Sb2(TeO4)(SO4): An Anhydrous Tellurite Sulfate with Large Birefringence and Enhanced Thermal Stability

The birefringence of birefringent materials is positively correlated with structural anisotropy. By introducing 5s2-electron-containing Sb3+ and Te4+ cations into a sulfate system, the anhydrous antimony tellurite sulfate Sb2(TeO4)(SO4) was obtained. Due to the stereochemically active lone pairs (SCALPs), both Sb3+ and Te4+ cations are four-coordinated with O atoms to form the distorted seesaw-shaped units. Benefiting from the strong polarizability anisotropy of the distorted polyhedra, Sb2(TeO4)(SO4) exhibits a large birefringence, as well as wide transparency range and high thermal stability, making the compound a potential UV birefringent material. Structural and theoretical calculations indicate that the optical properties mainly originate from the highly distorted SO4 and TeO4 units and their uniform alignment. This work will provide some useful insights into the development of SCALP-cation-based birefringent crystals.