Inducing large birefringence by enhancing asymmetric electron distribution of Y–O polyhedra

Abstract

Birefringent crystals, which can modulate the polarization of light, are particularly important in optical communication and the laser industry. Here we report the synthesis, structures, optical properties, and theoretical analyses of four rare-earth borates containing BO₃ groups, i.e. LiRb₂YB₂O₆, CaRbYB₂O₆, and Li₂MY₄B₅O₁₅ (M = Rb, Cs). Their structures are not all typical BO₃ group distributions with large birefringence values, e.g., being parallel with each other's distribution or being a regular polygon distribution with a specific form that is parallel with an optical axis. However, their birefringence values exhibit an obvious increase from LiRb₂YB₂O₆ (0.038@532 nm) to Li₂MY₄B₅O₁₅ (M = Rb, Cs) (0.103@532 nm). The origin of this birefringence increase was studied by structural analysis and theoretical calculation. First-principles calculations and structure comparisons indicate that the non-symmetry localization of electron density of the Y–O polyhedra has an important contribution to the large birefringence of Li₂MY₄B₅O₁₅ (M = Rb, Cs).