Relationships among optical and structural characteristics of ABSO4 crystals

Abstract We systematize the data for refractive indices and optical birefringences of ABSO4 crystals (A, B = Li, Na, K, NH4, Rb, Cs) and analyze their interrelations with structural and energy-band parameters. Besides of a number of empirical relationships that reveal low correlation coefficients and cannot be used for predictions, we find nearly functional links of the average refractive index n with the average lattice-cell parameter and the ionic radius of alkali cation for most of the ABSO4 crystals. The refractive index n correlates perfectly with the combinations of crystal density and average atomic mass that follow from Gladstone–Dale, Lorentz and Lorentz–Lorenz formulae. Moreover, we obtain a fair correlation of n with the unit-cell volume per one sulphur atom, which is associated with chemical-bond ionicity. We discuss also the links of the refractive index and the birefringence with the bandgap and its anisotropy, which follow from the Moss and Ravindra–Gupta formulae. Resources for ascertaining the birefringence size and availability of a zero-birefringence point in the ABSO4 crystals are poorer, when compared with the case of refractive indices. Our results suggest some physical and practical means for predicting basic optical properties of new materials.