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, Sb(TeO)(SO)-1̅ (I) and Sb(TeO)(SO)-2/ (II), synthesized using a dual lone pair strategy that incorporates Sb and Te 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 Sb and Te, 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.