ATOMIC DISPLACEMENT PARAMETER NOMENCLATURE - REPORT OF A SUBCOMMITTEEON ATOMIC DISPLACEMENT PARAMETER NOMENCLATURE

Modern X-ray and neutron diffraction techniques can give precise param eters that describe dynamic or static displacements of atoms in crysta ls. However, confusing and inconsistent terms and symbols for these qu antities occur in the crystallographic literature. This report discuss es various forms of these quantities, derived from probability density functions and based on Bragg diffraction data, both when the Gaussian approximation is appropriate and when it is not. The focus is especia lly on individual atomic anisotropic displacement parameters (ADPs), w hich may represent atomic motion and possible static displacive disord er. The first of the four sections gives background information, inclu ding definitions. The second concerns the kinds of parameter describin g atomic displacements that have most often been used in crystal struc ture analysis and hence are most commonly found in the literature on t he subject. It includes a discussion of graphical representations of t he Gaussian mean-square displacement matrix. The third section conside rs the expressions used when the Gaussian approximation is not adequat e. The final section gives recommendations for symbols and nomenclatur e.