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

Abstract

ABSTRACT 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, ATeF 5 (A = K, Rb, Cs, NH 4 ) featuring unique [TeF 5 ] − unit, exhibit significantly blue‐shifted cut‐off edges and ultrawide bandgaps exceeding 5 eV, indicating excellent solar‐blind UV transparency. Notably, NH 4 TeF 5 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 [TeF 5 ] − 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.