In-crystal ionic polarizabilities derived by combining experimental and ab initio results

I t is shown th a t an empirical model of in-crystal polarizabilities m ust incorporate the environm entally induced variation of anion polarizabili­ ties. Those of s1 2 *and p6 *cations can be taken to be constant. Such a model is combined with the ab initio calculation of small cation polarizabilities to ex trac t the polarizabilities of R b +, Cs+, T l+, Sr2+, B a2+ and P b 2+ from experim ental refractive index data . The deficiencies of previous estim ates are discussed. The function describing the environm ental dependence of halide polarizabilities is found to be transferable between different crystal structures. This does not hold for chalcogenide anions. I t is shown th a t the polarizabilities of d 10 cations are very sensitive to environm ent. The anisotropy of the purely electrostatic environm ent increases the polarizability of Ag+ in AgF by 4 % while model calculations suggest a much larger increase from anisotropic overlap effects. A comparison of the relative merits of an ionic polarizability and a charge-transfer picture of polarization in ionic solids is found to favour the former description.