Fluctuating dipoles and polarizabilities in ionic materials: Calculations on LiF

The physical mechanisms responsible for the fluctuating polarization which causes light scattering and infrared absorption in ionic materials are investigated by electronic structure calculations on distorted crystal lattices. The lattice is modeled at two levels which permit the separation of the effects of electrostatic and first-shell overlap interactions. The distortion-induced polarization may be regarded as the sum of an asymptotic term, which includes the electrostatically induced moments described by a multipole expansion, and the result of a change in shape of the confining potential well in which an ion sits. The latter is significant only for short-range distortions. The confining well is caused by both electrostatic and overlap interactions, and for first-shell distortions the two act in concert. Both the asymptotic and confining potential effects may be built into a computationally tractable model for the fluctuating polarization of crystalline LiF; extended schemes for more disordered situations, such as the melt, are considered.