Elasto-optic properties of K2SO4 crystals

The elastic stiffness coefficients of K2SO4 crystals at room temperature were calculated. The coefficient C66 is the smallest among the diagonal components, indicating a “soft” shear mode in the XY plane. The Born stability criteria were evaluated, and their fulfillment confirms the thermodynamic stability of the crystal. The Cauchy relations were also analyzed. The spatial distributions of Young’s modulus and Poisson’s ratio of K2SO4 crystals were constructed. The Young’s modulus surface is close to spherical in shape (a “rounded parallelepiped”) with pronounced protrusions along the [010] and [001] crystallographic directions. The values of Poisson’s ratio (within 0.23–0.27) are typical for ionic-covalent crystals of intermediate stiffness, indicating moderate transverse deformation. The elasto-optic parameters of K2SO4 crystals were analyzed. The baric (pressure-induced) changes in the refractive indices were calculated. It is shown that the temperature variation of the refractive index (dni/dT) in these crystals is governed by a complex interplay between volumetric and structural factors. The contributions to the temperature-induced refractive index changes arising from variations in atomic polarizability (≈ 32%) and crystal density (≈ 68%) were estimated.