How to set up polarization paramters correctly in GSAS (IPOLA,POLA) and Fullprof (monochromater polarization correction).

xujun16

2016/03/03

私聊

X射线衍射仪（XRD）

最近对极化率校正由较多的讨论，因为极化率对精修得影响很大，我把看到的讨论集中在了一起，希望有所帮助。
在参考文献中，你估计能注意我们版的专家lu jun大哥的也参与了2008帖子的讨论。
Among corrections like absorption, Lorentz, extinction, and polarization, Polarization correction is one of the most important correction to diffraction intensity due to that diffracted beam intensity varies with changing diffraction angle.
How to set up these value correctly?
Typical values for polarization factor correction:
there are mainly three cases in GSAS:
1) corrected on 12 of April 2016
IPOLA = 0 and POLA >0.9 used for polarized beam, like synchrotron radiation or other highly polarized beam.
For IPOLA = 1, it indicates that slightly different polarization function is used for correcting intensity (see GSAS manual). But both IPOLA 0 and IPOLA 1 are equivalent, and are different in scale.
for example: IPOLA 0 POLA 0.5. is equivalent to IPOLA 1, POLA 0.911. * (Therefore, my suggestion is always to use IPOLA 0 for different cases as listed here.)
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incorrect statement:
1) IPOLA = 1 and POLA 0.911 used for polarized beam, like synchrotron radiation or other highly polarized beam. For polarized beam, IPOLA = 1 indicates that Azarofffunction is used for correcting polarized beam.
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For lab diffractometer, typicalsettings are (refining POLA values is not recommended):
2) For non-polarized beam, setting IPOLA= 0 and POLA = 0.5 for instrument without no graphite monochromator.
3) For partially polarized beam, IPOLA= 0 and POLA = 0.7 for instrument equipped with graphite monochromator or others.
In Fullprof,
it use the same algorithm as IPOLA = 0, you just set up the polarization fraction from 0.5 to 1.
Monochromator polarization correction: 0.5 non polarized beam, 0.7 for partially polarized beam, 0.9 or above for polarized beam.
In reality, the values may vary a bit which depends on the instrument setting, in my opinion.
有不当之处，请大家指出。谢谢。
reference:
GSAS manual
Fullprof manual
https://www.mail-archive.com/rietveld_l@ill.fr/msg04524.html
* http://www.ccp14.ac.uk/solution/gsas/graphite_monochromator_and_gsas.html