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 > { > margin:0px;padding:0px;} > #yiv1270810026 { > font-size:10pt;font-family:Verdana;} > > > > 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. > Celebrate > a decade of Messenger with free winks, emoticons, display > pics, and more. Get > Them Now >