Correlation between Electronic Polarizabilities of Ions and Ratio of Their Radii in Alkali Halides

Abstract

Optical properties of ionic crystals are often described in terms of an approach based on the additivity rule. The approach assumes that the electronic polarizability assigned to each ion remains unchanged in all crystals containing this ion and that the electronic polarizability of the crystal is the sum of the polarizabilities of individual ions. If a crystal possesses the cubic symmetry, the optical susceptibility is related to polarizabilities of its constituent ions by the Clausius-Mossotti relation. The concept of additivity is very useful and there exist several sets of additive polarizabilities e. g. /1 to 3 1 . However, the additivity rule, which implies that the polarizability of a given ion remains constant in different crystals holds only in an approximate manner, even within the same crystal family. Theoretical values of electronic polarizabilities obtained employing different theoretical approaches predict also polarizabilities which change from crystal to crystal /2, 4 to 9/.The electronic polarizabilities are of great interest as related to ionic sizes (see, for example / 4 , lo/). Analogously to ionic polarizabilities, the additivity rule of ionic radii is often taken into consideration. However, as in the case of additive ionic polarizabilities one can observe the failure of this approach. Studies of X-ray determined electron density maps proved that within the alkali halides the radii change from crystal to crystal (see, for example values listed in /11/). Theoretical values of ionic radii obtained for alkali halides are also individual crystal dependent /12, 131.The aim of this note is to report the correlation between the polarizability ratio a / a where ac and aa are the polarizabilities of cation and anion, respectively, and the ratio of cation to anion radii rc/ra. It is usually assumed that the polarizability of cations is essentially unaffected by its environment while the polarizability of an anion Varies considerably as it is paired with different cations. Therefore, to derive aa it seems reasonable to subtract ac from the total molar polarizability . Recently, Johnson and Subbaswamy have computed polarizabilities of alkali halides in a local-density approximation scheme 181. The calculations have c a' ) U1. W6lczafiska 219, PL-93-005 Lodz, Poland. ') This work was carried out under Research Project CPBP 01.06,6.04

A study on the influence of the cations and anions on the electrical conductivity in thin films of potassium and rubidium halides

“…Table l presents values of electronic polarizabilities and effective charge parameter e*/e of the salts introduced by Szigeti (1949, 1950) for the potassium and rubidium halides. Kucharczyk (1990) has recently reported a correlation between the electronic polarizabilities of ions and their radii, in which the rubidium salts were not included. The Eorrelation follows a power law.…”