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 5<i>s</i>²-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.