Fluorination-Associated Symmetry Transformation and Band-Gap Widening in Antimony Tellurite-Based Birefringent Crystals

Balancing a wide optical band gap with large birefringence remains challenging for ultraviolet (UV) birefringent crystals. Herein, two antimony tellurite-based compounds, centrosymmetric (CS) Sb2O(TeO3)2 and noncentrosymmetric (NCS) SbTeO3F, were synthesized and compared to investigate fluorination-associated structural and optical regulation. Sb2O(TeO3)2 crystallizes in the CS space group P1̅ and features a layered framework of [SbO4], [SbO5], and [TeO3] units connected through Sb–O–Te linkages. In contrast, SbTeO3F adopts the NCS space group P212121, in which terminal Sb–F coordination accompanies changes in Sb coordination and Sb–Te–O framework connectivity. SbTeO3F exhibits improved UV transparency, shifting the absorption edge from 394 to 304 nm and increasing the experimental band gap from 3.15 to 4.08 eV. Both compounds retain large birefringence values of 0.29 and 0.30 at 546 nm, respectively. SbTeO3F also exhibits a measurable powder second-harmonic generation (SHG) response of approximately 0.6 times that of potassium dihydrogen phosphate (KDP, KH2PO4). First-principles calculations suggest that the band gap widening is related to modified band-edge electronic states, whereas large birefringence is associated with anisotropic lone-pair-active Sb3+ and Te4+ units. These findings highlight the potential of fluorination-associated structural regulation for improving UV transparency while preserving large birefringence in antimony tellurite-based crystals.