Li₆Lu(BO₃)₃: A Rare-Earth Borate Crystal Exhibiting Large Birefringence and Broad UV Transparency

Birefringent crystals have become increasingly appealing for versatile polarization devices owing to their ability to modulate the polarized light. However, the efficient synthesis of material with sufficient birefringent value and wide transparency range is still a significant challenge. Herein, a new rare-earth borate birefringent crystal, Li₆Lu(BO₃)₃ (LLBO), was successfully synthesized and comprehensively characterized. LLBO crystallizes in the monoclinic system with space group P2₁/c, featuring a robust three-dimensional framework constructed from distorted [LuO₆] octahedra and planar [BO₃] triangles. LLBO exhibits excellent thermal stability up to 1200 °C. Notably, LLBO displays a remarkably short ultraviolet (UV) cutoff edge at 204 nm supplemented by a wide optical band gap of 5.48 eV, demonstrating its excellent transparency in the near-UV regions. The experimentally measured birefringence (Δn = 0.057) displays a good agreement with the theoretical value (Δn = 0.056@1064 nm), confirming that LLBO exhibits pronounced optical anisotropy. The optical characteristics of LLBO derived from theoretical calculations are mainly reliant on the essential structural groups [LuO₆] polyhedra and [BO₃] triangles. Above results reveal the potential application of LLBO for light polarization modulation and provide novel insights for fabricating short-wave ultraviolet birefringent materials.