The atomic arrangement in the liquid and amorphous state represents a compromise between the long range order characteristic of crystalline solids and the total disorder observed in gases. The description of the atomic distribution in non-crystalline materials frequently employs the concept of the distribution function. Since the pair distribution function is obtained from diffraction data (Neutron, X-ray and electron diffraction) this experimental information plays a significant role in the discussion of the structure and properties of non-crystalline materials.

Typical experimental arrangements for neutron and X-ray scattering using synchrotron radiation are shown. A short introduction into the diffraction theory of multicomponent systems in terms of structure factor and pair distribution function will be given. We will discuss similarities and differences between neutron and X-ray diffraction and find out their relative merits and limitations.

However, diffraction methods suffer from the fact that each diffraction experiment averages the scattering contributions of the different atom types in the sample. To a certain extend XAFS-methods can overcome this disadvantage. In particular EXAFS is able to have a detailed look at the first neighbour sphere of a certain atom type chosen from the experimentator.

By combination of three independent diffraction experiments (isotopic substitution in neutron diffraction) partial structure factors and partial pair correlation functions can be extracted for two-component samples. In the case of ternary systems reverse Monte Carlo modelling by simultaneous fitting of at least three experiments may be a very helpful tool to estimate partial functions.