Introduction of the [B-O/F] Units Enhances the Band Gap and Birefringence from Na6Mg3B10O18F6 to K3NaB10O16F2

Abstract

The borate family is the main source of deep-ultraviolet (DUV) birefringent crystals, and it has attracted a lot of attention due to versatile [B-O] basic units. Herein, two new borate-based compounds Na6Mg3B10O18F6 and K3NaB10O16F2 were discovered. Their fundamental building blocks are [B5O11] and [B5O10F] units, respectively. The calculated results showed that the band gap and birefringence of K3NaB10O16F2 (Eg = 6.93 eV, Δn = 0.047 at 1064 nm) are greater than those of Na6Mg3B10O18F6 (Eg = 5.40 eV, Δn = 0.039 at 1064 nm). Furthermore, the effects of [B-O/F] units on band gap and birefringence were analyzed by the charge-transfer model and response electron distribution anisotropy method. The results show that introducing the [B-O/F] units can improve the band gap and birefringence. These findings will boost the exploration of DUV birefringent opticals.