Anisotropy of the lattice vibrations of 2HPbI2

In this paper an explanation is given of the observed large anisotropy of the static dielectric constant of 2HPbI₂ (?_0\t] = 26.4; ?_0| = 8.7). The optical modes of 2HPbI₂ are analysed in terms of long range Coulomb interactions. The results demonstrate the rather ionic nature of this compound. The polarizabilities of the I^? and the Pb²⁺ ions obtained from the optical data are 7.2 and 4.0 × 10^?24 cm³, respectively. The (Szigeti) effective charge is about 70% of the ionic charge. The anisotropy of the optical modes is caused mainly by static dipoles induced in the iodine ions by the anisotropic charge distribution in the crystal.