IONIC SIZES AND STRAIN DERIVATIVE OF ELECTRONIC SUSCEPTIBILITY IN ALKALI-HALIDES

An analysis of the electronic susceptibility of alkali halides related to ionic sizes is performed. The model based on the decomposition of the optical susceptibility into contributions of valence and core electrons is applied to obtain ionic radii, photoelastic constants p11 + 2p12, the strain derivative of electronic dieletric constants, and changes in the refractive index with density and strain polarizability constants. An attempt is made to predict p11 and p12 in some alkali halides. The relationship between the ratio of electronic polarizabilities of alkali and halide ions and the ratio of cation to anion radii r(c)/r(a) is observed. The photoelastic and strain polarizability constants are found to be nearly linearly dependent on r(c)/r(a). Correlations between crystal ionicity and effective ionic charge and their dependences on r(c)/r(a) are discussed. The approach relates photoelastic constants to the volume dependence of the effective ionic charge. The contribution of second-neighbour interactions is considered and the failure of the Cauchy relation is correlated with r(c)/r(a).