Dear Alex,

Quartz is not the best standard for the instrumental asymmetry determination 
since it does not have low-angle peaks. So, if you fixed the asymmetry 
parameter to that determined for quartz you may have problems in the low-angle 
area.
It is always better to refine the asymmetry parameter as it is not only 
instrument-dependent but also sample-dependent. The asymmetry may depend on the 
absorption coefficient, the powder packing density, the capillary diameter and 
other parameters.

Best regards,
Leonid

*******************************************************
 Leonid A. Solovyov
 Institute of Chemistry and Chemical Technology
 660049, K. Marx 42, Krasnoyarsk , Russia
 www.icct.ru/eng/content/persons/Sol_LA
 www.geocities.com/l_solovyov
*******************************************************

--- On Tue, 8/11/09, Alex Sinclair <sinclaira...@hotmail.com> wrote:

> From: Alex Sinclair <sinclaira...@hotmail.com>
> Subject: Asymmetry profile
> To: rietveld_l@ill.fr
> Date: Tuesday, August 11, 2009, 12:09 PM
> 
> 
> 
> #yiv1270810026 .hmmessage P
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> 
>  
> Dear mailing
> list,
>   
> Would anyone
> be able to help me with a problem I am having refining some
> data using TOPAS on a lab X-ray pattern?
>   
> The
> diffraction patterns collected show a large amount of
> asymmetry in the first couple of (low angle)
> peaks.
>   
> The
> diffractometer contribution to the profile was determined
> from refinement of a quartz standard and the sample
> dependent broadening was then modelled using the PV-II
> function within TOPAS. Axial convergence parameters were
> used to model the angle-dependent asymmetry.
>  
> Unfortunately the asymmetry of the first, lowest
> angle peak is still being fitted very poorly. Are there any
> other parameters or tricks that will help to model this
> asymmetry?
>   
> Many thanks
> for any help you have,
> Alex
> 
>   
> P.S.
> Additionally, the data was collected using a Bruker D8
> Advantix diffractometer in capillary geometry, with a Cu
> source, a Ge(111) monochromator, secondary soller slits (2.5
> degrees) and an linear PSD. 
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