THE 1ST-ORDER AND 2ND-ORDER STRAIN DERIVATIVES OF ELECTRONIC DIELECTRIC-CONSTANTS IN ALKALI-HALIDES

An analysis of the first- and second-order strain derivatives of the electronic dielectric constants of alkali halide crystals is performed in terms of a bond-polarizability approach and Penns' model. The derivatives V(depsilon(infinity)/dV) and V2(d2epsilon(infinity)/dV2) are related to the second- and third-order nonlinear polarizabilities of crystal bonds. V2(d2epsilon(infinity)/dV2) is considered in connection with the quadratic electro-optic effect. Results obtained suggest small changes in the third-order polarizability of the crystal bonds in different crystals. The assumption of the approximate constancy of the third-order polarizability provides a simple link of nonlinear phenomena with the crystal structure.