Dual-anion synergy drives remarkable birefringence in sulfate–nitrate crystals

Abstract

Wide-bandgap birefringent crystals are essential for polarization control in advanced optical systems; however, conventional sulfates exhibit inherently weak birefringence. Inspired by the strong anisotropic polarizability of the nitrate group and the wide bandgap characteristics of the sulfate group, we revisited the naturally occurring sulfate–nitrate hybrid mineral Darapskite, Na₃(NO₃)(SO₄)·H₂O. High-quality crystals were synthesized, and their optical properties were systematically investigated. This compound exhibits a notable birefringence of 0.14 at 546 nm, which is significantly higher than that of typical sulfate materials, while maintaining broad transparency and chemical stability. This study demonstrates the effectiveness of nature-derived structural prototypes in guiding the discovery of birefringent materials and highlights the potential of NO₃/SO₄ hybrid frameworks for ultraviolet (UV) optical materials.