You may have a look inside the Rietveld FAQ and discussions
gathered at :
    http://www.unige.ch/crystal/stxnews/riet/faq/rietfaqs.htm
             by Lachlan Cranswick.
   A starting point may be the "Low FWHM and Rp discussion". If 
you have not access to the WEB, it is available by anonymous ftp
at   aviion.univ-lemans.fr/pub/fluorlab/LOW_FWHM_AND_RP.TXT
as an ASCII file -28K-(consider the discussion is now one year old,
think to add your own conclusions).
   Armel                   http://fluo.univ-lemans.fr:8001/