Thanks all for the PDF explanations I think I'm beginning to understand. To summarise what we know:
- PDF for powder data is a Patterson function (as Alan Hewat stated) in one dimension that plots the histogram of atom separation - It will show quite nicely the short range order and then possible disorder longer range (eg. Rotated bucky balls) There seems to be a few issues: 1) PDF refinement on long range ordered crystals is the same as Rietveld refinement and little benefit if any is obtained by fitting to G(r) directly. It's useful however to view a PDF to ascertain whether there's disorder even in the event of a good Rietveld fit. 2) A nano-sized crystal, as Vincent mentioned, can be modelled by arranging the atoms such that it's G(r) matches the observed PDF. This can be done with disregard to lattice parameters and periodicity in general. 3) a mix of (1) and (2) as in regularly arranged bucky balls but with the balls randomly rotated. The ability to do number (2) is what can't be done with normal Rietveld refinement as a disordered object has no periodicity and no Bragg peaks. Arranging atoms in space to form an object with disregard to lattice parameters and space groups can yield a G(r) to match an observed PDF. However even very small crystals would have 10s of thousands of atoms. Its seems difficult to try and determine atomic positions of such an object from a PDF. It may be worthwhile to look at what the glass community has done (the Debye Formula - thanks Jon - still reading). The main point that is still confusing me is whether current PDF analysis considers an object (or should) or is it the case that most materials are periodic (with an enlarged unit cell) with parts of the cell being disordered. Cheers Alan
