Adding 2 cents to the discussion...
But I will try to convince myself otherwise :-)
Another reason which may preclude your self-convincing is the fact that all the very good PDF studies of materials that are not perfectly crystallized (producing diffuse scattering), though not being amorphous, are made by using synchrotron data or neutron data from spallation sources : not by using constant wavelength neutrons...
In the past, I have studied a few glasses at ILL by using the D4 instrument, at 0.5 A wavelength, allowing to attain modestly high Q values. The fact that the instrument resolution was very poor was not a problem for amorphous materials, but it was a problem for crystalline or partially crystalline materials : they also look like amorphous in the reciprocal space, due to the quite large instrumental contribution to the peak broadening. Is that improved now ? If not, you would not be able to apply both the Rietveld and the PDF methods from the same data, raw or Fourier transformed. The raw data would be too bad for applying the Rietveld method. So, no PDF at ILL ?-).
A different question is about size/strain effects, possibly anisotropic, which may have effects on the peak shapes and peaks broadening. We can more or less (progress are to be made), take account of these effects reflecting the deviation of the real sample from a model of infinite perfectly periodical structure. All that peak shape information is lost in the PDF... So, both PDF and Rietveld approaches may appear necessary and complementary, sometimes - for ill-crystallized compounds. However, is it really necessary to see on the PDF the difference between the Si-O and Al-O distances in respectively SiO4 and AlO4 ? This is already a well established fact (the same for any statistical substitution of atoms with different radii like In and Ga in (In/Ga)As). So, this PDF advantage does not impress me a lot (like opening an already open door), EXAFS reveals the same. Apart from being able to show such differences, I expect more from the PDF approach, but the more an ill-crystallized sample is close to be amorphous, and the more the structure hypothesis will be dubious...
Armel
