Hg₄(Te₂O₅)(SO₄): A Giant Birefringent Sulfate Crystal Triggered by a Highly Selective Cation

Sulfate crystals are often criticized for their low birefringence. The small anisotropic SO₄ group is becoming the biggest bottleneck hindering the application of sulfates in optical functional materials. In this study, we report a new method to significantly enhance the birefringence of sulfates. The title compound increases the birefringence recording of sulfates to 0.542@546 nm, which is significantly larger than that of the commercial birefringent crystal of TiO₂ (0.306@546.1 nm). At the infrared wavelength, the birefringence of Hg₄(Te₂O₅)(SO₄) can be up to 0.400@1064 nm, which is also much larger than the infrared birefringent crystal of YVO₄ (0.209@1064 nm). In addition, it also has a wide transparency range, high thermal stability, and excellent environmental stability, making it a potential birefringent material. Hg₄(Te₂O₅)(SO₄) features a novel two-dimensional layered structure composed of [Hg₄(Te₂O₅)]²⁺ layers separated by isolated (SO₄)^2- tetrahedra. This compound was designed by introducing a highly selective cation in a tellurite sulfate system. The low valence low coordination cations connect with tellurite groups only, making the sulfate isolated in the structure. The steric repulsive action of the isolated SO₄ tetrahedra may regulate the linear and lone pair groups arranged in a way that favors large birefringence. This method can be proven by theoretical calculations. PAWED studies showed that the large birefringence originated from the synergistic effect of (Hg₂O₂)^2-, (Te₂O₅)^2-, and (SO₄)^2- units, with a contribution ratio of 42.17, 37.92, and 19.88%, respectively. Our work breaks the limitation of low birefringence in sulfates and opens up new possibilities for their application as birefringent crystals.