Polarizability of the iodide ion in crystal

The electronic dipole polarizability of the iodide ion in solid cubic NaI and KI is derived from ab initio electronic structure computations as function of closest cation-anion separation for the four-coordinated zinc blende structure, the 6-fold coordinated rock salt structure, and the 8-fold coordinated phase having the CsCl structure. The contributions from electron correlation were computed using Moller-Plessett perturbation theory taken to the second order. For both NaI and KI, the anion polarizability predicted for the observed rock-salt polymorph at the equilibrium separation agrees well with the value deduced from experiment. The anion polarizabilities increase with both decreasing coordination number at constant distance and with increasing distance at constant coordination number. Both the polarizabilities and their contributions from electron correlation are suppressed in-crystal relative to those computed for the free iodide ion. The ab initio predictions for the derivatives of the anion polarizabilities with respect to closest cation-anion separation are slightly smaller than those deduced from the experimental observations. Reviews of both the latter and the dipole polarizability derivatives previously computed for other alkali halides are presented. The distance dependence of the polarizability of the iodide ion in the point charge representation of the rock-salt lattice, computed ab initio, differs in shape slightly but significantly from that for either NaI or KI. This difference prevents the in-crystal polarizabilities and their distance derivatives from being predicted by shifting the point lattice polarizability function as previously shown to be possible for the salts of the lighter halides. The applicability of the light scattering model description of anion polarizabilities in condensed phases is investigated.