Electronegativity Adaption Approach for Solar-Blind UV Birefringent Crystals With Ultrawide Bandgaps

Birefringent crystals are essential for modern optics for their ability to modulate light polarization. The steretochemically active lone-pair (SCALP) effect can typically confer high birefringence for materials by driving anisotropic distribution of an electronic clouds. However, a major limitation of SCALP-based birefringent crystals is the obvious red-shift of UV transparency window. Herein, we propose an electronegativity-adaption approach to widen the UV transparency of lone-pair birefringent crystals. We show that ternary tellurite fluorides, ATeF₅ (A = K, Rb, Cs, NH₄) featuring unique [TeF₅]^- unit, exhibit significantly blue-shifted cut-off edges and ultrawide bandgaps exceeding 5 eV, indicating excellent solar-blind UV transparency. Notably, NH₄TeF₅ achieves a very short UV cut-off edge of 218 nm (corresponding to bandgap of 5.69 eV) and a large birefringence value of 0.130@550 nm, making it the SCALP-based material with the largest bandgap among those with birefringence over 0.1. Electronic structures analysis reveals that the adaptable electronegativity between F ligands and the Te^IV center lowers the HOMO of [TeF₅]^- more rapidly than its LUMO energy compared to other SCALP-based units, thereby widening the bandgaps of resulting compounds. This work provides valuable insights for bandgap engineering in birefringent materials and opens up new avenues for designing solar-blind UV optical crystals with SCALP motifs.