Actually, V and W reflect instrumental broadening - so better to be refined globally or fixed at the values refined with a standard (LaB6 or similar). U is a mixture of instrumental contribution and Gaussian strain broadening term - so, can de different for three phases. But practically for such difficult multiphase situation maybe indeed it would be better to refine it globally, at least for intermediate refinements. Generally, it makes sense also to refine Gaussian size broadening [~1/cos^2(theta)], usually denoted P or Z (software-dependent).
Sincerely, Maxim. ------------------------------------------- Dr. Maxim Lobanov Head of R&D Department Huntsman-NMG mailto: m_loba...@huntsman-nmg.com -----Original Message----- From: gre...@unisi.it [mailto:gre...@unisi.it] Sent: Wednesday, February 03, 2010 6:10 PM To: rietveld_l@ill.fr Subject: Re: What is a good sequence for refinement of (U,V,W) of a multiphase sample ? you better refine uvw globally for the sample and then Lorentzian x and y individually for each phase. uvw should be constant for a given diffractometer setting and you might get it from an independent refinement of a standard. best miguel On 3 Feb 2010 at 15:41, Huy LE-QUOC wrote: > Dear Rietvelders, > > I'm currently doing Rietveld refinements (with FullProf) for a > multiphase sample whose 3 phases with space groups Fm3m, P63/m and > I41a/md. The problem is the reflections of these 3 phases are very > superposed and hence I have found that their (U,V,W) refined parameters > seem to be very correlated. > > I have read somewhere that for each phase we should begin refine W, then > V and finally U. But I don't know in my case whose 3 phases having their > own (U, V, W) we should follow which sequence ? First refine (U, V, W) > of the strongest phase and then for others phases? Or should we refine > simultaneously W for all phases and then V and U for all phases? > > > Do anyone of you have any experience on this case ? > > Thank you in advance for your kindly helps. > > Best regards, > > --- > Huy LE-QUOC, > Doctorant > LPSC/CNRS - Centre de Recherche Plasmas-Matériaux-Nanostructures > et Institut Néel/CNRS > 53 rue des Martyrs, Grenoble 38026, FRANCE > > -- Miguel Gregorkiewitz Dip Scienze della Terra, Università via Laterina 8, I-53100 Siena, Europe fon +39'0577'233810 fax 233938 email gre...@unisi.it
