> At the end it is all wrong as we don’t refine individual electrons...
I think we have heard enough when the argument turns to Reductio ad absurdum You might also say that Rietveld refinement by its nature is restrained because we restrain the peaks to positions that are determined by the the assumed average crystal lattice. We could otherwise, and do with PDF analysis, simply Fourier transform the diffraction pattern to obtain the pair correlation pattern, which "in the end" is the only real-space information available. As Leonid says, in some cases we might even try to determine the electron distribution. But there is a difference when we publish a structure as a "Rietveld Refinement", when in fact most of the "information" comes from restraints and not measured data. Yes if you have a complex structure and relatively poor data, you might want to restrain your model in order to say something about it. But the most you can then say is that the restrained model is consistent with the data. As Leonid says, when you restrain parameters in your model that would otherwise correlate with refined parameters (that is why you restrain them), the calculated standard deviations of the remaining refined parameters are underestimated (not "improved"). >FYI: The greater restraints to parameters ratio the better. AT the end >restraints are observations. S it is not about restraints... That was the statement that shocked me :-) and which started this unfortunate thread. As for x-ray powder refinements of heavy metal oxides, there are over 60 entries in ICSD for Mo-oxides. These oxides are often non-stoichiometric, probably due to mixed valence, have various phases or even mixed phases and even "non-commensurate" phases (actually block structures of different stoichiometry). Such materials are difficult to study with x-rays anyway, because of the heavy atoms as well as all kinds of x-ray powder experimental problems, including of course the powder averaging itself. When Armel praises such x-ray powder refinements as the most complex yet attempted, I suspect he is being a little sarcastic :-) Much as I admire Rietveld refinement, there are some problems for which it is simply not suitable. Hugo Rietveld himself thought that it would never be applied for x-rays because of the systematic errors, but of course he was wrong :-) People are happy now to apply "black box" Rietveld programs, refining all kinds of sophisticated sample-experiment corrections to improve their R-factor, and then having to add constraints to ensure that the resulting structure looks good enough to publish. OK, but when they say that the more restraints the better, some of us don't agree. Let's leave it at that. Alan ______________________________________________ Dr Alan Hewat, NeutronOptics, Grenoble, FRANCE <[email protected]> +33.476.98.41.68 http://www.NeutronOptics.com/hewat ______________________________________________
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