NaSb3O2(SO4)3·H2O: A New Alkali-Metal Antimony(III) Sulfate with a Unique Sb6O20H4 Unit and Moderate Birefringence

A new alkali-metal antimony(III) sulfate, NaSb3O2(SO4)3·H2O, has been discovered by introducing stereochemical active lone pair (SCALP) Sb3+ cations into sulfates. It exhibits a three-dimensional structure established from unique {Sb6} clusters and SO4 tetrahedra with Na+ ions located on the voids. A diffuse reflection spectrum shows that NaSb3O2(SO4)3·H2O possesses an experimental band gap of 3.80 eV. Theoretical calculation was executed to explain the structure–property relationships. The result reveals that NaSb3O2(SO4)3·H2O has a direct band gap with a theoretical value of 3.91 eV. Importantly, it also shows that the Sb3+ ions are stereochemically active, providing the main contribution to the birefringence. The work indicates that the introduction of SCALP cations in microscopic structures is an effective approach for exploring the new superior performing birefringent materials.