A User's Guide to Polarisabilities and Dispersion Coefficients for Ions in Crystals

Abstract

The definition, calculation, modelling and physical interpretation of static polarisabilities and dispersion coefficients for ions in crystals are reviewed. Ab initio calculations on clusters of ions embedded in point-charge lattices show that anions are sensitive to electrostatic and overlap interactions which reduce their polarisabilities from the free-ion values, sometimes by factors of two or more. By contrast, cations of s ² and p ⁶ configuration have almost constant polarisability. Models of optical and mechanical properties of ionic solids must take into account the variation of anion polarisability with lattice parameter. A semi-quantitative treatment of dispersion coefficients can be based on the Slater-Kirkwood formula which requires only the static polarisability and one adjustable parameter (fixed by reference to the isoelectronic inert gas).

Key Words:

• Polarisability
• dispersion energy
• ion
• crystal
• Ab initio