Dear all, so we are having a philosophical discussion - after being distracted all day it is evening, my users seem happy and I have some time to add my 5c worth to the discussion. For those who do not know me, I am running NPDF at the Lujan Center and we aim to bring PDF (neutron PDF) to the world !
(1) Rietveld vs. total scattering Although intentions might have been different and GSAS has the Debye formula implemented and provides a physical background to describe diffuse scattering, 99.999% of the cases use some background polynomial and Bragg intensities and positions are refined to an *average* structural model. Leaving profile analysis and texture aside, that is the structural picture. And although the Patterson can be extended to include non integer hkl's it usually does not and it gives a map of atom atom distances corresponding to the average structure. PDF Fourier transforms it all so you get an extended picture of the local-, medium range and long range atom correlations. Philosophy is nice - but there are many confused ideas about PDF and Rietveld and Bragg vs. diffuse scattering, that I think it helps to keep this distinction clear ! (2) Total scattering Now we are looking at all of it Bragg and diffuse scattering. Basically you can analyze in Q or r space - should be the same, except for the weights as pointed out by someone earlier. Some of the RMC studies refine data in Q and r simultaneously and more recently add the Bragg intensities as a constraint as well (using RMCprofile - www.isis.rl.ac.uk/rmc). So if you have a disordered system (disordered as in deviations from the average structure) Rietveld along will not give you the answer. Strange large adp's or their behavior as function of temperature will give you a clue as to 'there is something going on' but that is it. (3) Refining S(Q) vs. G(r) First on synchrotron beamlines or at spallation neutron sources where one can access high Q (> 40 A**-1), termination ripples from the Fourier transform are insignificant - at least in comparison to other systematic errors and noise. So the worry about truncation is many times unwarranted. Now one very nice thing about working in G(r) is that it is intuitive and in real space. The Si-O example was mentioned earlier. Many times one can get results from simple peak fitting to near neighbor PDF peaks - or study the width as function of temperatures (as beautifully demonstrated in Simon Billinge's 1996 PRL). More recently people exploit that one can restrict the refinement range and look at local geometries (e.g. Jahn-Teller distortions) and see where the cross over to the average behavior is. So it's cool, it's becoming easier - try your own PDF experiment ;-) Disclaimer: I am going to hit send without reading what I wrote. Thomas -- Thomas Proffen NPDF instrument scientist Lujan Neutron Scattering Center Los Alamos National Laboratory http://www/lansce.lanl.gov/
