. Enter EXPEDT and confirm creation of the EXP file. Enter P for Powder Data Preparation then P for "Phases - lattice and sp. group".

Entering Data Preparation


Enter I to insert a new phase and then an "Identifying name" (i.e., Y2O3), then enter the space group (I A -3), then the approximate cell constant (10.604) P for "Phases - lattice and sp. group". Type X to exit and you are prompted that you need to insert some diffraction data.

Enter H for "Select and Prepare Histograms", I to insert a new Histogram, then enter the histogram filename (y2o3.dat) and confirm this is correct. Then enter the Powder Instrument Parameter file for this data (xray.prm). We want to use Scan number 1 but don't wish to preview this file. Then enter D to set a minimum D-spacing (select "/" to go with the default), then 0 to exit.

Comments from Bob von Dreele on the above
Date: Tue, 07 Sep 1999 14:01:46 -0600
To: Lachlan Cranswick [l.m.d.cranswick@dl.ac.uk]
From: vondreele@popler.lansce.lanl.gov (Bob Von Dreele)

2. (Select & prepare histograms) Generally I use the "T" option to set max
2-theta at something just beyond (say 1deg) the end of the pattern. That
avoids some bad things that can happen due to reflections at nearly 180 deg
when you choose default.

Exit back to the EXPEDT main menu and enter L "Least Squares Refinement set up". A to "Edit Atom Parameters" then add a single dummy atom (i.e., a Hydrogen on a general position). Exit the "Edit Atom Parameters".

Select L - Edit least squares controls. Change number of Cycles to 40 using C 40

Now select E - "Change powder data Fobs extraction flags" and you should get a response that "Fo's will be extracted from this histogram"

Select C "Change Fo extraction method flags". Confirm that you wish to perform Le Bail extraction on histogram 1 and that you wish to "start LeBail extraction with all Fo's=1.0"

Select E - "Change powder data Fobs extraction flags" and you should get a response that "Fo's will be extracted from this histogram"

Comments from Bob von Dreele on the above
Date: Tue, 07 Sep 1999 14:01:46 -0600
To: Lachlan Cranswick [l.m.d.cranswick@dl.ac.uk]
From: vondreele@popler.lansce.lanl.gov (Bob Von Dreele)

3. Older version of GSAS don't extract Fo's by default, new versions do.

Setting the Le Bail Extraction Flag in GSAS

No go into Edit Overall Options and turn the H Histogram scale factor off.

By default GSAS uses the Finger, Cox & Jephcoat peak shape function (#3) without any values for S/L & H/L which can cause the present version of GSAS to crash. As there are no peaks at low angle, we will use profile shape function #2 (Pseudo Voight function).

While in Overall Options, enter P for "Profile Coefficients" then R ("R n - Reset profile coeffs. to default values for type "n"") to change the Profile function type on histogram 1. When prompted for the profile type, enter 2. If GSAS says no values found, continue on with Y and have them replaced. On being prompted with Enter new values for GU, GV and GW, enter 0 0 0.05. Then when prompted for values for the other parameters, enter / to accept them.

Comments from Bob von Dreele on the above
Date: Tue, 07 Sep 1999 14:01:46 -0600
To: Lachlan Cranswick [l.m.d.cranswick@dl.ac.uk]
From: vondreele@popler.lansce.lanl.gov (Bob Von Dreele)

4. The default profile function is determined by the contents of the iparm
file. The first set of profile coefficients for a "bank" is the default.
Others can also be given as well. I've attached inst_xry.prm from
\gsas\example to show how this is done.
Bob

            123456789012345678901234567890123456789012345678901234567890        
INS   BANK      1                                                               
INS   DTYPE   STND                                                              
INS   HTYPE   PXCR                                                              
INS  1 IRAD     3                                                               
INS  1 ICONS  1.540500  1.544300       0.0         0       0.5    0       0.5   
INS  1I HEAD  DUMMY INCIDENT SPECTRUM FOR X-RAY DIFFRACTOMETER                  
INS  1I ITYP    0    0.0000  180.0000         1                                 
INS  1PRCF1     2    6      0.01                                                
INS  1PRCF11   2.000000E+00  -2.000000E+00   5.000000E+00   0.000000E+00        
INS  1PRCF12   0.000000E+00   0.000000E+00                                      
INS  1PRCF1     3    8      0.01                                                
INS  1PRCF11   2.000000E+00  -2.000000E+00   5.000000E+00   0.000000E+00        
INS  1PRCF12   0.000000E+00   0.000000E+00   0.100000E-01   0.150000E-01        
INS  1PRCF1     4   12      0.01                                                
INS  1PRCF11   2.000000E+00  -2.000000E+00   5.000000E+00   0.000000E+00        
INS  1PRCF12   0.000000E+00   0.000000E+00   0.000000E+00   0.000000E+00        
INS  1PRCF13   0.000000E+00   0.100000E-01   0.150000E-01   0.000000E+00        

Now exit and set up the background parameters in O "Edit overall parameters". Change the background type to Chebyshev and set the refinement flag on. You may also like to set a damping factor for the background as well.


Exit EXPEDT, run POWPREF, then run GENLES. The fit will look something like the following (zoomed up region).

GSAS Rietveld Plot in Brian Toby's EXPGUI/LivePlot software


Continue on releasing parameters for the CELL, and Profile function; combined with Genles until the best fit is achieved.

If you have trouble fitting a powder XRD profile, refer to Difficulty in profiling Powder XRD data in GSAS, Peak Cutoff values and the influence of Powpref


From the Structure Solution from Powder Diffractometry List:
At 12:44 07/09/99 -0500, Jaap Vente wrote:
>On Tue, 7 Sep 1999, Peter Y. Zavalij wrote:
>
>> > Lachlan wrote :
>> > A belated reply on the topic of Le Bail extraction
>> > in GSAS:
>> > 
>> There is small problem with LeBail extraction in GSAS:
>> alpha1 and alpha2 components are refined independently...
>> 
>
>
>Yes I found that as well, but thought that I had made a mistake somewhere.
>I think it is a major problem though. Bob could you please fix that?

Not sure if Bob has subscribed to this list. The way this problem
was fixed in ARIT was to consider the two structure factor amplitudes
independently derived from the alpha 1 and 2 components, then
to make them both equal to the mean value, after each new
iteration. I did not find a better way. This is the reason why
Rp is more slowly decreasing when dealing with alpha1+2 than
with alpha 1 alone.

In FORTRAN, this operation corresponds to :
( see in ARIT source at http://sdpd.univ-lemans.fr/arit.html )

In ITER subroutine :
make both structure factors equal to the mean

       DO 1 I=1,NN
       IF(ICH(I).EQ.0)GO TO 1
       SOMH=FNUU(I)+FNUU(ICH(I))
       FNUU(I)=SOMH/2.
       FNUU(ICH(I))=FNUU(I)
1      CONTINUE

In the MAIN program, IHC is the order
of the alpha2 component in the hkl list :

       DO 4 I=1,NN-1
       IHC(I)=0
       IF(ICC(I).EQ.5)GO TO 4
       DO 3 J=I+1,NN
       IF(ICC(J).NE.5)GO TO 3
       IF(ICO(I).NE.ICO(J))GO TO 3
       IF(HHH(1,I).NE.HHH(1,J).OR.HHH(2,I).NE.HHH(2,J).OR.
     1HHH(3,I).NE.HHH(3,J))GO TO 3
       IHC(I)=J
       GO TO 4
3      CONTINUE
4      CONTINUE

Best,

Armel

PS : To subscribe, send an e-mail to sdpd-subscribe@egroups.com 


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