NaK 5 La 2 (SO 4 ) 6 : Enhanced Birefringence of Multiple-Alkali Metal Sulfate Systems via Rare Earth Metal-Centered Polyhedra

Polarization modulation of ultraviolet (UV) birefringent crystals is crucial for various applications. Here, we introduce distorted La-O polyhedra into alkali metal sulfates to synthesize a novel birefringent material with excellent UV transmission and birefringence. The incorporation of distorted La-O polyhedra significantly increases the birefringence to 0.0255 at 550 nm, surpassing that of many alkali metal sulfates while maintaining excellent UV transparency. The material exhibits excellent thermal stability up to 450 °C. Theoretical calculations show the connection between the crystal structure and optical functionality, confirming that the incorporation of La-O polyhedra enhances birefringence. This research provides novel insights into the discovery and design of outstanding birefringence materials.