> Two very good points by Armel: >>all the very good PDF studies ...are made by using synchrotron data or >>neutron data from spallation sources
And he is modest as well :-) But do you really have the resolution even on HRPD to see the diffuse scattering between Bragg peaks at high Q ? You may get better temperature factors with high-Q PDF refinement, but you will also do that with high-Q Rietveld. I also doubt that just because PDF uses data between the Bragg peaks, then it must be superior for seeing details not centered on atoms in real space in a crystal, eg the split atom sites in (In/Ga)As). You might do just as well with Bragg scattering if you use the result of Rietveld refinement to construct a Fourier map of the structure. Happily, a sampling of reciprocal space (Bragg peaks) is sufficient to re-construct the entire density of a periodic structure in real space, not just point atoms, to a resolution limited only by Q. > What I am saying is that if this > diffuse scattering is inelastic, then PDF will reflect istantaneous > correlations in a way that is missed by Bragg scattering. But you do agree that in a PDF experiment you integrate over energy, so you only see an instantaneous snapshot of the structure, ie the spatial correlations. If it is a periodic structure, you also average over unit cells, even though the spatial correlations are different from one cell to the next. (BTW, talk of inelastic scattering raises the question of different inelastic cross-sections for the very different neutron energies used on your TOF machine). So while I am convinced of the interest of PDF for non-crystalline materials, with short or intermediate range order, I am not yet convinced that you gain much from PDF refinement of crystalline materials, where you can also apply Rietveld refinement. A Rietveld refinement probably has fewer correlations between parameters, and can be used to construct a Fourier map of the complete scattering density, while PDF is only a Patterson map of the pair correlations, and a lot more difficult to interpret. Alan.
