New ultraviolet transparent rare-earth borates with enhanced birefringence induced by cation chemical substitution

Abstract

Birefringence determined by optical anisotropy is one of the most pivotal and fundamental performance of optical materials. However, optimizing the birefringence remains a significant challenge. Herein, a simple and effective method of cation chemical substitution for improving the birefringence has been accomplished and three Y-based borates, namely, LiNa₂Y(BO₃)₂, RbNa₂Y(BO₃)₂ and RbSrY(BO₃)₂, were successfully synthesized. They all have deep-ultraviolet (DUV) cutoff edges below 190 nm. Single-crystal analysis reveals that LiNa₂Y(BO₃)₂ and RbNa₂Y(BO₃)₂ possess three-dimensional (3D) frameworks with small channels filled by alkali metal cations, whereas RbSrY(BO₃)₂ features a two-dimensional (2D) layered structure separated by alkali metal and alkali-earth metal cations. The birefringence exhibits a progressive doubling increase from LiNa₂Y(BO₃)₂ (0.017@532 nm) to RbNa₂Y(BO₃)₂ (0.033@532 nm) and then to RbSrY(BO₃)₂ (0.070@532 nm). Using cation size arguments, coordination environment, and the arrangement of groups demonstrate that cation substitution have a decisive effect on the birefringence enhancement. In addition, other optical and thermal properties of the three title compounds were characterized. The structure–property relationships were analyzed by the first-principles calculations.