Bob,
This exactly what is needed when the sample is a mixture of amorphous and crystalline components. But what happens when the material is a single crystalline phase with some coherent defects? Don't the defect <-> average structure correlations start to dominate, and separating components is no longer possible...? Thinking of all the different kinds of rods and streaks you can see from a single crystal and then projecting them into one dimension gives a lot of possibilities! The difficulty is finding a way to describe all of the possible kinds of defect so that either the PDF or equally the diffraction pattern can be computed. Probably one would have to follow the route in PDFFit of giving the user a Turing complete command language to use. I'm just glad that the proteins don't seem have coherent defects! Can you fit that broad bump in the second half of the pattern which always seems to turn up in protein data via the Debye equations? I have to admit I haven't tried yet...
See you in Prague,
Jon
Von Dreele, Robert B. wrote:
Jon & others,
Well, there is an attempt at this in GSAS - the "diffuse scattering" functions for fitting these contributions separate from the "background" functions. These things have three forms related to the Debye equations formulated for glasses. The possibly neat thing about them is that they separate the diffuse scattering component from the Bragg component unlike PDF analysis. As a test of them I can fit neutron TOF "diffraction" data from fused silica quite nicely. I'm sure others have tried them - we all might want to hear about their experience.
Bob Von Dreele
