In-crystal polarizabilities of alkali and halide ions

Ab initio values for the polarizabilities of the ${\mathrm{F}}^{-}$, ${\mathrm{Cl}}^{-}$, ${\mathrm{H}}^{-}$, ${\mathrm{Li}}^{+}$, ${\mathrm{Na}}^{+}$, and ${\mathrm{K}}^{+}$ ions have been calculated, both for the isolated ions and in the crystalline environment appropriate to alkali halide crystals. The in-crystal values agree well with those deduced from refractive-index data. They are similar to the isolated ion values for the cations, but much smaller for the anions. The origins of this anion effect are resolved by the separate consideration of the influence of the electrostatic crystal field and of the overlap with the six nearest cation neighbors. The consequences of the effect on the transferability of in-crystal polarizabilities between different substances are discussed.