Hi Jon and Pamela, my very personal opinion regarding the "fundamental parameter" stuff: - fundamental is that the observed peak shape is a folding of contributions from (i) wavelength distribution, (ii) instrumental/geometrical aberations and (iii) microstructure of the sample. From this point of view all programs keeping constant (i) and (ii) and refining only a model for (iii) do use a FP approach. - The "fundamental parameters" in profile modelling could be defined as the "sample independent" or "fixed" contributions to the profile shape, for example the wavelength distribution function, slit widths/divergencies, distances, sample dimensions... So a program that uses such values for the calculation of (i) and especially (ii) could be called "FPA program in sensu strictu". This is true for Topas if the "full axial model" is chosen and for BGMN if the "raytracing simulation" is used. - each "fundamental parameter approach" contains, of course, a model, (sometimes an oversimplificated one). - 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 :-) - The main advantage of the fundamental parameter approach (in comparison to "learnt profiles") is that no standard measurement is required and that the whole angular range can be described without extrapolations. - Jon, I agree completely: any model what is able to describe adequately the profile shapes caused by the device INDEPENDENTLY of the microstructural broadening is "a priori" the better choice than a together fitting of any dubious parameters of physically meaningless analytical functions. - 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...
Reinhard 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 für Mineralogie Brennhausgasse 14 D-09596 Freiberg Germany Tel. +49 (0) 3731-39-3244 Fax. +49 (0) 3731-39-3129
