The Pair Distribution Function (PDF) is obtained from neutron or x-ray Total Scattering which in turn is obtained from a careful diffraction experiment designed to measure the Bragg and diffuse scattering from a powdered sample quantitatively. The PDF gives the probability of finding one atom a given distance away from another; peak positions therefore correspond to atom-atom mean separations and peak areas to atom-atom coordinations. Data may be analysed using 'small box methods', sometimes termed 'real-space Rietveld', or 'big box' methods such as reverse Monte Carlo (RMC) methods. In the latter, an ensemble of atoms are moved iteratively until good agrement is obtained with the total scattering data (in real and/or reciprocal space) and it is especially powerful for cases of substantial disorder, including amorphous materials. Further details are given here.
The three major differences between PDF and XAFS methods are
(a) the PDF is not atom specific. Although it is possible to use advanced techniques to effect some separation between contributions from different atom types, the PDF is a summation over all atom-atom correlations.
(b) the PDF is quantitative. Data corrections, particularly for neutron PDF, are robust and the PDF provides an unambiguous function for further study.
(c) the PDF describes atom-atom correlations over short (nearest neighbour) to medium (up to 10's nm) length scales.