Sn ⁴⁺ /Sn ²⁺ Aliovalent Substitution: A Strategy for Local Structure Engineering to Enhance Birefringence and Trigger Structural Symmetry Transition

Aliovalent substitution involving cations with lone pair electrons is a powerful strategy for designing novel optical materials with enhanced properties. Herein, by substituting Sn⁴⁺ with stereochemically active Sn²⁺ in the α/β-LiSn₂(PO₄)₃ parent structure, we successfully synthesized α/β-Li₂SnP₂O₇ pyrophosphates, achieving a remarkable 5.5 time enhancement in birefringence. This oxidation-state-driven transformation converts the symmetric [SnO₆] octahedra in α/β-LiSn₂(PO₄)₃ into highly distorted [SnO₄] tetrahedra in α/β-Li₂SnP₂O₇. This conversion leads to the bond length distortion index boosting (from 9.42/9.95 × 10⁻³ to 47.17/49.21 × 10⁻³) and the bond angle varying (from 26.97/29.65 deg² to 1054.2/1122.1 deg²), which triggers the [SnP₆O₂₄] clusters into [SnP₂O₉] three-membered rings, repectively. The increased distortion of Sn-O groups and the decreased symmetry of Sn-P-O blocks enhance the optical anisotropy of the compounds effectively. This aliovalent substitution strategy offers a valuable gudience for amplifying the intrinsically low birefringence of phosphates.