The only truly unique PDF information is about *correlations*. Let's say you have two bonded sites, both with anisotropic thermal ellipsoids along the bond, and let's assume that the motion is purely harmonic. A sharp PDF peak will indicate that the atoms move predominanly in-phase, a broad PDF peak that the atoms move predominantly out-of-phase. The two scenarios will give identical Fourier maps as reconstructed from the Bragg peaks, whatever the Qmax, so the additional width (or additional narrowness) of the peak with respect to an uncorrelated model arises purely from the non-Bragg scattering. You can make the same argument for static correlations. Ga(1-x)In(x)As is a typical case. It is not a split-site problem, in that both Ga/In and As will be slightly displaced locally depending on their surrounding, but the displacements are correlated in such a way as to give a shorter bond length for Ga-As and a longer one for In-As. Of course, PDF is also used to look at more general issues of static/dynamic disorder that could also be examined using Bragg scattering, and in many case it does quite well. PDF is not (yet) very good for structural refinements (so it is to be used only in desperate cases of highly disordered systems) and is pretty hopeless for weak ordered displacement patters, since the extra Bragg peaks in crystallography "lock-in" on the new modulation even in the presence of large unrelated displacements. For this very reason, PDF tends to miss phase transitions, particularly at higher temperatures, which led to some very wierd claims in the past literature. There is a lot of controversy about PDF being able to say something about weak disordered displacement patters (e.g., dynamic stripes), but I am personally very skeptical. PDF requires exquisite data and a true passion for data analysis. If you have a good problem, you can get (probably) the best PDF data worldwide "almost" routinely on my instrument GEM at the ISIS facility (see also the cited paper by Billinge).
Paolo Radaelli
