Dear Dreele: Your response about my comment 1 is convincing which i did not care much about. Thanks for your correction.
i appologize to all for this incorrect comment. Faithfully Jun Lu ---------- Lst. Prof. Lijie Qiao Department of Materials Physics and Chemistry University of Science and Technology Beijing 100083 Beijing P.R. China http://www.instrument.com.cn/ilog/handsomeland/ Lst. Prof. Loidl and Lunkenheimer Experimental Physics V Center for Electronic Correlations and Magnetism (EKM) University of Augsburg Universitaetsstr. 2 86159 Augsburg Germany http://www.physik.uni-augsburg.de/exp5 ----- Original Message ----- From: "Robert Von Dreele" <[EMAIL PROTECTED]> To: <[EMAIL PROTECTED]>; <[EMAIL PROTECTED]> Sent: Thursday, August 07, 2008 6:08 PM Subject: Re: confusion about some GSAS parameters > Hi, > The other note on this is not quite correct. POLA is the polarization > and depends on choice of monochromator/analyzer angle/material and the > wavelength. It is normally not refined but set to known values. > Typically for a laboratory instrument with graphite analyzer and CuKa > radiation POLA ~0.7 as given in the gsas\example\inst_xry.prm file. > For conventional Bragg-Brentano diffractometers the ZERO should NEVER be > refined because that is the thing you are setting to zero when you align > the diffractometer (or when it gets done by your service rep.) The > systematic effect you are seeing is the shift in the sample position > away from the diffracting circle; typically on the order of some tens of > microns and happens all the time. This is the "shft" parameter and is > appropriate for all but very transparent samples where the expected > x-ray penetration is only a few microns, i.e. anything with a metal (Na > or above) in it. Refining ZERO instead of shft will bias the resulting > lattice parameters because it does not describe the systematic effect > correctly. The other parameter "trns" is only needed for very > transparent samples (or harder radiation, e.g. MoKa) where the beam > penetration would be many microns. That shifts the effective sample > position to be below the surface and thus leads to a shift in peak > positions. Different math than for shft but not normally encountered; > they are usually mutually exclusive - no need to ever refine both > (despite the other remark!). Refinement of both would require a full > range scan (to ~140deg 2-theta!); anything less would give very nearly > singular refinements & probably crazy values for both. By the way the > shft parameter has been verified by placing shim stock under the sample; > the refined value of shft gave a sample displacement that matched the > thickness of the shim. trns has been used to determine sample packing > density from the known absorption coefficient for the bulk material. So > both are physically meaningful. > "asym" is used in peak shape function #2 in GSAS - I'd recommend using > #3 or #4 as those functions are more recent and use a better model for > the asymmetry seen at very low scattering angles -see paper by Finger, > et al. referenced in GSAS Manual on this. The GSAS Manual does also > discuss the other questions you had - I recommend reading it. > Best, > Bob Von Dreele > > [EMAIL PROTECTED] wrote: >> Dear all, >> I am very new with GSAS and some confusions arrised to me during >> refinement. I will be very much grateful if anyone helps me by giving the >> explanations of the following facts.. >> >> 1. what the parameter POLA stands for(is it polarization >> factor??)? is it necessary to refine POLA when we are refining the XRD >> patterns taken by a Brag-Brenteno diffractometer with fixed source? >> 2. Does 'shft' and 'trns' means sample shift and sample >> transperancy? what is the difference when we are refining 'zero' in place of >> refining 'shft' and 'trns'? >> 3. what physical explanation is behind the asymmetry(refined >> by 'asym') in the peak shape? >> >> >
