Li2AB5O8F2 (A = Na and K): two new deep-ultraviolet transparent fluorooxoborates with moderate birefringence designed by targeted dimensional modulation

Two mixed alkali-metal fluorooxoborates, Li2KB5O8F2 and Li2NaB5O8F2, were designed and synthesized by using the strategy of targeted dimensional modulation through adjusting component ratio in the sealed system. Li2KB5O8F2 exhibits a short ultraviolet cutoff edge (<196 nm), possessing moderate birefringence (about 0.05@1064 nm) which was mainly contributed by the [B5O8F2]∞2 wave layer. Further, the investigation on the relationship among (AI + F)/B (AI: alkali-metal cations) ratio and dimensionality of the anionic framework as well as birefringence has been conducted in available anhydrous alkali-metal fluorooxoborates for providing an idea in the synthesis of fluorooxoborates with desirable performances.