Li0.5Na0.5AlB2O4F2: Fluoroaluminoborate with Aligned 1∞[BO2] Chain Induced by Unprecedented [AlO3F3]6− Species Features Enhanced Birefringence

Abstract Birefringent materials have been used extensively owing to their indispensable role in creating and modulating polarized light, while birefringent materials for deep ultraviolet (DUV, λ < 200 nm) applications are still urgently needed. Herein, a strategy to induce the formation of birefringence‐active 1 ∞ [BO 2 ] chains driven by AlO m F n units is proposed for designing DUV birefringent materials, and a new fluoroaluminoborate, Li 0.5 Na 0.5 AlB 2 O 4 F 2 , with aligned 1 ∞ [BO 2 ] chains and an unprecedented [AlO 3 F 3 ] 6− motif is synthesized. The birefringence of Li 0.5 Na 0.5 AlB 2 O 4 F 2 achieves significant enhancement compared with maternal structure LiAlB 2 O 5 due to the optimization of functional units and arrangement via co‐substitution and fluorination tactics. The design strategy provides a propitious and feasible way to construct novel structures with excellent optical properties.