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 > No virus found in this incoming message. Checked by AVG - www.avg.com Version: 8.5.392 / Virus Database: 270.13.24/2255 - Release Date: 07/22/09 18:00:00
