Tricks and HintsMake 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:
To access the Marquardt damping option in GSAS, via EXPEDT:
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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".
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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.
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"
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.
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. |
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Exit EXPEDT, run POWPREF, then run GENLES. The fit will look
something like the following (zoomed up region).
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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 [email protected]
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