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Methods, Problems and Solutions

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

Setting up POLA or IPOLA and POLA correctly for a powder XRD system containing a diffracted beam Graphite Monochromator in GSAS

The CCP14 Homepage is at http://www.ccp14.ac.uk

[Back to Problems and Solutions] [Back to GSAS Hints/Resources]
[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).]

[The reference to cite in any resulting publications for using EXPGUI is: B. H. Toby, EXPGUI, a graphical user interface for GSAS, J. Appl. Cryst. (2001). 34, 210-213]

About POLA and IPOLA

(Summary: Differences between the "ideal" polarisation values for a Graphite Monochromator and "refined" values are expected due to other effects such as Mosaic spread. )

From: "Bob Von Dreele"
Subject: Difference between using POLA only and IPOLA/POLA
To: Lachlan Cranswick [l.m.d.cranswick@dl.ac.uk]
Date: Wed, 09 Oct 2002 08:58:49 -0700


The two functions are "identical" in effect but differ by 
a scaling factor. IPOLA=1 is in there to match the 
"traditional" Azaroff function for diffracted beam 
monochromators. The difference between theory & 
measurement for POLA is "expected". The IPOLA=0 function 
is my adaption so that POLA is the polarized fraction. 
POLA=0.50 for no polarization and POLA=1.0 for perfect 
polarization nearly realized in synchrotron radiation & 
vertical diffraction.


The following two methods show how to setup for the Graphite Monochromator by refining on a suitable standard. In this case, annealed Y2O3. Users of the DBW derived CSIRO Riet7 software or Koalariet would probably feel more at ease using the second IPOLA/POLA method. (In Koalariet, the 2-theta value is inserted).


Click here to obtain GSAS exp and data files of Y2O3 data for determining Polarisation by both methods. (Data from the IUCr CPD Quantitative Phase Analysis Round Robin)

     Philips 3020 Goniometer with PW3710 Controller (17.3cm Goniometer Radius). 
     Copper Long Fine Focus X-ray Tube set at 40kV and 40mA 
     Flat-plate Bragg-Brentano, reflection geometry 
     Sample backpacked into standard Philips sample holders 
     For non Pharmaceutical Samples: Step Scan from 5 to 150, 
        at 0.02 degree steps and 3 seconds per step 
     1 degree divergence slit 
     Incident beam Sollers Slit 
     Unspun sample 
     0.3mm receiving slit 
     Diffracted beam Sollers slit 
     1 degree scatter slit 
     Diffracted beam curved Graphite Monochromator
     Proportional Counter 

002 reflection of Graphite: 2-theta with Cu X-rays = 26.603 (d=3.3480). Calculated cos^2 2-theta = 0.80


Using "POLA only method" to handle a Graphite Monochomator within GSAS

The overall advice is to determine the best polarisation value by systematic refinement on an appropriate standard; where data collected up to the angular limit of the Powder XRD (140 to 160 degrees 2-theta). It can be advisable to fit the profile with a Le Bail fit, then fix profile parameters, "shft" offset correction and unit cell values. Be wary that there can be considerable correlation between the polarisation values, thermal values and the background. Using a standard (like cubic annealed Y2O3) where peaks are well separated is advised.

First you may want to fix the thermals at reasonable values, then systematically vary the POLA values and see what this does to the R-factors.

Then you may want to repeat the above but allow the thermals to vary.

Finally, set POLA at extreme values and see if it refines with the thermals to consistent values. If the refinement looks robust, then use this value of POLA for samples generated by this diffractometer. In the following table, POLA consistently refines to a value of 0.523(3) for this particular XRD configuration with a Graphite Monochromator.

GSAS via EXPGUI interface with POLA parameter

Results from refining cubic Y2O3 using the above described strategy

     POLA     R F**2     Rwp     Y1thermal  Y2thermal  O1Thermal     Thermals
    (fixed)
     0.40     0.0745     0.1148  0.00400     0.00400     0.00800     Fixed
     0.45     0.0525     0.1022  0.00400     0.00400     0.00800     Fixed
     0.50     0.0333     0.0952  0.00400     0.00400     0.00800     Fixed
     0.55     0.0270     0.0937  0.00400     0.00400     0.00800     Fixed
     0.60     0.0377     0.0972  0.00400     0.00400     0.00800     Fixed
     0.65     0.0497     0.1040  0.00400     0.00400     0.00800     Fixed
     0.70     0.0624     0.1130  0.00400     0.00400     0.00800     Fixed
     0.75     0.0751     0.1299  0.00400     0.00400     0.00800     Fixed
                                   
     0.40     0.0551     0.1051  0.00162     0.00065    -0.00379     Refined
     0.45     0.0370     0.0969  0.02763     0.00170    -0.00076     Refined
     0.50     0.0264     0.0933  0.00366     0.00276     0.00177     Refined
     0.55     0.0266     0.0933  0.00471     0.00382     0.00432     Refined
     0.60     0.0339     0.0961  0.00576     0.00488     0.00694     Refined
     0.65     0.0434     0.1006  0.00681     0.00593     0.00907     Refined
     0.70     0.0526     0.1062  0.00787     0.00698     0.01112     Refined
     0.75     0.0613     0.1124  0.00891     0.00800     0.01296     Refined

