In-crystal ionic polarizabilities derived by combining experimental and <i>ab initio</i> results

Abstract

Abstract It is shown that an empirical model of in-crystal polarizabilities must incorporate the environmentally induced variation of anion polarizabilities. Those of s2 and p6 cations can be taken to be constant. Such a model is combined with the ab initio calculation of small cation polarizabilities to extract the polarizabilities of Rb+, Cs+, Tl+, Sr2+, Ba2+ and Pb2+ from experimental refractive index data. The deficiencies of previous estimates are discussed. The function describing the environmental dependence of halide polarizabilities is found to be transferable between different crystal structures. This does not hold for chalcogenide anions. It is shown that the polarizabilities of d10 cations are very sensitive to environment. The anisotropy of the purely electrostatic environment 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.