Well... the situation with LP is not so simple. Using TOPAS for refinement
data collected on D8 advance with Ni-filter and LynxEye detector I observe
the following: 
- For all samples LP=0 is OK and gives the best fit as it should be by the
book.
- HOWEVER for LaB6 standard LP=0 yields very poor fit for several high angle
reflections (>120 deg. 2theta) while LP=90 gives perfect fit. The difference
in R factors 12% and 4% cannot be simply ignored...

Can anyone explain this?


Peter Zavalij 

X-ray Crystallographic Center
University of Maryland
College Park, MD

Office: (301)405-1861
Lab: (301)405-3230
Fax: (301)314-9121




-----Original Message-----
From: Ross Williams [mailto:ross.willi...@curtin.edu.au] 
Sent: Wednesday, July 22, 2009 8:43 PM
To: Jon Wright; xiu...@ualberta.ca
Cc: rietveld_l@ill.fr
Subject: RE: LP factor in the Rietveld refinement

Hi Xiujun,

Jon is correct, but to answer your question fully, the angle is used in an
equation to scale the peaks as function of 2theta. 

If you look in the Technical Reference Manual of TOPAS states that the LP
factor (for x-rays) is given by

LP = (1 + cos(2th)^2 cos(2th_m)^2) / (cos(theta) sun(theta)^2)

2th_m is the angle mentioned by Jon, ie 26.4° when using Cu Kalpha  with a
graphic monochromator, 0° when using unpolarised beam, and 90° for full
polarised.

Or in TOPAS macro language : scale_pks = (1 + Cos(CeV(c,v) Deg)^2 Cos(2
Th)^2) /(Sin(Th)^2 Cos(Th));

The Technical Reference has a derivation of the LP equation above and
compares it to parameters used in GSAS and Fullprof.

Kind Regards,

Ross 



+++++++++++++++++++++++++

Ross Williams
PhD Student
Centre for Materials Research
Department of Imaging and Applied Physics
Curtin University of Technology
GPO Box U1987 Perth WA 6845
Western Australia
Phone: +61 (0)8 9266 4219
Fax:     +61 (0)8 9266 2377
Email:   ross.willi...@curtin.edu.au






-----Original Message-----
From: Jon Wright [mailto:wri...@esrf.fr] 
Sent: Thursday, 23 July 2009 5:26 AM
To: xiu...@ualberta.ca
Cc: rietveld_l@ill.fr
Subject: Re: LP factor in the Rietveld refinement

Sounds like the parameter is the monochromator angle you would need to 
use to convert an unpolarised beam into a beam with the polarisation 
state you have (eg, 90 degrees gives 100% polarised). Don't confuse this 
with the actual monochromator angle at the synchrotron, as the bean is 
usually polarised before it reaches the monochromator anyway. With some 
packages you can set the monochromator "roll" angle to put the 
polarisation in the right plane, depending which way up an area detector 
was mounted.

Good luck,

Jon

xiu...@ualberta.ca wrote:
> Hello, everyone,
>
> I have some questions about the refinement in Topas.
>
> When we put the instrument parameters, we always include the LP 
> factor, and set it to a constant value. I thought LP factor is a 
> function of theta and not a constant value, so my question is what 
> exact the constant value means. Why for unpolarized radiation, it is 
> equal to 0, and for synchrotron radiation it is equal to 90. Sorry to 
> throw so many questions.
>
> Thank a lot for any help.
>
> Xiujun Li
> Master Student
> Advanced Materials and Processing Laboratory
> Chemical and Materials Engineering
> University of Alberta
> Edmonton, Alberta, Canada T6G 2G6
> Phone: 1-780-492-0701
>


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