Anisotropic bond polarizabilities in birefringent crystals

Based on the concept of bond polarizabilities the optical properties of a crystal are traced back to the orientation of its bonds and to the individual bond parameters β and Δβ. A test with various sulfate compounds shows that the average bond parameters β describe the average refractive indices with a higher accuracy than the traditional model of ionic polarizabilities. The anisotropic part of a bond polarizability, Δβ, is considered to be the origin of optical birefringence. Values for Δβ are evaluated in MCO3 and MSO4 (M = Mg, Ca, Sr, Ba). In different crystals the same values for Δβ have been discovered for the C-O and the S-O bonds. Δβ of the M-O bonds depends in a characteristic way on the bond length. A simple method is used to calculate local field effects in accordance with the model of bond polarizabilities. Possible applications of bond polarizabilities for the determination of structural quantities are discussed. (orig.)