Oooo, picking up where Friday left off...... Must go and put the coffee pot on :-)
- refining of "fundamental parameters" is nonsense, unless one is not familiar with his diffractometer or want/must "absorb" any unkown effects or weaknesses of his model :-) In a perfect world this would be the case, but given that very few people run systems that conform to the requirements for true fundamental parameters (Bragg-Brentano with NO monochromator, mirrors, etc), then one is not really using fundamental parameters per se, which is why using the term convolution-based profile fitting is a better terminology. Some of the effects of things like monochromators are amenable to modelling - they act in the same way as a Soller slit, as well as having a slight effect on the radiation characteristics reaching the detector. The only thing that the fundamental parameters does is to convolute certain functions into the source profiles. In the case of the conventional Bragg-Brentano, these functions are known, and their dependence on the geometry known. For other systems is it a good start, but other, user defined functions (or modifications of the existing ones) may be necessary to correctly model the instrument function, which, at the end of the day is what you need. I don't know if you've any experience with them, but if you can model the effects of two mirrors on the profiles of a system (including minor errors in alignment in both) then you are a better person than I. I can say for certain that I tried for a long time to get GSAS to fit these profiles without much success. Unless you can model the profiles properly then you are wasting your time doing refinements as the intensities are going to be all wrong. I can say that I'm extremely happy with the fits I get. The only thing I don't look forward to is realigning my system! :-) - Pamela, there are surely additional reasons (besides the FPA peak shape modelling) for the high stability and the easy use of the programs like BGMN and Topas in comparision to traditional DBWS and similar codes: a more stable minimization algorithm, the extensive and easy use of restraints/constraints... This is true - I don't know about BGMN, but Topas has a very stable algorithm. However, Topas will also use more conventional profile characteristics, and can suffer from some similar and some different problems from conventional programs. I have been a long time user of GSAS and appreciate the stability of Topas, but one must be careful to make sure the parameters make sense at the end of the day. Only having one or two variables in the profile shape is a constraint in itself, additionally contributing to a stable refinement in complex systems. Pam At 15:11 04.06.2004 +0200, you wrote: > >>Is the fundamental parameter approach better than >>mathematical approach used in most of the Rietveld >>refinement programs? >> >Perhaps someone is about to explain the difference is between >"fundamental parameters" and anything else? I used to think it might >mean convoluting something which was actually measured into the >peakshape description, but this doesn't always seem to be the case? I'm >guessing it has to be more than choosing suitable equations for peak >width parameters and peak positions as a function of scattering >variable, otherwise all programs are using fundamental parameters >already, just some are better approximations for certain diffractometers >than others. > >In any case, if the calculated peakshape matches the observed peakshape >then it makes no difference for refinement of a crystal structure. For >deriving microstructural parameters, like "size" and "strain", then a >better description of the instrument can help, and can be a good >indicator of diffractometer misalignment. In that sense, zero shift is a >fundamental parameter, but does not seem to be unique to "fundamental >parameters" programs. Perhaps the difference is that programs which >don't do "fundamental parameters" make you compute "size" and "strain" >from the peakshape parameters yourself. > >Jon > Dr. R. Kleeberg TU Bergakademie Freiberg Institut f�r Mineralogie Brennhausgasse 14 D-09596 Freiberg Germany Tel. +49 (0) 3731-39-3244 Fax. +49 (0) 3731-39-3129
