Dramatically improved optical anisotropy by realizing stereochemically active lone pairs in a sulfate system, K2SO4·HIO3

Abstract

Obtaining sulfate-based materials with large birefringence is of great challenge due to the weak polarizability anisotropy of the constituting spherical SO₄²⁻ tetrahedron. Here, we have implemented a polyhedron of stereoactive lone pair cations, IO₃⁻ to the system to tackle the issue. Centimetre-sized single crystals of an alkali metal sulfate iodate, K₂SO₄·HIO₃, have been readily grown by an accessible solution–evaporation method. Optical measurements on K₂SO₄·HIO₃ single crystals reveal that the reported material exhibits a 70-fold increase in birefringence (ca. 0.14 at 589.3 nm) compared to that of the singular sulfate, K₂SO₄. In addition, K₂SO₄·HIO₃ exhibits a blue-shifted UV cutoff edge of 275 nm, 25 nm shorter than that of α-HIO₃, highlighting the key role of the SO₄²⁻ tetrahedron in broadening the transmittance window.