Refining POLA from (and Thermals starting at Y1=0.004, Y2=0.004, O1=0.008)                                   
0.40 0.523    0.0247     0.0930  0.00415     0.00326     0.00291     Refined
0.50 0.523    0.0248     0.0929  0.00415     0.00326     0.00292     Refined
0.75 0.523    0.0248     0.0929  0.00415     0.00326     0.00292     Refined

Background      Refining POLA from 0.5 (and Thermals starting at Y1=0.004, Y2=0.004, O1=0.008)   
                        POLA     R F**2     Rwp     Y1thermal  Y2thermal  O1Thermal    Thermals
 2 term Shifted Cheby  0.472    0.0345     0.1420  0.00203     0.00087    -0.00018     Refined
 3 term Shifted Cheby  0.555    0.0309     0.1215  0.00537     0.00451     0.00319     Refined
 4 term Shifted Cheby  0.529    0.0335     0.1124  0.00462     0.00417     0.00307     Refined
 5 term Shifted Cheby  0.517    0.0327     0.1025  0.00460     0.00370     0.00268     Refined
 6 term Shifted Cheby  0.532    0.0307     0.0932  0.00494     0.00452     0.00238     Refined
 7 term Shifted Cheby  0.527    0.0291     0.0877  0.00490     0.00394     0.00326     Refined
 8 term Shifted Cheby  0.530    0.0263     0.0850  0.00497     0.00423     0.00358     Refined
 9 term Shifted Cheby  0.529    0.0252     0.0844  0.00491     0.00415     0.00307     Refined
10 term Shifted Cheby  0.529    0.0268     0.0841  0.00490     0.00415     0.00293     Refined
11 term Shifted Cheby  0.530    0.0264     0.0831  0.00495     0.00422     0.00348     Refined
12 term Shifted Cheby  0.529    0.0247     0.0829  0.00492     0.00419     0.00356     Refined
13 term Shifted Cheby  0.528    0.0250     0.0826  0.00483     0.00413     0.00383     Refined
14 term Shifted Cheby  0.530    0.0226     0.0821  0.00498     0.00424     0.00329     Refined
15 term Shifted Cheby  0.530    0.0243     0.0820  0.00507     0.00431     0.00309     Refined
16 term Shifted Cheby  0.529    0.0247     0.0809  0.00491     0.00419     0.00294     Refined
17 term Shifted Cheby  0.529    0.0245     0.0804  0.00489     0.00416     0.00308     Refined
18 term Shifted Cheby  0.528    0.0247     0.0804  0.00488     0.00415     0.00310     Refined
19 term Shifted Cheby  0.530    0.0249     0.0795  0.00501     0.00447     0.00223     Refined
20 term Shifted Cheby  0.529    0.0243     0.0793  0.00500     0.00437     0.00244     Refined

--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

Using neutron derived thermals from Smrcok L, Duris P (1989) P. 375-378 - 8 term Cheby Background
     POLA     R F**2     Rwp     Y1thermal  Y2thermal  O1Thermal     Thermals
     Refined value
0.40 0.549    0.0295     0.0862  0.00760     0.00510     0.00460     Fixed  
0.45 0.549    0.0295     0.0862  0.00415     0.00326     0.00291     Fixed  
0.50 0.549    0.0295     0.0862  0.00415     0.00326     0.00292     Fixed  
0.55 0.549    0.0295     0.0862  0.00415     0.00326     0.00292     Fixed  
0.60 0.549    0.0295     0.0862  0.00415     0.00326     0.00292     Fixed  
0.65 0.549    0.0295     0.0862  0.00415     0.00326     0.00292     Fixed  
0.70 0.549    0.0295     0.0862  0.00415     0.00326     0.00292     Fixed  
0.75 0.549    0.0295     0.0862  0.00415     0.00326     0.00292     Fixed  


Using POLA and IPOLA=1 to handle a Graphite Monochomator within GSAS

As explained above by Bob, set IPOLA to 1 (numeral one), then enter the calculated POLA value as "cos^2 2-theta" for the monochromator. (coefficient in formula for polarisation correction when using a monochromator.)

Then for a graphite monochromator, calculate the "cos^2 2-theta" of the 002 graphite reflection (For Cu X-rays this is at an angle of 26.6 degrees 2-theta). The calculated value should be 0.80. Enter this value into the POLA field and refine the structure. Near the end of the refinement, allow POLA to refine (which in this case refines to 0.91(1)).

Similarly to the above "POLA only method", it would be best to do similar systematics to ensure you have a robust and reliable result.

GSAS with IPOLA set and POLA refining


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[Generating a VRML based 3D Structure and Electron Density Contour Map in GSAS]
[Viewing GSAS Fourier Contour Maps in 3rd party programs Using Scott Belmonte's GPL'd FOUE software]

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