Design and Synthesis of Two Sn-Centered Mixed Halide Crystals with Enhanced Birefringence

Enhancing the optical anisotropy of compounds has attracted significant interest in the optical field. Sn-centered crystals, containing stereochemically active lone pairs, are widely regarded as promising birefringent materials. In this study, we successfully synthesized two novel Sn-centered mixed halide birefringent crystals, NaSn₂F₄Br and Na₂Sn₂F₅I. These crystals, composed of Sn-centered mixed halide polyhedra, exhibit more than a 2-fold increase in birefringence from NaSn₂F₄Br to Na₂Sn₂F₅I. Among all known Sn-centered halides, the Na₂Sn₂F₅I crystal demonstrates the highest birefringence (Δn = 0.408@546 nm). Theoretical calculations indicate that the exceptional optical properties of Na₂Sn₂F₅I arise from the large polarization anisotropy induced by the Sn-I bond within the Sn-centered polyhedra. These findings provide valuable insights for the development of high-quality birefringent crystal materials in Sn-centered mixed halide systems.