Li_0.5Na_0.5AlB₂O₄F₂: 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_∞[BO₂] chains driven by AlO_mF_n units is proposed for designing DUV birefringent materials, and a new fluoroaluminoborate, Li_0.5Na_0.5AlB₂O₄F₂, with aligned ¹_∞[BO₂] chains and an unprecedented [AlO₃F₃]⁶⁻ motif is synthesized. The birefringence of Li_0.5Na_0.5AlB₂O₄F₂ achieves significant enhancement compared with maternal structure LiAlB₂O₅ 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.