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

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).