Dual Lone Pair Strategy for High Birefringence in Antimony(III)-Based Tellurite Sulfates with Short-Wave UV Transparency

We present two novel antimony(III)-based tellurite sulfate crystals, Sb2(TeO3)(SO4)2-P1̅ (I) and Sb2(TeO3)(SO4)2-P21/c (II), synthesized using a dual lone pair strategy that incorporates Sb3+ and Te4+ ions into a sulfate framework. This approach significantly enhances the birefringence of these compounds, with values of 0.11 and 0.10 at 546 nm, respectively, surpassing those of many previously reported mixed alkali metal/alkaline earth metal sulfates and tellurites. Both compounds also exhibit excellent UV transparency with cutoff edges at 292 and 294 nm, making them promising candidates for short-wave UV optical applications. Structural and theoretical analyses reveal that the enhanced birefringence arises from the synergistic effects of the stereochemically active lone pairs of Sb3+ and Te4+, which contribute to the optical anisotropy. This work highlights the potential of dual SCALP elements in improving the optical properties of sulfate-based crystals and provides new insights into designing high-performance optical materials with tailored properties for advanced UV technologies.