Influence of static dipole moments on the lattice dynamics of PbI2

We propose an extended shell model for ionic layer compounds which includes both static and dynamically induced electronic dipoles. The model is applied to PbI₂. In particular we find that the static dipoles at the iodine ions are important for the E_uA_2u splitting at the zone center, the anomalously large and anisotropic effective charges, the electronic susceptibility σ^∞, the Davydov splitting in the polytypes, and the behavior of transverse dispersion curves throughout the Brillouin zone.