Atomic displacement parameters from diffraction studies: The experimental evidence
X-ray and neutron single crystal diffraction, now paralleled by powder diffraction techniques, is the principal tool for the experimental investigation of crystal structures. For the most part the information obtained is the volume averaged model of the atom arrangement in the crystal, so that only a small subset of the published structural studies advance beyond the determination of atomic positions and site occupancies in the crystallographic cell. However, it is well proven that the analysis of accurately measured diffraction intensities may yield important physico-chemical information, such as the electron density distribution, the vibrational behaviour of the atoms about their average positions, or the potential energy of the molecular conformation in the crystal.
The main scope of this contribution is to emphasise the importance of the experimentally derived atomic displacement parameters obtained by diffraction methods from ordered crystal structures.
A few concepts will be illustrated using cuprite-type structures as case studies:
the advantage of neutron nuclear scattering over X-ray scattering by electrons in discriminating the vibrational contribution from the charge density contribution to the apparent atomic displacement behaviour;
the use of the temperature dependence of the atomic displacement parameters in the detection of site disorder effects;
the comparison of atomic displacement parameters as extracted from diffraction and spectroscopic data;
the use of neutron single crystal diffraction in detecting the anharmonic contribution to the overall harmonic atomic vibrational behaviour.
It is argued that the accurate measurement