The determination of anharmonic probability densities from static and dynamic disorder by neutron powder diffraction

Atomic probability densities as derived from a careful analysis of the Debye-Waller factors contain important information about the average static and dynamic disorder in crystals. Although the inclusion of anharmonic terms often requires single crystal data, in favourable cases, in particular at high temperatures, it is possible or even necessary to rely on neutron powder experiments too. This is illustrated by several examples including the anharmonic motion of the atoms in ZnS, the anionic diffusion and the static disorder in zirconia based super-ionic conductors, and the critical behaviour at the structural phase transition in PbTiO3. Anharmonic and split models are compared on the example of Zr-Mg-O-N.