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?
>>
>>   
>

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