Ions in Crystals:  The Topology of the Electron Density in Ionic Materials. III. Geometry and Ionic Radii

We present the application of Bader's topological analysis of the electron density to geometric properties in ionic materials. Particular attention is paid to the concept of ionic radius, in relation to the shapes of ions in crystals, and to the various correlations among atomic properties, i.e., electronegativities, deformabilities, etc., that it induces. Using a simple model to fit our results to a theoretical frame, we show that ionic bonds display properties in complete parallelism to those known in covalent bonds. This allows us to define unambiguosly the strength of an ionic bond, which is found to correspond to Pauling's bond valence.