Dynamic processes and disorder in crystal structures as seen by temperature-dependent diffraction experiments

Single crystal diffraction experiments with X-rays or neutrons routinely provide average atomic positions and atomic mean square displacement parameters (ADPs). Atomic positions are easily converted to interatomic distances, angles, conformational descriptors and other quantities of interest in structural science. ADPs which encode information on atomic motion and disorder are more difficult to decipher. This is because the interatomic or correlation ADPs, which describe the coupling of atomic displacements, are lost in Bragg diffraction. The missing information can be retrieved, however, from the temperature dependence of ADPs. Measurements in the (low temperature) quantum and the (high temperature) classical regimes provide the relative and absolute atomic displacements associated with low frequency molecular vibrations on the one hand, and the contributions from high frequency vibrations and positional disorder on the other. Diffraction as a function of temperature is often a simpler alternative to vibrational spectroscopy and diffuse scattering studies. It is a potentially rich source of information on dynamic processes and disorder phenomena in crystals. The information is relatively easy to obtain and extends the scope of crystal structure analysis. (C) 1999 Elsevier Science B.V. All rights reserved.